研究者データベース

太田 裕道(オオタ ヒロミチ)
電子科学研究所 物質科学研究部門
教授

基本情報

所属

  • 電子科学研究所 物質科学研究部門

職名

  • 教授

学位

  • 修士(工学)(名古屋大学)
  • 博士(工学)(東京工業大学)

ホームページURL

科研費研究者番号

  • 80372530

J-Global ID

プロフィール

  • 材料研究には大きく分けて二通りの研究パターンがあります。一つはコツコツ長い時間をかけて新物質を探す探索研究であり、もう一つは既存の物質を「材料」として使える品質にするプロセス研究です。家具職人の父が家具を製作するプロセスを幼少の頃からすぐ近くで見てきた影響なのか、私は良い商品を作るためのプロセス研究のほうが得意なようです。現在は、原子レベルで平坦な高品質の薄膜にすることで機能性酸化物のポテンシャルを最大限引き出し、世の中で役に立つデバイス開発を目指して研究しています。


    主要論文


    [1] Y. Zhang, H. Ohta et al., Nature Communications 9, 2224 (2018).


    [2] H. Jeen, H. Ohta, H-N. Lee et al., Nature Materials 12, 1057 (2013).


    [3] H. Ohta et al., Nature Communications 1, 118 (2010).


    [4] H. Ohta et al., Nature Materials 6, 129 (2007).


    [5] K. Nomura, H. Ohta, H. Hosono et al., Nature 432, 488 (2004).


    [6] K. Nomura, H. Ohta, H. Hosono et al., Science 300, 1269 (2003).

研究キーワード

  • 薄膜トランジスタ   電界効果トランジスタ   電子デバイス   単結晶基板   パルスレーザー堆積   熱電変換   エピタキシャル薄膜   金属酸化物   無機材料   無機工業薬品   

研究分野

  • ナノテク・材料 / 無機材料、物性 / 電気伝導性セラミックス
  • ナノテク・材料 / 薄膜、表面界面物性 / 酸化物エレクトロニクス
  • ナノテク・材料 / 応用物性 / 熱電変換

職歴

  • 2012年09月 - 現在 北海道大学 電子科学研究所 教授
  • 2003年10月 - 2012年08月 名古屋大学大学院工学研究科 助教授/准教授
  • 2008年10月 - 2012年03月 科学技術振興機構さきがけ「ナノ製造技術の探索と展開」領域研究員(兼務)
  • 1999年11月 - 2004年09月 科学技術振興機構(JST) 創造科学技術推進事業(ERATO)細野透明電子活性プロジェクト
  • 1998年01月 - 2003年09月 HOYA株式会社
  • 1996年04月 - 1997年12月 三洋電機株式会社

学歴

  • 1998年10月 - 2001年09月   東京工業大学   大学院総合理工学研究科   物質科学創造専攻
  • 1994年04月 - 1996年03月   名古屋大学   大学院工学研究科   応用化学専攻
  • 1990年04月 - 1994年03月   埼玉大学   工学部   応用化学科

所属学協会

  • 電気化学会   Materials Research Society   日本熱電学会   日本セラミックス協会   応用物理学会   

研究活動情報

論文

  • Ichiro Terasaki, Isuzu Sano, Kosuke Toda, Shuji Kawasaki, Akitoshi Nakano, Hiroki Taniguchi, Hai Jun Cho, Hiromichi Ohta, Fumihiko Nakamura
    Journal of the Physical Society of Japan 89 9 093707 - 093707 2020年09月15日 [査読有り][通常論文]
  • Fabian Krahl, Yuzhang Wu, Hai Jun Cho, Maarit Karppinen, Hiromichi Ohta
    Advanced Electronic Materials 6 2000404 - 2000404 2020年09月11日 [査読有り][通常論文]
  • Gowoon Kim, Bin Feng, Yu-Miin Sheu, Hai Jun Cho, Yuichi Ikuhara, Hiromichi Ohta
    ACS Applied Electronic Materials 2 8 2507 - 2513 2020年08月25日 [査読有り][通常論文]
  • Qian Yang, Joonhyuk Lee, Bin Feng, Yuichi Ikuhara, Gowoon Kim, Hai Jun Cho, Hyoungjeen Jeen, Hiromichi Ohta
    ACS Applied Electronic Materials 2 7 2250 - 2256 2020年07月28日 [査読有り][通常論文]
  • Mian Wei, Lizhikun Gong, Dou‐dou Liang, Hai Jun Cho, Hiromichi Ohta
    Advanced Electronic Materials 6 7 2000100 - 2000100 2020年07月 [査読有り][通常論文]
  • Kohei Fujiwara, Miho Kitamura, Daisuke Shiga, Yasuhiro Niwa, Koji Horiba, Tsutomu Nojima, Hiromichi Ohta, Hiroshi Kumigashira, Atsushi Tsukazaki
    Chemistry of Materials 32 12 5272 - 5279 2020年06月23日 [査読有り][通常論文]
  • M. Timpel, M. V. Nardi, B. Wegner, G. Ligorio, L. Pasquali, M. Pätzel, S. Hecht, H. Ohta, N. Koch
    Adv. Mater. Interfaces 1902114  2020年05月04日 [査読有り][通常論文]
  • Dou-dou Liang, Yu-qiao Zhang, Hai Jun Cho, Hiromichi Ohta
    Appl. Phys. Lett. 116 143503  2020年04月08日 [査読有り][通常論文]
  • Effect of lattice distortions on the electron and thermal transport properties of transparent oxide semiconductor Ba1-xSrxSnO3 solid solution films
    Hai Jun Cho, Koichi Sato, Mian Wei, Gowoon Kim, Hiromichi Ohta
    J. Appl. Phys. 127 115701  2020年03月 [査読有り][通常論文]
  • Thermoelectric properties of a semicrystalline polymer doped beyond the insulator-to-metal transition by electrolyte gating
    Hisaaki Tanaka, Kaito Kanahashi, Naoya Takekoshi, Hiroaki Mada, Hiroshi Ito, Yukihiro Shimoi, Hiromichi Ohta, Taishi Takenobu
    Science Adv. 6 eaay8065  2020年02月14日 [査読有り][通常論文]
  • Arbitrary control of the diffusion potential between a plasmonic metal and a semiconductor by an angstrom-thick interface dipole layer
    Tomoya Oshikiri, Hiroki Sawayanagi, Keisuke Nakamura, Kosei Ueno, Takayoshi Katase, Hiromichi Ohta, Hiroaki Misawa
    J. Chem. Phys. 152 934705  2020年01月17日 [査読有り][通常論文]
  • Takaki Onozato, Hai Jun Cho, Hiromichi Ohta
    Japanese Journal of Applied Physics 59 024002  2020年01月16日 [査読有り][通常論文]
  • Current-induced effective magnetic field in La0.67Sr0.33MnO3/LaAlO3/SrTiO3 structures
    Michihiko Yamanouchi, Tatsuro Oyamada, Hiromichi Ohta
    AIP Advances 10 015129  2020年01月14日 [査読有り][通常論文]
  • Mian Wei, Anup V. Sanchela, Bin Feng, Yuichi Ikuhara, Hai Jun Cho, Hiromichi Ohta
    Applied Physics Letters 116 022103  2020年01月13日 [査読有り][通常論文]
  • Phase Instability amid Dimensional Crossover in Artificial Oxide Crystal
    Seung Gyo Jeong, Taewon Min, Sungmin Woo, Jiwoong Kim, Yu-Qiao Zhang, Seong Won Cho, Jaeseok Son, Young-Min Kim, Jung Hoon Han, Sungkyun Park, Hu Young Jeong, Hiromichi Ohta, Suyoun Lee, Tae Won Noh, Jaekwang Lee, Woo Seok Choi, Seung Gyo Jeong, Taewon Min, Sungmin Woo, Jiwoong Kim, Yu-Qiao Zhang, Seong Won Cho, Jaeseok Son, Young-Min Kim, Jung Hoon Han, Sungkyun Park, Hu Young Jeong, Hiromichi Ohta, Suyoun Lee, Tae Won Noh, Jaekwang Lee* and Woo Seok Choi*
    Phys. Rev. Lett. 124 026401  2020年01月13日 [査読有り][通常論文]
  • Hai Jun Cho, Yugo Takashima, Yukio Nezu, Takaki Onazato, Hiromichi Ohta
    Advanced Materials Interfaces 7 1901816  2020年01月01日 [査読有り][通常論文]
  • Ferroelectric oxide thin film with an out-of-plane electrical conductivity
    Tingting Yao, Yixiao Jiang, Chunlin Chen, Xuexi Yan, Ang Tao, Lixin Yang, Chuihong Li, Kenyu Sugo, Hiromichi Ohta, Hengqiang Ye, Yuichi Ikuhara, Xiuliang Ma, Tingting Yao, Yixiao Jiang, Chunlin Chen*, Xuexi Yan, Ang Tao, Lixin Yang, Chuihong Li, Kenyu Sugo, Hiromichi Ohta, Hengqiang Ye, Yuichi Ikuhara, and Xiuliang Ma*
    Nano Lett. 2019年12月26日 [査読有り][通常論文]
  • Qian Yang, Hai Jun Cho, Hyoungjeen Jeen, Hiromichi Ohta
    Advanced Materials Interfaces 6 1901260 - 1901260 2019年12月22日 [査読有り][通常論文]
  • Surface charge accumulation and electrochemical protonation of transition metal oxides using water-infiltrated nanoporous glass
    Takayoshi Katase, Hiromichi Ohta
    Semiconductor Science and Technology 34 123001  2019年11月15日 [査読有り][招待有り]
  • Giant power factors in p- and n-type large-area graphene films on a flexible plastic substrate
    Kaito Kanahashi, Masatou Ishihara, Masataka Hasegawa, Hiromichi Ohta, Taishi Takenobu, Kaito Kanahashi, Masatou Ishihara, Masataka Hasegawa, Hiromichi Ohta, and Taishi Takenobu*
    npj 2D Mater. Appl. 3 44  2019年11月08日 [査読有り][通常論文]
  • High electron and thermal transport properties of La-doped BaSnO3 films fabricated under ozone atmosphere
    Hai Jun Cho, Bin Feng, Takaki Onozato, Mian Wei, Anup Sanchela, Yuichi Ikuhara, Hiromichi Ohta, Hai Jun Cho*, Bin Feng, Takaki Onozato, Mian Wei, Anup Sanchela, Yuichi Ikuhara, and Hiromichi Ohta*
    Phys. Rev. Materials 3 094601  2019年09月 [査読有り][通常論文]
  • Yuqiao Zhang, Kenyu Sugo, Hai Jun Cho, Hiromichi Ohta
    J. Appl. Phys. 126 075104  2019年08月21日 [査読有り][通常論文]
  • Electrical, optical and thermal transport properties of oxygen deficient amorphous WOx (2.5 < x < 3) films
    Gowoon Kim, Hai Jun Cho, Yu-Miin Sheu, Hiromichi Ohta
    The Journal of Physical Chemistry C 123 15419  2019年05月 [査読有り][通常論文]
  • Oxide-based optical, electrical and magnetic properties switching devices with water-incorporated gate insulator
    Takayoshi Katase, Hiromichi Ohta
    Japanese Journal of Applied Physics 58 090501  2019年04月11日 [査読有り][招待有り]
  • Room-temperature ferroelectric ferromagnet in 1D tetrahedral chain network
    Kyeong Tae Kang, Chang Jae Roh, Jinyoung Lim, Taewon Min, Jun Han Lee, Kyoungjun Lee, Tae Yoon Lee, Seunghun Kang, Daehee Seol, Jiwoong Kim, Hiromichi Ohta, Amit Khare, Sungkyun Park, Yunseok Kim, Seung Chul Chae, Yoon Seok Oh, Jaekwang Lee, Jaejun Yu, Jong Seok Lee, Woo Seok Choi
    Adv. Mater. 31 1808104  2019年04月 [査読有り][通常論文]
  • Electron sandwich doubles the thermoelectric power factor of SrTiO3
    Yuqiao Zhang, Hiromichi Ohta
    Phys. Status Solidi A 1800832  2019年03月 [査読有り][招待有り]
  • Thermal conductivity tensor of NbO2
    Hai Jun Cho, Gowoon Kim, Takaki Onozato, Hyoungjeen Jeen, Hiromichi Ohta
    International Journal of Heat and Mass Transfer 137 263  2019年03月 [査読有り][通常論文]
  • Peculiar magnetotransport properties in La0.67Sr0.33MnO3/LaAlO3/SrTiO3
    Michihiko Yamanouchi, Tatsuro Oyamada, Hiromichi Ohta
    AIP Advances 9 035129  2019年03月 [査読有り][通常論文]
  • Current-induced modulation of coercive field in the ferromagnetic oxide SrRuO3
    Michihiko Yamanouchi, Tatsuro Oyamada, Koichi Sato, Hiromichi Ohta, Jun’ichi Ieda
    IEEE Transactions on Magnetics 1400604  2019年02月 [査読有り][通常論文]
  • 薄い電子層を絶縁体でサンドイッチ:熱電変換特性を高める方法
    太田裕道, 張 雨橋
    車載テクノロジー 6 5 38  2019年02月 [査読無し][招待有り]
  • Fast operation of a WO3-based solid-state electrochromic transistor
    Takaki Onozato, Yukio Nezu, Hai Jun Cho, Hiromichi Ohta
    AIP Advances 9 025122  2019年02月 [査読有り][通常論文]
  • Formation of environmentally stable hole-doped graphene films: Instantaneous and high-density carrier doping by a boron-based oxidant
    Kaito Kanahashi, Naoki Tanaka, Yoshiaki Shoji, Mina Maruyama, Il Jeon, Kenji Kawahara, Masatou Ishihara, Masataka Hasegawa, Hiromichi Ohta, Hiroki Ago, Yutaka Matsuo, Susumu Okada, Takanori Fukushima, Taishi Takenobu
    npj 2D Materials and Applications 3 7  2019年02月 [査読有り][通常論文]
  • Anup Sanchela, Mian Wei, Joonhyuk Lee, Gowoon Kim, Hyoungjeen Jeen, Bin Feng, Yuichi Ikuhara, Hai Jun Cho, Hiromichi Ohta
    Journal of Materials Chemistry C 7 5797 - 5802 2019年 [査読有り][通常論文]
     

    Hall mobility of La-doped BaSnO3 films was improved without any buffer layers if the films are grown under O3 atmospheres.

  • Thermopower modulation clarification of the operating mechanism in wide bandgap BaSnO3−SrSnO3 solid-solution based thin film transistors
    Anup V. Sanchela, Mian Wei, Hai Jun Cho, Hiromichi Ohta
    Small 15 1805394  2019年01月 [査読有り][通常論文]
  • Effects of vacuum annealing on the electron mobility of epitaxial La-doped BaSnO3 films
    Hai Jun Cho, Takaki Onozato, Mian Wei, Anup Sanchela, Hiromichi Ohta
    APL Mater. 7 022507  2018年12月 [査読有り][通常論文]
  • Extremely light carrier effective mass in a distorted simple metal oxide
    Gowoon Kim, Yu-Qiao Zhang, Taewon Min, Hoyoung Suh, Jae Hyuck Jang, Hyeonjun Kong, Joonhyuk Lee, Jaekwang Lee, Tae-Yeol Jeon, Inwon Lee, Jinhyung Cho, Hiromichi Ohta, Hyoungjeen Jeen
    Adv. Electron. Mater. 5 1800504  2018年12月 [査読有り][通常論文]
  • Oxygen vacancies allow tuning the work function of vanadium dioxide
    Rongbin Wang, Takayoshi Katase, Ke-Ke Fu, Tianshu Zhai, Jiacheng Yang, Qiankun Wang, Hiromichi Ohta, Norbert Koch, Steffen Duhm
    Adv. Mater. Interfaces 2018年10月 [査読有り][通常論文]
  • Crystallographic orientation – surface energy – wetting property relationships of rare earth oxides
    太Jason Tam, Bin Feng, Yuichi Ikuhara, Hiromichi Ohta, Uwe Erb
    J. Mater. Chem. A 2018年09月 [査読有り][通常論文]
  • Double thermoelectric power factor of a 2D electron system
    Yuqiao Zhang, Bin Feng, Hiroyuki Hayashi, Cheng-Ping Chang, Yu-Miin Sheu, Isao Tanaka, Yuichi Ikuhara, Hiromichi Ohta
    Nature Communications 9 2224  2018年07月 [査読有り][通常論文]
  • Anup V. Sanchela, Mian Wei, Haruki Zensyo, Bin Feng, Joonhyuk Lee, Gowoon Kim, Hyoungjeen Jeen, Yuichi Ikuhara, Hiromichi Ohta
    Applied Physics Letters 112 23 232102  2018年06月04日 [査読有り][通常論文]
     
    In this study, we report that the carrier mobility of 2%-La-doped BaSnO3 (LBSO) films on (001) SrTiO3 and (001) MgO substrates strongly depends on the thickness, whereas it is unrelated to the film/substrate lattice mismatch (+5.4% for SrTiO3 and -2.3% for MgO). The films exhibited large differences in lattice parameters, lateral grain sizes (∼85 nm for SrTiO3 and ∼20 nm for MgO), surface morphologies, threading dislocation densities, and misfit dislocation densities. However, the mobility dependences on the film thickness in both cases were almost the same, saturating at ∼100 cm2 V-1 s-1, while the charge carrier densities approached the nominal carrier concentration (=[2% La3+]). Our study clearly indicates that the carrier mobility of LBSO films strongly depends on the thickness. These results would be beneficial for understanding the carrier transport properties and fruitful to further enhance the mobility of LBSO films.
  • Plasmon-Assisted Polarity Switching of a Photoelectric Conversion Device by UV and Visible Light Irradiation
    Keisuke Nakamura, Tomoya Oshikiri, Kosei Ueno, Takayoshi Katase, Hiromichi Ohta, Hiroaki Misawa
    J. Phys. Chem. C 122 25 14064 - 14071 2018年04月 [査読有り][通常論文]
  • Keke Fu, Rongbin Wang, Takayoshi Katase, Hiromichi Ohta, Norbert Koch, Steffen Duhm
    ACS Applied Materials and Interfaces 10 12 10552 - 10559 2018年03月28日 [査読有り][通常論文]
     
    Using photoemission spectroscopy, we show that the surface electronic structure of VO2 is determined by the temperature-dependent metal-insulator phase transition and the density of oxygen vacancies, which depends on the temperature and ultrahigh vacuum (UHV) conditions. The atomically clean and stoichiometric VO2 surface is insulating at room temperature and features an ultrahigh work function of up to 6.7 eV. Heating in UHV just above the phase transition temperature induces the expected metallic phase, which goes in hand with the formation of oxygen defects (up to 6% in this study), but a high work function > 6 eV is maintained. To demonstrate the suitability of VO2 as hole injection contact for organic semiconductors, we investigated the energy-level alignment with the prototypical organic hole transport material N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB). Evidence for strong Fermi-level pinning and the associated energy-level bending in NPB is found, rendering an Ohmic contact for holes.
  • Hirofumi Matsuoka, Kaito Kanahashi, Naoki Tanaka, Yoshiaki Shoji, Lain-Jong Li, Jiang Pu, Hiroshi Ito, Hiromichi Ohta, Takanori Fukushima, Taishi Takenobu
    Japanese Journal of Applied Physics 57 2 02CCB15  2018年02月01日 [査読有り][通常論文]
     
    Hole carrier doping into single-crystalline transition metal dichalcogenide (TMDC) films can be achieved with various chemical reagents. However, large-area polycrystalline TMDC monolayers produced by a chemical vapor deposition (CVD) growth method have yet to be chemically doped. Here, we report that a salt of a two-coordinate boron cation, Mes2B+ (Mes: 2,4,6-trimethylphenyl group), with a chemically stable tetrakis(pentafluorophenyl)borate anion, [(C6F5)4B]-, can serve as an efficient hole-doping reagent for large-area CVD-grown tungsten diselenide (WSe2) films. Upon doping, the sheet resistance of large-area polycrystalline WSe2 monolayers decreased from 90 GΩ/sq to 3.2 kΩ/sq.
  • Hiromichi Ohta, Hidenori Hiramatsu
    Nanoinformatics 207 - 235 2018年01月15日 [査読有り][通常論文]
     
    Regions of a few nanometers at the surface or interface of a material exhibit various functional properties, which differ from those of the bulk because the electrons and/or ions receive different potentials due to the incoherent atomic arrangement. High-quality epitaxial films of functional materials called "nanolayers" are important to utilize such functional properties. However, fabrication of high-quality nanolayers of complex materials with complicated crystal structures is usually challenging due to the difference in the thermochemical properties of the constituents. In this chapter, epitaxial growth techniques, especially "reactive solid-phase epitaxy" of functional oxides and chalcogenides, are reviewed based on the authors' efforts. Additionally, this chapter reviews several modulation methods of optical, electrical, and magnetic properties of functional oxide nanolayers.
  • Hiromichi Ohta, Sung Wng Kim, Shota Kaneki, Atsushi Yamamoto, Tamotsu Hashizume
    Advanced Science 5 1 2018年01月01日 [査読有り][通常論文]
     
    Thermoelectric conversion is an energy harvesting technology that directly converts waste heat from various sources into electricity by the Seebeck effect of thermoelectric materials with a large thermopower (S), high electrical conductivity (σ), and low thermal conductivity (κ). State-of-the-art nanostructuring techniques that significantly reduce κ have realized high-performance thermoelectric materials with a figure of merit (ZT = S2∙σ∙T∙κ−1) between 1.5 and 2. Although the power factor (PF = S2∙σ) must also be enhanced to further improve ZT, the maximum PF remains near 1.5–4 mW m−1 K−2 due to the well-known trade-off relationship between S and σ. At a maximized PF, σ is much lower than the ideal value since impurity doping suppresses the carrier mobility. A metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) structure on an AlGaN/GaN heterostructure is prepared. Applying a gate electric field to the MOS-HEMT simultaneously modulates S and σ of the high-mobility electron gas from −490 µV K−1 and ≈10−1 S cm−1 to −90 µV K−1 and ≈104 S cm−1, while maintaining a high carrier mobility (≈1500 cm2 V−1 s−1). The maximized PF of the high-mobility electron gas is ≈9 mW m−1 K−2, which is a two- to sixfold increase compared to state-of-the-art practical thermoelectric materials.
  • Amit Khare, Jaekwang Lee, Jaeseoung Park, Gi-Yeop Kim, Si-Young Choi, Takayoshi Katase, Seulki Roh, Tae Sup Yoo, Jungseek Hwang, Hiromichi Ohta, Junwoo Son, Woo Seok Choi
    ACS Applied Materials and Interfaces 10 5 4831 - 4837 2018年 [査読有り][通常論文]
     
    Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO2.5 are stabilized along two crystallographic directions of the epitaxial thin films. The distinctively orientated phases are found to be highly stable and exhibit a considerable difference in their electronic structures and optical properties, which could be understood in terms of orbital anisotropy. The control of the OVC orientation further leads to modifications in the hydrogenation of the BM-SrFeO2.5 thin films. The results demonstrate a strong correlation between crystallographic orientations, electronic structures, and ionic motion in the BM structure.
  • High thermoelectric power factor of high-mobility two-dimensional electron gas
    Hiromichi Ohta, Sung Wng Kim, Shota Kaneki, Atsushi Yamamoto, Tamotsu Hashizume
    Adv. Sci. 4 1700696-1 - 1700696-6 2017年11月 [査読有り][通常論文]
  • Yukio Nezu, Yu-Qiao Zhang, Chunlin Chen, Yuichi Ikuhara, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 122 13 135305  2017年10月 [査読有り][通常論文]
     
    High-quality epitaxial films of a ferroelectric oxide Sr2Nb2O7 were successfully fabricated by solid phase epitaxy (SPE) on (110) LaAlO3 single crystal substrates. In the SPE method, amorphous Sr-Nb-O films are first deposited by pulsed laser deposition at room temperature and then annealed in vacuum at elevated temperatures, resulting in the crystallization of Sr2Nb2O7 with highly ordered atomic arrangement and an atomically flat surface. The refractive index of the resultant film was 2.1, indicating that the dielectric permittivity of the film was in between 20 and 80, which corresponds well with that of single crystal Sr2Nb2O7, demonstrating the effectiveness of the SPE method for the fabrication of high-quality epitaxial films of Sr2Nb2O7. Published by AIP Publishing.
  • Takayoshi Katase, Yuki Suzuki, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 122 13 135303  2017年10月 [査読有り][通常論文]
     
    The electrochemical switching of SrCoOx-based non-volatile memory with a thin-film-transistor structure was examined by using liquid-leakage-free electrolytes with different conductivities (sigma) as the gate insulator. We first examined leakage-free water, which is incorporated in the amorphous (a-) 12CaO center dot 7Al(2)O(3) film with a nanoporous structure (Calcium Aluminate with Nanopore), but the electrochemical oxidation/reduction of the SrCoOx layer required the application of a high gate voltage (V-g) up to 20V for a very long current-flowing-time (t) similar to 40min, primarily due to the low sigma [2.0 x 10(-8) S cm(-1) at room temperature (RT)] of leakage-free water. We then controlled the r of the leakage-free electrolyte, infiltrated in the alpha-NaxTaO(3) film with a nanopillar array structure, from 8.0 x 10(-8) S cm(-1) to 2.5 x 10(-6) S cm(-1) at RT by changing the x = 0.01-1.0. As the result, the t, required for the metallization of the SrCoOx layer under small Vg = -3V, becomes two orders of magnitude shorter with increase of the r of the a-NaxTaO3 leakage-free electrolyte. These results indicate that the ion migration in the leakage-free electrolyte is the rate-determining step for the electrochemical switching, compared to the other electrochemical process, and the high r of the leakage-free electrolyte is the key factor for the development of the non-volatile SrCoOx-based electro-magnetic phase switching device. Published by AIP Publishing.
  • Jae-Yeol Hwang, Young-Min Kim, Kyu Hyoung Lee, Hiromichi Ohta, Sung Wng Kim
    NANO LETTERS 17 10 6140 - 6145 2017年10月 [査読有り][通常論文]
     
    Demands on high-quality layer structured two-dimen sional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.
  • Amit Khare, Dongwon Shin, Tae Sup Yoo, Minu Kim, Tae Dong Kang, Jaekwang Lee, Seulki Roh, In-Ho Jung, Jungseek Hwang, Sung Wng Kim, Tae Won Noh, Hiromichi Ohta, Woo Seok Choi
    ADVANCED MATERIALS 29 37 1606566  2017年10月 [査読有り][通常論文]
     
    Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeOx epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeOx thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.
  • Shao-Pin Chiu, Michihiko Yamanouchi, Tatsuro Oyamada, Hiromichi Ohta, Juhn-Jong Lin
    PHYSICAL REVIEW B 96 8 085143  2017年08月 [査読有り][通常論文]
     
    Epitaxial La2/3Sr1/3MnO3 (LSMO) films have been grown on SrTiO3 (001) substrates via pulsed laser deposition. In a 22-nm-thick LSMO film with a low residual resistivity of rho(0) approximate to 59 mu Omega cm, we found a zero-field dip in the magnetoresistance (MR) below 10 K, manifesting the weak antilocalization (WAL) effect due to strong spin-orbit coupling (SOC). We have analyzed the MR data by including the D'yakonov-Perel' spin-relaxation mechanism in the WAL theory. We explain that the delocalized spin-down electron sub-band states play a crucial role for facilitating marked SOC in clean LSMO. Moreover, we find that the SOC strength and gate voltage tunability are similar to those in a two-dimensional electron gas at the LaAlO3/SrTiO3 interface, indicating the presence of an internal electric field near the LSMO/SrTiO3 interface. In a control measurement on a 5-nm-thick high resistivity (rho(0) approximate to 280 mu Omega cm) LSMO film, we observe only a small zero-field peak in MR from weak localization effect, indicating negligible SOC.
  • Anup V. Sanchela, Takaki Onozato, Bin Feng, Yuichi Ikuhara, Hiromichi Ohta
    PHYSICAL REVIEW MATERIALS 1 3 034603  2017年08月 [査読有り][通常論文]
     
    The exact intrinsic carrier effective mass m* of a well-studied transparent oxide semiconductor BaSnO3 is unknown because the reported m* values are scattered from 0.06m(0) to 3.7m(0). This paper identifies the intrinsic m* of BaSnO3, m* = 0.40 +/- 0.01m(0), by the thermopower modulation clarification method and determines the threshold of the degenerate/nondegenerate semiconductor. At the threshold, the thermopower values of both the La-doped BaSnO3 and BaSnO3 thin-film transistor structures are 240 mu V K-1, the bulk carrier concentration is 1.4 x 10(19) cm(-3), and the two-dimensional sheet carrier concentration is 1.8 x 10(12) cm(-2). When the Fermi energy E-F is located above the parabolic shaped conduction band bottom, the mobility is rather high. In contrast, E-F below the threshold exhibits a very low carrier mobility, most likely because the tail states suppress the carrier mobility. The present results are useful to further develop BaSnO3-based oxide electronics.
  • Yuqiao Zhang, Bin Feng, Hiroyuki Hayashi, Tetsuya Tohei, Isao Tanaka, Yuichi Ikuhara, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 121 18 185102-1 - 185102-7 2017年05月 [査読有り][通常論文]
     
    Here, we present a thermoelectric phase diagram for the SrTi1-xNbxO3 (0.05 <= x <= 1) solid solution system, which we derived from the characterization of epitaxial films. We observed two thermoelectric phase boundaries in the system, which originate from the step-like decrease in carrier effective mass at x similar to 0.3 and from a local minimum in carrier relaxation time at x similar to 0.5. The origins of these phase boundaries are considered to be related to isovalent/heterovalent B-site substitution: parabolic Ti 3d orbitals dominate electron conduction for compositions with x < 0.3, whereas the Nb 4 d orbital dominates when x > 0.3. At x similar to 0.5, a tetragonal distortion of the lattice, in which the B-site is composed of Ti4+ and Nb4+ ions, leads to the formation of tail-like impurity bands, which maximizes the electron scattering. These results provide a foundation for further research into improving the thermoelectric performance of SrTi1-xNbxO3. Published by AIP Publishing.
  • Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    APL MATERIALS 5 5 056105  2017年05月 [査読有り][通常論文]
     
    Infrared (IR) transmittance tunable metal-insulator conversion was demonstrated on a glass substrate by using thermochromic vanadium dioxide (VO2) as the active layer in a three-terminal thin-film-transistor-type device withwater-infiltrated glass as the gate insulator. Alternative positive/negative gate-voltage applications induce the reversible protonation/deprotonation of a VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49% modulation of IR-transmittance were simultaneously demonstrated at room temperature by the metal-insulator phase conversion of VO2 in a non-volatile manner. The present device is operable by the room-temperature protonation in an all-solid-state structure, and thus it will provide a new gateway to future energy-saving technology as an advanced smart window. (C) 2017 Author(s).
  • Shota Katayama, Takayoshi Katase, Tetsuya Tohei, Bin Feng, Yuichi Ikuhara, Hiromichi Ohta
    CRYSTAL GROWTH & DESIGN 17 4 1849 - 1853 2017年04月 [査読有り][通常論文]
     
    Layered alkali ion-containing metal oxides (LAMOs) are promising as candidate material for energy storage and conversion applications. Although high-quality epitaxial films are useful to clarify their physical properties, it is very difficult to fabricate LAMO films by a conventional vapor phase epitaxy method because of the high vapor pressure of alkali metals at elevated temperatures. Here we report reactive solid-phase epitaxy (R-SPE) [Cryst. Growth Des. 2005,S, 25] of layered sodium manganese oxide, which is a candidate as cathode active material for Na-ion batteries. In the R-SPE method, manganese oxide epitaxial film with spinel structure was first grown on sapphire substrate, and then the film was changed into Na approximate to 2/3MnO2 epitaxial film by heating the film at 700 degrees C with Na2CO3 powder. Hydrated Na approximate to 0.61MnO2 center dot approximate to 0.42H(2)O film was also prepared by immersing a Na,approximate to 2/3MnO2 film into water at room temperature. The electron hopping conductivity of Na approximate to 2/3MnO2 and Na approximate to 0.61MnO2 approximate to 0.42H2O films was clarified as similar to 1 and similar to 0.1 mS cm(-1) at room temperature, respectively, with an activation energy of 0.47 eV, demonstrating the usefulness of our R-SPE method for clarifying the physical properties of LAMO.
  • Kazuma Funahashi, Naoki Tanaka, Yoshiaki Shoji, Naoki Imazu, Ko Nakayama, Kaito Kanahashi, Hiroyuki Shirae, Suguru Noda, Hiromichi Ohta, Takanori Fukushima, Taishi Takenobu
    APPLIED PHYSICS EXPRESS 10 3 2017年03月 [査読有り][通常論文]
     
    Hole doping into carbon nanotubes can be achieved. However, the doped nanotubes usually suffer from the lack of air and moisture stability, thus, they eventually lose their improved electrical properties. Here, we report that a salt of the two-coordinate boron cation Mes(2)B(+) (Mes: 2,4,6-trimethylphenyl group) can serve as an efficient hole-doping reagent to produce nanotubes with markedly high stability in the presence of air and moisture. Upon doping, the resistances of the nanotubes decreased, and these states were maintained for one month in air. The hole-doped nanotube films showed a minimal increase in resistance even upon humidification with a relative humidity of 90%. (C) 2017 The Japan Society of Applied Physics
  • 特別記事・注目をあびるエレクトロクロミック材料とその可能性-窓ガラスがメモリーとして利用可能に-
    太田 裕道
    工業材料 65 1 78 - 82 2017年02月 [査読無し][招待有り]
  • Remarkably air- and moisture-stable hole-doped carbon nanotube films by a boron-based oxidant
    Kazuma Funahashi, Naoki Tanaka, Yoshiaki Shoji, Naoki Imazu, Ko Nakayama, Kaito Kanahashi, Hiroyuki Shirae, Suguru Noda, Hiromichi Ohta, Takanori Fukushima, Taishi Takenobu
    Appl. Phys. Express 10 035101  2017年02月 [査読有り][通常論文]
  • Eun Sung Kim, Jae-Yeol Hwang, Kyu Hyoung Lee, Hiromichi Ohta, Young Hee Lee, Sung Wng Kim
    ADVANCED MATERIALS 29 8 1604899  2017年02月 [査読有り][通常論文]
  • Infrared-transmittance tunable metal-insulator switching device on a glass substrate
    Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    APL Materials 5 056105 2017年 [査読有り][通常論文]
  • Takayoshi Katase, Hiromichi Ohta
    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 125 8 608 - 615 2017年 [査読無し][招待有り]
     
    Using the flexible valence states of transition-metal-oxides (TMOs), the optical, electronic, and magnetic properties can be simultaneously controlled by electrochemical oxidation and reduction. Herein we review our recent works on the electrochemically switchable functional thin-film device, which has a three-terminal thin-film-transistor (TFT) structure with the TMO as an active channel layer and the liquid-leakage-free electrolyte as a gate insulator. Thin films of vanadium dioxide, tungsten trioxide, and strontium cobaltite were selected as the channel layer. By applying the gate voltage at room temperature in air, the protonation/deprotonation or oxidation/deoxidation occurs in the TMO channels and their opto-electronic and electro-magnetic properties are reversibly switched by using the mobile ions in the solid TFT structure. The present device with liquid-leakage-free electrolyte provides a novel design concept for the development of future multifunctional switching devices based on TMOs. (C)2017 The Ceramic Society of Japan. All rights reserved.
  • Ning Li, Takayoshi Katase, Yanbei Zhu, Takao Matsumoto, Tomonari Umemura, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS EXPRESS 9 12 125501  2016年12月 [査読有り][通常論文]
     
    A thin film of Li4Ti5O12, a candidate anode material for solid-state Li-ion batteries, was heteroepitaxially grown on a (001) SrTiO3 substrate using solid-liquid phase epitaxy. An amorphous Li4Ti5O12 film deposited at room temperature was first heated with LiNO3 powder in air and then washed with distilled water. The Li4Ti5O12 epitaxial film was obtained by heating with molten LiNO3 at 600 degrees C; the liquid LiNO3 completely covered the film, suppressing the formation of Li deficiencies and enhancing the low-temperature crystal growth. Solid-liquid phase epitaxy is a powerful approach to grow Li-containing-oxide films, which are difficult to fabricate because of the loss of Li species at high temperature. (C) 2016 The Japan Society of Applied Physics
  • Jiang Pu, Kaito Kanahashi, Nguyen Thanh Cuong, Chang-Hsiao Chen, Lain-Jong Li, Susumu Okada, Hiromichi Ohta, Taishi Takenobu
    PHYSICAL REVIEW B 94 1 014312  2016年07月 [査読有り][通常論文]
     
    The carrier-density-dependent conductance and thermoelectric properties of large-area MoS2 and WSe2 monolayers are simultaneously investigated using the electrolyte gating method. The sign of the thermoelectric power changes across the transistor off-state in the ambipolar WSe2 transistor as the majority carrier density switches from electron to hole. The thermopower and thermoelectric power factor of monolayer samples are one order of magnitude larger than that of bulk materials, and their carrier-density dependences exhibit a quantitative agreement with the semiclassical Mott relation based on the two-dimensional energy band structure, concluding the thermoelectric properties are enhanced by the low-dimensional effect.
  • Takaki Onozato, Takayoshi Katase, Akira Yamamoto, Shota Katayama, Koichi Matsushima, Naho Itagaki, Hisao Yoshida, Hiromichi Ohta
    JOURNAL OF PHYSICS-CONDENSED MATTER 28 25 25501  2016年06月 [査読有り][通常論文]
     
    In order to understand the optoelectronic properties of amorphous niobium oxide (a-NbOx), we have investigated the valence states, local structures, electrical resistivity, and optical absorption of a-NbOx thin films with various oxygen contents. It was found that the valence states of Nb ion in a-NbOx films can be controlled from 5+ to 4+ by reducing oxygen pressure during film deposition at room temperature, together with changing the oxide-ion arrangement around Nb ion from Nb2O5-like to NbO2-like local structure. As a result, a four orders of magnitude reduction in the electrical resistivity of a-NbOx films was observed with decreasing oxygen content, due to the carrier generation caused by the appearance and increase of an oxygen-vacancy-related subgap state working as an electron donor. The tunable optoelectronic properties of a-NbOx films by valence-state-control with oxygen-vacancy formation will be useful for potential flexible optoelectronic device applications.
  • Takayoshi Katase, Yuki Suzuki, Hiromichi Ohta
    ADVANCED ELECTRONIC MATERIALS 2 6 2016年06月 [査読有り][通常論文]
     
    A reversibly switchable electromagnetic oxide device is demonstrated by using a thin-film transistor structure with a newly developed leakage-free electrolyte as a gate insulator. The electrical and magnetic behavior of the device can be switched from antiferromagnetic insulation to ferromagnetic metal electrically under DC voltage of +3 V at room temperature. The result provides a novel design concept for practical memory devices.
  • Takayoshi Katase, Takaki Onozato, Misako Hirono, Taku Mizuno, Hiromichi Ohta
    SCIENTIFIC REPORTS 6 25819  2016年05月 [査読有り][通常論文]
     
    Proton and hydroxyl ion play an essential role for tuning functionality of oxides because their electronic state can be controlled by modifying oxygen off-stoichiometry and/or protonation. Tungsten trioxide (WO3), a well-known electrochromic (EC) material for smart window, is a wide bandgap insulator, whereas it becomes a metallic conductor HxWO3 by protonation. Although one can utilize electrochromism together with metal-insulator (MI) switching for one device, such EC-MI switching cannot be utilized in current EC devices because of their two-terminal structure with parallel-plate configuration. Here we demonstrate a transparent EC-MI switchable device with three-terminal TFT-type structure using amorphous (a-) WO3 channel layer, which was fabricated on glass substrate at room temperature. We used water-infiltrated nano-porous glass, CAN (calcium aluminate with nanopores), as a liquid-leakage-free solid gate insulator. At virgin state, the device was fully transparent in the visible-light region. For positive gate voltage, the active channel became dark blue, and electrical resistivity of the a-WO3 layer drastically decreased with protonation. For negative gate voltage, deprotonation occurred and the active channel returned to transparent insulator. Good cycleability of the present transparent EC-MI switching device would have potential for the development of advanced smart windows.
  • Bin Feng, Issei Sugiyama, Hajime Hojo, Hiromichi Ohta, Naoya Shibata, Yuichi Ikuhara
    SCIENTIFIC REPORTS 6 20288  2016年02月 [査読有り][通常論文]
     
    Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.
  • Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    OXIDE-BASED MATERIALS AND DEVICES VII 9749 974916  2016年 [査読有り][通常論文]
     
    Compared to state-of-the-art modulation techniques, protonation is the most ideal to control the electrical and optical properties of transition metal oxides (TMOs) due to its intrinsic non-volatile operation. However, the protonation of TMOs is not typically utilized for solid-state devices because of imperative high-temperature annealing treatment in hydrogen source. Although one solution for room temperature (RT) protonation of TMOs is liquid-phase electrochemistry, it is unsuited for practical purposes due to liquid-leakage problem. Herein we demonstrate solid-state RT-protonation of vanadium dioxide (VO2), which is a well-known thermochromic TMO. We fabricated the three terminal thin-film-transistor structure on an insulating VO2 film using a water-infiltrated nanoporous glass, which serves as a solid electrolyte. For gate voltage application, water electrolysis and protonation/deprotonation of VO2 film surface occurred, leading to reversible metal-insulator phase conversion of similar to 11-nm-thick VO2 layer. The protonation was clearly accompanied by the structural change from an insulating monoclinic to a metallic tetragonal phase. Present results offer a new route for the development of electro-optically active solid-state devices with TMO materials by engineering RT protonation.
  • Koji Yokoyama, Shun Yokoyama, Yoshinori Sato, Kazutaka Hirano, Shinji Hashiguchi, Kenichi Motomiya, Hiromichi Ohta, Hideyuki Takahashi, Kazuyuki Tohji, Yoshinori Sato
    JOURNAL OF MATERIALS CHEMISTRY A 4 23 9184 - 9195 2016年 [査読有り][通常論文]
     
    Nitrogen-doped carbon nanomaterials are known to be excellent electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells. In this study, we used a new and simple post-doping synthesis method to prepare nitrogen-doped single-walled carbon nanotubes (SWCNTs) by reacting fluorinated SWCNTs with ammonia gas at 300-600 degrees C. The structure and morphology of the N-x degrees C-doped SWCNTs (x: reaction temperature) synthesized by the defluorination-assisted nanotube-substitution reaction were characterized. Their levels of nitrogen doping (1.38-3.04 at%) are fairly high, with enriched pyridinic- and pyrrolic-nitrogen species. Their electrochemical catalytic activity for ORR in 0.5 M H2SO4 was evaluated by cyclic voltammetry and linear sweep voltammetry, and their catalytic durability was assessed in load-potential cycle tests. For the N-x degrees C-doped SWCNTs, the peak and the onset potential (E-peak and E-onset, respectively) shifted towards the positive and the current density (j@E-1/2) at the half-wave potential (E-1/2) increased with increasing reaction temperature. The Eonset values of N-500 degrees C- and N-600 degrees C-doped SWCNTs were +0.51 V while that of the non-fluorinated hc-SWCNTs was +0.16 V. The number of electrons transferred per oxygen molecule (n) in ORR was determined to be 2.32-3.64 at the potential of -0.3 V. The ORR catalytic activity was evaluated comprehensively through the measured parameters E-peak, E-onset, n, and j@E-1/2. The N-500 degrees C-doped SWCNTs possessed the highest ORR catalytic activity. After 11 000 cycles in the load-potential test, its current density remained at 93% of the initial value, indicating better durability than platinum nanoparticles supported on carbon black (Pt-C).
  • Ichiro Terasaki, Ryuji Okazaki, Hiromichi Ohta
    SCRIPTA MATERIALIA 111 23 - 28 2016年01月 [査読有り][通常論文]
     
    Since thermoelectrics is an energy-conversion technology from heat flow to electrical flow, or vice versa, it is essentially a non-equilibrium phenomenon in the sense that energy and particle flows are involved. We have recently focused on this feature in order to overcome theoretical predictions of the upper limit of the conversion efficiency in thermal equilibrium conditions. One trial is the thermoelectric phenomena in the range of non-ohmic conduction. The second approach is the thermoelectric phenomena shown by the photo-doped carriers. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Takayoshi Katase, Hidefumi Takahashi, Tetsuya Tohei, Yuki Suzuki, Michihiko Yamanouchi, Yuichi Ikuhara, Ichiro Terasaki, Hiromichi Ohta
    ADVANCED ELECTRONIC MATERIALS 1 12 1500199  2015年12月 [査読有り][通常論文]
     
    The oxygen-deficient Sr4-xErxCo4O12-delta (SECO), one of the ordered perovskite oxides, is a room-temperature (RT) ferrimagnetic semiconductor that arises from the A-site-ordered structure. Development of such a material can lead to the realization of spintronic heterojunction devices; however, to this point there have been difficulties in achieving the A-site-ordered structure in SECO thin films. Here, single-crystalline film growth of SECO with perfectly aligned A-site-ordered structure on (LaAlO3)(0.3)(Sr2TaAlO6)(0.7) substrates by solid-phase epitaxy (SPE) method is demonstrated. The brownmillerite-type, A-site-disordered structure of the as-grown epitaxial film clearly changes into well-aligned A-site-ordered structure after heat treatment at 1050 degrees C in air; the aligned ordered structure in the film is clearly visualized at atomic level. This ordering induces ferrimagnetism with Curie temperature (T-c) approximate to 310 K and it is found that the SECO film with p-type semiconductivity exhibits anomalous Hall effect at the temperature up to 300 K, which is suitable for the test bench to demonstrate advanced spintronic heterojunction devices, operating at RT. The present SPE method is expected to serve as a powerful technique for the fabrication of thin films and exploration of potential characteristics of A-site-ordered perovskite oxides.
  • Koji Yokoyama, Yoshinori Sato, Kazutaka Hirano, Hiromichi Ohta, Kenichi Motomiya, Kazuyuki Tohji, Yoshinori Sato
    CARBON 94 1052 - 1060 2015年11月 [査読有り][通常論文]
     
    Nitrogen-doped single-walled carbon nanotubes (N-doped SWCNTs) were synthesized by reacting fluorinated SWCNTs with ammonia gas at reaction temperatures of 300-600 degrees C. In the N-doped SWCNTs synthesized at 400 degrees C (N-400 degrees C-doped SWCNTs), nitrogen atoms were introduced into the nanotube framework at a concentration of 3.04 at.%, the concentrations of the pyridinic-, pyrrolic-, graphitic-, and pyridine-N-oxide-type nitrogen species were 1.22, 1.36, 0.34, and 0.08 at.%, respectively, and their percentages with respect to the total nitrogen species were 40.1, 44.7, 11.2, and 2.6%, respectively. The volume resistivity of films of the N-400 degrees C-doped SWCNTs was 2.3 +/- 1.1 Omega.cm, indicating that they can be classified as semiconductors. Finally, it was found that the thermo power of the N-400 degrees C-doped SWCNTs was negative at room temperature in air, indicating that they exhibited n-type conduction. (C) 2015 Elsevier Ltd. All rights reserved.
  • Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    PHYSICAL REVIEW B 92 3 035302  2015年07月 [査読有り][通常論文]
     
    Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films with controlled lattice distortion was investigated by thermopower measurements. VO2 epitaxial films with different crystallographic orientations, grown on (0001) alpha-Al2O3, (11 (2) over bar0) alpha-Al2O3, and (001) TiO2 substrates, showed significant decrease of absolute value of Seebeck coefficient (S) from similar to 200 to 23 mu VK-1, along with a sharp drop in electrical resistivity (rho), due to the transition from an insulator to a metal. The MI transition temperatures observed both in rho(T-rho) and S(T-S) for the VO2 films systematically decrease with lattice shrinkage in the pseudorutile structure along the c axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset T-S, where the insulating phase starts to become metallic, is much lower than the onset T-rho. This difference is attributed to the sensitivity of S for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in rho measurements. Consequently, S measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in VO2.
  • Takayoshi Katase, Kenji Endo, Tetsuya Tohei, Yuichi Ikuhara, Hiromichi Ohta
    ADVANCED ELECTRONIC MATERIALS 1 7 1500063  2015年07月 [査読有り][通常論文]
     
    Room-temperature protonation and metal-insulator conversion of a transition metal oxide (TMO) is demonstrated by all-solid-state thin-film transistor with vanadium dioxide using a water-infiltrated nanoporous glass as a solid electrolyte. This promising result can provide a novel route for TMO-based solid-state electro-optical devices, in particular, smart windows for on-demand infrared shielding.
  • Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 123 1437 307 - 311 2015年05月 [査読有り][通常論文]
     
    Electronic structure across the metal-insulator (MI) transition of electron-doped V1-x WxO2 epitaxial films (x = 0-0.06) grown on alpha-Al2O3 substrates was studied by means of thermopower (S) measurements. Significant increase of vertical bar S vertical bar-values accompanied by MI transition was observed, and the transition temperatures of S (T-S) decreased with x in good linear relation with MI transition temperatures. vertical bar S vertical bar values of V1-x WxO2 films at T > T-S were constant at low values of 23 mu VK-1 independently of x, which reflects a metallic electronic structure, whereas, those at T < T-S almost linearly decreased with logarithmic W-concentrations. The gradient of -213 mu VK-1 agrees well with -k(B)/e.ln10 (-198 mu VK-1), suggesting that V1-x WxO2 films have insulating electronic structures with a parabolic density of state around the conduction band bottom. (C) 2015 The Ceramic Society of Japan. All rights reserved.
  • Woo Seok Choi, Hyang Keun Yoo, Hiromichi Ohta
    ADVANCED FUNCTIONAL MATERIALS 25 5 799 - 804 2015年02月 [査読有り][通常論文]
     
    The electrodynamic properties of La-doped SrTiO3 thin films with controlled elemental vacancies are investigated using optical spectroscopy and thermopower measurement. In particular, a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films is observed. With decreasing oxygen partial pressure during the film growth (P(O-2)), a systematic lattice expansion is observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, an absorption in the mid-infrared photon energy range is found, which is attributed to the polaron formation in the doped SrTiO3 system. Thermopower of the La-doped SrTiO3 thin films can be largely modulated from -120 to -260 V K-1, reflecting an enhanced polaronic mass of approximate to 3 < m(polron)/m < approximate to 4. The elemental vacancies generated in the TMO films grown at various P(O-2) influences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.
  • ナノ構造熱電材料の開発動向
    太田 裕道
    高分子 63 11 785  2014年11月 [査読無し][招待有り]
  • Woo Seok Choi, Hiromichi Ohta, Ho Nyung Lee
    ADVANCED MATERIALS 26 39 6701 - 6705 2014年10月 [査読有り][通常論文]
  • Takayoshi Katase, Kenji Endo, Hiromichi Ohta
    PHYSICAL REVIEW B 90 16 161105(R)  2014年10月 [査読有り][通常論文]
     
    The electronic structure across the metal-insulator (MI) transition of electron-doped V1-xWxO2 epitaxial films (x = 0-0.06) grown on alpha-Al2O3 substrates was studied by means of thermopower (S) measurements. Significant increase of vertical bar S vertical bar values accompanied by MI transition was observed, and the transition temperatures of S (T-S) decreased with x in a good linear relation with MI transition temperatures. vertical bar S vertical bar values of V1-xWxO2 films at T > T-S were constant at low values of 23 mu VK-1 independently of x, which reflects a metallic electronic structure, whereas those at T < T-S almost linearly decreased with logarithmic W concentrations. The gradient of -213 mu VK-1 agrees well with -k(B)/e ln10 (-198 mu VK-1), suggesting that V1-xWxO2 films have insulating electronic structures with a parabolic density of state around the conduction band bottom.
  • 単層遷移金属ダイカルコゲナイドにおける熱電効果の電場変調(7pAH 領域4,領域7合同グラフェン関連(新物質・原子層),領域4(半導体,メゾスコピック系・局在))
    蒲, 江, 金橋, 魁利, Nguyen, Thanh Cuong, Li, Lain-Jong, 岡田, 晋, 太田, 裕道, 竹延, 大志
    日本物理学会講演概要集 69 2 470  一般社団法人日本物理学会 2014年08月 [査読無し][通常論文]
  • Hyoungjeen Jeen, Woo Seok Choi, Michael D. Biegalski, Chad M. Folkman, I-Cheng Tung, Dillon D. Fong, John W. Freeland, Dongwon Shin, Hiromichi Ohta, Matthew F. Chisholm, Ho Nyung Lee
    NATURE MATERIALS 12 11 1057 - 1063 2013年11月 [査読有り][通常論文]
     
    Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation's oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoOx) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO3-delta or the brownmillerite SrCoO2.5. Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200-300 degrees C) in a considerably short time (<1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO3-delta is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials.
  • H. Jeen, W. S. Choi, J. W. Freeland, H. Ohta, C. U. Jung, H. N. Lee
    Adv. Mater. 25 27 3651 - 3656 2013年07月 [査読有り][通常論文]
  • Hiromichi Ohta, Teruyasu Mizoguchi, Noriyuki Aoki, Takashi Yamamoto, Akhmad Sabarudin, Tomonari Umemura
    APPLIED PHYSICS LETTERS 102 8 2013年02月 [査読有り][通常論文]
  • Shijian Zheng, Craig A. J. Fisher, Takeharu Kato, Yuki Nagao, Hiromichi Ohta, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 101 19 191602  2012年11月 [査読有り][通常論文]
     
    Using advanced Cs-corrected scanning transmission electron microscopy, we have observed the atomic structures of coherent TiO2/LaAlO3 heterointerfaces in anatase TiO2 thin films deposited on single-crystal LaAlO3. Two epitaxial relationships are identified: (001)[100] TiO2//(001)[100] LaAlO3 and (001)[010] TiO2//(001)[100] LaAlO3. These epitaxial relationships and the terrace steps on the substrate surface result in the formation of multidomains in the anatase TiO2 thin films. Three types of domain boundaries are identified, namely, 90 degrees rotation domain boundaries, translation domain boundaries, and mixtures of the two. These domain boundaries are expected to have a profound effect on the properties of the thin film. (C) 2012 American Institute of Physics. [http://dx. doi. org/10.1063/1.4766338]
  • Hiromichi Ohta, Kunihito Koumoto
    Multifunctional Oxide Heterostructures 9780199584123 2012年08月30日 [査読有り][通常論文]
     
    Thermoelectric energy conversion technology to convert waste heat into electricity has received much attention. In addition, metal oxides have recently been considered as thermoelectric power generation materials that can operate at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness. This chapter fabricates high quality epitaxial films composed of oxide thermoelectric materials, which are suitable for clarifying the intrinsic properties. It focuses on the thermoelectric properties of two representative oxide epitaxial films, ptype Ca3Co4O9 and n-type SrTiO3, which exhibit the best thermoelectric figures of merit, ZT(=S2·σT·κ-1, S: Seebeck coefficient, σ: electrical conductivity, κ: thermal conductivity, and T: absolute temperature) among oxide thermoelectric materials reported to date. In addition, it introduces the recently discovered giant S of two-dimensional electrons confined within a unit cell layer thickness (~0.4 nm) of SrTiO3.
  • H. Ohta, T. Mizoguchi, N. Aoki, T. Yamamoto, A. Sabarudin, T. Umemura
    Appl. Phys. Lett. 100 17 173107  2012年04月 [査読有り][通常論文]
     
    La2/3-xLi3xTiO3 (LLT, x - 0.1 and 0.167) thin films were grown on (001)-oriented SrTiO3 (STO) substrates by pulsed laser deposition. The resultant LLT films were heteroepitaxially grown with atomically flat surfaces. Although the films considered to be single crystalline, they were composed of 90 degrees-rotated multi domains (similar to 5 nm in size) with epitaxial relationships of (100) [001] LLT parallel to (001) [100] STO and (100) [010] LLT parallel to (001) [100] STO. The bulk conductivity of the LLT (x - 0.1) films was 2.5 x 10(-2) S cm(-1) at 190 degrees C, comparable to that of a bulk single crystalline LLT. Single crystalline LLT thin films with atomically flat surfaces should help clarify interfacial lithium-ion conductivity. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709402]
  • Hiromichi Ohta, Taku Mizuno, Shijian Zheng, Takeharu Kato, Yuichi Ikuhara, Katsumi Abe, Hideya Kumomi, Kenji Nomura, Hideo Hosono
    ADVANCED MATERIALS 24 6 740 - + 2012年02月 [査読有り][通常論文]
     
    Unusually large thermopower modulation is demonstrated using a field effect transistor structure on a thermoelectric material. An electric field application provides an extremely thin (similar to 2 nm) two-dimensional electron gas (2DEG), which exhibits unusually large thermopower, approximately five times larger than that of the bulk. The present electric field induced 2DEG approach may accelerate the development of nanostructures for high performance thermoelectric materials.
  • B. Feng, H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, Y. Sato, N. Shibata, T. Yamamoto, Y. Ikuhara
    APPLIED PHYSICS LETTERS 100 7 073109  2012年02月 [査読有り][通常論文]
     
    The atomic structure of a Sigma 3 [110]/(111) grain boundary in CeO2 was studied by scanning transmission electron microscopy, electron energy loss spectroscopy, and the first-principles calculations. It was revealed that this grain boundary does not promote the formation of oxygen vacancies and keeps oxygen stoichiometry, which is different from that of Sigma 5 CeO2 grain boundary studied previously [H. Hojo, T. Mizoguchi, H. Ohta, S. D. Findlay, N. Shibata, T. Yamamoto, and Y. Ikuhara, Nano Lett. 10, 4668 (2010)]. It was found that the difference in grain boundary oxygen stoichiometry is correlated with the grain boundary atomic structure. (C) 2012 American Institute of Physics. [doi:10.1063/1.3682310]
  • Munetoshi Seki, Hitoshi Tabata, Hiromichi Ohta, Katsuhiko Inaba, Shintaro Kobayashi
    APPLIED PHYSICS LETTERS 99 24 242504  2011年12月 [査読有り][通常論文]
     
    Epitaxial thin films of Ge-substituted Fe3O4 were grown on alpha-Al2O3 (001) using pulsed laser deposition, and their electrical and magnetic properties were investigated. A target with excess Ge compositions was employed for the film growth to compensate for the volatilization of germanium oxides during the high-temperature deposition. P-type conduction was achieved for the Ge-rich films. The Neel temperature of all the films was above 300 K. Furthermore, the films exhibited an anomalouos Hall effect at 300 K, suggesting that the carriers in the films are spin-polarized. (C) 2011 American Institute of Physics. [doi:10.1063/1.3670045]
  • Taku Mizuno, Yuki Nagao, Akira Yoshikawa, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 110 6 063719  2011年09月 [査読有り][通常論文]
     
    Electric field modulation analysis of thermopower (S)-carrier concentration (n) relation of a bilayer laminate structure composed of a 1.5-nm-thick conducting layer, probably TinO2n-1 (n=2, 3, ...) Magneli phase, and rutile TiO2, was performed. The results clearly showed that both the rutile TiO2 and the thin interfacial layer contribute to carrier transport: the rutile TiO2 bulk region (mobility mu similar to 0.03 cm(2) V-1 s(-1)) and the 1.5-nm-thick interfacial layer (mu similar to 0.3 cm(2) V-1 s(-1)). The effective thickness of the interfacial layer, which was obtained from the S-n relation, was below similar to 3 nm, which agrees well with that of the TEM observation (similar to 1.5 nm), clearly showing that electric field modulation measurement of S-n relation can effectively clarify the carrier transport properties of a bilayer laminate structure. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3633217]
  • Teruyasu Mizoguchi, Hiromichi Ohta, Hak-Sung Lee, Nobuaki Takahashi, Yuichi Ikuhara
    ADVANCED FUNCTIONAL MATERIALS 21 12 2258 - 2263 2011年06月 [査読有り][通常論文]
     
    By combining state-of-the-art microscopy, spectrosccopy, and first-principles calculations, atomic-scale intermixing behavior at heterointerfaces in SrTiO3-based superlattices is investigated. It is found that Nb is confined to a unit-cell thickness without intermixing, whereas Ba diffuses only to the adjoining Nb-doped SrTiO3 layer. It is revealed that the intermixing behaviors at the heterointerfaces are determined by not only the migration energy, but also by the vacancy-formation energy and the Fermi energy of each layer. Based on these results, we find a method to control the atomic-scale intermixing at the nonpolar heterointerfaces and clearly demonstrate the property improvements obtained by constructing an abrupt heterointerface.
  • Hajime Hojo, Eita Tochigi, Teruyasu Mizoguchi, Hiromichi Ohta, Naoya Shibata, Bin Feng, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 98 15 153104  2011年04月 [査読有り][通常論文]
     
    Threading dislocations in CeO2 thin films grown on yttria-stabilized ZrO2 substrates were investigated by transmission electron microscopy (TEM), high-resolution TEM, and scanning TEM. It is revealed that there are two kinds of threading dislocations with the Burgers vector of b = 1/2 < 110 >: one is pure edge-type and the other is mixed-type. Comparing the strain field of the mixed-type dislocations with that of the Peierls-Nabarro and the Foreman dislocation models, we find that the Foreman model better describes it in CeO2. (C) 2011 American Institute of Physics. [doi:10.1063/1.3575566]
  • Hajime Hojo, Teruyasu Mizoguchi, Hiromichi Ohta, Scott D. Findlay, Naoya Shibata, Takahisa Yamamoto, Yuichi Ikuhara
    Nano Letters 10 11 4668 - 4672 2010年11月10日 [査読有り][通常論文]
     
    Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO2 grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations. © 2010 American Chemical Society.
  • Hiromichi Ohta, Yukio Sato, Takeharu Kato, SungWng Kim, Kenji Nomura, Yuichi Ikuhara, Hideo Hosono
    NATURE COMMUNICATIONS 1 118  2010年11月 [査読無し][通常論文]
     
    Water is composed of two strong electrochemically active agents, H+ and OH- ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO3, using water-infiltrated nanoporous glass-amorphous 12CaO center dot 7Al(2)O(3)-as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature. This leads to the formation of a thin (similar to 3 nm) metal layer with an extremely high electron concentration (10(15)-10(16) cm(-2)), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage.
  • Hirotaka Koide, Yuki Nagao, Kunihito Koumoto, Yuka Takasaki, Tomonari Umemura, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS LETTERS 97 18 182105  2010年11月 [査読無し][通常論文]
     
    To clarify the electronic density of states (DOS) around the conduction band bottom for state of the art transparent amorphous oxide semiconductors (TAOSs), InGaZnO4 and In2MgO4, we fabricated TAOS-based transparent thin film transistors (TTFTs) and measured their gate voltage dependence of thermopower (S). TAOS-based TTFTs exhibit an unusual S behavior. The vertical bar S vertical bar-value abruptly increases but then gradually decreases as V-g increases, clearly suggesting the antiparabolic shaped DOS is hybridized with the original parabolic shaped DOS around the conduction band bottom. (c) 2010 American Institute of Physics. [doi:10.1063/1.3512870]
  • Hiromichi Ohta, Yukio Sato, Takeharu Kato, SungWng Kim, Kenji Nomura, Yuichi Ikuhara, Hideo Hosono
    NATURE COMMUNICATIONS 1 118  2010年11月 [査読無し][通常論文]
     
    Water is composed of two strong electrochemically active agents, H+ and OH- ions, but has not been used as an active electronic material in oxide semiconductors. In this study, we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from insulator to metal. We fabricated a field-effect transistor structure on an oxide semiconductor, SrTiO3, using water-infiltrated nanoporous glass-amorphous 12CaO center dot 7Al(2)O(3)-as the gate insulator. Positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature. This leads to the formation of a thin (similar to 3 nm) metal layer with an extremely high electron concentration (10(15)-10(16) cm(-2)), which exhibits exotic thermoelectric behaviour. The electron activity of water as it infiltrates nanoporous glass may find many useful applications in electronics or in energy storage.
  • Hirotaka Koide, Yuki Nagao, Kunihito Koumoto, Yuka Takasaki, Tomonari Umemura, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS LETTERS 97 18 182105  2010年11月 [査読無し][通常論文]
     
    To clarify the electronic density of states (DOS) around the conduction band bottom for state of the art transparent amorphous oxide semiconductors (TAOSs), InGaZnO4 and In2MgO4, we fabricated TAOS-based transparent thin film transistors (TTFTs) and measured their gate voltage dependence of thermopower (S). TAOS-based TTFTs exhibit an unusual S behavior. The vertical bar S vertical bar-value abruptly increases but then gradually decreases as V-g increases, clearly suggesting the antiparabolic shaped DOS is hybridized with the original parabolic shaped DOS around the conduction band bottom. (c) 2010 American Institute of Physics. [doi:10.1063/1.3512870]
  • Y. Kozuka, M. Kim, H. Ohta, Y. Hikita, C. Bell, H. Y. Hwang
    APPLIED PHYSICS LETTERS 97 22 222115  2010年11月 [査読有り][通常論文]
     
    We fabricated high-mobility delta-doped structures in SrTiO3 thin films in order to investigate the low temperature electronic transport properties of confined carriers in this system. An enhancement of the electron mobility above the bulk value was observed as the doped layer thickness decreased. High-field Hall measurements revealed that this mobility enhancement originates from higher-mobility electrons in the undoped clean regions, which have quantum-mechanically broadened from the doped layer. Because of the absence of apparent lattice misfit between the layers, this structure is highly suitable for investigating two-dimensional electron gases in SrTiO3. (C) 2010 American Institute of Physics. [doi:10.1063/1.3524198]
  • Yuki Nagao, Akira Yoshikawa, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS LETTERS 97 17 172112  2010年10月 [査読無し][通常論文]
     
    Thermopower (S) for anatase TiO2 epitaxial films (n(3D): 10(17)-10(21) cm(-3)) and the gate voltage (V-g) dependence of S for thin film transistors (TFTs) based on TiO2 films were investigated to clarify the electronic density of states around the conduction band bottom. The slope of the vertical bar S vertical bar-log n(3D) plots was -20 mu V K-1, which is an order magnitude smaller than that of semiconductors (-198 mu V K-1), and the vertical bar S vertical bar values for the TFTs increased with V-g in the low V-g region, suggesting that the extra tail states are hybridized with the original conduction band bottom. (C) 2010 American Institute of Physics. [doi:10.1063/1.3507898]
  • Yuki Nagao, Akira Yoshikawa, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS LETTERS 97 17 172112  2010年10月 [査読無し][通常論文]
     
    Thermopower (S) for anatase TiO2 epitaxial films (n(3D): 10(17)-10(21) cm(-3)) and the gate voltage (V-g) dependence of S for thin film transistors (TFTs) based on TiO2 films were investigated to clarify the electronic density of states around the conduction band bottom. The slope of the vertical bar S vertical bar-log n(3D) plots was -20 mu V K-1, which is an order magnitude smaller than that of semiconductors (-198 mu V K-1), and the vertical bar S vertical bar values for the TFTs increased with V-g in the low V-g region, suggesting that the extra tail states are hybridized with the original conduction band bottom. (C) 2010 American Institute of Physics. [doi:10.1063/1.3507898]
  • Y. Ishida, A. Mizutani, K. Sugiura, H. Ohta, K. Koumoto
    PHYSICAL REVIEW B 82 7 075325  2010年08月 [査読無し][通常論文]
     
    We investigate the transport properties of LixCoO2 thin films whose resistivities are nearly an order of magnitude lower than those of the bulk polycrystals. A metal-nonmetal transition occurs at x similar to 0.8 in a biphasic domain and the Seebeck coefficient (S) is drastically increased at similar to 140 K(=T*) with increasing the Li concentration to show a peak of magnitude similar to 120 mu V/K in the S-T curve of x= 0.87. We show that T* corresponds to a crossover temperature in the conduction, most likely reflecting the correlation-induced temperature dependence in the low-energy excitations.
  • Y. Ishida, A. Mizutani, K. Sugiura, H. Ohta, K. Koumoto
    PHYSICAL REVIEW B 82 7 075325  2010年08月 [査読有り][通常論文]
     
    We investigate the transport properties of LixCoO2 thin films whose resistivities are nearly an order of magnitude lower than those of the bulk polycrystals. A metal-nonmetal transition occurs at x similar to 0.8 in a biphasic domain and the Seebeck coefficient (S) is drastically increased at similar to 140 K(=T*) with increasing the Li concentration to show a peak of magnitude similar to 120 mu V/K in the S-T curve of x= 0.87. We show that T* corresponds to a crossover temperature in the conduction, most likely reflecting the correlation-induced temperature dependence in the low-energy excitations.
  • Woo Seok Choi, Hiromichi Ohta, Soon Jae Moon, Yun Sang Lee, Tae Won Noh
    PHYSICAL REVIEW B 82 2 024301  2010年07月 [査読無し][通常論文]
     
    Using optical spectroscopy, we investigated the electrodynamic properties of Nb:SrTiO3/SrTiO3 superlattices. In these superlattices, a large enhancement of the Seebeck coefficient (S) has been reported with decreasing Nb:SrTiO3 layer thickness [H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing Nb: SrTiO3 layer thickness from 11 to 1 unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of the polaron from three-dimensional to quasi-two-dimensional. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing S and preventing the decrease in carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching for efficient thermoelectric materials.
  • Woo Seok Choi, Hiromichi Ohta, Soon Jae Moon, Yun Sang Lee, Tae Won Noh
    PHYSICAL REVIEW B 82 2 024301  2010年07月 [査読有り][通常論文]
     
    Using optical spectroscopy, we investigated the electrodynamic properties of Nb:SrTiO3/SrTiO3 superlattices. In these superlattices, a large enhancement of the Seebeck coefficient (S) has been reported with decreasing Nb:SrTiO3 layer thickness [H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing Nb: SrTiO3 layer thickness from 11 to 1 unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of the polaron from three-dimensional to quasi-two-dimensional. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing S and preventing the decrease in carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching for efficient thermoelectric materials.
  • Kosuke Uchida, Akira Yoshikawa, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 107 9 096103  2010年05月 [査読無し][通常論文]
     
    We report herein fabrication and characterization of a thin film transistor (TFT) using single crystalline strontium titanate (SrTiO3) film, which was grown by a pulsed laser deposition technique followed by the thermal annealing treatment in an oxygen atmosphere. Although TFTs on the as-deposited SrTiO3 films (polycrystalline epitaxial films) exhibited poor transistor characteristics, the annealed single crystalline SrTiO3 TFT exhibits transistor characteristics comparable with those of bulk single crystal SrTiO3 field-effect transistor: an on/off current ratio >10(5), subthreshold swing similar to 2.1 V decade(-1), and field-effect mobility similar to 0.8 cm(2) V-1 s(-1). This demonstrates the effectiveness of the appropriate thermal annealing treatment of epitaxial SrTiO3 films. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3407568]
  • Kosuke Uchida, Akira Yoshikawa, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    JOURNAL OF APPLIED PHYSICS 107 9 096103  2010年05月 [査読有り][通常論文]
     
    We report herein fabrication and characterization of a thin film transistor (TFT) using single crystalline strontium titanate (SrTiO3) film, which was grown by a pulsed laser deposition technique followed by the thermal annealing treatment in an oxygen atmosphere. Although TFTs on the as-deposited SrTiO3 films (polycrystalline epitaxial films) exhibited poor transistor characteristics, the annealed single crystalline SrTiO3 TFT exhibits transistor characteristics comparable with those of bulk single crystal SrTiO3 field-effect transistor: an on/off current ratio >10(5), subthreshold swing similar to 2.1 V decade(-1), and field-effect mobility similar to 0.8 cm(2) V-1 s(-1). This demonstrates the effectiveness of the appropriate thermal annealing treatment of epitaxial SrTiO3 films. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3407568]
  • 太田裕道
    粉体および粉末冶金 57 4 232 - 236 2010年04月 [査読無し][通常論文]
     
    Here I review the thermoelectric properties of electron doped SrTiO 3. Since density of states effective mass of electron carrier in SrTiO3 crystal is rather large, heavily electron doped SrTiO 3 crystal exhibits rather large Seebeck coefficient (S). The reliable thermoelectric figure of merit, ZT is ∼0.08 at room temperature and ∼0.3 at 1,000 K, which is highest among metallic oxides. Recently, we found that high-density ( ∼-1021 cm-3) two-dimensional (2D) electron layers in SrTiO3 crystal exhibits giant S, which is ∼5 times larger than that of simple bulk, when the layer thickness is as thin as one unit cell (0.3905 nm). The 2D electron layer is realized by superlattices of SrTiO 3/ SrTi0.8Nb0.2o3 or TiO 2/SrTiO3 heterointerface. Very recently, we fabricated high performance field effect transistors (FETs) on SrTiO3 single crystal to modulate the Seebeck coefficient by applying the gate voltage.
  • Yifeng Wang, Kyu Hyoung Lee, Hideki Hyuga, Hideki Kita, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF ELECTROCERAMICS 24 2 76 - 82 2010年04月 [査読有り][通常論文]
     
    Aiming at the realization of the enhancement of thermoelectric performance through structural modification, the present work has clarified the significant effects of rare earth (RE=Gd, Sm, Nd, and La) doping at Sr-sites in Sr3Ti2O7, both on the structural restoration of distorted TiO6 octahedra and on the Seebeck coefficient, especially at high temperatures. The preferential substitution of RE (3+) at the nine-coordinate Sr-sites can facilitate the degeneration of the conduction band (Ti 3d-t (2g) ) orbital, owing to its special capability in restoring TiO6 octahedra to a higher state of symmetry and thus enhance the density of states (DOS) effective mass of the carriers, which gives rise to a rather large increase in the Seebeck coefficient. The present findings have affirmed the effectiveness of structural restoration in enhancing the Seebeck coefficient by Sr-site-doping, which will help establish a useful solution for Ti-based thermoelectric oxides with inherently distorted TiO6 octahedra to achieve high thermoelectric performance.
  • Takayoshi Katase, Kenji Nomura, Hiromichi Ohta, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    CRYSTAL GROWTH & DESIGN 10 3 1084 - 1089 2010年03月 [査読無し][通常論文]
     
    Atomically flat single-crystalline thin films or ScAlMgO4 (SCAM) were fabricated on yttria-stabilized zirconia (YSZ) (111)substrates by reactive solid-phase epitaxy using an ablation ceramic target with an optimized chemical composition. Owing to the good lattice matching, the SCAM layers enhanced two-dimensional growth of ZnO epitaxial films, suppressed the formation of threading dislocations at step edges, and provided larger Hall mobilities than those of ZnO films on YSZ and Al2O3 single-crystal substrates especially at low deposition temperatures < 700 degrees C. The ZnO films exhibited Hall voltage anomaly at temperatures < 140 K, which is explained by a two-layer model with a degenerated layer.
  • T. Katase, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono
    Cryst. Growth Des. 10 1084  2010年 [査読有り][通常論文]
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hideo Hosono, Kunihito Koumoto
    ADVANCES IN ELECTRONIC CERAMICS II 30 9 95 - + 2010年 [査読有り][通常論文]
     
    We have measured the thermoelectric properties of layered CaxCoO2 epitaxial films when the Ca-content was varied from x = 0.33 to 0.45. The Ca0.33CoO2 film, which was fabricated by ion-exchange of Na0.7CoO2 film, successfully incorporated the additional Ca-ions in-between the CoO2 layers by the thermal annealing with Ca(OH)(2) powder. The thermal annealing caused the Ca-content to increase up to x = 0.45 after the transformation of Ca-arrangements. The electrical conductivity of the film changed from metallic behavior to semiconducting behavior by the transformation of Ca-arrangement, and became more than two orders of magnitude lower with increasing x. The Seebeck coefficient of the film gradually increased with increasing x, and reached to similar to 210 mu V.K-1 at x = 0.45.
  • Akira Yoshikawa, Kosuke Uchida, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS EXPRESS 2 12 121103  2009年12月 [査読無し][通常論文]
     
    We show herein fabrication and field-modulated thermopower for KTaO3 single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field-effect mobility of similar to 8 cm(2) V-1 s(-1). which is similar to 4 times larger than that of SrTiO3 FETs. The thermopower of the KTaO3 FET decreased from 600 to 220 mu V K-1 by the application of gate electric field up to 1.5 MV cm(-1), similar to 400 mu V K-1 below that of an SrTiO3 FET, clearly reflecting the smaller carrier effective mass of KTaO3. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.121103
  • Akira Yoshikawa, Kosuke Uchida, Kunihito Koumoto, Takeharu Kato, Yuichi Ikuhara, Hiromichi Ohta
    APPLIED PHYSICS EXPRESS 2 12 121103  2009年12月 [査読無し][通常論文]
     
    We show herein fabrication and field-modulated thermopower for KTaO3 single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field-effect mobility of similar to 8 cm(2) V-1 s(-1). which is similar to 4 times larger than that of SrTiO3 FETs. The thermopower of the KTaO3 FET decreased from 600 to 220 mu V K-1 by the application of gate electric field up to 1.5 MV cm(-1), similar to 400 mu V K-1 below that of an SrTiO3 FET, clearly reflecting the smaller carrier effective mass of KTaO3. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.121103
  • Hiromichi Ohta, Yumi Masuoka, Ryoji Asahi, Takeharu Kato, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono
    APPLIED PHYSICS LETTERS 95 11 113505  2009年09月 [査読無し][通常論文]
     
    We report transistor characteristics and field-modulated thermopower (S) for single crystal SrTiO3-based field-effect transistors (FETs). We use 150-nm-thick amorphous 12CaO center dot 7Al(2)O(3) glass as the gate insulator of the SrTiO3-FET. The resulting SrTiO3-FET exhibits excellent transistor characteristics at room temperature: on-to-off current ratio greater than 10(6), threshold gate voltage of +1.1 V, subthreshold swing of approximately 0.3 V decade(-1), and effective mobility of 2 cm(2) V-1 s(-1). The field-modulated S-value of the SrTiO3-FET varied from -900 to -580 mu V K-1 with electric fields of up to 2 MV cm(-1), demonstrating the effectiveness of the FET structure for the exploration of thermoelectric materials. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3231873]
  • Hiromichi Ohta, Yumi Masuoka, Ryoji Asahi, Takeharu Kato, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono
    APPLIED PHYSICS LETTERS 95 11 113505  2009年09月 [査読無し][通常論文]
     
    We report transistor characteristics and field-modulated thermopower (S) for single crystal SrTiO3-based field-effect transistors (FETs). We use 150-nm-thick amorphous 12CaO center dot 7Al(2)O(3) glass as the gate insulator of the SrTiO3-FET. The resulting SrTiO3-FET exhibits excellent transistor characteristics at room temperature: on-to-off current ratio greater than 10(6), threshold gate voltage of +1.1 V, subthreshold swing of approximately 0.3 V decade(-1), and effective mobility of 2 cm(2) V-1 s(-1). The field-modulated S-value of the SrTiO3-FET varied from -900 to -580 mu V K-1 with electric fields of up to 2 MV cm(-1), demonstrating the effectiveness of the FET structure for the exploration of thermoelectric materials. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3231873]
  • Sung Wng Kim, Ryuichi Tarumi, Hideo Iwasaki, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    PHYSICAL REVIEW B 80 7 075201  2009年08月 [査読無し][通常論文]
     
    Thermal conductivity (kappa) and Seebeck coefficient (alpha) of electron-doped light-metal oxide 12CaO center dot 7Al(2)O(3) (C12A7 electride) with a subnanometer-sized cage structure are reported on single crystals with various electron concentrations (N-e). The semiconducting C12A7 electride exhibits n-type conduction with the highest alpha value of -100 mu V K-1 at 300 K. The kappa exhibits an amorphouslike T-2 dependence at low temperatures and varies between 2.3 and 4.5 Wm(-1) K-1 at 300 K. This is an order-of-magnitude lower than that of the constituents, CaO (15 Wm(-1) K-1) and Al2O3 (30 Wm(-1) K-1). These properties are attributed to the cage structure, suggesting that the semiconducting electride should be regarded as a phonon glass and electron crystal material. The thermoelectric performance of electrides evaluated by a dimensionless figure of merit (ZT) shows an optimized value of 2 x 10(-3) at 300 K for the semiconducting electride with N-e of 5 x 10(20) cm(-3).
  • Sung Wng Kim, Ryuichi Tarumi, Hideo Iwasaki, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    PHYSICAL REVIEW B 80 7 075201  2009年08月 [査読無し][通常論文]
     
    Thermal conductivity (kappa) and Seebeck coefficient (alpha) of electron-doped light-metal oxide 12CaO center dot 7Al(2)O(3) (C12A7 electride) with a subnanometer-sized cage structure are reported on single crystals with various electron concentrations (N-e). The semiconducting C12A7 electride exhibits n-type conduction with the highest alpha value of -100 mu V K-1 at 300 K. The kappa exhibits an amorphouslike T-2 dependence at low temperatures and varies between 2.3 and 4.5 Wm(-1) K-1 at 300 K. This is an order-of-magnitude lower than that of the constituents, CaO (15 Wm(-1) K-1) and Al2O3 (30 Wm(-1) K-1). These properties are attributed to the cage structure, suggesting that the semiconducting electride should be regarded as a phonon glass and electron crystal material. The thermoelectric performance of electrides evaluated by a dimensionless figure of merit (ZT) shows an optimized value of 2 x 10(-3) at 300 K for the semiconducting electride with N-e of 5 x 10(20) cm(-3).
  • Wilfried Wunderlich, Hiromichi Ohta, Kunihito Koumoto
    PHYSICA B-CONDENSED MATTER 404 16 2202 - 2212 2009年08月 [査読有り][通常論文]
     
    The effective mass is one of the main factors determining the Seebeck coefficient and electrical conductivity of thermo-electrics. In this ab-initio LDA-GGA study the effective mass is estimated from the curvature of electronic bands by one-band-approximation and is in excellent agreement with experimental data of Nb- and La-doped SrTiO3. It is clarified that the deformation of SrTiO3 crystals has a significant influence on the bandgap, effective electronic DOS-mass and band-mass, but the electronic effect due to the e(g)-band flattening near the Gamma-point due to Nb-doping up to 0.2 at% is the main factor for the effective mass increase. Doping of La shows a linear decrease of the effective mass; this can be explained by the different surroundings of A- and B-sites in perovskite. Substitution with other elements such as Ba on the A-site and V on the B-site in SrTiO3 increases the effective mass as well. (C) 2009 Published by Elsevier B.V.
  • Yifeng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 105 10 103701  2009年05月 [査読無し][通常論文]
     
    Crystal structure and thermoelectric properties of Nb5+- and Ln(3+)- (rare earth: La3+, Nd3+, Sm3+, and Gd3+) doped SrO(SrTiO3)(n) (n=1,2) ceramics, which were fabricated by conventional hot-pressing, were measured to clarify the effects of Ti4+- and Sr2+- site substitution on the thermoelectric properties. The thermal conductivities are very close between the n=1 and 2 phases either doped with Nb5+ or Ln(3+) and decreased by similar to 60% at room temperature and similar to 30% at 1000 K as compared to that of SrTiO3, which is likely due to an enhanced phonon scattering at the SrO/(SrTiO3)(n) (n=1,2) interfaces. The density of states effective mass m(d)*(1.8-2.4 m(0)) and consequently the Seebeck coefficient |S| in Nb5+- doped samples are fairly smaller than those reported for SrTiO3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a(1g) (Ti 3d(xy)) orbital as the conduction band bottom, which should be induced by the distortion of TiO6 octahedra in (SrTiO3) n layers. However, in the Ln(3+)- doped SrO (SrTiO3)(2), the TiO6 octahedra were found to be restored, in contrast to the Nb5+- doped, with a gradually increasing O-Ti-O bond angle in the (100) plane at high temperatures, which would lead to the formation of triply degenerate Ti 3d-t(2g) (d(xy), d(yz), and d(xz)) orbitals to cause a significant enhancement in m(d)* (similar to 7.5 m(0) at 1000 K) and consequently in |S|. Accordingly, the maximum dimensionless figure of merit ZT similar to 0.24 obtained in 5%-Gd3+-doped SrO(rTiO(3))(2) at 1000 K is about 70% larger than that of Nb-doped SrO(SrTiO3)(2) (ZT(1000) (K) similar to 0.14). (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3117943]
  • Yifeng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 105 10 103701  2009年05月 [査読無し][通常論文]
     
    Crystal structure and thermoelectric properties of Nb5+- and Ln(3+)- (rare earth: La3+, Nd3+, Sm3+, and Gd3+) doped SrO(SrTiO3)(n) (n=1,2) ceramics, which were fabricated by conventional hot-pressing, were measured to clarify the effects of Ti4+- and Sr2+- site substitution on the thermoelectric properties. The thermal conductivities are very close between the n=1 and 2 phases either doped with Nb5+ or Ln(3+) and decreased by similar to 60% at room temperature and similar to 30% at 1000 K as compared to that of SrTiO3, which is likely due to an enhanced phonon scattering at the SrO/(SrTiO3)(n) (n=1,2) interfaces. The density of states effective mass m(d)*(1.8-2.4 m(0)) and consequently the Seebeck coefficient |S| in Nb5+- doped samples are fairly smaller than those reported for SrTiO3, which probably resulted from a deterioration of DOS due to the formation of the singly degenerate a(1g) (Ti 3d(xy)) orbital as the conduction band bottom, which should be induced by the distortion of TiO6 octahedra in (SrTiO3) n layers. However, in the Ln(3+)- doped SrO (SrTiO3)(2), the TiO6 octahedra were found to be restored, in contrast to the Nb5+- doped, with a gradually increasing O-Ti-O bond angle in the (100) plane at high temperatures, which would lead to the formation of triply degenerate Ti 3d-t(2g) (d(xy), d(yz), and d(xz)) orbitals to cause a significant enhancement in m(d)* (similar to 7.5 m(0) at 1000 K) and consequently in |S|. Accordingly, the maximum dimensionless figure of merit ZT similar to 0.24 obtained in 5%-Gd3+-doped SrO(rTiO(3))(2) at 1000 K is about 70% larger than that of Nb-doped SrO(SrTiO3)(2) (ZT(1000) (K) similar to 0.14). (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3117943]
  • Takayoshi Katase, Kenji Nomura, Hiromichi Ohta, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS 161 1-3 66 - 70 2009年04月 [査読無し][通常論文]
     
    A homologous series compound ScAlO3(MgO) (SCAM) has a superior lattice matching as small as similar to 1.4% in a-axis with GaN. This paper reports an efficient fabrication process of a single-crystalline SCAM buffer layer on a (1 1 1) yttria-stabilized zirconia (YSZ) substrate using pulsed laser deposition (PLD). A 10-nm thick ZnO epitaxial layer was used to induce solid-phase epitaxial growth of an amorphous (a-) SCAM layer formed at room temperature on (1 1 1) YSZ. It was found that the addition of excess Sc2O3 and ZnO to a SCAM target used for PLD was needed to obtain single-crystalline SCAM films with atomically flat terraces-and-steps surfaces. The resulting single-crystalline SCAM films were examined as buffer layers to grow GaN by molecular beam epitaxy with a plasma nitrogen source. The GaN films were grown epitaxially on the SCAM/YSZ substrates with the epitaxial relationship of [0 0 0 1] GaN parallel to[0 0 0 1] SCAM parallel to[1 1 1] YSZ and [1 0 0] GaN parallel to [11-20] SCAM parallel to [1-10] YSZ. The SCAM buffer layers enhanced lateral growth of the GaN films owing to the good lattice matching. (C) 2009 Elsevier B.V. All rights reserved.
  • Kenji Sugiura, Hiromichi Ohta, Shin-ichi Nakagawa, Rong Huang, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 94 15 142105  2009年04月 [査読無し][通常論文]
     
    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 epitaxial film grown on the m-plane of an alpha-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroepitaxially grown with the CoO2 conducting planes inclined by similar to 43 degrees against the alpha-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was similar to 1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.
  • Takayoshi Katase, Kenji Nomura, Hiromichi Ohta, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS 161 1-3 66 - 70 2009年04月 [査読無し][通常論文]
     
    A homologous series compound ScAlO3(MgO) (SCAM) has a superior lattice matching as small as similar to 1.4% in a-axis with GaN. This paper reports an efficient fabrication process of a single-crystalline SCAM buffer layer on a (1 1 1) yttria-stabilized zirconia (YSZ) substrate using pulsed laser deposition (PLD). A 10-nm thick ZnO epitaxial layer was used to induce solid-phase epitaxial growth of an amorphous (a-) SCAM layer formed at room temperature on (1 1 1) YSZ. It was found that the addition of excess Sc2O3 and ZnO to a SCAM target used for PLD was needed to obtain single-crystalline SCAM films with atomically flat terraces-and-steps surfaces. The resulting single-crystalline SCAM films were examined as buffer layers to grow GaN by molecular beam epitaxy with a plasma nitrogen source. The GaN films were grown epitaxially on the SCAM/YSZ substrates with the epitaxial relationship of [0 0 0 1] GaN parallel to[0 0 0 1] SCAM parallel to[1 1 1] YSZ and [1 0 0] GaN parallel to [11-20] SCAM parallel to [1-10] YSZ. The SCAM buffer layers enhanced lateral growth of the GaN films owing to the good lattice matching. (C) 2009 Elsevier B.V. All rights reserved.
  • Kenji Sugiura, Hiromichi Ohta, Shin-ichi Nakagawa, Rong Huang, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 94 15 142105  2009年04月 [査読無し][通常論文]
     
    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 epitaxial film grown on the m-plane of an alpha-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroepitaxially grown with the CoO2 conducting planes inclined by similar to 43 degrees against the alpha-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was similar to 1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.
  • Kenji Sugiura, Hiromichi Ohta, Yukiaki Ishida, Rong Huang, Tomohiro Saito, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS EXPRESS 2 3 035503  2009年03月 [査読無し][通常論文]
     
    The electrical properties of Ca0.33CoO2 epitaxial films are investigated as the Ca2+-arrangement is transformed. Despite identical Ca-content, annealing the film caused the Ca-arrangement to change from a root 3a x root 3a hexagonal type to a 2a x root 3a orthorhombic type, which was confirmed by X-ray diffraction and scanning transmission electron microscopy. A hexagonal Ca-arrangement type, Ca0.33CoO2, exhibited a metallic resistivity as low as 7.5 x 10(-4) Omega.cm and a moderately large thermopower (similar to 100 mu V.K-1), whereas the orthorhombic Ca-arrangement type displayed a thermally-activated resistivity (greater than or similar to 10(-2) Omega.cm) with a slightly increase in thermopower, which led to about a one order of magnitude difference in the thermoelectric power factor. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.035503
  • Kenji Sugiura, Hiromichi Ohta, Yukiaki Ishida, Rong Huang, Tomohiro Saito, Yuichi Ikuhara, Kenji Nomura, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS EXPRESS 2 3 035503  2009年03月 [査読無し][通常論文]
     
    The electrical properties of Ca0.33CoO2 epitaxial films are investigated as the Ca2+-arrangement is transformed. Despite identical Ca-content, annealing the film caused the Ca-arrangement to change from a root 3a x root 3a hexagonal type to a 2a x root 3a orthorhombic type, which was confirmed by X-ray diffraction and scanning transmission electron microscopy. A hexagonal Ca-arrangement type, Ca0.33CoO2, exhibited a metallic resistivity as low as 7.5 x 10(-4) Omega.cm and a moderately large thermopower (similar to 100 mu V.K-1), whereas the orthorhombic Ca-arrangement type displayed a thermally-activated resistivity (greater than or similar to 10(-2) Omega.cm) with a slightly increase in thermopower, which led to about a one order of magnitude difference in the thermoelectric power factor. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.035503
  • Rong Huang, Teruyasu Mizoguchi, Kenji Sugiura, Shin-ichi Nakagawa, Hiromichi Ohta, Tomohiro Saito, Kunihito Koumoto, Tsukasa Hirayama, Yuichi Ikuhara
    JOURNAL OF MATERIALS RESEARCH 24 1 279 - 287 2009年01月 [査読無し][通常論文]
     
    Microstructures of epitaxial Ca0.33CoO2 thin films, which were grown oil m(1 (1) over bar 00) plane and c(0001) plane of alpha-Al2O3 by the reactive solid-phase epitaxy (R-SPE) method and the subsequent ion-exchange treatment, were investigated in detail by using selected-area electron diffraction, high-resolution transmission electron microcopy, spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (Cs-corrected HAADF-STEM), and electron energy-loss spectroscopy (EELS). Detailed electron diffraction analyses reveal that the orientation relationships between Ca0.33CoO2 thin film and substrate are [11 (2) over bar0](Ca0.33CoO2)//[0001](Al2O3) and (0001)(Ca0.33CoO2), having an angle of about 43 degrees with (1 (1) over bar 00)(Al2O3) for the film deposited oil m(1 (1) over bar 00) plane, and [1 (1) over bar 00](Ca0.33CoO2)//[11 (2) over bar0](Al2O3) and (0001)(Ca0.33CoO2)//(0001)(Al2O3) to the film deposited on c(0001) plane though a Ca-Al-O amorphous layer formed between them. CoO seed layer near the interface and residual Co3O4 phase inside the films were observed and identified by HAADF-STEM and EELS in both samples. Such microstructural configuration indicates that the processes of film growth during R-SPE are (i) oxidation of CoO into Co3O4 With residual CoO layer near the interface and (ii) intercalation of Na+ layer into Co3O4 to achieve the layered NaxCoO2 film while forming Na-Al-O amorphous layer at the interface.
  • Microstructure and electron states of the dielectric TiO2/SrTiO3 heterointerface
    Materials Integration 22 9 10,38-42  2009年 [査読無し][通常論文]
  • Rong Huang, Teruyasu Mizoguchi, Kenji Sugiura, Shin-ichi Nakagawa, Hiromichi Ohta, Tomohiro Saito, Kunihito Koumoto, Tsukasa Hirayama, Yuichi Ikuhara
    JOURNAL OF MATERIALS RESEARCH 24 1 279 - 287 2009年01月 [査読無し][通常論文]
     
    Microstructures of epitaxial Ca0.33CoO2 thin films, which were grown oil m(1 (1) over bar 00) plane and c(0001) plane of alpha-Al2O3 by the reactive solid-phase epitaxy (R-SPE) method and the subsequent ion-exchange treatment, were investigated in detail by using selected-area electron diffraction, high-resolution transmission electron microcopy, spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (Cs-corrected HAADF-STEM), and electron energy-loss spectroscopy (EELS). Detailed electron diffraction analyses reveal that the orientation relationships between Ca0.33CoO2 thin film and substrate are [11 (2) over bar0](Ca0.33CoO2)//[0001](Al2O3) and (0001)(Ca0.33CoO2), having an angle of about 43 degrees with (1 (1) over bar 00)(Al2O3) for the film deposited oil m(1 (1) over bar 00) plane, and [1 (1) over bar 00](Ca0.33CoO2)//[11 (2) over bar0](Al2O3) and (0001)(Ca0.33CoO2)//(0001)(Al2O3) to the film deposited on c(0001) plane though a Ca-Al-O amorphous layer formed between them. CoO seed layer near the interface and residual Co3O4 phase inside the films were observed and identified by HAADF-STEM and EELS in both samples. Such microstructural configuration indicates that the processes of film growth during R-SPE are (i) oxidation of CoO into Co3O4 With residual CoO layer near the interface and (ii) intercalation of Na+ layer into Co3O4 to achieve the layered NaxCoO2 film while forming Na-Al-O amorphous layer at the interface.
  • 誘電体TiO2/SrTiO3ヘテロ界面の構造と電子状態
    マテリアルインテグレーション 22 9 10,38-42  2009年 [査読無し][通常論文]
  • Yi Feng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Koumoto
    Ceramics International 35 1 537  2009年01月 [査読有り][通常論文]
  • Kenji Nomura, Toshio Kamiya, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    APPLIED PHYSICS LETTERS 93 19 192107  2008年11月 [査読無し][通常論文]
     
    Roles of H(2)O addition to an annealing atmosphere were investigated for amorphous In-Ga-Zn-O thin-film transistors fabricated at room temperature. Although dry O(2) annealing improved saturation mobility (mu(sat)) and subthreshold voltage swings (S), wet O(2) annealing further improved them to mu(sat)similar to 12 cm(2)(V s)(-1) and S < 0.12 V decade(-1) along with improvement of their uniformity. Desorption of OH-related species caused conductivity increase during thermal annealing at < 310 degrees C. Zn-O components started to desorb at similar to 300 degrees C for the unannealed and the dry O(2) annealed films, while these were suppressed remarkably by the wet O(2) annealing.
  • Rong Huang, Teruyasu Mizoguchi, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto, Tsukasa Hirayama, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 93 18 181907  2008年11月 [査読無し][通常論文]
     
    Ca-ion superstructures such as root 3a X root 3a (hexagonal) and 2a X root 3a (orthorhombic) in layered Ca0.33CoO2 crystalline film, which were prepared by the reactive solid-phase epitaxy and the subsequent ion-exchange treatment, were directly distinguished by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The fluctuation of Ca intensities in the HAADF-STEM image indicates the existence of many Ca vacancies in the 2a X root 3a orthorhombic superstructure, which is consistent with the partial occupation of Ca (x=0.33) in the cation sites of x=0.5. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3021369]
  • Kenji Nomura, Toshio Kamiya, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    APPLIED PHYSICS LETTERS 93 19 192107  2008年11月 [査読無し][通常論文]
     
    Roles of H(2)O addition to an annealing atmosphere were investigated for amorphous In-Ga-Zn-O thin-film transistors fabricated at room temperature. Although dry O(2) annealing improved saturation mobility (mu(sat)) and subthreshold voltage swings (S), wet O(2) annealing further improved them to mu(sat)similar to 12 cm(2)(V s)(-1) and S < 0.12 V decade(-1) along with improvement of their uniformity. Desorption of OH-related species caused conductivity increase during thermal annealing at < 310 degrees C. Zn-O components started to desorb at similar to 300 degrees C for the unannealed and the dry O(2) annealed films, while these were suppressed remarkably by the wet O(2) annealing.
  • Rong Huang, Teruyasu Mizoguchi, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto, Tsukasa Hirayama, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 93 18 181907  2008年11月 [査読無し][通常論文]
     
    Ca-ion superstructures such as root 3a X root 3a (hexagonal) and 2a X root 3a (orthorhombic) in layered Ca0.33CoO2 crystalline film, which were prepared by the reactive solid-phase epitaxy and the subsequent ion-exchange treatment, were directly distinguished by using spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The fluctuation of Ca intensities in the HAADF-STEM image indicates the existence of many Ca vacancies in the 2a X root 3a orthorhombic superstructure, which is consistent with the partial occupation of Ca (x=0.33) in the cation sites of x=0.5. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3021369]
  • Hiromichi Ohta
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 245 11 2363 - 2368 2008年11月 [査読無し][通常論文]
     
    This review provides the origin of the unusually large thermoelectric Seebeck coefficient |S| of a two-dimensional electron gas confined within a unit cell layer thickness (similar to 0.4 nm) of a SrTi0.8Nb0.2O3 layer of artificial superlattices of SrTiO3/SrTi0.8Nb0.2O3 [H. Ohta et al., Nature Mater 6, 129, (2007)]. The |S|(2D) values of the [(SrTiO3)(17)/SrTi0.8Nb0.2O3)(y)](20) superlattice increase proportional y(0.5), and reach 290 mu VK-1 (y = 1) at room temperature, which is similar to 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S|(3D) = 61 mu VK-1) proving that the density of states in the ground state of SrTiO3 increases in inverse proportion to y. The critical barrier thickness for quantum electron confinement is also clarified to be 6.25 nm (16 unit cells of SrTiO3). Significant structural changes are not observed in the superlattice after annealing at 900 K in a vacuum. The value of |S|(2D) of the superlattice gradually increases with temperature and reaches 450 mu VK-1 at 900 K, which is similar to 3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence than the [(SrTiO3)(17)/SrTi0.8Nb0.2O3)](20) superlattice is stable and exhibits a giant |S| even at high temperature.
  • Hiromichi Ohta, Kenji Sugiura, Kunihito Koumoto
    INORGANIC CHEMISTRY 47 19 8429 - 8436 2008年10月 [査読無し][通常論文]
     
    Thermoelectric energy conversion technology to convert waste heat into electricity has received much attention. In addition, metal oxides have recently been considered as thermoelectric power generation materials that can operate at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness. We have fabricated high-quality epitaxial films composed of oxide thermoelectric materials that are suitable for clarifying the intrinsic "real" properties. This review focuses on the thermoelectric properties of two representative oxide epitaxial films, p-type Ca3CO4O9 and n-type SrTiO3, which exhibit the best thermoelectric figures of merit, ZT(=S-2 sigma T kappa(-1), S = Seebeck coefficient, a = electrical conductivity, K = thermal conductivity, and T = absolute temperature) among oxide thermoelectric materials reported to date. In addition, we introduce the recently discovered giant S of two-dimensional electrons confined within a unit cell layer thickness (similar to 0.4 nm) of SrTiO3.
  • Hiromichi Ohta, Kenji Sugiura, Kunihito Koumoto
    INORGANIC CHEMISTRY 47 19 8429 - 8436 2008年10月 [査読無し][通常論文]
     
    Thermoelectric energy conversion technology to convert waste heat into electricity has received much attention. In addition, metal oxides have recently been considered as thermoelectric power generation materials that can operate at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness. We have fabricated high-quality epitaxial films composed of oxide thermoelectric materials that are suitable for clarifying the intrinsic "real" properties. This review focuses on the thermoelectric properties of two representative oxide epitaxial films, p-type Ca3CO4O9 and n-type SrTiO3, which exhibit the best thermoelectric figures of merit, ZT(=S-2 sigma T kappa(-1), S = Seebeck coefficient, a = electrical conductivity, K = thermal conductivity, and T = absolute temperature) among oxide thermoelectric materials reported to date. In addition, we introduce the recently discovered giant S of two-dimensional electrons confined within a unit cell layer thickness (similar to 0.4 nm) of SrTiO3.
  • K. Nomura, T. Kamiya, H. Ohta, K. Shimizu, M. Hirano, H. Hosono
    PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 205 8 1910 - 1914 2008年08月 [査読無し][通常論文]
  • K. Nomura, T. Kamiya, H. Ohta, K. Shimizu, M. Hirano, H. Hosono
    PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 205 8 1910 - 1914 2008年08月 [査読無し][通常論文]
  • Takayoshi Katase, Kenji Noinura, Hirornichi Ohta, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    THIN SOLID FILMS 516 17 5842 - 5846 2008年07月 [査読無し][通常論文]
     
    Single-crystalline thin films of ScAlMgO4 (SCAM) were fabricated on YSZ (I 11) substrates by reactive solid-phase epitaxy (R-SPE) using ScGaO3(ZnO)(m) (SGZO) single-crystalline buffer layers, which suppress interface reactions between the SCAM layers and the YSZ substrates. First, post-annealing of the SCAM layers fabricated by the R-SPE process was examined. When annealing temperature was raised to > 1200 degrees C, the formation of a spinel MgAl2O4 phase was observed. This is due probably to out-diffusion of Sc ions from the SCAM layers to the YSZ substrates. By introducing an SGZO buffer layer, a single-phase SCAM layer with a step-and-terrace surface was obtained by annealing at 1450 degrees C without the spinel formation. The SCAM layer was grown heteroepitaxially with an epitaxial relationship of (000 1)SCAM // (111)ysz and 010)(SCAM) // (110)ysz. Atomically flat surfaces were eventually formed by chemical-mechanical polishing. The SCAM layer was applied to a lattice-matched buffer layer for the growth of ZnO film, leading to marked lateral growth of ZnO domains. (C) 2007 Published by Elsevier B.V.
  • Takayoshi Katase, Kenji Noinura, Hirornichi Ohta, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    THIN SOLID FILMS 516 17 5842 - 5846 2008年07月 [査読無し][通常論文]
     
    Single-crystalline thin films of ScAlMgO4 (SCAM) were fabricated on YSZ (I 11) substrates by reactive solid-phase epitaxy (R-SPE) using ScGaO3(ZnO)(m) (SGZO) single-crystalline buffer layers, which suppress interface reactions between the SCAM layers and the YSZ substrates. First, post-annealing of the SCAM layers fabricated by the R-SPE process was examined. When annealing temperature was raised to > 1200 degrees C, the formation of a spinel MgAl2O4 phase was observed. This is due probably to out-diffusion of Sc ions from the SCAM layers to the YSZ substrates. By introducing an SGZO buffer layer, a single-phase SCAM layer with a step-and-terrace surface was obtained by annealing at 1450 degrees C without the spinel formation. The SCAM layer was grown heteroepitaxially with an epitaxial relationship of (000 1)SCAM // (111)ysz and 010)(SCAM) // (110)ysz. Atomically flat surfaces were eventually formed by chemical-mechanical polishing. The SCAM layer was applied to a lattice-matched buffer layer for the growth of ZnO film, leading to marked lateral growth of ZnO domains. (C) 2007 Published by Elsevier B.V.
  • Hiromichi Ohta, Yoriko Mune, Kunihito Koumoto, Teruyasu Mizoguchi, Yuichi Ikuhara
    THIN SOLID FILMS 516 17 5916 - 5920 2008年07月 [査読無し][通常論文]
     
    Seebeck coefficient (vertical bar S vertical bar) of two-dimensional electron gas (2DEG) confined within (SrTiO3)(LB)/(SrTi0.8Nb0.2O3)(LW) superlattices were measured at room temperature to clarify the critical thicknesses of barrier SrTiO3 (L-B) and well SrTi0.8Nb0.2O3 (L-W) for giant vertical bar S vertical bar [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The vertical bar S vertical bar values of the superlattices increased proportionally to L-W(-1/2) due to increasing of the density of states near the conduction band edge (quantum size effect), and reached 300 mu V K-1 at L-W = 1 unit cell SrTi0.8Nb0.2O3 (0.39 nm), which is similar to 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (60 mu V K-1). The critical thickness of L-B and L-W for giant vertical bar S vertical bar was clarified to be 16 unit cells (6.25 nm). The best thermoelectric performance can be obtained at (L-B, L-W)=(16, 1). (C) 2007 Elsevier B.V. All rights reserved.
  • Yi Feng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Kournoto
    CERAMICS INTERNATIONAL 34 4 849 - 852 2008年05月 [査読無し][通常論文]
     
    To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT= S-2 sigma T kappa(-1)) of Ruddlesden-Popper phase SrO(SrTiO3)(n) (or Srn+1TinO3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (a), Seebeck coefficient (S) and thermal conductivity (kappa), of (Sr1-xREx)(n+1)TinO3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr1-xREx)(n+1) TinO3n+1, evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the a values simply depended on the dopant concentration, indicating that both La3+ and Nd3+ ions act as electron donors. The vertical bar S vertical bar values increased with temperature due to decrease in the chemical potential. Significant reduction of the K values was observed as compared to cubic-perovskite SrTiO3. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr0.95La0.05)(3)Ti2O7. (C) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
  • Yi Feng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Kournoto
    CERAMICS INTERNATIONAL 34 4 849 - 852 2008年05月 [査読無し][通常論文]
     
    To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT= S-2 sigma T kappa(-1)) of Ruddlesden-Popper phase SrO(SrTiO3)(n) (or Srn+1TinO3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (a), Seebeck coefficient (S) and thermal conductivity (kappa), of (Sr1-xREx)(n+1)TinO3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr1-xREx)(n+1) TinO3n+1, evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the a values simply depended on the dopant concentration, indicating that both La3+ and Nd3+ ions act as electron donors. The vertical bar S vertical bar values increased with temperature due to decrease in the chemical potential. Significant reduction of the K values was observed as compared to cubic-perovskite SrTiO3. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr0.95La0.05)(3)Ti2O7. (C) 2007 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
  • Atsushi Mizutani, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    CRYSTAL GROWTH & DESIGN 8 3 755 - 758 2008年03月 [査読無し][通常論文]
     
    High-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of alpha-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. The average x-value of the LixCoO2 films was controlled from 0.60 to 0.93 by immersing in a K2S2O8 aqueous solution. The metal (x(met). <= 0.72)/insulator (x(ins). <= 0.83) phase separation was clearly observed in the range of 0.72 < x < 0.87. Seebeck coefficient (S) for the Li0.6CoO2 film was +38 mu V center dot K-1, and it increased gradually with the x-value (x <= 0.72), while no significant x-value dependence of S was observed at x > 0.72, indicating that the metal phase dominates the S-value of the metal/insulator mixture.
  • Atsushi Mizutani, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    CRYSTAL GROWTH & DESIGN 8 3 755 - 758 2008年03月 [査読無し][通常論文]
     
    High-quality epitaxial films of Li0.93CoO2 were successfully grown on the (0001)-face of alpha-Al2O3 substrates via the topotactic ion exchange of Na0.8CoO2 epitaxial films. The average x-value of the LixCoO2 films was controlled from 0.60 to 0.93 by immersing in a K2S2O8 aqueous solution. The metal (x(met). <= 0.72)/insulator (x(ins). <= 0.83) phase separation was clearly observed in the range of 0.72 < x < 0.87. Seebeck coefficient (S) for the Li0.6CoO2 film was +38 mu V center dot K-1, and it increased gradually with the x-value (x <= 0.72), while no significant x-value dependence of S was observed at x > 0.72, indicating that the metal phase dominates the S-value of the metal/insulator mixture.
  • Yukiaki Ishida, Ritsuko Eguchi, Masaharu Matsunami, Koji Horiba, Munetaka Taguchi, Ashish Chainani, Yasunori Senba, Haruhiko Ohashi, Hiromichi Ohta, Shik Shin
    PHYSICAL REVIEW LETTERS 100 5 056401  2008年02月 [査読無し][通常論文]
     
    Resonant photoemission at the Ti 2p and O 1s edges on a Nb-doped SrTiO(3) thin film revealed that the coherent state (CS) at the Fermi level (E(F)) had a mainly Ti 3d character whereas the incoherent in-gap state (IGS) positioned similar to 1.5 eV below E(F) had a mixed character of Ti 3d and O 2p states. This indicates that the IGS is formed by a spectral-weight transfer from the CS and subsequent spectral-weight redistribution through d-p hybridization. We discuss the evolution of the excitation spectrum with 3d band filling and rationalize the IGS through a mechanism similar to that proposed by Haldane and Anderson.
  • Yukiaki Ishida, Ritsuko Eguchi, Masaharu Matsunami, Koji Horiba, Munetaka Taguchi, Ashish Chainani, Yasunori Senba, Haruhiko Ohashi, Hiromichi Ohta, Shik Shin
    PHYSICAL REVIEW LETTERS 100 5 056401  2008年02月 [査読無し][通常論文]
     
    Resonant photoemission at the Ti 2p and O 1s edges on a Nb-doped SrTiO(3) thin film revealed that the coherent state (CS) at the Fermi level (E(F)) had a mainly Ti 3d character whereas the incoherent in-gap state (IGS) positioned similar to 1.5 eV below E(F) had a mixed character of Ti 3d and O 2p states. This indicates that the IGS is formed by a spectral-weight transfer from the CS and subsequent spectral-weight redistribution through d-p hybridization. We discuss the evolution of the excitation spectrum with 3d band filling and rationalize the IGS through a mechanism similar to that proposed by Haldane and Anderson.
  • Hiromichi Ohta, Rong Huang, Yuichi Ikuhara
    Physica Status Solidi - Rapid Research Letters 2 3 105 - 107 2008年 [査読無し][通常論文]
     
    Herein we demonstrate that amorphous oxide semiconductor (AOS) superlattices composed of a-In-Zn-O (well) and a-In-Ga-Zn-O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, exhibited an enhanced Seebeck coefficient |S |. The |S | value increases drastically with decreasing a-In-Zn-O thickness (dIZO) when dIZO < ∼5 nm, and reached 73 uV K (dIZO = 0.3 nm), which is ∼4 times larger than that of bulk |S|3D(19 uV K '), while it kept its high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • Kyu Hyoung Lee, Yoriko Mune, Hiromichi Ohta, Kunihito Koumoto
    APPLIED PHYSICS EXPRESS 1 1 015007  2008年01月 [査読無し][通常論文]
     
    Herein we report the carrier transport properties of [(SrTi0O(3))(x)/(SrTi0.8Nb0.2O3)(1)](20) (x = 0-50) superlattices at high temperatures (T = 300-900K). Significant structural changes were not observed in the superlattices after annealing at 900K in a vacuum. The Seebeck coefficient of the [(SrTiO3)(20)/(SrTi0.8Nb0.2O3)(1)](20) superlattice, which was 300 mu V.K-1 at room temperature, gradually increased with temperature and reached 450 mu V.K-1 at 900 K, which is similar to 3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence that the superlattice is stable and exhibits a giant Seebeck coefficient even at high temperature. (C) 2008 The Japan Society of Applied Physics.
  • Critical thickness for giant thermoelectric Seebeck coefficient of 2DEG confined in SrTiO3/SrTi0.8Nb0.2O3 superlattices
    Thin Solid Films 8 755  2008年 [査読無し][通常論文]
  • Two-dimensional thermoelectric Seebeck coefficient of the SrTiO3 based superlattices
    phys. stat. sol. (b) 245 2363  2008年 [査読無し][通常論文]
  • Hiromichi Ohta, Rong Huang, Yuichi Ikuhara
    Physica Status Solidi - Rapid Research Letters 2 3 105 - 107 2008年 [査読無し][通常論文]
     
    Herein we demonstrate that amorphous oxide semiconductor (AOS) superlattices composed of a-In-Zn-O (well) and a-In-Ga-Zn-O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, exhibited an enhanced Seebeck coefficient |S |. The |S | value increases drastically with decreasing a-In-Zn-O thickness (dIZO) when dIZO < ∼5 nm, and reached 73 uV K (dIZO = 0.3 nm), which is ∼4 times larger than that of bulk |S|3D(19 uV K '), while it kept its high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • Kyu Hyoung Lee, Yoriko Mune, Hiromichi Ohta, Kunihito Koumoto
    APPLIED PHYSICS EXPRESS 1 1 015007  2008年01月 [査読無し][通常論文]
     
    Herein we report the carrier transport properties of [(SrTi0O(3))(x)/(SrTi0.8Nb0.2O3)(1)](20) (x = 0-50) superlattices at high temperatures (T = 300-900K). Significant structural changes were not observed in the superlattices after annealing at 900K in a vacuum. The Seebeck coefficient of the [(SrTiO3)(20)/(SrTi0.8Nb0.2O3)(1)](20) superlattice, which was 300 mu V.K-1 at room temperature, gradually increased with temperature and reached 450 mu V.K-1 at 900 K, which is similar to 3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence that the superlattice is stable and exhibits a giant Seebeck coefficient even at high temperature. (C) 2008 The Japan Society of Applied Physics.
  • 熱電コバルト酸化物のキャリア・エントロピーの光電子分光による直接観測
    固体物理 43 343  2008年 [査読無し][通常論文]
  • 反応性固相エピタキシャル成長法
    機能材料 28 27  2008年 [査読無し][通常論文]
  • 電気学会誌 128 5 2008年 [査読無し][通常論文]
  • 酸化物熱電変換材料の開発動向
    セラミックス 43 966  2008年 [査読無し][通常論文]
  • 厚さ0.4ナノ㍍の極薄SrTiO3シートが発生する巨大熱起電力- 酸化物熱電変換材料の設計
    未来材料 8 2008年 [査読無し][通常論文]
  • Kyu Hyoung Lee, Yi Feng Wang, Hiromichi Ohta, Kunihito Koumoto
    ADVANCES IN ELECTRONIC CERAMICS 28 8 193 - + 2008年 [査読有り][通常論文]
     
    We have investigated the thermoelectric properties and crystallographic features of Ruddlesden-Popper phase Ca- and/or Nb-doped SrO(SrTiO3)(n) (n = 1, 2) to elucidate their potential as thermoelectric materials and clarify the influence of crystal structure on the carrier effective mass in Ti-based oxides containing TiO6 octahedra. Significant reduction in the lattice thermal conductivity by suppression of the mean free path of phonons possibly associated with the presence of internal sublattice SrO/(SrTiO3)n interfaces was observed. It was also found that large Seebeck coefficients of Ti-based oxides can be attributed to the high value of carrier effective mass, which originates from the high symmetry of the TiO6 octahedra. The overall ZT values obtained in the present study were in the range of 0.09 - 0.15 at 1000 K.
  • R. Funahashi, A. Kosuga, N. Miyasou, E. Takeuchi, S. Urata, K. Lee, H. Ohta, K. Koumoto
    PROCEEDINGS ICT 07: TWENTY-SIXTH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 124 - 128 2008年 [査読有り][通常論文]
     
    Ca1-xRExMnO3 (RE: Nd, Tb, Ho, Yb, Lu, x = 0.1 and 0.2) bulks were prepared by a solid state reaction. They show metallic behavior at temperatures higher than about 400 K and electrical resistivity rho is lower than 12 m Omega cm at 1000 K in air. Absolute values of Seebeck coefficient S increases with temperature up to 973 K for all samples except Ca0.9Yb0.1MnO3 at 973 K. For Ca0.9Lu0.1MnO3, S value reaches -130 mu V/K at 973 K. Both thermoelectric properties are dominated mainly by crystallographic structure. Power factor PF (=S-2/rho) increases with decreasing cell volume. Thermal conductivity kappa of Yb- and Lu-substituted samples is as low as 1.5 W/m-K-2 and dimension-less figure of merit ZT reaches 0.16 at 973 K for Ca0.8Lu0.2MnO3 in air.
  • Tomohito Nakano, Wataru Kobayashi, Ryoji Funahashi, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto, Ichiro Terasaki
    PROCEEDINGS ICT 07: TWENTY-SIXTH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 121 - 123 2008年 [査読有り][通常論文]
     
    We synthesized whisker-crystal and thin-film samples of the layered cobalt oxide (Bi2Sr2O4)(x)CoO2 well known as a candidate of a thermoelectric material, and measured their resistivity, voltage-current characteristics and magnetoresistance with pulsed currents. Resistivity of the whisker sample with a small current density of 2 mA/cm(2) increases with decreasing temperature below 100 K, suggesting development of a pseudogap on the density of states. This increase is suppressed by a large current density of 110 A/cm(2). The nonlinear voltage-current characteristics are observed both for the whisker and thin-film samples. The negative magnetoresistance of the thin-film sample is suppressed by current density. These suggest that the density of states recovers owing to the suppression of the pseudogap by applying large current density.
  • Hiromichi Ohta, Rong Huang, Yuichi Ikuhara
    THERMOELECTRIC POWER GENERATION 1044 395 - + 2008年 [査読有り][通常論文]
     
    We propose herein that amorphous oxide semiconductor (AOS) superlattices, which can be deposited on various substrate including glasses or plastics without any substrate heating, are appropriate for the realization of superlattice thermoelectric devices. As an example, thermoelectric properties of AOS superlattices composed of a-In-Zn-O (well) and a-In-Ga-Zn-O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, were measured to clarify whether enhancement of Seebeck coefficient |S| occurs or not. The |S|(2D) value increases drastically with decreasing a-In-Zn-O thickness (d(IZO)) when the d(IZO) is < similar to 5 nm, and reached 73 mu V.K-1 (d(IZO) = 0.3 nm), which is similar to 4 times larger than that of bulk |S|(3D) (19 mu V.K-1), while it kept high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices.
  • Yuki Nakanishi, Hiromichi Ohta, Teruyasu Mizoguchi, Yuichi Ikuhara, Kunihito Koumoto
    THERMOELECTRIC POWER GENERATION 1044 375 - + 2008年 [査読有り][通常論文]
     
    Carrier transport properties such as carrier concentration (n(e)), Hall mobility (mu(Hall)) and Seebeck coefficient (S) for the superlattices of [(BaTiO3)(x)/(SrTi0.7Nb0.3O3)(1)](z) (x = 1, 4, 6, 9 and 15) were measured to clarify the critical barrier thickness for the electron confinement in a unit cell layer thickness of SrTi0.7Nb0.3O3. The mu(Hall) values drastically decreased with the x-value due to the Ba-alloying occurred. We observed enhancement of |S| value in BaTiO3/SrTi1-xNbxO3 superlattices due to the quantum size effect, which is similar to SrTiO3/SrTi0.8Nb0.2O3 superlattices. Critical BaTiO3 layer thickness for fully confinement of the conduction electron significantly decreased (6 unit cells) as compared to that of SrTiO3/SrTi0.8Nb0.2O3 superlattices.
  • Yoriko Mune, Hiromichi Ohta, Teruyasu Mizoguchi, Yuichi Ikuhara, Kunihito Koumoto
    THERMOELECTRIC POWER GENERATION 1044 369 - + 2008年 [査読有り][通常論文]
     
    We report two-dimensional Seebeck coefficients (|S|(2D)) of [(SrTio(3))(x)/(SrTi0.8Nb0.2O3)(y)](20) (x 1-50, y = 1-16) superlattices, which were grown on the (100)-face of insulating LaAlO3 substrates, to clarify the origin of the giant |S|(2D) values of the SrTiO3 superlattices [H. Ohta et al., Nature Materials 6, 129 (2007)]. The |S|(2D) values of the [(SrTiO3)(17)/(SrTi0.8Nb0.2O3)(y)](20) superlattices increased proportionally to Y-05, and reached 320 mu V K-1 (y = 1), which is similar to 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S|(3D) = 61 mu V K-1). The slope of the log |S|(2D)-log y plots was -0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. Further, the |S|(2D) value monotonically increases with x-value and is saturated when x-value > 16 (6.25 nm). We clarified that the critical barrier thickness for electron tunneling in [(SrTiO3)(x)/(SrTi0.8Nb0.2O3)(y)](z) superlattice is 6.25 nm (16 unit cell layers of SrTiO3).
  • Keisuke Kato, Masahiro Yamamoto, Shingo Ohta, Hiroaki Muta, Ken Kurosaki, Shinsuke Yamanaka, Hideo Iwasaki, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 102 11 116107  2007年12月 [査読無し][通常論文]
     
    Herein we demonstrate that Eu2+ substitution of SrTi0.8Nb0.2O3 is effective in reducing the thermal conductivity, with no reduction in the thermoelectric power factor (S-2 sigma; S, Seebeck coefficient; sigma, electrical conductivity). We prepared cubic (Sr,Eu)Ti0.8Nb0.2O3 crystals having the same lattice parameter (a=0.395 nm) over the whole composition range. Although no dependence upon the Eu content was observed for the electron transport properties, including the density of states effective mass and carrier relaxation time for (Sr,Eu)Ti0.8Nb0.2O3, the mean free path of the phonon was reduced similar to 12% at room temperature when half of the Sr2+ was substituted by Eu2+. (C) 2007 American Institute of Physics.
  • Keisuke Kato, Masahiro Yamamoto, Shingo Ohta, Hiroaki Muta, Ken Kurosaki, Shinsuke Yamanaka, Hideo Iwasaki, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 102 11 116107  2007年12月 [査読無し][通常論文]
     
    Herein we demonstrate that Eu2+ substitution of SrTi0.8Nb0.2O3 is effective in reducing the thermal conductivity, with no reduction in the thermoelectric power factor (S-2 sigma; S, Seebeck coefficient; sigma, electrical conductivity). We prepared cubic (Sr,Eu)Ti0.8Nb0.2O3 crystals having the same lattice parameter (a=0.395 nm) over the whole composition range. Although no dependence upon the Eu content was observed for the electron transport properties, including the density of states effective mass and carrier relaxation time for (Sr,Eu)Ti0.8Nb0.2O3, the mean free path of the phonon was reduced similar to 12% at room temperature when half of the Sr2+ was substituted by Eu2+. (C) 2007 American Institute of Physics.
  • Yifeng Wang, Kyu Hyoung Lee, Hideki Hyuga, Hideki Kita, Katsuhiko Inaba, Hiromichi Ohta, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 91 24 2007年12月 [査読有り][通常論文]
     
    We found that Sm3+ substitution in SrO (SrTiO3)(2) is effective in improving the Seebeck coefficient (S). The vertical bar S vertical bar value increases notably with temperature, benefiting from an enhancement of the density of states (DOS) effective mass m(d)(*) from similar to 3m(0) (300 K) to similar to 7.5m(0) (1000 K), due to an improvement of the local symmetry of TiO6 octahedra, enhancing the degeneracy in the Ti 3d orbitals, which form the conduction band (CB), and also to an accompanying lattice expansion, which gives rise to a higher DOS at the bottom of the CB and, consequently, a larger m(d)(*). (c) 2007 American Institute of Physics.
  • Yoriko Mune, Hiromichi Ohta, Kunihito Koumoto, Teruyasu Mizoguchi, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 91 19 192105  2007年11月 [査読無し][通常論文]
     
    We report two-dimensional Seebeck coefficients (parallel to S parallel to(2D)) of [(SrTiO3)(x)/(SrTi0.8Nb0.2O3)(y)](20) (x=1-60, y=1-20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant parallel to S parallel to(2D) values of the SrTiO3 superlattices [H. Ohta , Nat. Mater. 6, 129 (2007)]. The parallel to S parallel to(2D) values of the [(SrTiO3)(17)/(SrTi0.8Nb0.2O3)(y)](20) superlattices increased proportionally to y(-0.5) and reached 320 mu V K-1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (parallel to S parallel to(3D)=61 mu V K-1). The slope of the log parallel to S parallel to(2D)-log y plots was -0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25 nm (16 unit cells of SrTiO3).
  • Yoriko Mune, Hiromichi Ohta, Kunihito Koumoto, Teruyasu Mizoguchi, Yuichi Ikuhara
    APPLIED PHYSICS LETTERS 91 19 192105  2007年11月 [査読無し][通常論文]
     
    We report two-dimensional Seebeck coefficients (parallel to S parallel to(2D)) of [(SrTiO3)(x)/(SrTi0.8Nb0.2O3)(y)](20) (x=1-60, y=1-20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant parallel to S parallel to(2D) values of the SrTiO3 superlattices [H. Ohta , Nat. Mater. 6, 129 (2007)]. The parallel to S parallel to(2D) values of the [(SrTiO3)(17)/(SrTi0.8Nb0.2O3)(y)](20) superlattices increased proportionally to y(-0.5) and reached 320 mu V K-1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (parallel to S parallel to(3D)=61 mu V K-1). The slope of the log parallel to S parallel to(2D)-log y plots was -0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25 nm (16 unit cells of SrTiO3).
  • Yukiaki Ishida, Hiromichi Ohta, Atsushi Fujimori, Hideo Hosono
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 76 10 103709  2007年10月 [査読無し][通常論文]
     
    We have performed a temperature-dependent photoemission study of a NaxCoO(2) (x similar to 0.8) epitaxial thin film prepared by the reactive solid-phase epitaxy method. The chemical potential shift as a function of temperature was derived from the Co 3d peak shift, and revealed a crossover from the degenerate Fermion state at low temperatures to the correlated hopping state of Co3+/Co4+ mixed-valence at high temperatures. This suggests that the large thermoelectric power at high temperatures should be considered in the correlated hopping picture.
  • Yukiaki Ishida, Hiromichi Ohta, Atsushi Fujimori, Hideo Hosono
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 76 10 103709  2007年10月 [査読無し][通常論文]
     
    We have performed a temperature-dependent photoemission study of a NaxCoO(2) (x similar to 0.8) epitaxial thin film prepared by the reactive solid-phase epitaxy method. The chemical potential shift as a function of temperature was derived from the Co 3d peak shift, and revealed a crossover from the degenerate Fermion state at low temperatures to the correlated hopping state of Co3+/Co4+ mixed-valence at high temperatures. This suggests that the large thermoelectric power at high temperatures should be considered in the correlated hopping picture.
  • Hiromichi Ohta
    Materials Today 10 10 44 - 49 2007年10月 [査読無し][通常論文]
     
    Thermoelectric energy conversion technology is attracting great attention for the conversion of waste heat into electricity. For practical thermoelectric applications, the figure of merit for thermoelectric performance ZT needs to be > 1. Although heavy-metal-based materials such as Bi2Te3 and PbTe exhibit ZT ∼ 1, they are not attractive for applications because they are mostly toxic and decomposition, vaporization, or melting of the constituents can easily occur at high temperatures. We review a new class of thermoelectric material based on a metal oxide: a two-dimensional electron gas (2DEG) in SrTiO3. The 2DEG demonstrates a Seebeck coefficient S that is enhanced by a factor of ∼5 compared with the bulk and an optimized ZT that reaches 2.4, twice that of conventional thermoelectric materials. © 2007 Elsevier Ltd. All rights reserved.
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Tomohiro Saito, Yuichi Ikuhara, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    MATERIALS TRANSACTIONS 48 8 2104 - 2107 2007年08月 [査読無し][通常論文]
     
    To clarify the thermoelectric properties of the layered cobalt oxide Ca3Co4O9 epitaxial film fabricated by the topotactic ion-exchange method [K. Sugiura et al., Appl. Phys. Lett. 89, 032111 (2006)], the electrical resistivity (rho), Hall coefficient (R-H), and Seebeck coefficient (S) of high quality Ca3Co4O9 epitaxial film were examined over the temperature range 10-1000 K. The film exhibited a low rho of 4.0 x 10(-3) Omega cm and a large S of similar to+200 mu V K-1, which leads to the thermoelectric power factor of 1.0 x 10(-3) W m(-1) K-2, at 1000 K. Furthermore, the film exhibited good thermal stability at 1000 K in air.
  • Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    International Journal of Applied Ceramic Technology 4 4 308 - 317 2007年08月 [査読無し][通常論文]
     
    This article reviews high-quality epitaxial film growth of layered cobalt oxides by reactive solid-phase epitaxy (R-SPE) with topotactic ion-exchange methods. Epitaxial film of Na0.8CoO2 was grown on a (0001)-oriented α-Al2O3 substrate by R-SPE using CoO film as the starting material. The Na0.8 CoO2 epitaxial films were converted into high-quality epitaxial films of Sr0.32Na0.21 CoO2 and [Ca2 CoO3]x CoO2 by topotactic ion-exchange methods. The Sr0.32 Na0.21CoO2 film exhibited better stability against moisture than that of the Na0.8 CoO2 film, while it retained the good thermoelectric properties of Na0.8 CoO2. The [Ca2CoO3]x CoO2 film exhibited a high electrical conductivity of 2.95 × 102S/cm and a large Seebeck coefficient of +125μV/ K at 300K. © 2007 The American Ceramic Society.
  • Kyu Hyoung Lee, Yi Feng Wang, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    International Journal of Applied Ceramic Technology 4 4 326 - 331 2007年08月 [査読無し][通常論文]
     
    We report herein on Ruddlesden-Popper-type doped Srn+1 TinO3n+1 (n =2) as a potential candidate for n-type thermoelectric (TE) oxides. The TE properties of 5at.% La-, Nd-, and Nb-doped Sr3Ti2O7 polycrystalline ceramics were investigated and the origin of Seebeck coefficient was discussed from the viewpoint of the symmetry of TiO6 octahedra. A significant reduction in the lattice thermal conductivity was observed by the enhancement of phonon scattering at SrO/(SrTiO3)n interfaces originating from the inherent superlattice structure, and the maximum dimensionless figure of merit, ZT ∼0.15, at a 1000K value was obtained in 5at.% La-doped Sr3Ti2O7. © 2007 The American Ceramic Society.
  • Kyu Hyoung Lee, Akihiro Ishizaki, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 102 3 033702  2007年08月 [査読無し][通常論文]
     
    We report the epitaxial film growth, by pulsed laser deposition, of c-axis-oriented Nb-doped SrO(SrTiO3)(1) films having a K2NiF4-type structure and their thermoelectric properties over a wide temperature range, from 20 to 800 K. High-resolution x-ray diffraction and reflection high energy electron diffraction patterns revealed that the films were grown heteroepitaxially on the (001) face of the LaAlO3 substrate. The thermoelectric properties of the Nb-doped SrO(SrTiO3)(1) epitaxial films were improved in terms of power factor compared to those for polycrystalline Nb-doped SrO(SrTiO3)(1). A theoretical analysis of the transport parameters revealed that the intrinsic transport occurred predominantly in the SrTiO3 layers. (c) 2007 American Institute of Physics.
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Tomohiro Saito, Yuichi Ikuhara, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    MATERIALS TRANSACTIONS 48 8 2104 - 2107 2007年08月 [査読無し][通常論文]
     
    To clarify the thermoelectric properties of the layered cobalt oxide Ca3Co4O9 epitaxial film fabricated by the topotactic ion-exchange method [K. Sugiura et al., Appl. Phys. Lett. 89, 032111 (2006)], the electrical resistivity (rho), Hall coefficient (R-H), and Seebeck coefficient (S) of high quality Ca3Co4O9 epitaxial film were examined over the temperature range 10-1000 K. The film exhibited a low rho of 4.0 x 10(-3) Omega cm and a large S of similar to+200 mu V K-1, which leads to the thermoelectric power factor of 1.0 x 10(-3) W m(-1) K-2, at 1000 K. Furthermore, the film exhibited good thermal stability at 1000 K in air.
  • Kyu Hyoung Lee, Yi Feng Wang, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    International Journal of Applied Ceramic Technology 4 4 326 - 331 2007年08月 [査読無し][通常論文]
     
    We report herein on Ruddlesden-Popper-type doped Srn+1 TinO3n+1 (n =2) as a potential candidate for n-type thermoelectric (TE) oxides. The TE properties of 5at.% La-, Nd-, and Nb-doped Sr3Ti2O7 polycrystalline ceramics were investigated and the origin of Seebeck coefficient was discussed from the viewpoint of the symmetry of TiO6 octahedra. A significant reduction in the lattice thermal conductivity was observed by the enhancement of phonon scattering at SrO/(SrTiO3)n interfaces originating from the inherent superlattice structure, and the maximum dimensionless figure of merit, ZT ∼0.15, at a 1000K value was obtained in 5at.% La-doped Sr3Ti2O7. © 2007 The American Ceramic Society.
  • Kyu Hyoung Lee, Akihiro Ishizaki, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 102 3 033702  2007年08月 [査読無し][通常論文]
     
    We report the epitaxial film growth, by pulsed laser deposition, of c-axis-oriented Nb-doped SrO(SrTiO3)(1) films having a K2NiF4-type structure and their thermoelectric properties over a wide temperature range, from 20 to 800 K. High-resolution x-ray diffraction and reflection high energy electron diffraction patterns revealed that the films were grown heteroepitaxially on the (001) face of the LaAlO3 substrate. The thermoelectric properties of the Nb-doped SrO(SrTiO3)(1) epitaxial films were improved in terms of power factor compared to those for polycrystalline Nb-doped SrO(SrTiO3)(1). A theoretical analysis of the transport parameters revealed that the intrinsic transport occurred predominantly in the SrTiO3 layers. (c) 2007 American Institute of Physics.
  • Hidenori Hiramatsu, Kazushige Ueda, Hiromichi Ohta, Masahiro Hirano, Maiko Kikuchi, Hiroshi Yanagi, Toshio Kamiya, Hideo Hosono
    APPLIED PHYSICS LETTERS 91 1 012104  2007年07月 [査読無し][通常論文]
     
    The high density hole doping (1.7x10(21) cm(-3)) for a wide gap (E-g=similar to 2.8 eV) p-type semiconductor was achieved on 40 nm thick Mg-doped LaCuOSe epitaxial films. These films exhibited distinct free carrier absorption, and the effective mass and momentum relaxation time were analyzed. Its small hole mobility [similar to 3.5 cm(2)/(V s)] compared to the electron mobilities of wide gap n-type semiconductors is attributed to a heavy effective mass of 1.6 +/- 0.2m(e). Regardless of the heavy hole doping, a band filling effect was not observed. These results are discussed with a rigid band model and an acceptor band model. (c) 2007 American Institute of Physics.
  • Hidenori Hiramatsu, Kazushige Ueda, Hiromichi Ohta, Masahiro Hirano, Maiko Kikuchi, Hiroshi Yanagi, Toshio Kamiya, Hideo Hosono
    APPLIED PHYSICS LETTERS 91 1 012104  2007年07月 [査読無し][通常論文]
     
    The high density hole doping (1.7x10(21) cm(-3)) for a wide gap (E-g=similar to 2.8 eV) p-type semiconductor was achieved on 40 nm thick Mg-doped LaCuOSe epitaxial films. These films exhibited distinct free carrier absorption, and the effective mass and momentum relaxation time were analyzed. Its small hole mobility [similar to 3.5 cm(2)/(V s)] compared to the electron mobilities of wide gap n-type semiconductors is attributed to a heavy effective mass of 1.6 +/- 0.2m(e). Regardless of the heavy hole doping, a band filling effect was not observed. These results are discussed with a rigid band model and an acceptor band model. (c) 2007 American Institute of Physics.
  • Kyu Hyoung Lee, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 101 8 083707  2007年04月 [査読無し][通常論文]
     
    The thermoelectric properties and crystallographic features of layered perovskite-type Ca-substituted (Sr1-xCax)(3)(Ti1-yNby)(2)O-7 (y=0-0.2), Ruddlesden-Popper phases were investigated. The influence of crystal structure and carrier concentration on the carrier effective mass for Ti-based metal oxides based on TiO6 octahedra and having a significant Ca substitution effect are discussed. It was found from structural analysis that the large Seebeck coefficients of Ca-substituted compounds can be attributed to the high values of carrier effective mass, which originates from the enhancement of the symmetry of the TiO6 octahedra. Low thermal conductivities, 3.6-3.8 W m(-1) K-1 at 300 K, were obtained for Ca-substituted compositions due to phonon scattering at SrO/(SrTiO3)(2) interfaces of the inherent superlattice structure and to mass-defect phonon scattering between Ca (M-Ca=40) and Sr (M-Sr=88). The highest ZT value obtained was 0.15 at 1000 K, for 10 at % Ca-substituted Sr-3(Ti0.95Nb0.05)(2)O-7. (c) 2007 American Institute of Physics.
  • Kyu Hyoung Lee, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 101 8 083707  2007年04月 [査読無し][通常論文]
     
    The thermoelectric properties and crystallographic features of layered perovskite-type Ca-substituted (Sr1-xCax)(3)(Ti1-yNby)(2)O-7 (y=0-0.2), Ruddlesden-Popper phases were investigated. The influence of crystal structure and carrier concentration on the carrier effective mass for Ti-based metal oxides based on TiO6 octahedra and having a significant Ca substitution effect are discussed. It was found from structural analysis that the large Seebeck coefficients of Ca-substituted compounds can be attributed to the high values of carrier effective mass, which originates from the enhancement of the symmetry of the TiO6 octahedra. Low thermal conductivities, 3.6-3.8 W m(-1) K-1 at 300 K, were obtained for Ca-substituted compositions due to phonon scattering at SrO/(SrTiO3)(2) interfaces of the inherent superlattice structure and to mass-defect phonon scattering between Ca (M-Ca=40) and Sr (M-Sr=88). The highest ZT value obtained was 0.15 at 1000 K, for 10 at % Ca-substituted Sr-3(Ti0.95Nb0.05)(2)O-7. (c) 2007 American Institute of Physics.
  • Hiromichi Ohta, Sungwng Kim, Yorika Mune, Teruyasu Mizoguchi, Kenji Nomura, Shingo Ohta, Takashi Nomura, Yuki Nakanishi, Yuichi Ikuhara, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    NATURE MATERIALS 6 2 129 - 134 2007年02月 [査読無し][通常論文]
     
    Enhancement of the Seebeck coeffcient (S) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT = S-2 (.) s (.) T (.) kappa(-1)) exceeding 2, where T is the absolute temperature and. is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO3 yields unusually large vertical bar S vertical bar, approximately five times larger than that of SrTiO3 bulks, while maintaining a high sigma(2DEG). In the best case, we observe vertical bar S vertical bar = 850 mu VK-1 and sigma(2DEG) = 1.4 x 10(3) S cm(-1). In addition, by using the kappa of bulk single-crystal SrTiO3 at room temperature, we estimate ZT similar to 2.4 for the 2DEG, corresponding to ZT similar to 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.
  • Masahiro Yamamoto, Hiromichi Ohta, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 90 7 072101  2007年02月 [査読無し][通常論文]
     
    Herein the authors propose a compositional thermoelectric phase diagram, which includes the density of states effective mass (m(d)(*)) carrier relaxation time (tau), and thermoelectric power factor (S-2 sigma, where S and sigma are Seebeck coefficient and electrical conductivity) for high-quality epitaxial films composed of 20%-Nb-doped CaTiO3-SrTiO3-BaTiO3 solid solutions [(Ca,Sr,Ba)(Ti0.8Nb0.2)O-3]. The m(d)(*) values almost proportionally increase with the in-plane lattice parameter of (Ca,Sr,Ba)(Ti0.8Nb0.2)O-3 epitaxial film, while the tau values drastically decrease when Ca and/or Ba are substituted for Sr in Sr(Ti0.8Nb0.2)O-3, indicating that the Sr-site substitution negatively affects the thermoelectric performance of Nb-doped SrTiO3. (c) 2007 American Institute of Physics.
  • Hiromichi Ohta, Sungwng Kim, Yorika Mune, Teruyasu Mizoguchi, Kenji Nomura, Shingo Ohta, Takashi Nomura, Yuki Nakanishi, Yuichi Ikuhara, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    NATURE MATERIALS 6 2 129 - 134 2007年02月 [査読無し][通常論文]
     
    Enhancement of the Seebeck coeffcient (S) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT = S-2 (.) s (.) T (.) kappa(-1)) exceeding 2, where T is the absolute temperature and. is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO3 yields unusually large vertical bar S vertical bar, approximately five times larger than that of SrTiO3 bulks, while maintaining a high sigma(2DEG). In the best case, we observe vertical bar S vertical bar = 850 mu VK-1 and sigma(2DEG) = 1.4 x 10(3) S cm(-1). In addition, by using the kappa of bulk single-crystal SrTiO3 at room temperature, we estimate ZT similar to 2.4 for the 2DEG, corresponding to ZT similar to 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.
  • Masahiro Yamamoto, Hiromichi Ohta, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 90 7 072101  2007年02月 [査読無し][通常論文]
     
    Herein the authors propose a compositional thermoelectric phase diagram, which includes the density of states effective mass (m(d)(*)) carrier relaxation time (tau), and thermoelectric power factor (S-2 sigma, where S and sigma are Seebeck coefficient and electrical conductivity) for high-quality epitaxial films composed of 20%-Nb-doped CaTiO3-SrTiO3-BaTiO3 solid solutions [(Ca,Sr,Ba)(Ti0.8Nb0.2)O-3]. The m(d)(*) values almost proportionally increase with the in-plane lattice parameter of (Ca,Sr,Ba)(Ti0.8Nb0.2)O-3 epitaxial film, while the tau values drastically decrease when Ca and/or Ba are substituted for Sr in Sr(Ti0.8Nb0.2)O-3, indicating that the Sr-site substitution negatively affects the thermoelectric performance of Nb-doped SrTiO3. (c) 2007 American Institute of Physics.
  • Kenji Nomura, Toshio Kamiya, Hiromichi Ohta, Tomoya Uruga, Masahiro Hirano, Hideo Hosono
    PHYSICAL REVIEW B 75 3 035212  2007年01月 [査読無し][通常論文]
     
    Ionic amorphous oxide semiconductors (IAOSs) are new materials for flexible thin film transistors that exhibit field-effect mobilities of similar to 10 cm(2) V(-1) s(-1) [K. Nomura , Nature 488, 432 (2004)]. The local coordination structure in an IAOS, In-Ga-Zn-O (a-IGZO), was examined using extended x-ray absorption fine structure analysis combined with ab initio calculations. The short-range ordering and coordination structures in a-IGZO are similar to those in the corresponding crystalline phase, InGaZnO(4), and edge-sharing structures consisting of In-O polyhedra remain in the amorphous structure. The In(3+) 5s orbitals form an extended state with a band effective mass of similar to 0.2m(e) at the conduction band bottom.
  • Materials Today 10 44  2007年 [査読無し][通常論文]
  • Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    INTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY 4 4 308 - 317 2007年 [査読無し][通常論文]
     
    This article reviews high-quality epitaxial film growth of layered cobalt oxides by reactive solid-phase epitaxy (R-SPE) with topotactic ion-exchange methods. Epitaxial film of Na0.8CoO2 was grown on a (0001)-oriented alpha-Al2O3 substrate by R-SPE using CoO film as the starting material. The Na0.8CoO2 epitaxial films were converted into high-quality epitaxial films of Sr0.32Na0.21CoO2 and [Ca2CoO3](x)CoO2 by topotactic ion-exchange methods. The Sr0.32Na0.21CoO2 film exhibited better stability against moisture than that of the Na0.8CoO2 film, while it retained the good thermoelectric properties of Na0.8CoO2. The [Ca2CoO3](x)CoO2 film exhibited a high electrical conductivity of 2.95 x 10(2) S/cm and a large Seebeck coefficient of +125 mu V/K at 300 K.
  • Kenji Nomura, Toshio Kamiya, Hiromichi Ohta, Tomoya Uruga, Masahiro Hirano, Hideo Hosono
    PHYSICAL REVIEW B 75 3 035212  2007年01月 [査読無し][通常論文]
     
    Ionic amorphous oxide semiconductors (IAOSs) are new materials for flexible thin film transistors that exhibit field-effect mobilities of similar to 10 cm(2) V(-1) s(-1) [K. Nomura , Nature 488, 432 (2004)]. The local coordination structure in an IAOS, In-Ga-Zn-O (a-IGZO), was examined using extended x-ray absorption fine structure analysis combined with ab initio calculations. The short-range ordering and coordination structures in a-IGZO are similar to those in the corresponding crystalline phase, InGaZnO(4), and edge-sharing structures consisting of In-O polyhedra remain in the amorphous structure. The In(3+) 5s orbitals form an extended state with a band effective mass of similar to 0.2m(e) at the conduction band bottom.
  • 毒性物質を使わない高性能酸化物熱電材料の開発
    熱電学会誌 3 2  2007年 [査読無し][通常論文]
  • 酸化物熱電変換材料-誘電体中に閉じ込められた二次元電子の巨大熱起電力-
    セラミックス 42 592  2007年 [査読無し][通常論文]
  • 誘電体チタン酸ストロンチウム結晶に閉じ込めた電子の巨大熱起電力
    機能材料 27 69 2007年 [査読無し][通常論文]
  • 脱・重金属!!ありふれた金属酸化物で廃熱を電気に変える
    化学 62 31  2007年 [査読無し][通常論文]
  • 熱電現象の新発見-2DEGが発生する巨大熱起電力
    セラミックデータブック2007(工業製品技術協会) 35 84  2007年 [査読無し][通常論文]
  • K. H. Lee, A. Ishizaki, H. Ohta, K. Koumoto
    International Conference on Thermoelectrics, ICT, Proceedings 136 - 139 2007年 [査読有り][通常論文]
     
    Thermoelectric properties of the heavily Nb-doped SrO(SrTiO 3)1 were investigated over a wide temperature range (20 - 800 K) by the preparation of c-axis-oriented epitaxial films. The films were grown heteroepitaxially on the (001)-face of the LaAlO3 substrate with an epitaxial relationship of (001) [100] Nb-doped SrO(SrTiO 3)1 || (001) [100] LaAlO3. The thermoelectric properties of the Nb-doped SrO(SrTiO3)1 epitaxial films were enhanced compared with those of the polycrystalline ceramics, and the electronic transport occurred predominantly in the SrTiO3 layers. © 2008 IEEE.
  • Y. F. Wang, K. H. Lee, H. Ohta, K. Koumoto
    International Conference on Thermoelectrics, ICT, Proceedings 157 - 160 2007年 [査読有り][通常論文]
     
    We herein report the significant relaitonship between the thermoelectric properties and the crystallogrphic characteristics, especially the local symmetry of TiO6 octahedra, in the n = 2 Ruddlesden-Popper (RP)-type phases of 5 at.%-rare earth (RE3+ = La3+, Nd3+, Sm3+ and Gd3+)-doped SrO(SrTiO3)2. It was observed that doped RE3+ ions occupied preferentially the Sr-site (CN = 9) in SrO layer, which had a favorable restoration effect on the distorted TiO6 octahedra which was found more effective in smaller-sized RE3+ -doped compounds, as partially proven by the increase in O3-Ti-O3 bond angle with decreasing in ionic radius of RE 3+. The Sm3+ and Gd3+-doped SrO(SrTiO 3)2 exhibited larger Seebeck coefficient (|S|) than those of La3+ and Nd3+-doped SrO(SrTiO3)2 over the whole measured temperature range (300~1000 K). This is considered to be due to the enhancement of the density of states (DOS) in Ti 3d orbitals originated from improved symmetry in TiO6 octahedra by smaller RE3+ doping. © 2008 IEEE.
  • Hiromichi Ohta, Shingo Ohta, Kunihito Koumoto
    HIGH-PERFORMANCE CERAMICS IV, PTS 1-3 336-338 809 - + 2007年 [査読有り][通常論文]
     
    Nb-content dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on (100)-face of LaAlO3 single crystal substrates by a pulsed-laser deposition. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT reached 0.37 (20%-Nb-doped SrTiO3 epitaxial film) at 1000 K, which is the largest value among n-type oxide semiconductors ever reported.
  • Toshio Kamiya, Yujiro Takeda, Kenji Nomura, Hiromichi Ohta, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono
    CRYSTAL GROWTH & DESIGN 6 11 2451 - 2456 2006年11月 [査読無し][通常論文]
     
    Effects of a lattice-matched buffer layer on growth, structure, and transport properties of ZnO thin films were examined using a homologous series layered compound, InGaO3(ZnO)(5) (IGZO5). High-quality single-crystalline IGZO5 layers were fabricated on yttria-stabilized zirconia (YSZ) (111) substrates by reactive solid-phase epitaxy. ZnO layers deposited on the IGZO5 buffer layers showed smooth and uniform surface morphologies compared to those deposited on YSZ (111) without the IGZO5 buffer layer. Then postgrowth annealing was examined to convert the ZnO surface to an atomically flat surface. When the IGZO5 buffer layer was used, the annealing temperature was reduced from 1000 to 700 degrees C, which was attributable to the smooth surface morphology and good lattice matching of the buffer layer. The annealed films exhibited rather large Hall mobilities 73-82 cm(2) (Vs)(-1), which are larger than those reported for epitaxial ZnO films fabricated at similar temperatures with similar thicknesses. Cross-sectional high-resolution transmission electron microscopy revealed that the step height of the IGZO5 buffer layer is automatically adjusted to the c-axis period of the ZnO crystal lattice. The present findings demonstrate the IGZO5 layer works as a three-dimensional lattice-matching substrate for ZnO.
  • Daisuke Kurita, Shingo Ohta, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 100 9 096105  2006年11月 [査読無し][通常論文]
     
    To compare the intrinsic thermoelectric (TE) properties of heavily Nb-doped TiO2 to those of heavily Nb-doped SrTiO3 [S. Ohta , Appl. Phys. Lett. 87, 092108 (2005)], the electrical conductivity (sigma), carrier concentration (n(e)), Hall mobility (mu(Hall)), and Seebeck coefficient (S) of heavily Nb-doped TiO2 (anatase) epitaxial films were measured at high temperatures (300-900 K). The epitaxial films were grown on the (100)-face of LaAlO3 single-crystalline substrates by a pulsed-laser deposition technique at 800 degrees C. The carrier effective mass (m(*)) of the anatase TiO2 epitaxial films was similar to 1m(0), which is an order of magnitude smaller than that of Nb-doped SrTiO3 (similar to 10m(0)). The estimated TE power factor (S-2 sigma) of the similar to 2%-Nb-doped anatase TiO2 film (n(e)similar to 5x10(20) cm(-3)) was similar to 2.5x10(-4) W m(-1) K-2 at 900 K, which is approximately 15% of the 20%-Nb-doped SrTiO3 (1.5x10(-3) W m(-1) K-2). The present findings will help establish a future TE material design concept for Ti-based metal oxides. (c) 2006 American Institute of Physics.
  • Toshio Kamiya, Yujiro Takeda, Kenji Nomura, Hiromichi Ohta, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono
    Cryst. Growth Des. 6 11 2451 - 2456 2006年11月 [査読無し][通常論文]
     
    Effects of a lattice-matched buffer layer on growth, structure, and transport properties of ZnO thin films were examined using a homologous series layered compound, InGaO3(ZnO)(5) (IGZO5). High-quality single-crystalline IGZO5 layers were fabricated on yttria-stabilized zirconia (YSZ) (111) substrates by reactive solid-phase epitaxy. ZnO layers deposited on the IGZO5 buffer layers showed smooth and uniform surface morphologies compared to those deposited on YSZ (111) without the IGZO5 buffer layer. Then postgrowth annealing was examined to convert the ZnO surface to an atomically flat surface. When the IGZO5 buffer layer was used, the annealing temperature was reduced from 1000 to 700 degrees C, which was attributable to the smooth surface morphology and good lattice matching of the buffer layer. The annealed films exhibited rather large Hall mobilities 73-82 cm(2) (Vs)(-1), which are larger than those reported for epitaxial ZnO films fabricated at similar temperatures with similar thicknesses. Cross-sectional high-resolution transmission electron microscopy revealed that the step height of the IGZO5 buffer layer is automatically adjusted to the c-axis period of the ZnO crystal lattice. The present findings demonstrate the IGZO5 layer works as a three-dimensional lattice-matching substrate for ZnO.
  • Daisuke Kurita, Shingo Ohta, Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    J. Appl. Phys. 100 9 096105  2006年11月 [査読無し][通常論文]
     
    To compare the intrinsic thermoelectric (TE) properties of heavily Nb-doped TiO2 to those of heavily Nb-doped SrTiO3 [S. Ohta , Appl. Phys. Lett. 87, 092108 (2005)], the electrical conductivity (sigma), carrier concentration (n(e)), Hall mobility (mu(Hall)), and Seebeck coefficient (S) of heavily Nb-doped TiO2 (anatase) epitaxial films were measured at high temperatures (300-900 K). The epitaxial films were grown on the (100)-face of LaAlO3 single-crystalline substrates by a pulsed-laser deposition technique at 800 degrees C. The carrier effective mass (m(*)) of the anatase TiO2 epitaxial films was similar to 1m(0), which is an order of magnitude smaller than that of Nb-doped SrTiO3 (similar to 10m(0)). The estimated TE power factor (S-2 sigma) of the similar to 2%-Nb-doped anatase TiO2 film (n(e)similar to 5x10(20) cm(-3)) was similar to 2.5x10(-4) W m(-1) K-2 at 900 K, which is approximately 15% of the 20%-Nb-doped SrTiO3 (1.5x10(-3) W m(-1) K-2). The present findings will help establish a future TE material design concept for Ti-based metal oxides. (c) 2006 American Institute of Physics.
  • D. Flahaut, T. Mihara, R. Funahashi, N. Nabeshima, K. Lee, H. Ohta, K. Koumoto
    JOURNAL OF APPLIED PHYSICS 100 8 084911  2006年10月 [査読無し][通常論文]
     
    CaMnO3 is an electron-doped compound which belongs to the perovskite family. Despite its high Seebeck coefficient S value, the figure of merit at high temperature remains low due to its large resistivity rho(rho(300 K)=2 Omega cm). To optimize the performance of this material in terms of thermoelectric properties, several substitutions have been attempted on the Ca site to decrease the rho. Structure and thermoelectric properties of polycrystalline samples Ca(1-x)A(x)MnO(3) (A=Yb, Tb, Nd, and Ho) have been investigated. Although rho strongly depends on the ionic radius < r(A)> and carrier concentration, we have shown that the thermal conductivity kappa is mainly driven by the atomic weight of the A site and decreases with it. Therefore, it seems that the S, rho, and kappa could be controlled separately. For instance, the highest dimensionless ZT (=0.16) has been obtained at 1000 K in the air for Ca0.9Yb0.1MnO3. (c) 2006 American Institute of Physics.
  • Yukiaki Ishida, Atsushi Fujimori, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    APPLIED PHYSICS LETTERS 89 15 153502  2006年10月 [査読無し][通常論文]
     
    The authors have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO/p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV. (c) 2006 American Institute of Physics.
  • D. Flahaut, T. Mihara, R. Funahashi, N. Nabeshima, K. Lee, H. Ohta, K. Koumoto
    JOURNAL OF APPLIED PHYSICS 100 8 084911  2006年10月 [査読無し][通常論文]
     
    CaMnO3 is an electron-doped compound which belongs to the perovskite family. Despite its high Seebeck coefficient S value, the figure of merit at high temperature remains low due to its large resistivity rho(rho(300 K)=2 Omega cm). To optimize the performance of this material in terms of thermoelectric properties, several substitutions have been attempted on the Ca site to decrease the rho. Structure and thermoelectric properties of polycrystalline samples Ca(1-x)A(x)MnO(3) (A=Yb, Tb, Nd, and Ho) have been investigated. Although rho strongly depends on the ionic radius < r(A)> and carrier concentration, we have shown that the thermal conductivity kappa is mainly driven by the atomic weight of the A site and decreases with it. Therefore, it seems that the S, rho, and kappa could be controlled separately. For instance, the highest dimensionless ZT (=0.16) has been obtained at 1000 K in the air for Ca0.9Yb0.1MnO3. (c) 2006 American Institute of Physics.
  • Yukiaki Ishida, Atsushi Fujimori, Hiromichi Ohta, Masahiro Hirano, Hideo Hosono
    Appl. Phys. Lett. 89 15 153502  2006年10月 [査読無し][通常論文]
     
    The authors have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO/p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV. (c) 2006 American Institute of Physics.
  • Kyu Hyoung Lee, Sung Wng Kim, Hiromichi Ohta, Kunihito Koumoto
    JOURNAL OF APPLIED PHYSICS 100 6 063717  2006年09月 [査読無し][通常論文]
     
    A class of materials known as superlattices has shown substantial promise for potential thermoelectric (TE) applications because of its low thermal conductivity. We have investigated natural superlattice Ruddlesden-Popper (RP) phases [S. N. Ruddlesden and P. Popper, Acta Crystallogr. 10, 538 (1957)] to elucidate their potential as TE materials. The TE properties of Nb-doped SrO(SrTiO3)(n) (n=1,2) with a RP structure were measured, and the origin of the TE properties is discussed from the viewpoint of the structure of the TiO6 octahedron. Compared with the cubic perovskite-type Nb-doped SrTiO3, the lattice thermal conductivity decreased by more than 50% (4.4-5 W m(-1) K-1) at room temperature and by 30% (1.9-2.2 W m(-1) K-1) at 1000 K. There was a decrease in electrical conductivity owing to the randomly distributed insulating SrO layers in polycrystalline RP phases, and it was found that large TE power can be obtained in conjunction with high symmetry TiO6 octahedra. The largest dimensionless figure of merit (ZT), 0.14 at 1000 K, was obtained in 5 at. % Nb-doped SrO(SrTiO3)(2). (c) 2006 American Institute of Physics.
  • Hidenori Hiramatsu, Hayato Kamioka, Kazushige Ueda, Hiromichi Ohta, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 203 11 2800 - 2811 2006年09月 [査読無し][通常論文]
     
    Electronic and optical properties of widegap oxychalcogenides, LaCuOCh (Ch = chalcogen) and La2CdO2Se2, are reviewed with a focus on those relevant to their layered crystal structures, including high hole mobility, degenerate p-type conduction, room temperature exciton, and large third order optical nonlinearity. In particular, the widegap p-type metallic conduction was realized in Mg-doped LaCuOSe: the first demonstration among any class of widegap materials including GaN: Mg. Furthermore, we demonstrate the room temperature operation of a blue light-emitting diode using a pn hetero-junction composed of a LaCuOSe epilayer and an n-type amorphous InGaZn5O8. Those results strongly suggest that a series of the layered oxychalcogenides are applicable to the light-emitting layers in opto-electronic devices that operate in the ultraviolet-blue region as well as to transparent p-type conductors. (c) 2006 WILEY-VCH Verlag GmbH & Co. KG&A, Weinheim.
  • Hidenori Hiramatsu, Hayato Kamioka, Kazushige Ueda, Hiromichi Ohta, Toshio Kamiya, Masahiro Hirano, Hideo Hosono
    Phys. Stat. Sol. (a) 203 11 2800 - 2811 2006年09月 [査読無し][通常論文]
     
    Electronic and optical properties of widegap oxychalcogenides, LaCuOCh (Ch = chalcogen) and La2CdO2Se2, are reviewed with a focus on those relevant to their layered crystal structures, including high hole mobility, degenerate p-type conduction, room temperature exciton, and large third order optical nonlinearity. In particular, the widegap p-type metallic conduction was realized in Mg-doped LaCuOSe: the first demonstration among any class of widegap materials including GaN: Mg. Furthermore, we demonstrate the room temperature operation of a blue light-emitting diode using a pn hetero-junction composed of a LaCuOSe epilayer and an n-type amorphous InGaZn5O8. Those results strongly suggest that a series of the layered oxychalcogenides are applicable to the light-emitting layers in opto-electronic devices that operate in the ultraviolet-blue region as well as to transparent p-type conductors. (c) 2006 WILEY-VCH Verlag GmbH & Co. KG&A, Weinheim.
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 89 3 032111  2006年07月 [査読無し][通常論文]
     
    Epitaxial film of a layered cobalt oxide, Ca3Co4O9, was fabricated on a (0001) face of an alpha-Al2O3 substrate by a topotactic ion-exchange method using a gamma-Na0.8CoO2 epitaxial film as a precursor. High-resolution x-ray diffraction and atomic force microscope measurements revealed that the film was high-quality (001)-oriented Ca3Co4O9 with stepped and terraced surface morphology. The film exhibits a high electrical conductivity of 2.95x10(2) S cm(-1) and a large Seebeck coefficient of similar to+125 mu V K-1, which leads to the thermoelectric power factor (TPF) of 4.5x10(-4) W m(-1) K-2 at 300 K, potentially usable as a building block of the multilayered film structure with an enhanced TPF value. (c) 2006 American Institute of Physics.
  • Hiromichi Ohta, Atsushi Mizutani, Kenji Sugiura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    ADVANCED MATERIALS 18 13 1649 - + 2006年07月 [査読無し][通常論文]
     
    An epitaxially grown NaxCoO2 film can be peeled off from its alpha-Al2O3 substrate by dissolving the interfacial layer between the film and the substrate in water, as shown in the figure. The freestanding film thus formed can be pasted on an amorphous glass substrate and used for growing an epitaxial ZnO film. This method can be used to impart atomic regularity to amorphous glass, polymer, and metal surfaces.
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    APPLIED PHYSICS LETTERS 89 3 032111  2006年07月 [査読無し][通常論文]
     
    Epitaxial film of a layered cobalt oxide, Ca3Co4O9, was fabricated on a (0001) face of an alpha-Al2O3 substrate by a topotactic ion-exchange method using a gamma-Na0.8CoO2 epitaxial film as a precursor. High-resolution x-ray diffraction and atomic force microscope measurements revealed that the film was high-quality (001)-oriented Ca3Co4O9 with stepped and terraced surface morphology. The film exhibits a high electrical conductivity of 2.95x10(2) S cm(-1) and a large Seebeck coefficient of similar to+125 mu V K-1, which leads to the thermoelectric power factor (TPF) of 4.5x10(-4) W m(-1) K-2 at 300 K, potentially usable as a building block of the multilayered film structure with an enhanced TPF value. (c) 2006 American Institute of Physics.
  • Hiromichi Ohta, Atsushi Mizutani, Kenji Sugiura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    ADVANCED MATERIALS 18 13 1649 - + 2006年07月 [査読無し][通常論文]
     
    An epitaxially grown NaxCoO2 film can be peeled off from its alpha-Al2O3 substrate by dissolving the interfacial layer between the film and the substrate in water, as shown in the figure. The freestanding film thus formed can be pasted on an amorphous glass substrate and used for growing an epitaxial ZnO film. This method can be used to impart atomic regularity to amorphous glass, polymer, and metal surfaces.
  • K Nomura, A Takagi, T Kamiya, H Ohta, M Hirano, H Hosono
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 5B 4303 - 4308 2006年05月 [査読無し][通常論文]
  • K Nomura, A Takagi, T Kamiya, H Ohta, M Hirano, H Hosono
    Jpn. J. Appl. Phys. 45 5B 4303 - 4308 2006年05月 [査読無し][通常論文]
  • K Sugiura, H Ohta, K Nomura, H Yanagi, M Hirano, H Hosono, K Koumoto
    INORGANIC CHEMISTRY 45 5 1894 - 1896 2006年03月 [査読無し][通常論文]
     
    We have developed a unique multistep film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate epitaxial films of superconducting sodium-cobalt oxyhydrate, Na(0.3)CoO(2)center dot 1.3H(2)O. An epitaxial film of Na0.8CoO2 grown on an alpha-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na(0.3)CoO(2)center dot 1.3H(2)O epitaxial film. The film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the epitaxial growth of superconducting Na(0.3)CoO(2)center dot 1.3H(2)O.
  • K Sugiura, H Ohta, K Nomura, H Yanagi, M Hirano, H Hosono, K Koumoto
    Inorg. Chem. (communication) 45 5 1894 - 1896 2006年03月 [査読無し][通常論文]
     
    We have developed a unique multistep film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate epitaxial films of superconducting sodium-cobalt oxyhydrate, Na(0.3)CoO(2)center dot 1.3H(2)O. An epitaxial film of Na0.8CoO2 grown on an alpha-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na(0.3)CoO(2)center dot 1.3H(2)O epitaxial film. The film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the epitaxial growth of superconducting Na(0.3)CoO(2)center dot 1.3H(2)O.
  • Y Ogo, K Nomura, H Yanagi, H Ohta, T Kamiya, M Hirano, H Hosono
    THIN SOLID FILMS 496 1 64 - 69 2006年02月 [査読無し][通常論文]
     
    A reactive solid-phase epitaxy (R-SPE) method combines deposition of a thick amorphous or polycrystalline layer with a desired chemical composition and post-deposition solid-phase epitaxial growth. The solid-phase epitaxial growth is invoked by thermal annealing with all assistance of a sacrificial layer working as an epitaxial template. Thereby it enables Lis to grow high-quality epitaxial films of complex oxides whose epitaxial films are not grown by conventional high-temperature growth techniques. It was reported that 2-nm-thick ZnO layers worked as template for growing InGaO3(ZnO)(m) (m = integer) epitaxial films. The present study extended the R-SPE technique to growth of various complex oxides with chemical compositions of RAO(3)(MO)(m) and to use of various epitaxial template layers. We found that mono oxide epitaxial layers such as In2O3 and Ga2O3 work as template layers as well. Alternatively, a ZnO epitaxial layer is also applicable to ZnO-free compounds. The films obtained were grown heteroepitaxially on YSZ(111) and single-crystalline when the fabrication conditions are optimized. (c) 2005 Elsevier B.V. All rights reserved.
  • K Sugiura, H Ohta, K Nomura, M Hirano, H Hosono, K Koumoto
    APPLIED PHYSICS LETTERS 88 8 082109  2006年02月 [査読無し][通常論文]
     
    Thermoelectric properties of gamma-Sr0.32Na0.21CoO2 epitaxial films, which were fabricated on the (0001)-face of alpha-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with gamma-Na0.8CoO2 epitaxial films, were measured to clarify the effect of Sr2+-ion exchange of gamma-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (sigma), Seebeck coefficient (S), and thermoelectric power factor (PF=S-2 sigma) of the gamma-Sr0.32Na0.21CoO2 film (300 K) were 8.9x10(2) S cm(-1), +120 mu V K-1, and 1.2x10(-3) W m(-1) K-2, respectively. These values were relatively large compared to those of gamma-Na0.8CoO2 epitaxial films (sigma=1.2x10(3) S cm(-1), S=+95 mu V K-1, and PF=1.1x10(-3) W m(-1) K-2). The gamma-Sr0.32Na0.21CoO2 epitaxial film exhibited much improved chemical stability against moisture compared to the gamma-Na0.8CoO2 epitaxial film. The Sr2+-ion exchange of gamma-Na0.8CoO2 is vital for practical thermoelectric applications. (c) 2006 American Institute of Physics.
  • Y Ogo, K Nomura, H Yanagi, H Ohta, T Kamiya, M Hirano, H Hosono
    Thin Solid Films 496 1 64 - 69 2006年02月 [査読無し][通常論文]
     
    A reactive solid-phase epitaxy (R-SPE) method combines deposition of a thick amorphous or polycrystalline layer with a desired chemical composition and post-deposition solid-phase epitaxial growth. The solid-phase epitaxial growth is invoked by thermal annealing with all assistance of a sacrificial layer working as an epitaxial template. Thereby it enables Lis to grow high-quality epitaxial films of complex oxides whose epitaxial films are not grown by conventional high-temperature growth techniques. It was reported that 2-nm-thick ZnO layers worked as template for growing InGaO3(ZnO)(m) (m = integer) epitaxial films. The present study extended the R-SPE technique to growth of various complex oxides with chemical compositions of RAO(3)(MO)(m) and to use of various epitaxial template layers. We found that mono oxide epitaxial layers such as In2O3 and Ga2O3 work as template layers as well. Alternatively, a ZnO epitaxial layer is also applicable to ZnO-free compounds. The films obtained were grown heteroepitaxially on YSZ(111) and single-crystalline when the fabrication conditions are optimized. (c) 2005 Elsevier B.V. All rights reserved.
  • K Sugiura, H Ohta, K Nomura, M Hirano, H Hosono, K Koumoto
    Appl. Phys. Lett. 88 8 082109  2006年02月 [査読無し][通常論文]
     
    Thermoelectric properties of gamma-Sr0.32Na0.21CoO2 epitaxial films, which were fabricated on the (0001)-face of alpha-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with gamma-Na0.8CoO2 epitaxial films, were measured to clarify the effect of Sr2+-ion exchange of gamma-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (sigma), Seebeck coefficient (S), and thermoelectric power factor (PF=S-2 sigma) of the gamma-Sr0.32Na0.21CoO2 film (300 K) were 8.9x10(2) S cm(-1), +120 mu V K-1, and 1.2x10(-3) W m(-1) K-2, respectively. These values were relatively large compared to those of gamma-Na0.8CoO2 epitaxial films (sigma=1.2x10(3) S cm(-1), S=+95 mu V K-1, and PF=1.1x10(-3) W m(-1) K-2). The gamma-Sr0.32Na0.21CoO2 epitaxial film exhibited much improved chemical stability against moisture compared to the gamma-Na0.8CoO2 epitaxial film. The Sr2+-ion exchange of gamma-Na0.8CoO2 is vital for practical thermoelectric applications. (c) 2006 American Institute of Physics.
  • H Ohta
    JOURNAL OF THE CERAMIC SOCIETY OF JAPAN 114 1326 147 - 154 2006年02月 [査読無し][通常論文]
     
    High-quality epitaxial film growth for several complex oxides having layered structure by "Reactive Solid-Phase-Epitaxy (R-SPE1))" is reviewed. Materials focused are InGaO3 (ZnO)(m) (m=integer), LaCuOS and NaxCoO2 (x similar to 0.8). For the epitaxial film growths of the complex systems with a complicated layered crystal structure, it is necessary to form an epitaxial template layer before the film deposition, followed by the thermal annealing of the bilayer films. The layers subsequently deposited on the template layer may be amorphous, polycrystalline or powder, which allows control of the epitaxially grown films in the R-SPE method.
  • Reactive Solid-Phase Epitaxy: A powerful method for epitaxial film growth of complex layered oxides [Review]
    J. Ceram. Soc. Japan 114 147  2006年 [査読無し][通常論文]
  • Shingo Ohta, Hiromichi Ohta, Kunihito Koumoto
    Journal of the Ceramic Society of Japan 114 1325 102 - 105 2006年 [査読無し][通常論文]
     
    We investigated thermoelectric properties of 20-%-Nb-doped SrTiO 3 at high-temperature (300-1000 K) using single-crystalline epitaxial film, polycrystalline film (average grain size ∼200 nm) and ceramic (average grain size ∼20 μm) samples to clarify the effect of grain boundaries on the thermoelectric performance. Although carrier concentration and Seebeck coefficient of all samples showed no significant differences, Hall mobility of the polycrystalline film was extremely small (≪10-1 cm2V-1S-1 at 300 K) as compared to those of the epitaxial film and the ceramic sample (∼3 cm 2V-1S-1). However, it drastically increased with increasing temperature and exhibited the value similar to those of the epitaxial film and the ceramic sample above 700 K. The thermoelectric figure of merit for 20%-Nb-doped SrTiO3 was found to reach 0.35 at 1000 K irrespective of the grain size.
  • S Ohta, H Ohta, K Koumoto
    J. Ceram. Soc. Japan 114 1325 102 - 105 2006年01月 [査読無し][通常論文]
     
    We investigated thermoelectric properties of 20-%-Nb-doped SrTiO3 at high-temperature (300-1000 K) using single-crystalline epitaxial film, polycrystalline film (average grain size similar to 200 nm) and ceramic (average grain size similar to 20 mu m) samples to clarify the effect of grain boundaries on the thermoelectric performance. Although carrier concentration and Seebeck coefficient of all samples showed no significant differences, Hall mobility of the polycrystalline film was extremely small (<< 10(-1) cm(2)V(-1)s(-1) at 300 K) as compared to those of the epitaxial film and the ceramic sample (similar to 3 cm(2)V(-1)s(-1)). However, it drastically increased with increasing temperature and exhibited the value similar to those of the epitaxial film and the ceramic sample above 700 K. The thermoelectric figure of merit for 20%-Nb-doped SrTiO3 was found to reach 0.35 at 1000 K irrespective of the grain size.
  • 反応性固相エピタキシャル成長法
    リガクジャーナル 37 3  2006年 [査読無し][通常論文]
  • K. H. Lee, H. Ohta, K. Koumoto
    International Conference on Thermoelectrics, ICT, Proceedings 81 - 84 2006年 [査読有り][通常論文]
     
    Thermoelectric (TE) properties of natural superlattice Ruddlesden-Popper (RP) phases were measured to elucidate their potential as TE materials, and the origin of the TE properties was discussed from the viewpoint of the shape of the TiO6 octahedron. Compared with the cubic perovskite-type phase Nb-doped SrTiO3, efficient reduction of the lattice thermal conductivity was observed by more than 50% at room temperature and 30% at 1000 K. There was a decrease in electrical conductivity owing to the randomly distributed insulating SrO layers in polycrystalline RP phases, and it was found that large TE power can be obtained in conjunction with high symmetry TiO 6 octahedra. © 2006 IEEE.
  • D. Flahaut, R. Funahashi, K. Lee, H. Ohta, K. Koumoto
    ICT'06: XXV INTERNATIONAL CONFERENCE ON THERMOELECTRICS, PROCEEDINGS 103 - + 2006年 [査読有り][通常論文]
     
    Ca1-xYbxMnO3 (x = 0.05) samples were prepared via solid state reaction in air. Electrical and thermoelectric properties have been investigated up to 1000K. The measurements reveal that the resistivity values are strongly affected by the charge carrier content and the octahedral distortion. The lowest p reaches 3m Omega.cm for x=0.15. Whereas the Seebeck coefficient depends only on the charge carrier concentration, the thermal conductivity of Ca1-xYbxMnO3 is mainly governed by the mass difference between the Yb and Ca cations. The best ZT value, ZT=0.2, is obtained for x=0.05 at 1000K and demonstrates the good potentialities of these oxides as high temperature thermoelectric material.
  • Y. Ishida, H. Ohta, A. Fujimori, H. Hosono
    LOW TEMPERATURE PHYSICS, PTS A AND B 850 1215 - + 2006年 [査読有り][通常論文]
     
    The electronic structure of NaxCoO2 photoemission spectroscopy (PES). We present the first successful angle-resolved PES (ARPES) measurements on a NaxCoO2 epitaxial thin film prepared by the reactive solid-phase epitaxy method. Dispersions in ARPES and a 6-fold low-energy electron-diffraction pattern were observed. The Co 3d-derived peak (located similar to 0.8 eV below Fermi level) showed an anomalous shift to higher binding energies at elevated temperatures, which indicates the importance of large spin entropy.
  • Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Tomohiro Saito, Yuichi Ikuhara, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    ICT'06: XXV INTERNATIONAL CONFERENCE ON THERMOELECTRICS, PROCEEDINGS 99 - + 2006年 [査読有り][通常論文]
     
    Epitaxial film of a layered cobalt oxide, Ca3Co4O9, was fabricated on a (0001)-face of alpha-Al2O3 substrate by a topotactic ion exchange method using a Na0.8CoO2 epitaxial film as a precursor. High-resolution X-ray diffraction and atomic force microscope measurements revealed that the film was high-quality (001)-oriented Ca3Co4O9 with stepped & terraced surface morphology. The film exhibits a high electrical conductivity of 2.95 x 10(2) Scm(-1) and a large Seebeck coefficient of similar to+125 mu VK-1, which leads to the thermoelectric power factor of 4.5 x 10(-4) Wm(-1)K(-2) at 300 K.
  • H Hiramatsu, K Ueda, H Ohta, T Kamiya, M Hirano, H Hosono
    APPLIED PHYSICS LETTERS 87 21 211107  2005年11月 [査読無し][通常論文]
     
    A hetero p/n junction diode was fabricated by laminating an amorphous n-type InGaZn5O8 layer to a p-type LaCuOSe film epitaxially grown on a MgO (001) substrate. It exhibited a relatively sharp blue electroluminescence (EL) that peaked at similar to 430 nm at room temperature when a forward bias voltage above 8 V was applied. The wavelength and bandwidth of the EL band agreed well with those of the excitonic photoluminescence band in LaCuOSe, which indicates that the EL band originates from the exciton in LaCuOSe. This experiment strongly suggests that layered compounds, LnCuOCh (Ln=lanthanide, Ch=chalcogen), are promising as the light-emitting layer in optoelectronic devices that operate in the blue-ultraviolet region. (c) 2005 American Institute of Physics.
  • H Hiramatsu, K Ueda, H Ohta, T Kamiya, M Hirano, H Hosono
    Appl. Phys. Lett. 87 21 211107  2005年11月 [査読無し][通常論文]
     
    A hetero p/n junction diode was fabricated by laminating an amorphous n-type InGaZn5O8 layer to a p-type LaCuOSe film epitaxially grown on a MgO (001) substrate. It exhibited a relatively sharp blue electroluminescence (EL) that peaked at similar to 430 nm at room temperature when a forward bias voltage above 8 V was applied. The wavelength and bandwidth of the EL band agreed well with those of the excitonic photoluminescence band in LaCuOSe, which indicates that the EL band originates from the exciton in LaCuOSe. This experiment strongly suggests that layered compounds, LnCuOCh (Ln=lanthanide, Ch=chalcogen), are promising as the light-emitting layer in optoelectronic devices that operate in the blue-ultraviolet region. (c) 2005 American Institute of Physics.
  • Y Takeda, K Nomura, H Ohta, H Yanagi, T Kamiya, M Hirano, H Hosono
    THIN SOLID FILMS 486 1-2 28 - 32 2005年08月 [査読無し][通常論文]
     
    Effects of lattice-matched substrates on growth of ZnO epitaxial films were studied. ZnO thin films were grown on single-crystalline InGaO3(ZnO)(6) (IGZO) layers, which have small lattice mismatches of similar to 0.8% and similar to 2.2% in a- and c-axes, respectively. Epitaxial ZnO films were grown with the epitaxial relationship between the ZnO film and the single-crystalline IGZO of [0001](ZnO)//[111](IGZO)//[111](YSZ) and [1120](ZnO)//[1201](IGZO)//[110](YSZ). The use of the lattice-matched substrate and optimization of film microstructure and post-annealing condition led to atomically flat surfaces at maximum process temperatures as low as 700 degrees C. A large Hall electron mobility -80 cm(2) (V s)(-1) (N-e: similar to 2.8 X 10(18) cm(-3)) was obtained even if the film thickness was only 150 nm although comparable mobilities have been reported on films having much larger thicknesses (similar to 1000 nm) fabricated at higher temperatures similar to 1000 degrees C. (c) 2004 Published by Elsevier B.V.
  • A Takagi, K Nomura, H Ohta, H Yanagi, T Kamiya, M Hirano, H Hosono
    THIN SOLID FILMS 486 1-2 38 - 41 2005年08月 [査読無し][通常論文]
     
    Carrier transport properties in amorphous oxide semiconductor InGaZnO4 (a-IGZO) thin films were investigated in detail using temperature dependence of Hall measurements. It was found that Hall mobility increased distinctly as carrier concentration increased. Unlikely conventional amorphous semiconductors such as a-Si/H, definite normal Hall voltage signals were observed on the films with carrier concentrations (Nc)> 10(16) cm(-3), and Hall mobilities as large as 15 cm(2) (Vs)(-1) were attained in the films with Ne > 10(20) cm(-3). When N, was less than 10 19 cm(-3), the temperature dependence of Hall mobility showed thermally-activated behavior in spite that carrier concentration was independent of temperature. While, it changed to almost degenerate conduction at N-c > 10(18) cm(-3). These behaviors are similar to those observed in single-crystal line IGZO, and are explained by percolation conduction through distributed potential barriers which are formed in the vicinity of the conduction band bottom due to the randomness of the amorphous structure. The effective mass of a-IGZO was estimated to be similar to 0.34 m(e) (m(e) is the mass of free electron) from optical data, which is almost the same as that of crystalline IGZO (-0.32 m(c)). (c) 2004 Published by Elsevier B.V.
  • T Kamiya, K Ueda, H Hiramatsu, H Kamioka, H Ohta, M Hirano, H Hosono
    THIN SOLID FILMS 486 1-2 98 - 103 2005年08月 [査読無し][通常論文]
     
    Electronic structures of layered oxychalcogenides LaCuOCh (Ch-S, Se, Te) were studied using relativistic ab initio band calculations to understand their optical and electronic properties. Step-like structures terminated with one or two sharp peaks were observed in low-temperature (10 K) optical absorption spectra. Third optical nonlinearity measurements supported that the sharp peaks came from split excitonic levels. The ab initio calculations reproduced well these characteristic structures in the spectra and proved that the step-like optical absorption structures originated from two-dimensional nature of the electronic structures associated with the layered crystal structure of LaCuOCh. The split energies of the excitonic levels were quantitatively explained by spin-orbit interaction in the chalcogen ions. (c) 2004 Elsevier B.V. All rights reserved.
  • S Ohta, T Nomura, H Ohta, M Hirano, H Hosono, K Koumoto
    APPLIED PHYSICS LETTERS 87 9 092108  2005年08月 [査読無し][通常論文]
     
    Carrier concentration dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT of Nb-doped SrTiO3 increases with Nb concentration and it reaches similar to 0.37 (20% Nb doped), which is the largest value among n-type oxide semiconductors ever reported.
  • Y Takeda, K Nomura, H Ohta, H Yanagi, T Kamiya, M Hirano, H Hosono
    Thin Solid Films 486 1-2 28 - 32 2005年08月 [査読無し][通常論文]
     
    Effects of lattice-matched substrates on growth of ZnO epitaxial films were studied. ZnO thin films were grown on single-crystalline InGaO3(ZnO)(6) (IGZO) layers, which have small lattice mismatches of similar to 0.8% and similar to 2.2% in a- and c-axes, respectively. Epitaxial ZnO films were grown with the epitaxial relationship between the ZnO film and the single-crystalline IGZO of [0001](ZnO)//[111](IGZO)//[111](YSZ) and [1120](ZnO)//[1201](IGZO)//[110](YSZ). The use of the lattice-matched substrate and optimization of film microstructure and post-annealing condition led to atomically flat surfaces at maximum process temperatures as low as 700 degrees C. A large Hall electron mobility -80 cm(2) (V s)(-1) (N-e: similar to 2.8 X 10(18) cm(-3)) was obtained even if the film thickness was only 150 nm although comparable mobilities have been reported on films having much larger thicknesses (similar to 1000 nm) fabricated at higher temperatures similar to 1000 degrees C. (c) 2004 Published by Elsevier B.V.
  • A Takagi, K Nomura, H Ohta, H Yanagi, T Kamiya, M Hirano, H Hosono
    Thin Solid Films 486 1-2 38 - 41 2005年08月 [査読無し][通常論文]
     
    Carrier transport properties in amorphous oxide semiconductor InGaZnO4 (a-IGZO) thin films were investigated in detail using temperature dependence of Hall measurements. It was found that Hall mobility increased distinctly as carrier concentration increased. Unlikely conventional amorphous semiconductors such as a-Si/H, definite normal Hall voltage signals were observed on the films with carrier concentrations (Nc)> 10(16) cm(-3), and Hall mobilities as large as 15 cm(2) (Vs)(-1) were attained in the films with Ne > 10(20) cm(-3). When N, was less than 10 19 cm(-3), the temperature dependence of Hall mobility showed thermally-activated behavior in spite that carrier concentration was independent of temperature. While, it changed to almost degenerate conduction at N-c > 10(18) cm(-3). These behaviors are similar to those observed in single-crystal line IGZO, and are explained by percolation conduction through distributed potential barriers which are formed in the vicinity of the conduction band bottom due to the randomness of the amorphous structure. The effective mass of a-IGZO was estimated to be similar to 0.34 m(e) (m(e) is the mass of free electron) from optical data, which is almost the same as that of crystalline IGZO (-0.32 m(c)). (c) 2004 Published by Elsevier B.V.
  • T Kamiya, K Ueda, H Hiramatsu, H Kamioka, H Ohta, M Hirano, H Hosono
    Thin Solid Films 486 1-2 98 - 103 2005年08月 [査読無し][通常論文]
     
    Electronic structures of layered oxychalcogenides LaCuOCh (Ch-S, Se, Te) were studied using relativistic ab initio band calculations to understand their optical and electronic properties. Step-like structures terminated with one or two sharp peaks were observed in low-temperature (10 K) optical absorption spectra. Third optical nonlinearity measurements supported that the sharp peaks came from split excitonic levels. The ab initio calculations reproduced well these characteristic structures in the spectra and proved that the step-like optical absorption structures originated from two-dimensional nature of the electronic structures associated with the layered crystal structure of LaCuOCh. The split energies of the excitonic levels were quantitatively explained by spin-orbit interaction in the chalcogen ions. (c) 2004 Elsevier B.V. All rights reserved.
  • S Ohta, T Nomura, H Ohta, M Hirano, H Hosono, K Koumoto
    Appl. Phys. Lett. 87 9 092108  2005年08月 [査読無し][通常論文]
     
    Carrier concentration dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient, and thermal conductivity of Nb-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. ZT of Nb-doped SrTiO3 increases with Nb concentration and it reaches similar to 0.37 (20% Nb doped), which is the largest value among n-type oxide semiconductors ever reported.
  • Peixin Zhu, Takahiro Takeuchi, Hiromichi Ohta, Won-Seon Seo, Kunihito Koumoto
    Materials Transactions 46 7 1453 - 1455 2005年07月 [査読無し][通常論文]
     
    Na xCoO 2/Co 3O 4 layered nano-composite was prepared through the exfoliation, stacking, and sintering processes. Although almost no electrical conduction was observed at low temperatures, a significant increase in conductivity was observed above 750 K, and it reached 1.2 × 10 2 Scm -1 at 1200 K, which is comparable to that of Na 0.7CoO 2. Seebeck coefficient also largely increased above 750 K. This observation might be associated with the electrical behaviors of Co 3O 4 at high temperature. © 2005 The Thermoelectrics Society of Japan.
  • PX Zhu, T Takeuchi, H Ohta, WS Seo, K Koumoto
    Mater. Trans. 46 7 1453 - 1455 2005年07月 [査読無し][通常論文]
     
    NaxCoO2/Co3O4 layered nano-composite was prepared through the exfoliation, stacking, and sintering processes. Although almost no electrical conduction was observed at low temperatures, a significant increase in conductivity was observed above 750K, and it reached 1.2 x 10(2) Scm(-1) at 1200 K, which is comparable to that of Na0.7CoO2. Seebeck coefficient also largely increased above 750 K. This observation might be associated with the electrical behaviors Of Co3O4 at high temperature.
  • T Kamiya, S Narushima, H Mizoguchi, K Shimizu, K Ueda, H Ohta, M Hirano, H Hosono
    ADVANCED FUNCTIONAL MATERIALS 15 6 968 - 974 2005年06月 [査読無し][通常論文]
     
    p-Type conduction in amorphous oxide was firstly found in zinc rhodium oxide (ZnO-Rh2O3) (Adv. Mater 2003,15,1409), and it is still the only p-type amorphous oxide to date. It was reported that an ordered structure at the nanometer scale was contained and its electronic structure is not clear yet. In this paper, optoelectronic and structural properties are reported in detail for xZnO center dot Rh2O3 thin films (x = 0.5-2.0) in relation to the chemical composition x. All the films exhibit positive Seebeck coefficients, confirming p-type conduction. Local network structure strongly depends on the chemical composition. Transmission electron microscopic observations reveal that lattice-like structures made of edge-sharing RhO6 network exist in 2-3 nm sized grains for rhodium-rich films (x = 0.5 and 1.0), while the zinc-rich film (x = 2) is completely amorphous. This result indicates that excess Zn assists to form an amorphous network in the ZnO-Rh2O3 system since Zn ions tend to form corner-sharing networks. The electronic structure of an all-amorphous oxide p-ZnO center dot Rh2O3/n-InGaZnO4 junction is discussed with reference to electrical characteristics and results of photoelectron emission measurements, suggesting that the p/n junction has large band offsets at the conduction and valence bands, respectively.
  • T Kamiya, S Narushima, H Mizoguchi, K Shimizu, K Ueda, H Ohta, M Hirano, H Hosono
    Adv. Funct. Mater. 15 6 968 - 974 2005年06月 [査読無し][通常論文]
     
    p-Type conduction in amorphous oxide was firstly found in zinc rhodium oxide (ZnO-Rh2O3) (Adv. Mater 2003,15,1409), and it is still the only p-type amorphous oxide to date. It was reported that an ordered structure at the nanometer scale was contained and its electronic structure is not clear yet. In this paper, optoelectronic and structural properties are reported in detail for xZnO center dot Rh2O3 thin films (x = 0.5-2.0) in relation to the chemical composition x. All the films exhibit positive Seebeck coefficients, confirming p-type conduction. Local network structure strongly depends on the chemical composition. Transmission electron microscopic observations reveal that lattice-like structures made of edge-sharing RhO6 network exist in 2-3 nm sized grains for rhodium-rich films (x = 0.5 and 1.0), while the zinc-rich film (x = 2) is completely amorphous. This result indicates that excess Zn assists to form an amorphous network in the ZnO-Rh2O3 system since Zn ions tend to form corner-sharing networks. The electronic structure of an all-amorphous oxide p-ZnO center dot Rh2O3/n-InGaZnO4 junction is discussed with reference to electrical characteristics and results of photoelectron emission measurements, suggesting that the p/n junction has large band offsets at the conduction and valence bands, respectively.
  • S Ohta, T Nomura, H Ohta, K Koumoto
    JOURNAL OF APPLIED PHYSICS 97 3 034106  2005年02月 [査読無し][通常論文]
     
    Electron and thermal transport properties, i.e., electrical conductivity, carrier concentration, Hall mobility, Seebeck coefficient, thermal conductivity, of heavily La- or Nb-doped SrTiO3 (STO) bulk single crystals were measured at high temperatures, (300-1050 K) to clarify the influence of doping upon the thermoelectric performance of STO. The temperature dependence of Hall mobility and Seebeck coefficient changed at similar to750 K in all samples because the dominant mechanism for carrier scattering changed with increasing temperature from coupled scattering by polar optical phonons and acoustic phonons to mere acoustic phonon scattering. The density-of-states effective mass of Nb-doped STO, which was estimated from the carrier concentration and Seebeck coefficient, was larger than that of La-doped STO. Thermal conductivity of the samples, which was similar to that of undoped STO single crystal, decreased proportionally to T-1, indicating that the phonon conduction takes place predominantly and the electronic contribution to thermal conductivity is negligible. (C) 2005 American Institute of Physics.
  • S Ohta, T Nomura, H Ohta, K Koumoto
    J. Appl. Phys. 97 3 034106  2005年02月 [査読無し][通常論文]
     
    Electron and thermal transport properties, i.e., electrical conductivity, carrier concentration, Hall mobility, Seebeck coefficient, thermal conductivity, of heavily La- or Nb-doped SrTiO3 (STO) bulk single crystals were measured at high temperatures, (300-1050 K) to clarify the influence of doping upon the thermoelectric performance of STO. The temperature dependence of Hall mobility and Seebeck coefficient changed at similar to750 K in all samples because the dominant mechanism for carrier scattering changed with increasing temperature from coupled scattering by polar optical phonons and acoustic phonons to mere acoustic phonon scattering. The density-of-states effective mass of Nb-doped STO, which was estimated from the carrier concentration and Seebeck coefficient, was larger than that of La-doped STO. Thermal conductivity of the samples, which was similar to that of undoped STO single crystal, decreased proportionally to T-1, indicating that the phonon conduction takes place predominantly and the electronic contribution to thermal conductivity is negligible. (C) 2005 American Institute of Physics.
  • Effect of boundary plane on the atomic structure of [0001] S7 tilt grain boundaries in ZnO (共著)
    J. Mater. Sci. 40 3067  2005年 [査読無し][通常論文]
  • H Ohta, SW Kim, S Ohta, K Koumoto, M Hirano, H Hosono
    CRYSTAL GROWTH & DESIGN 5 1 25 - 28 2005年01月 [査読無し][通常論文]
     
    High-quality epitaxial films of layered cobaltite Na(0.8)3CoO(2) were successfully grown on (0001)-oriented alpha-Al2O3 substrate by reactive solid-phase epitaxy using CoO epitaxial layer and NaHCO3 powder. First, the thermal oxidation of the CoO film into Co3O4 occurs at a lower temperature (300-500 degreesC), then a Na0.83CoO2 epitaxial layer formed via lateral diffusion of Na+ ions together with oxide ions into Co3O4 layers.
  • Effect of boundary plane on the atomic structure of [0001] S7 tilt grain boundaries in ZnO (共著)
    J. Mater. Sci. 40 3067  2005年 [査読無し][通常論文]
  • T Kambayashi, H Ohta, H Hoshi, M Hirano, H Hosono, H Takezoe, K Ishikawa
    Cryst. Growth Des. 5 1 143 - 146 2005年01月 [査読無し][通常論文]
     
    A copper-phthalocyanine (CuPc) thin film was heteroepitaxially grown by the organic molecular beam epitaxy method at 200 degreesC on an indium-tin oxide (ITO) film with an atomically flat terraced and stepped surface. Two different epitaxial domains with a 6-fold symmetry of CuPc were grown on the ITO substrate with relationships of (100){001}CuPc parallel to (111){112}ITO (major domain) and (100){001}CuPc parallel to (111){110}ITO (minor domain). The orientation of the major domain was better than that of the minor domain. The difference in orientation may be understood by considering geometrical matching between the CuPc molecules and the ITO surface.
  • 透明酸化物半導体の高品質薄膜成長
    機能材料 25 22  2005年 [査読無し][通常論文]
  • 特集 世界の熱電変換研究 「n型酸化物熱電変換材料の設計指針~ペロブスカイト型SrTiO3~」
    マテリアルインテグレーション 18 9 2005年 [査読無し][通常論文]
  • W Wunderlich, S Ohta, H Ohta, K Koumoto
    ICT: 2005 24TH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 237 - 240 2005年 [査読有り][通常論文]
     
    The Ruddlesden-Popper phase (SrTiO3)(n)(SrO)(m), with n=2, m=1 is considered as an advanced thermoelectric layered material based on ceramic semiconductors and combines the high electric conductivity of SrTiO3 with the low thermal conductivity of SrO due to the concept of nanoblock integration. However, it still need to be improved: In SrTiO3 the effective mass as one of main factors for the Seebeck coefficient can be enlarged through substitution by heavy elements as calculated from the curvature of the electronic band structure. The effective mass increases with the substitution of Ba on the A-site and V on the B-site in SrTiO3, For the RP-phases STO327, STO214 it shows anisotropy of more than a factor of two for in-plane and out-of-plane-directions and the Sr-O and Ti-O bonds show a stronger localization at the SrO/SrTiO3 interface than in SrTiO3-bulk, but the average effective mass is rather low.
  • S Ohta, T Nomura, H Ohta, M Hirano, H Hosono, K Koumoto
    ICT: 2005 24TH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 168 - 171 2005年 [査読有り][通常論文]
     
    To clarify the potential of n-type conductive SrTiO3 as a high temperature thermoelectric material, Carrier concentration dependence of the thermoelectric figure of merit, ZT of SrTiO3 at high-temperature (1000 K) is clarified using heavily Nb- or La-doped SrTiO3 epitaxial films, which were grown on insulating (100)-oriented LaAlO3 single-crystalline substrates by a pulsed-laser deposition method. Carrier concentration, Hall mobility, Seebeck coefficient and thermal conductivity of Nb- or La-doped SrTiO3 epitaxial films were experimentally evaluated at 1000 K with an aid of theoretical analysis. The maximum ZT value was obtained ZT = 0.37 for 4 x 10(21) cm(-3) Nb-doped SrTiO3 at 1000 K, which is the largest value among n-type oxide semiconductors ever reported.
  • K Nomura, H Ohta, A Takagi, T Kamiya, M Hirano, H Hosono
    NATURE 432 7016 488 - 492 2004年11月 [査読無し][通常論文]
     
    Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers(1). So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H)(2-5) and organic semiconductors(2,6-10) have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material-namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)-for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm(2) V-1 s(-1), which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6-9 cm(2) V-1 s(-1), and device characteristics are stable during repetitive bending of the TTFT sheet.
  • K Nomura, H Ohta, A Takagi, T Kamiya, M Hirano, H Hosono
    NATURE 432 7016 488 - 492 2004年11月 [査読無し][通常論文]
     
    Transparent electronic devices formed on flexible substrates are expected to meet emerging technological demands where silicon-based electronics cannot provide a solution. Examples of active flexible applications include paper displays and wearable computers(1). So far, mainly flexible devices based on hydrogenated amorphous silicon (a-Si:H)(2-5) and organic semiconductors(2,6-10) have been investigated. However, the performance of these devices has been insufficient for use as transistors in practical computers and current-driven organic light-emitting diode displays. Fabricating high-performance devices is challenging, owing to a trade-off between processing temperature and device performance. Here, we propose to solve this problem by using a novel semiconducting material-namely, a transparent amorphous oxide semiconductor from the In-Ga-Zn-O system (a-IGZO)-for the active channel in transparent thin-film transistors (TTFTs). The a-IGZO is deposited on polyethylene terephthalate at room temperature and exhibits Hall effect mobilities exceeding 10 cm(2) V-1 s(-1), which is an order of magnitude larger than for hydrogenated amorphous silicon. TTFTs fabricated on polyethylene terephthalate sheets exhibit saturation mobilities of 6-9 cm(2) V-1 s(-1), and device characteristics are stable during repetitive bending of the TTFT sheet.
  • H Hiramatsu, K Ueda, T Kamiya, H Ohta, M Hirano, H Hosono
    JOURNAL OF PHYSICAL CHEMISTRY B 108 45 17344 - 17351 2004年11月 [査読有り][通常論文]
     
    The optical and electronic properties of a wide-gap (similar to3.3 eV) layered oxychalcogenide, La2CdO2Se2, were examined using epitaxial thin films prepared by a reactive solid-phase epitaxy. Two optical absorption peaks due to exciton split by the spin-orbit interaction were observed at 3.43 and 3.61 eV at 10 K near the absorption edge. A sharp ultraviolet photoluminescence was observed even at room temperature, indicating that the free exciton had a large binding energy (estimated value = similar to40 meV) similar to Cu-based, layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen). The optical properties were compared to those of the Cu-based oxychalcogenides and low-dimensional CdSe-based materials. It is concluded that the two-dimensional crystal structure, which remarkably reduces the bandwidth of the conduction band, is a major origin for the wide band gap. Energy band calculations indicate that the holes are confined in the two-dimensional (CdSe2)(2-) layer, which is most likely responsible for the large binding energy of the exciton.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 79 4-6 1517 - 1520 2004年09月 [査読無し][通常論文]
     
    Heteroepitaxial thin films of Cu-based oxychalcogenides, LnCuOCh (Ln = La, Pr and Nd; Ch = mixture of S and Se), were fabricated and their resulting film structures were characterized. A reactive solid-phase epitaxy method successfully yielded heteroepitaxial films on MgO(001) substrates. A high-resolution electron microscopic examination of a LaCuOS film revealed a sharp film-substrate interface. Four-axes high-resolution X-ray diffraction measurements revealed that crystalline lattices in the films are fully relaxed and that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO. Furthermore, systematic variations in the lattice constant by lanthanide or chalcogen ion substitutions were observed.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 79 4-6 1521 - 1523 2004年09月 [査読無し][通常論文]
     
    Electrical and photoluminescence properties were investigated for epitaxial films of layered oxychalcogenides, LnCuOCh (Ln = La, Pr, and Nd, Ch = S or Se). Epitaxial films of Mg 10 at. % doped LaCuOS1-xSex are the first demonstration of degenerate conduction with high hole concentration >10(20) cm(-3) in wide gap p-type semiconductors. Ion substitution varied the excitonic emission energy from 3.21 eV to 2.89 eV while lanthanide and chalcogenide ion substitutions displayed the opposite tendency against cell volume. These unique properties are discussed with respect to the electronic structure originating from the layered crystal structure.
  • K Nomura, T Kamiya, H Ohta, K Ueda, M Hirano, H Hosono
    APPLIED PHYSICS LETTERS 85 11 1993 - 1995 2004年09月 [査読無し][通常論文]
     
    We have investigated carrier transport in a crystalline oxide semiconductor InGaO3(ZnO)(5) using single-crystalline thin films. When carrier concentration is less than 2x10(18) cm(-3), logarithm of electrical conductivity decreases in proportion to T (-1/4) and room-temperature Hall mobility was as low as similar to1 cm(2)(V s)(-1). When carrier concentration was increased to 4x10(18) cm(-3), the conduction mechanism changed to degenerate conduction and room-temperature Hall mobility was steeply increased to >10 cm(2)(Vs)(-1), showing metal-insulator transition behavior. These results are explained by percolation conduction over distribution of potential barriers formed around conduction band edge. The potential distribution is a consequence of potential modulation originating from random distribution of Ga3+ and Zn2+ ions in the crystal structure of InGaO3(ZnO)(5). (C) 2004 American Institute of Physics.
  • F Oba, H Ohta, Y Sato, H Hosono, T Yamamoto, Y Ikuhara
    PHYSICAL REVIEW B 70 12 125415  2004年09月 [査読無し][通常論文]
     
    The atomic structure of [0001]-tilt grain boundaries in ZnO was investigated using high-resolution transmission electron microscopy (HRTEM) and atomistic calculations. HRTEM observation was conducted for [0001] fiber-textured ZnO thin films grown on quartz-glass substrates by the pulsed-laser deposition. The [0001]-tilt boundaries observed in the films can be classified into three types: low-angle boundaries composed of irregular dislocation arrays, boundaries with {10 (1) over bar0} facet structures, and near-low Sigma boundaries represented by symmetric periodicity units. The atomic structure of the boundaries is discussed with a focus on a Sigma=7 boundary in conjunction with atomistic calculations and HRTEM image simulations. The Sigma=7 boundary consists of multiple structural units that are very similar to the core structures of edge dislocations. Straight or zigzag arrangements of the dislocationlike structural units constitute other high-angle boundaries with symmetric and {10 (1) over bar0} facet structures as well. It is suggested that [0001]-tilt boundaries in ZnO are generally described as an array of the dislocationlike units.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    Appl. Phys. A 79 4-6 1517 - 1520 2004年09月 [査読無し][通常論文]
     
    Heteroepitaxial thin films of Cu-based oxychalcogenides, LnCuOCh (Ln = La, Pr and Nd; Ch = mixture of S and Se), were fabricated and their resulting film structures were characterized. A reactive solid-phase epitaxy method successfully yielded heteroepitaxial films on MgO(001) substrates. A high-resolution electron microscopic examination of a LaCuOS film revealed a sharp film-substrate interface. Four-axes high-resolution X-ray diffraction measurements revealed that crystalline lattices in the films are fully relaxed and that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO. Furthermore, systematic variations in the lattice constant by lanthanide or chalcogen ion substitutions were observed.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    Appl. Phys. A 79 4-6 1521 - 1523 2004年09月 [査読無し][通常論文]
     
    Electrical and photoluminescence properties were investigated for epitaxial films of layered oxychalcogenides, LnCuOCh (Ln = La, Pr, and Nd, Ch = S or Se). Epitaxial films of Mg 10 at. % doped LaCuOS1-xSex are the first demonstration of degenerate conduction with high hole concentration >10(20) cm(-3) in wide gap p-type semiconductors. Ion substitution varied the excitonic emission energy from 3.21 eV to 2.89 eV while lanthanide and chalcogenide ion substitutions displayed the opposite tendency against cell volume. These unique properties are discussed with respect to the electronic structure originating from the layered crystal structure.
  • K Nomura, T Kamiya, H Ohta, K Ueda, M Hirano, H Hosono
    Appl. Phys. Lett. 85 11 1993 - 1995 2004年09月 [査読無し][通常論文]
     
    We have investigated carrier transport in a crystalline oxide semiconductor InGaO3(ZnO)(5) using single-crystalline thin films. When carrier concentration is less than 2x10(18) cm(-3), logarithm of electrical conductivity decreases in proportion to T (-1/4) and room-temperature Hall mobility was as low as similar to1 cm(2)(V s)(-1). When carrier concentration was increased to 4x10(18) cm(-3), the conduction mechanism changed to degenerate conduction and room-temperature Hall mobility was steeply increased to >10 cm(2)(Vs)(-1), showing metal-insulator transition behavior. These results are explained by percolation conduction over distribution of potential barriers formed around conduction band edge. The potential distribution is a consequence of potential modulation originating from random distribution of Ga3+ and Zn2+ ions in the crystal structure of InGaO3(ZnO)(5). (C) 2004 American Institute of Physics.
  • Fumiyasu Oba, Hiromichi Ohta, Yukio Sato, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara
    Phys. Rev. B 70 12 1 - 125415 2004年09月 [査読無し][通常論文]
     
    The atomic structure of [0001]-tilt grain boundaries in ZnO was investigated using high-resolution transmission electron microscopy (HRTEM) and atomistic calculations. HRTEM observation was conducted for [0001] fiber-textured ZnO thin films grown on quartz-glass substrates by the pulsed-laser deposition. The [0001]-tilt boundaries observed in the films can be classified into three types: low-angle boundaries composed of irregular dislocation arrays, boundaries with {1010} facet structures, and near-low ∑ boundaries represented by symmetric periodicity units. The atomic structure of the boundaries is discussed with a focus on a ∑=7 boundary in conjunction with atomistic calculations and HRTEM image simulations. The ∑=7 boundary consists of multiple structural units that are very similar to the core structures of edge dislocations. Straight or zigzag arrangements of the dislocationlike structural units constitute other high-angle boundaries with symmetric and {1010} facet structures as well. It is suggested that [0001]-tilt boundaries in ZnO are generally described as an array of the dislocationlike units.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    JOURNAL OF MATERIALS RESEARCH 19 7 2137 - 2143 2004年07月 [査読無し][通常論文]
     
    Processes and preparation conditions for growing epitaxial thin films of Cu-based, layered oxychalcogenides LnCuOCh (Ln = La, Ce, Pr or Nd; Ch = S,,Se, or Se1-yTey) are reported. Epitaxial thin films on MgO (001) substrates were prepared by a reactive solid-phase epitaxy method. Four-axes high-resolution x-ray diffraction measurements revealed that the crystallographic orientation is (001)[110] LnCuOCh parallel to (001)[110] MgO and the internal stress of the crystalline lattices in the films are relaxed during thermal-annealing process of the reactive solid-phase epitaxy. Furthermore, except for CeCuOS, systematic variations in the lattice constant by chalcogen or lanthanide ion substitutions were observed. These results demonstrated that the reactive solid-phase epitaxy is an efficient technique for fabricating LnCuOCh epitaxial films.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    J. Mater. Res. 19 7 2137 - 2143 2004年07月 [査読無し][通常論文]
     
    Processes and preparation conditions for growing epitaxial thin films of Cu-based, layered oxychalcogenides LnCuOCh (Ln = La, Ce, Pr or Nd; Ch = S,,Se, or Se1-yTey) are reported. Epitaxial thin films on MgO (001) substrates were prepared by a reactive solid-phase epitaxy method. Four-axes high-resolution x-ray diffraction measurements revealed that the crystallographic orientation is (001)[110] LnCuOCh parallel to (001)[110] MgO and the internal stress of the crystalline lattices in the films are relaxed during thermal-annealing process of the reactive solid-phase epitaxy. Furthermore, except for CeCuOS, systematic variations in the lattice constant by chalcogen or lanthanide ion substitutions were observed. These results demonstrated that the reactive solid-phase epitaxy is an efficient technique for fabricating LnCuOCh epitaxial films.
  • T Kamiya, H Ohta, H Hiramatsu, K Hayashi, K Nomura, S Matsuishi, K Ueda, M Hirano, H Hosono
    MICROELECTRONIC ENGINEERING 73-4 620 - 626 2004年06月 [査読無し][通常論文]
     
    Nanostructures naturally formed in oxide semiconductor crystals are discussed in relation to their electronic structure and properties. Three different crystals are studied: (i) layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen), (ii) homologous series layered oxides RMO3(ZnO)(m) (R = In, Lu etc., M = In, Ga etc., m = integer), and (iii) nanoporous crystal 12CaO7 . Al2O3 (C12A7). Coexistence of moderately large mobility, intense photoluminescence arising from room-temperature exciton and high-density hole is explained by a natural modulation doping structure in LnCuOCh. C12A7 exhibits different properties by replacing free O2- ions anions clathrated in its subnanometer-sized cages with O-, H-, and electron. The electronic structures of the clathrated anions were investigated by ab initio calculations and discussed in terms of possible quantum structures and applications. (C) 2004 Elsevier B.V. All rights reserved.
  • T Kamiya, H Ohta, H Hiramatsu, K Hayashi, K Nomura, S Matsuishi, K Ueda, M Hirano, H Hosono
    Microelectron. Eng. 73-4 620 - 626 2004年06月 [査読無し][通常論文]
     
    Nanostructures naturally formed in oxide semiconductor crystals are discussed in relation to their electronic structure and properties. Three different crystals are studied: (i) layered oxychalcogenides LnCuOCh (Ln = lanthanide, Ch = chalcogen), (ii) homologous series layered oxides RMO3(ZnO)(m) (R = In, Lu etc., M = In, Ga etc., m = integer), and (iii) nanoporous crystal 12CaO7 . Al2O3 (C12A7). Coexistence of moderately large mobility, intense photoluminescence arising from room-temperature exciton and high-density hole is explained by a natural modulation doping structure in LnCuOCh. C12A7 exhibits different properties by replacing free O2- ions anions clathrated in its subnanometer-sized cages with O-, H-, and electron. The electronic structures of the clathrated anions were investigated by ab initio calculations and discussed in terms of possible quantum structures and applications. (C) 2004 Elsevier B.V. All rights reserved.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Orita, M Hirano, T Suzuki, C Honjyo, Y Ikuhara, H Hosono
    JOURNAL OF APPLIED PHYSICS 95 10 5532 - 5539 2004年05月 [査読無し][通常論文]
     
    The growth mechanism for a single-crystalline film of InGaO3(ZnO)(5) (IGZO) on a (111)-oriented yttria-stabilized-zirconia substrate by reactive solid-phase epitaxy was studied by high-resolution x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We focused on the role of the 2-nm-thick ZnO epitaxial layer during the structural evolution of the 200-nm-thick polycrystalline- (poly-) IGZO layer while thermal annealing. When a thin ZnO epitaxial layer was formed before depositing the poly-IGZO layer, thin single-crystalline epitaxial IGZO seeds were initially formed only on the substrate surface or on the thin ZnO epitaxial layer through a solid-phase reaction between the ZnO epitaxial layer and the poly-IGZO layer. A single-crystalline IGZO layer grew from the IGZO seeds toward the top surface of the film along with a much faster lateral grain growth, which formed a large-area single-crystalline IGZO film with an atomically flat terraced and stepped surface. On the other hand, an epitaxial film was not obtained unless a ZnO epitaxial layer was used, demonstrating that the ZnO epitaxial layer plays a crucial role as the seed for subsequent crystallite growth and a template for determining the crystallographic orientation. (C) 2004 American Institute of Physics.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Orita, M Hirano, T Suzuki, C Honjyo, Y Ikuhara, H Hosono
    J. Appl. Phys. 95, 5532 (2004) 95 10 5532 - 5539 2004年05月 [査読無し][通常論文]
     
    The growth mechanism for a single-crystalline film of InGaO3(ZnO)(5) (IGZO) on a (111)-oriented yttria-stabilized-zirconia substrate by reactive solid-phase epitaxy was studied by high-resolution x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We focused on the role of the 2-nm-thick ZnO epitaxial layer during the structural evolution of the 200-nm-thick polycrystalline- (poly-) IGZO layer while thermal annealing. When a thin ZnO epitaxial layer was formed before depositing the poly-IGZO layer, thin single-crystalline epitaxial IGZO seeds were initially formed only on the substrate surface or on the thin ZnO epitaxial layer through a solid-phase reaction between the ZnO epitaxial layer and the poly-IGZO layer. A single-crystalline IGZO layer grew from the IGZO seeds toward the top surface of the film along with a much faster lateral grain growth, which formed a large-area single-crystalline IGZO film with an atomically flat terraced and stepped surface. On the other hand, an epitaxial film was not obtained unless a ZnO epitaxial layer was used, demonstrating that the ZnO epitaxial layer plays a crucial role as the seed for subsequent crystallite growth and a template for determining the crystallographic orientation. (C) 2004 American Institute of Physics.
  • K Ueda, H Hiramatsu, H Ohta, M Hirano, T Kamiya, H Hosono
    PHYSICAL REVIEW B 69 15 155305  2004年04月 [査読無し][通常論文]
     
    LnCuOCh (Ln=lanthanide, Ch=chalcogen) layered oxychalcogenides are wide-gap p-type semiconductors composed of alternately stacked (Ln(2)O(2))(2+) oxide layers and (Cu(2)Ch(2))(2-) chalcogenide layers. Energy band calculations revealed that Cu-Ch hybridized bands only spread in the (Cu(2)Ch(2))(2-) layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the (Ln(2)O(2))(2+) layers. Stepwise absorption spectra of a series of LnCuOCh experimentally verified that an exciton in the (Cu(2)Ch(2))(2-) layers shows a two-dimensional behavior. These theoretical and experimental results indicate that LnCuOCh has "natural multiple quantum wells" built into its layered structure.
  • K Nomura, H Ohta, HA Masahiro, K Ueda, T Kamiya, M Hirano, H Hosono
    MICROELECTRONIC ENGINEERING 72 1-4 294 - 298 2004年04月 [査読無し][通常論文]
     
    Metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated using a n-type transparent oxide semiconductor, InGaO3(ZnO)(5), single-crystalline layer as n-channel. High-k dielectric materials, amorphous Al2Ox and HfOx, were examined for gate insulators. The deposition and post-annealing conditions of the gate insulators were optimized for leakage current and MISFET performances. The MISFETs exhibited good performances such as normally off characteristics, an on/off current ratio as large as 10(5) and insensitivity to visible light. A field-effect mobility for oxide MISFETs (similar to80 cm(2) V-1 s(-1)) was obtained when amorphous HfOx was used for gate insulator. (C) 2004 Elsevier B.V. All rights reserved.
  • K Ueda, H Hiramatsu, H Ohta, M Hirano, T Kamiya, H Hosono
    Phys. Rev. B 69 15 155305  2004年04月 [査読無し][通常論文]
     
    LnCuOCh (Ln=lanthanide, Ch=chalcogen) layered oxychalcogenides are wide-gap p-type semiconductors composed of alternately stacked (Ln(2)O(2))(2+) oxide layers and (Cu(2)Ch(2))(2-) chalcogenide layers. Energy band calculations revealed that Cu-Ch hybridized bands only spread in the (Cu(2)Ch(2))(2-) layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the (Ln(2)O(2))(2+) layers. Stepwise absorption spectra of a series of LnCuOCh experimentally verified that an exciton in the (Cu(2)Ch(2))(2-) layers shows a two-dimensional behavior. These theoretical and experimental results indicate that LnCuOCh has "natural multiple quantum wells" built into its layered structure.
  • K Nomura, H Ohta, HA Masahiro, K Ueda, T Kamiya, M Hirano, H Hosono
    Miroelectron. Eng. 72 1-4 294 - 298 2004年04月 [査読無し][通常論文]
     
    Metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated using a n-type transparent oxide semiconductor, InGaO3(ZnO)(5), single-crystalline layer as n-channel. High-k dielectric materials, amorphous Al2Ox and HfOx, were examined for gate insulators. The deposition and post-annealing conditions of the gate insulators were optimized for leakage current and MISFET performances. The MISFETs exhibited good performances such as normally off characteristics, an on/off current ratio as large as 10(5) and insensitivity to visible light. A field-effect mobility for oxide MISFETs (similar to80 cm(2) V-1 s(-1)) was obtained when amorphous HfOx was used for gate insulator. (C) 2004 Elsevier B.V. All rights reserved.
  • Kazushige Ueda, Hidenori Hiramatsu, Hiromichi Ohta, Masahiro Hirano, Toshio Kamiya, Hideo Hosono
    Physical Review B - Condensed Matter and Materials Physics 69 15 1 - 155305 2004年04月 [査読有り][通常論文]
     
    LnCuOCh (Ln = lanthanide, Ch = chalcogen) layered oxychalcogenides are wide-gap p-type semiconductors composed of alternately stacked (Ln 2O2)2+ oxide layers and (Cu2Ch 2)2- chalcogenide layers. Energy band calculations revealed that Cu-Ch hybridized bands only spread in the (Cu2Ch 2)2- layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the (Ln2O 2)2+ layers. Stepwise absorption spectra of a series of LnCuOCh experimentally verified that an exciton in the (Cu2Ch 2)2- layers shows a two-dimensional behavior. These theoretical and experimental results indicate that LnCuOCh has "natural multiple quantum wells" built into its layered structure.
  • T Kamiya, H Ohta, M Kamiya, K Nomura, K Ueda, M Hirano, H Hosono
    JOURNAL OF MATERIALS RESEARCH 19 3 913 - 920 2004年03月 [査読無し][通常論文]
     
    Li-doped NiO epitaxial films with high electrical conductivity and atomically flat stepped surfaces were fabricated by a combined technique of pulsed laser deposition and subsequent annealing. It was determined that subsequently annealing at temperatures as low as 600degreesC significantly decreased electrical conductivity due to Li evaporation when the film surface was not protected from Li evaporation. To suppress Li evaporation, a yttria-stabilized-zironia plate was used to cover the film surface, which raised the annealing temperature up to 1300degreesC while maintaining a high Li concentration and electrical conductivity. Thermally annealing at this temperature also improved crystal quality and formed epitaxial films with atomically flat stepped surfaces. The films were single crystalline at least in observation areas, 10 Pm x 10 mum. A reasonably large Hall mobility approximately 0.05 cm(2)/Vs similar to that reported for bulk single-crystal NiO and a visible-light transmission in excess of 75% were obtained on 120-nm-thick films. Although annealing at higher temperatures such as 1400degreesC can further improve the structural and optical properties, the Li concentration in the films was decreased to < 3% of the as-deposited film.
  • H Hiramatsu, H Ohta, T Suzuki, C Honjo, Y Ikuhara, K Ueda, T Kamiya, M Hirano, H Hosono
    CRYSTAL GROWTH & DESIGN 4 2 301 - 307 2004年03月 [査読無し][通常論文]
     
    A unique epitaxial growth method, reactive solid-phase epitaxy, was used to fabricate heteroepitaxial thin films of LaCuOS, a transparent p-type semiconductor with layered structure. The epitaxial growth mechanism is examined through microscopic observations. A thin metallic Cu layer deposited between the amorphous LaCuOS (a-LaCuOS) and the yttria-stabilized zirconia (YSZ) single-crystal substrate before thermally annealing is a key to realizing the epitaxial LaCuOS films in this process. To grow the epitaxial films, it is critical for the Cu layer to have a discontinuous structure with triple junctions among the Cu, a-LaCuOS layer, and the YSZ substrate. The Cu layer is needed to create seed grains for the epitaxial LaCuOS at the junctions. The resulting seed grains work as an epitaxy template for subsequent epitaxial growth from the substrate to the film's top surface, and high temperatures such as 1000 degreesC are necessary to completely convert the a-LaCuOS layer to an epitaxial layer.
  • T Kamiya, H Ohta, M Kamiya, K Nomura, K Ueda, M Hirano, H Hosono
    J. Mater. Res. 19 3 913 - 920 2004年03月 [査読無し][通常論文]
     
    Li-doped NiO epitaxial films with high electrical conductivity and atomically flat stepped surfaces were fabricated by a combined technique of pulsed laser deposition and subsequent annealing. It was determined that subsequently annealing at temperatures as low as 600degreesC significantly decreased electrical conductivity due to Li evaporation when the film surface was not protected from Li evaporation. To suppress Li evaporation, a yttria-stabilized-zironia plate was used to cover the film surface, which raised the annealing temperature up to 1300degreesC while maintaining a high Li concentration and electrical conductivity. Thermally annealing at this temperature also improved crystal quality and formed epitaxial films with atomically flat stepped surfaces. The films were single crystalline at least in observation areas, 10 Pm x 10 mum. A reasonably large Hall mobility approximately 0.05 cm(2)/Vs similar to that reported for bulk single-crystal NiO and a visible-light transmission in excess of 75% were obtained on 120-nm-thick films. Although annealing at higher temperatures such as 1400degreesC can further improve the structural and optical properties, the Li concentration in the films was decreased to < 3% of the as-deposited film.
  • H Hiramatsu, H Ohta, T Suzuki, C Honjo, Y Ikuhara, K Ueda, T Kamiya, M Hirano, H Hosono
    Cryst. Growth. Des. 4 2 301 - 307 2004年03月 [査読無し][通常論文]
     
    A unique epitaxial growth method, reactive solid-phase epitaxy, was used to fabricate heteroepitaxial thin films of LaCuOS, a transparent p-type semiconductor with layered structure. The epitaxial growth mechanism is examined through microscopic observations. A thin metallic Cu layer deposited between the amorphous LaCuOS (a-LaCuOS) and the yttria-stabilized zirconia (YSZ) single-crystal substrate before thermally annealing is a key to realizing the epitaxial LaCuOS films in this process. To grow the epitaxial films, it is critical for the Cu layer to have a discontinuous structure with triple junctions among the Cu, a-LaCuOS layer, and the YSZ substrate. The Cu layer is needed to create seed grains for the epitaxial LaCuOS at the junctions. The resulting seed grains work as an epitaxy template for subsequent epitaxial growth from the substrate to the film's top surface, and high temperatures such as 1000 degreesC are necessary to completely convert the a-LaCuOS layer to an epitaxial layer.
  • H Kamioka, H Hiramatsu, H Ohta, M Hirano, K Ueda, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 84 6 879 - 881 2004年02月 [査読無し][通常論文]
     
    We have studied the third-order optical nonlinearity (chi((3))) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The chi((3)) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2-4x10(-9) esu with a fast time response of 250-300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers. (C) 2004 American Institute of Physics.
  • H Ohta, T Kambayashi, K Nomura, M Hirano, K Ishikawa, H Takezoe, H Hosono
    ADVANCED MATERIALS 16 4 312 - + 2004年02月 [査読無し][通常論文]
     
    An improved-performance transparent organic thin-film transistor (OTFT) is described, which has a vanadyl-phthalocyanine (VOPc) film as an active p-channel, lattice-matched (Sc0.7Y0.3)(2)O-3 film as a high-k gate dielectric, and atomically flat indium tin oxide (ITO) film as the bottom contact (see Figure). These features-lattice matching and atomic flatness-are expected to lead to further improvements in transparent OTFTs.
  • H Kamioka, H Hiramatsu, H Ohta, M Hirano, K Ueda, T Kamiya, H Hosono
    Appl. Phys. Lett. 84 6 879 - 881 2004年02月 [査読無し][通常論文]
     
    We have studied the third-order optical nonlinearity (chi((3))) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The chi((3)) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2-4x10(-9) esu with a fast time response of 250-300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers. (C) 2004 American Institute of Physics.
  • H Ohta, T Kambayashi, K Nomura, M Hirano, K Ishikawa, H Takezoe, H Hosono
    ADVANCED MATERIALS 16 4 312 - + 2004年02月 [査読無し][通常論文]
     
    An improved-performance transparent organic thin-film transistor (OTFT) is described, which has a vanadyl-phthalocyanine (VOPc) film as an active p-channel, lattice-matched (Sc0.7Y0.3)(2)O-3 film as a high-k gate dielectric, and atomically flat indium tin oxide (ITO) film as the bottom contact (see Figure). These features-lattice matching and atomic flatness-are expected to lead to further improvements in transparent OTFTs.
  • J. Ceram. Soc. Jpn. Suppl. 112 S602-S609  2004年 [査読無し][通常論文]
  • Takeo Sasaki, Katsuyuki Matsunaga, Hiromichi Ohta, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara
    Materials Transactions 45 7 2137 - 2143 2004年 [査読無し][通常論文]
     
    Thin Ni films were deposited on the (111) surface of YSZ at 1073 K by a pulsed laser deposition technique. The interfacial atomic structure of Ni/YSZ was investigated by high-resolution transmission electron microscopy (HRTEM). It was found that Ni was epitaxially oriented to the YSZ surface, and the following orientation relationship (OR) was observed: (111)Ni ∥ (111)YSZ, ∥ (11̄0)Ni, [11̄0]YSZ · Geometrical coherency of the Ni/YSZ system was also evaluated by the coincidence of reciprocal lattice points (CRLP) method. It was found that the most coherent OR predicted by CRLP method was (705)Ni ∥ (111)YSZ, [01̄0]Ni [11̄0]YSZ, which was not consistent with the experimentally observed OR. To understand the detailed atomic structure. HRTEM image simulations were performed. However, simulated images based on both O-terminated and Zr-terminated interface models were quite similar to the experimental image, and thus it was hard to determine which model is comparable with the actual interface only by the HRTEM image simulations. In order to clarify the termination layer at the interface, electronic structures of the Ni/YSZ interface were investigated by electron energy-loss spectroscopy. It was found that significant differences were observed in O-K edge spectra between the interface and the YSZ crystal interior, and the spectrum from the interface showed similar features to the reference spectrum of bulk NiO. This indicates that the Ni-O interaction occurs at the interface to terminate the oxygen {111} plane of YSZ at the Ni/YSZ interface. In addition, the density of Ni-O bonds across the interface in the experimental OR was larger than that in the most coherent OR predicted by CRLP method, which also suggests that the on-top Ni-O bonds stabilize the Ni/YSZ(111) interface.
  • Transparent Oxide Optoelectronics
    Materials Today 7 42  2004年 [査読無し][通常論文]
  • J. Ceram. Soc. Jpn. Suppl. 112 S602-S609  2004年 [査読無し][通常論文]
  • Takeo Sasaki, Katsuyuki Matsunaga, Hiromichi Ohta, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara
    Materials Transactinos 45 7 2137 - 2143 2004年 [査読無し][通常論文]
     
    Thin Ni films were deposited on the (111) surface of YSZ at 1073 K by a pulsed laser deposition technique. The interfacial atomic structure of Ni/YSZ was investigated by high-resolution transmission electron microscopy (HRTEM). It was found that Ni was epitaxially oriented to the YSZ surface, and the following orientation relationship (OR) was observed: (111)Ni ∥ (111)YSZ, ∥ (11̄0)Ni, [11̄0]YSZ · Geometrical coherency of the Ni/YSZ system was also evaluated by the coincidence of reciprocal lattice points (CRLP) method. It was found that the most coherent OR predicted by CRLP method was (705)Ni ∥ (111)YSZ, [01̄0]Ni [11̄0]YSZ, which was not consistent with the experimentally observed OR. To understand the detailed atomic structure. HRTEM image simulations were performed. However, simulated images based on both O-terminated and Zr-terminated interface models were quite similar to the experimental image, and thus it was hard to determine which model is comparable with the actual interface only by the HRTEM image simulations. In order to clarify the termination layer at the interface, electronic structures of the Ni/YSZ interface were investigated by electron energy-loss spectroscopy. It was found that significant differences were observed in O-K edge spectra between the interface and the YSZ crystal interior, and the spectrum from the interface showed similar features to the reference spectrum of bulk NiO. This indicates that the Ni-O interaction occurs at the interface to terminate the oxygen {111} plane of YSZ at the Ni/YSZ interface. In addition, the density of Ni-O bonds across the interface in the experimental OR was larger than that in the most coherent OR predicted by CRLP method, which also suggests that the on-top Ni-O bonds stabilize the Ni/YSZ(111) interface.
  • 透明酸化物半導体とデバイスへの展開(分担)
    オプトロニクス 10 2004年 [査読無し][通常論文]
  • Transparent Oxide Optoelectronics
    Materials Today 7 42  2004年 [査読無し][通常論文]
  • H Hiramatsu, K Ueda, T Kamiya, H Ohta, M Hirano, H Hosono
    JOURNAL OF MATERIALS CHEMISTRY 14 19 2946 - 2950 2004年 [査読有り][通常論文]
     
    A single-phase La2CdO2Se2 powder was successfully synthesized and its optical and electrical properties were examined. Single-phase La2CdO2Se2 was obtained by preserving the stoichiometric composition of the initial ingredients, especially Cd and Se, during the sintering process. Exhibiting a yellowish white color, the synthesized sample has a large optical bandgap of similar to3.3 eV. The crystal structure, refined by a Rietveld method taking into account the site occupancies and isotropic thermal vibration parameters of Cd and Se, indicates that La2CdO2Se2 belongs to space group P4(2)/nmc. Instability against Cd or Se vacancy formation makes it difficult to generate carriers through vacancies or allo-valent ions doping, leading to electrical insulating properties both in as-prepared and intentionally doped La2CdO2Se2 form.
  • T Kamiya, K Ueda, H Hiramatsu, H Ohta, M Hirano, H Hosono
    INTEGRATION OF ADVANCED MICRO-AND NANOELECTRONIC DEVICES-CRITICAL ISSUES AND SOLUTIONS 811 457 - 462 2004年 [査読有り][通常論文]
     
    Electronic structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, Te) and La2CdO2Se2, were studied using ab-initio band calculations in relation to their optical and electronic properties. It was found that the dispersions of the top valence bands are much smaller in F-Z direction than in F-X direction, indicating that the electronic structure is highly two-dimensional, and that holes are confined in the CuCh or CdSe layers. The two-dimensional electronic structure is supported experimentally by staircase-like structure observed in optical absorption spectra at 10 K associated with two excitonic absorption peaks split by spin-orbit interaction of Ch ions. La2CdO2Se2 has the largest bandgap due to the two-dimensional network structure of CdSe tetrahedra.
  • H Ohta, M Kamiya, T Kamaiya, M Hirano, H Hosono
    THIN SOLID FILMS 445 2 317 - 321 2003年12月 [査読無し][通常論文]
     
    A transparent ultraviolet (UV)-detector was fabricated using a high-quality pn-heterojunction diode composed of transparent oxide semiconductors, p-type NiO and n-type ZnO, and its UV-response was measured at room temperature. Transparent tri-layered oxide films of ZnO/NiO/ITO were heteroepitaxially grown on an YSZ (1 1 1) substrate by a pulsed-laser-deposition combined with a solid-phase-epitaxy technique and they were processed to fabricate a p-NiO/n-ZnO diode. The diodes exhibited a clear rectifying I-V characteristic with an ideality factor of similar to2 and a forward threshold voltage of similar to1 V. Although the photoresponsivity was fairly weak at the zero bias voltage, it was enhanced up to similar to0.3 AW(-1) by applying a reverse bias of -6 V under an irradiation of 360-nm light, which is comparable to that of commercial devices. (C) 2003 Elsevier B.V. All rights reserved.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, H Hosono
    THIN SOLID FILMS 445 2 322 - 326 2003年12月 [査読無し][通常論文]
     
    We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO3(ZnO)(m) (m = integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as similar to1 nA with a positive threshold voltage of similar to3 V and on/off current ratio of 10(5). The field-effect mobility increased from similar to1 to similar to10 cm(2) (V s)(-1) as the m-value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds. (C) 2003 Elsevier B.V. All rights reserved.
  • H Hiramatsu, K Ueda, H Ohta, M Hirano, T Kamiya, H Hosono
    Thin Solid Films 445 2 304 - 308 2003年12月 [査読無し][通常論文]
     
    Hole transport and optical properties were investigated on undoped and Mg-doped LaCuOS1-xSex (x = 0-1) epitaxial films. Both electrical conductivity and Hall mobility were found to increase monotonously with increasing Se content in the films. The increase in Hall mobility is considered to be associated with the increase in valence band dispersion. Mg ion doping increased hole concentrations in the undoped films by an order of magnitude to similar to2 x 10(20) cm(-3), while Mg doping reduced mobility to merely half that of undoped films. The,results suggest that hole scattering due to Mg impurity ions is suppressed by natural modulation doping originating from the layered structure of LaCuOS1-xSex. Hole concentrations showed no temperature dependence, indicating degenerate conduction. The largest value for conductivity, 140 S cm(-1), was obtained with Mg-doped LaCuOSe epitaxial film. Accompanying characteristics included moderately high optical transparency in the visible region and blue photoluminescence. (C) 2003 Elsevier B.V. All rights reserved.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, H Hosono
    Thin Solid Films 445 2 322 - 326 2003年12月 [査読無し][通常論文]
     
    We have investigated the characteristics of transparent metal-insulator-semiconductor field-effect transistors (MISFETs) fabricated using InGaO3(ZnO)(m) (m = integer) single-crystalline thin films as n-channel layers and amorphous alumina as gate insulator films. The MISFETs exhibit good characteristics such as insensitivity to visible light illumination, off-current as low as similar to1 nA with a positive threshold voltage of similar to3 V and on/off current ratio of 10(5). The field-effect mobility increased from similar to1 to similar to10 cm(2) (V s)(-1) as the m-value increased. Room temperature Hall mobility also increased. However, unexpectedly these values were lower than the field-effect mobility. It is explained by existence of shallow localized state in the homologous compounds. (C) 2003 Elsevier B.V. All rights reserved.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    JOURNAL OF APPLIED PHYSICS 94 9 5805 - 5808 2003年11月 [査読無し][通常論文]
     
    The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and similar to30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below similar to80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu(2)Ch(2))(2-) layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices. (C) 2003 American Institute of Physics.
  • H Hiramatsu, K Ueda, K Takafuji, H Ohta, M Hirano, T Kamiya, H Hosono
    J. Appl. Phys. 94 9 5805 - 5808 2003年11月 [査読無し][通常論文]
     
    The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and similar to30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below similar to80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu(2)Ch(2))(2-) layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices. (C) 2003 American Institute of Physics.
  • K Ueda, K Takafuji, H Hiramatsu, H Ohta, T Kamiya, M Hirano, H Hosono
    CHEMISTRY OF MATERIALS 15 19 3692 - 3695 2003年09月 [査読無し][通常論文]
     
    A series of layered oxysulfides, LnCuOS (Ln = Lasimilar toNd), was studied to examine the influence of these lanthanide ions on their optical and electrical transport properties. PrCuOS and NdCuOS showed wide-gap p-type semiconducting properties similar to LaCuOS, which is a p-type semiconductor with an energy gap of 3.1 eV. A sharp photoluminescence peak near the absorption edge, which originates from excitons, was observed in LnCuOS (Ln = La, Pr, and Nd), indicating that these lanthanide ions do not influence the fundamental optical properties and excitons are confined in (Cu2S2)(2-) sulfide layers sandwiched between (Ln(2)O(2))(2+) oxide layers. On the other hand, CeCuOS showed p-type degenerate semiconducting behavior and its color was black. The electronic structures of LnCuOS (Ln = Lasimilar toNd) were investigated by ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy to understand these differences derived from the lanthanide ions. Although no remarkable difference in the electronic structures was found among LnCuOS (Ln = Lasimilar toNd), only an inverse photoemission spectrum of CeCuOS clearly showed an unoccupied band near the Fermi level in the conduction band. It is considered that the formation of this unoccupied band at the conduction band minimum is responsible for the unique electrical and optical features of CeCuOS.
  • S Narushima, H Mizoguchi, K Shimizu, K Ueda, H Ohta, M Hirano, T Kamiya, H Hosono
    ADVANCED MATERIALS 15 17 1409 - 1413 2003年09月 [査読無し][通常論文]
  • K Ueda, K Takafuji, H Hiramatsu, H Ohta, T Kamiya, M Hirano, H Hosono
    Chem. Mater. 15 19 3692 - 3695 2003年09月 [査読無し][通常論文]
     
    A series of layered oxysulfides, LnCuOS (Ln = Lasimilar toNd), was studied to examine the influence of these lanthanide ions on their optical and electrical transport properties. PrCuOS and NdCuOS showed wide-gap p-type semiconducting properties similar to LaCuOS, which is a p-type semiconductor with an energy gap of 3.1 eV. A sharp photoluminescence peak near the absorption edge, which originates from excitons, was observed in LnCuOS (Ln = La, Pr, and Nd), indicating that these lanthanide ions do not influence the fundamental optical properties and excitons are confined in (Cu2S2)(2-) sulfide layers sandwiched between (Ln(2)O(2))(2+) oxide layers. On the other hand, CeCuOS showed p-type degenerate semiconducting behavior and its color was black. The electronic structures of LnCuOS (Ln = Lasimilar toNd) were investigated by ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy to understand these differences derived from the lanthanide ions. Although no remarkable difference in the electronic structures was found among LnCuOS (Ln = Lasimilar toNd), only an inverse photoemission spectrum of CeCuOS clearly showed an unoccupied band near the Fermi level in the conduction band. It is considered that the formation of this unoccupied band at the conduction band minimum is responsible for the unique electrical and optical features of CeCuOS.
  • S Narushima, H Mizoguchi, K Shimizu, K Ueda, H Ohta, M Hirano, T Kamiya, H Hosono
    Adv. Mater. 15 17 1409 - 1413 2003年09月 [査読無し][通常論文]
  • Hiromichi Ohta, Takuya Kambayashi, Masahiro Hirano, Hajime Hoshi, Ken Ishikawa, Hideo Takezoe, Hideo Hosono
    Advanced Materials 15 15 1258 - 1262 2003年08月05日 [査読無し][通常論文]
     
    A laterally grown vanadyl phthalocyanine (VOPc) layer on an epitaxial indium tin oxide (ITO) film surface composed of atomically flat terraces and 0.29 nm steps using a molecular beam epitaxy (MBE) method was obtained. As such, crystallographic orientation of the film differs distinctly from those of films on other substrates such as alkali halides. Atomic force microscopy (AFM) images revealed that six kinds of two-dimensional VOPc domains grew heteroepitaxially horizontally on the ITO surface, in contact with each other, forming domain boundaries.
  • H Ohta, M Hirano, K Nakahara, H Maruta, T Tanabe, M Kamiya, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 83 5 1029 - 1031 2003年08月 [査読無し][通常論文]
     
    Transparent trilayered oxide films of ZnO/NiO/indium tin oxide were heteroepitaxially grown on a YSZ (111) substrate by pulsed-laser deposition combined with a solid-phase-epitaxy technique, and were processed to fabricate a p-NiO/n-ZnO diode. The diode exhibited a clear rectifying I-V characteristic with an ideality factor of similar to2 and a forward threshold voltage of similar to1 V. Although the photoresponsivity was fairly weak at the zero-bias voltage, it was enhanced up to similar to0.3 A W-1 through the application of a reverse bias of -6 V under the irradiation of 360 nm light, a value comparable to that of commercial devices. (C) 2003 American Institute of Physics.
  • H Ohta, T Kambayashi, M Hirano, H Hoshi, K Ishikawa, H Takezoe, H Hosono
    ADVANCED MATERIALS 15 15 1258 - + 2003年08月 [査読無し][通常論文]
     
    The lateral growth of an organic molecule on a transparent conductive substrate-important for molecular electronics-is demonstrated using vanadyl phthalocyanine (VOPc) on epitaxial indium tin oxide (ITO) film with an atomically flat and stepped surface. The occurrence of two-dimensional lateral growth is proved by the heights of the square-shaped domains of VOPc (see Figure) stacked parallel to the ITO substrate.
  • H Ohta, M Hirano, K Nakahara, H Maruta, T Tanabe, M Kamiya, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 83 5 1029 - 1031 2003年08月 [査読無し][通常論文]
     
    Transparent trilayered oxide films of ZnO/NiO/indium tin oxide were heteroepitaxially grown on a YSZ (111) substrate by pulsed-laser deposition combined with a solid-phase-epitaxy technique, and were processed to fabricate a p-NiO/n-ZnO diode. The diode exhibited a clear rectifying I-V characteristic with an ideality factor of similar to2 and a forward threshold voltage of similar to1 V. Although the photoresponsivity was fairly weak at the zero-bias voltage, it was enhanced up to similar to0.3 A W-1 through the application of a reverse bias of -6 V under the irradiation of 360 nm light, a value comparable to that of commercial devices. (C) 2003 American Institute of Physics.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, H Hosono
    SCIENCE 300 5623 1269 - 1272 2003年05月 [査読無し][通常論文]
     
    We report the fabrication of transparent field-effect transistors using a single-crystalline thin-film transparent oxide semiconductor, InGaO3(ZnO)(5), as an electron channel and amorphous hafnium oxide as a gate insulator. The device exhibits an on-to-off current ratio of similar to10(6) and a field-effect mobility of similar to80 square centimeters per volt per second at room temperature, with operation insensitive to visible light irradiation. The result provides a step toward the realization of transparent electronics for next-generation optoelectronics.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, H Hosono
    SCIENCE 300 5623 1269 - 1272 2003年05月 [査読無し][通常論文]
     
    We report the fabrication of transparent field-effect transistors using a single-crystalline thin-film transparent oxide semiconductor, InGaO3(ZnO)(5), as an electron channel and amorphous hafnium oxide as a gate insulator. The device exhibits an on-to-off current ratio of similar to10(6) and a field-effect mobility of similar to80 square centimeters per volt per second at room temperature, with operation insensitive to visible light irradiation. The result provides a step toward the realization of transparent electronics for next-generation optoelectronics.
  • H Hiramatsu, K Ueda, H Ohta, M Hirano, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 82 7 1048 - 1050 2003年02月 [査読無し][通常論文]
     
    Epitaxial films of LaCuOS1-xSex (x=0-1) solid solution were grown on MgO (001) substrates and their electrical and optical properties were examined. Sharp emission due to room-temperature exciton with binding energy of similar to50 meV is observed for all x values. Hall mobility becomes large with an increase in the Se content and it reaches 8.0 cm(2)V(-1)s(-1) in LaCuOSe, a comparable value to that of p-type GaN:Mg. Doping of Mg2+ ions at La3+ sites enhances a hole concentration up to 2.2x10(20) cm(-3), while maintaining the Hall mobility as large as 4.0 cm(2)V(-1)s(-1). Consequently, a degenerate p-type electrical conduction with a conductivity of 140 S cm(-1) was achieved. (C) 2003 American Institute of Physics.
  • H Ohta, H Mizoguchi, M Hirano, S Narushima, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 82 5 823 - 825 2003年02月 [査読無し][通常論文]
     
    A reactive solid-phase epitaxy technique was applied to fabricate all-oxide transparent p-n heterojunctions composed of p-ZnRh2O4 and n-ZnO thin layers. Polycrystalline ZnRh2O4 was deposited on a ZnO epitaxial layer at room temperature. Thermal annealing of the bilayer sample at 950 degreesC in air converts the polycrystalline ZnRh2O4 layer to an epitaxial single-crystalline layer. The resultant p-n heterojunctions have an abrupt interface and exhibit a distinct rectifying I-V characteristic. The threshold voltage is similar to2 V, agreeing well with the band-gap energy of ZnRh2O4. It also exhibits photovoltage with UV-light illumination, which originates mainly from the n-ZnO layer. (C) 2003 American Institute of Physics.
  • H Ohta, K Nomura, M Orita, M Hirano, K Ueda, T Suzuki, Y Ikuhara, H Hosono
    ADVANCED FUNCTIONAL MATERIALS 13 2 139 - 144 2003年02月 [査読無し][通常論文]
     
    Single-crystalline thin films of the homologous series InGaO3(ZnO)(m) (where m is an integer) are fabricated by the reactive solid-phase epitaxy (R-SPE) method. Specifically, the role of ZnO as epitaxial initiator layer for the growth mechanism is clarified High-temperature annealing of bilayer films consisting of an amorphous InGaO3(ZnO)(5) layer deposited at room temperature and an, epitaxial ZnO layer on yttria-stabilized zirconia (YSZ) substrate allows for the growth of single-crystalline film with controlled chemical composition. The epitaxial ZnO thin layer plays an essential role in determining the crystallographic orientation, while the ratio of the thickness of both layers controls the film composition.
  • H Hiramatsu, K Ueda, H Ohta, M Hirano, T Kamiya, H Hosono
    Appl. Phys. Lett. 82 7 1048 - 1050 2003年02月 [査読無し][通常論文]
     
    Epitaxial films of LaCuOS1-xSex (x=0-1) solid solution were grown on MgO (001) substrates and their electrical and optical properties were examined. Sharp emission due to room-temperature exciton with binding energy of similar to50 meV is observed for all x values. Hall mobility becomes large with an increase in the Se content and it reaches 8.0 cm(2)V(-1)s(-1) in LaCuOSe, a comparable value to that of p-type GaN:Mg. Doping of Mg2+ ions at La3+ sites enhances a hole concentration up to 2.2x10(20) cm(-3), while maintaining the Hall mobility as large as 4.0 cm(2)V(-1)s(-1). Consequently, a degenerate p-type electrical conduction with a conductivity of 140 S cm(-1) was achieved. (C) 2003 American Institute of Physics.
  • H Ohta, H Mizoguchi, M Hirano, S Narushima, T Kamiya, H Hosono
    APPLIED PHYSICS LETTERS 82 5 823 - 825 2003年02月 [査読無し][通常論文]
     
    A reactive solid-phase epitaxy technique was applied to fabricate all-oxide transparent p-n heterojunctions composed of p-ZnRh2O4 and n-ZnO thin layers. Polycrystalline ZnRh2O4 was deposited on a ZnO epitaxial layer at room temperature. Thermal annealing of the bilayer sample at 950 degreesC in air converts the polycrystalline ZnRh2O4 layer to an epitaxial single-crystalline layer. The resultant p-n heterojunctions have an abrupt interface and exhibit a distinct rectifying I-V characteristic. The threshold voltage is similar to2 V, agreeing well with the band-gap energy of ZnRh2O4. It also exhibits photovoltage with UV-light illumination, which originates mainly from the n-ZnO layer. (C) 2003 American Institute of Physics.
  • H Ohta, K Nomura, M Orita, M Hirano, K Ueda, T Suzuki, Y Ikuhara, H Hosono
    ADVANCED FUNCTIONAL MATERIALS 13 2 139 - 144 2003年02月 [査読無し][通常論文]
     
    Single-crystalline thin films of the homologous series InGaO3(ZnO)(m) (where m is an integer) are fabricated by the reactive solid-phase epitaxy (R-SPE) method. Specifically, the role of ZnO as epitaxial initiator layer for the growth mechanism is clarified High-temperature annealing of bilayer films consisting of an amorphous InGaO3(ZnO)(5) layer deposited at room temperature and an, epitaxial ZnO layer on yttria-stabilized zirconia (YSZ) substrate allows for the growth of single-crystalline film with controlled chemical composition. The epitaxial ZnO thin layer plays an essential role in determining the crystallographic orientation, while the ratio of the thickness of both layers controls the film composition.
  • Takeo Sasaki, Katsuyuki Matsunaga, Hiromichi Ohta, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara
    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS 4 6 575 - 584 2003年 [査読無し][通常論文]
     
    Interfacial atomic structures of Cu/Al2O3(0001) and Cu/Al2O3(11 (2) over bar0) systems prepared by a pulsed-laser deposition technique have been characterized by using high-resolution transmission electron microscopy (HRTEM). It was found that Cu metals were epitaxially oriented to the surface of Al2O3 substrates, and the following orientation relationships (ORs) were found to be formed: (111)(Cu)//(0001)Al2O3, [1 (1) over bar0](Cu)//[1 (1) over bar 00]Al2O3 in the Cu/Al2O3(0001) interface and (001)(Cu)//(11 (2) over bar0)Al2O3, [1 (1) over bar0](Cu)//[0001]Al2O3 in the Cu/Al2O3(11 (2) over bar0) interface. Geometrical coherency of the Cu/Al2O3 system has been evaluated by the coincidence of reciprocal lattice points method, and the result showed that the most coherent ORs were (111)(Cu)//(0001)Al2O3, [11 (2) over bar](Cu)//[1 (1) over bar 00]Al2O3 and (1 (1) over bar0)(Cu)//(11 (2) over bar0)Al2O3, [111](Cu)//[0001]Al2O3, which are equivalent to each other. These ORs were not consistent with the experimentally observed ORs, and it was possible that crucial factors to determine the ORs between Cu and Al2O3 were not only geometrical coherency, but also other factors such as chemical bonding states. Therefore, to understand the nature of the interface atomic structures, the electronic structures of the Cu/Al2O3 interfaces have been investigated by electron energy-loss spectroscopy. It was found that the pre-edge at the lower energy part of the main peak appeared in the OK edge spectra at the interface region in both the Cu/Al2O3(0001) and Cu/Al2O3(11 (2) over bar0) systems. This indicates the existence of Cu-O interactions at the interface. In fact, HRTEM simulation images based on O-terminated interface models agreed well with the experimental images, indicating that O-terminated interfaces were formed in both systems. Since the overlapped Cu atomic density in the experimental ORs were larger than that in the most coherent OR, it is considered that the on-top Cu-O bonds stabilize the O-terminated Cu/Al2O3 interfaces. (C) 2004 Elsevier Ltd. All rights reserved.
  • Thin Solid Films 445 304  2003年 [査読無し][通常論文]
  • Takeo Sasaki, Katsuyuki Matsunaga, Hiromichi Ohta, Hideo Hosono, Takahisa Yamamoto, Yuichi Ikuhara
    Science and Technology of Advanced Materials 4 6 575 - 584 2003年 [査読無し][通常論文]
     
    Interfacial atomic structures of Cu/Al2O3(0001) and Cu/Al2O3(11 (2) over bar0) systems prepared by a pulsed-laser deposition technique have been characterized by using high-resolution transmission electron microscopy (HRTEM). It was found that Cu metals were epitaxially oriented to the surface of Al2O3 substrates, and the following orientation relationships (ORs) were found to be formed: (111)(Cu)//(0001)Al2O3, [1 (1) over bar0](Cu)//[1 (1) over bar 00]Al2O3 in the Cu/Al2O3(0001) interface and (001)(Cu)//(11 (2) over bar0)Al2O3, [1 (1) over bar0](Cu)//[0001]Al2O3 in the Cu/Al2O3(11 (2) over bar0) interface. Geometrical coherency of the Cu/Al2O3 system has been evaluated by the coincidence of reciprocal lattice points method, and the result showed that the most coherent ORs were (111)(Cu)//(0001)Al2O3, [11 (2) over bar](Cu)//[1 (1) over bar 00]Al2O3 and (1 (1) over bar0)(Cu)//(11 (2) over bar0)Al2O3, [111](Cu)//[0001]Al2O3, which are equivalent to each other. These ORs were not consistent with the experimentally observed ORs, and it was possible that crucial factors to determine the ORs between Cu and Al2O3 were not only geometrical coherency, but also other factors such as chemical bonding states. Therefore, to understand the nature of the interface atomic structures, the electronic structures of the Cu/Al2O3 interfaces have been investigated by electron energy-loss spectroscopy. It was found that the pre-edge at the lower energy part of the main peak appeared in the OK edge spectra at the interface region in both the Cu/Al2O3(0001) and Cu/Al2O3(11 (2) over bar0) systems. This indicates the existence of Cu-O interactions at the interface. In fact, HRTEM simulation images based on O-terminated interface models agreed well with the experimental images, indicating that O-terminated interfaces were formed in both systems. Since the overlapped Cu atomic density in the experimental ORs were larger than that in the most coherent OR, it is considered that the on-top Cu-O bonds stabilize the O-terminated Cu/Al2O3 interfaces. (C) 2004 Elsevier Ltd. All rights reserved.
  • 自然超格子構造を有する透明半導体―エピタキシャル成長と光・電子物性―(共著)
    月刊ディスプレイ 5 78 - 83 2003年 [査読無し][通常論文]
  • K Ueda, K Takafuji, H Hiramatsu, H Ohta, M Hirano, H Hosono, H Kawazoe
    CRYSTALLINE OXIDE-SILICON HETEROSTRUCTURES AND OXIDE OPTOELECTRONICS 747 223 - 233 2003年 [査読有り][通常論文]
     
    Several Cu(I)-containing layered oxysulfides were selected as candidates for wide-gap p-type semiconductors by extending a concept of a materials design for transparent p-type conducting oxides. The electrical and optical properties of the selected oxysulfides were investigated, and their electronic structures were analyzed by energy band calculations. LaCuOS, Sr2Cu2ZnO2S2 and Sr2CuGaO3S were found to be wide-gap p-type semiconductors, and LaCuOS showed the largest energy gap (E-g=3.1eV) among these layered oxysulfides. It was also found that LaCuOS shows band edge emission under uv excitation at room temperature, which is consistent with the results of the energy band calculations that LaCuOS has a direct-allowed-type energy gap at F point. In further materials research, analogous layered oxychalcogenides such as LaCuOSe and LnCuOS (Ln=Pr, Nd) were found to show similar optical and electrical properties to those of LaCuOS. Therefore, it is considered that the layered crystal structure and the electronic structure are responsible for the wide-gap p-type conductive properties in these materials.
  • S Narushima, H Mizoguchi, H Ohta, M Hirano, K Shimizu, K Ueda, T Kamiya, H Hosono
    CRYSTALLINE OXIDE-SILICON HETEROSTRUCTURES AND OXIDE OPTOELECTRONICS 747 235 - 240 2003年 [査読有り][通常論文]
     
    An amorphous p-type conductive oxide semiconductor was created based on a mother crystalline material, a p-type conductive ZnRh2O4 spinel. The amorphous film of ZnRh2O4 was deposited by an rf sputtering method. Seebeck coefficient was positive, +78 muVK(-1), indicating that major carrier is a positive hole. A moderate electrical conductivity (2 S cm(-1) at room temperature) for a p-type semiconductor was observed. Optical band gap was estimated to be 2.1 eV. P-n junction diodes with a structure of Au / a-ZnRh2O4 / a-InGaZnO4 / ITO fabricated on glass substrates, operated with a good rectifying characteristics, a rectification current ratio at +/- 5V of similar to10(3). The threshold voltage was 2.1 eV, which corresponds to the band gap energy of the amorphous ZnRh2O4. This is the first discovery of a p-type amorphous oxide and the demonstration of p-n junction all composed of amorphous oxide semiconductors.
  • H Hiramatsu, K Ueda, H Ohta, M Hirano, H Hosono
    CRYSTALLINE OXIDE-SILICON HETEROSTRUCTURES AND OXIDE OPTOELECTRONICS 747 359 - 364 2003年 [査読有り][通常論文]
     
    Epitaxial films of LaCuOS, a wide gap p-type semiconductor, were grown on yittria- stabilized-zirconia (YSZ) (001) or MgO (001) substrates by a reactive solid phase epitaxy (R-SPE) method. Crystal quality, electrical and optical properties on the epitaxial films on each substrate are examined in this paper. Achievement of the heteroepitaxial growth of LaCuOS on the MgO (001) substrate improves optical properties of LaCuOS such as spectral bandwidths and emission intensity, suggesting that the MgO (001) substrate is more preferable than the YSZ (001) for epitaxial growth substrate for LaCuOS.
  • H Hosono, H Ohta, M Orita, K Ueda, M Hirano
    VACUUM 66 3-4 419 - 425 2002年08月 [査読無し][通常論文]
     
    The recent advancement of transparent conductive oxide thin films is reviewed using our strategy. Topics focused are the lowest resistive indium tin oxide films, deep-ultraviolet (deep-UV) transparent conductive oxides (TCO) thin film, p-n homo junction based on bipolar CuInO2, and UV-light emitting diode based on p-n hetero-junction. A frontier of transparent oxide semiconductors is opening as a consequence of discovery of p-type TCO. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • H Hosono, H Ohta, M Orita, K Ueda, M Hirano
    Vacuum 66 3-4 419 - 425 2002年08月 [査読無し][通常論文]
     
    The recent advancement of transparent conductive oxide thin films is reviewed using our strategy. Topics focused are the lowest resistive indium tin oxide films, deep-ultraviolet (deep-UV) transparent conductive oxides (TCO) thin film, p-n homo junction based on bipolar CuInO2, and UV-light emitting diode based on p-n hetero-junction. A frontier of transparent oxide semiconductors is opening as a consequence of discovery of p-type TCO. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • H Hiramatsu, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    APPLIED PHYSICS LETTERS 81 4 598 - 600 2002年07月 [査読無し][通常論文]
     
    High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 degreesC results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films. (C) 2002 American Institute of Physics.
  • H Hiramatsu, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    Appl. Phys. Lett. 81 4 598 - 600 2002年07月 [査読無し][通常論文]
     
    High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 degreesC results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films. (C) 2002 American Institute of Physics.
  • H Hiramatsu, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    THIN SOLID FILMS 411 1 125 - 128 2002年05月 [査読無し][通常論文]
     
    Single phase thin films of a layered oxysulfide, (La1-xSrxO)CuS (x = 0, and 0.03), were prepared by r.f. sputtering technique followed by a post-annealing treatment in an evacuated silica tube containing a small amount of (LaO)CuS powder. Key growth parameters for obtaining a single-phase film were the r.f. power and the substrate temperature in r.f. sputtering under an H,S atmosphere. ICP and FP-XRF measurements revealed that the chemical compositions of post-annealed films prepared under optimum conditions were in significant agreement with those of the target. Sr2+ ion doping in the films was performed by adding SrS in the targets, which enhanced the p-type electrical conductivity from 6.4x10(-5) (x=0) to 20 S cm(-1) (x=0.03). Optical transmission spectra of the films showed high optical transmission (>60%) in the visible-near IR region. (C) 2002 Elsevier Science B.V. All rights reserved.
  • K Nomura, H Ohta, K Ueda, M Orita, M Hirano, H Hosono
    THIN SOLID FILMS 411 1 147 - 151 2002年05月 [査読無し][通常論文]
     
    Single crystalline thin films of ln(2)O(3)(ZnO)(5) were grown by a novel solid state diffusion technique using a ZnO epitaxial layer as solid template. Crystal quality of the film was evaluated by high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). Multilayered films composed of amorphous In2O3(ZnO)(4) and epitaxial ZnO were grown on (111) yttrium-stabilized-zirconia (YSZ) by a pulsed-laser-deposition method, followed by annealing the films fully covered by YSZ plate at 1450 degreesC in air. HR-XRD measurement revealed In2O3(ZnO)(5) to be a single crystalline film. Sharp diffraction peaks of (0001) due to superlattices were seen in the out-of-plane HR-XRD pattern. Out-of-plane and in-plane orientations of the film and the substrate were In2O3(ZnO)(5) (0001)//YSZ (111) and In2O3(ZnO)(5) (11 (2) over bar0)//YSZ(1 (1) over bar0), respectively Step and terrace structure was clearly observed in an AFM image of the In2O3(ZnO)(5) film. (C) 2002 Elsevier Science B.V. All rights reserved.
  • M Orita, H Hiramatsu, H Ohta, M Hirano, H Hosono
    THIN SOLID FILMS 411 1 134 - 139 2002年05月 [査読無し][通常論文]
     
    beta-Ga2O3 is a unique oxide that exhibits deep ultraviolet transparency as well as good electric conductivity when dopants are introduced. With the purpose of realizing highly conductive films using this material, tin doped Ga2O3 films were deposited on Al2O3 (0001) substrates by the pulsed laser deposition method. ((2) over bar 01) oriented beta-phase films were grown at substrate temperatures of between 380 degreesC and 435 degreesC, and deep ultraviolet transparent films with a conductivity as high as 8.2 S cm(-1) were obtained at 380 degreesC under laser ablation with a low repetition rate of 1 Hz. With a further increase in the deposition temperature, the films underwent a crystalline phase transition, accompanied by an abrupt decrease in conductivity. (C) 2002 Elsevier Science B.V All rights reserved.
  • H Hiramatsu, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    Thin Solid Films 411 1 125 - 128 2002年05月 [査読無し][通常論文]
     
    Single phase thin films of a layered oxysulfide, (La1-xSrxO)CuS (x = 0, and 0.03), were prepared by r.f. sputtering technique followed by a post-annealing treatment in an evacuated silica tube containing a small amount of (LaO)CuS powder. Key growth parameters for obtaining a single-phase film were the r.f. power and the substrate temperature in r.f. sputtering under an H,S atmosphere. ICP and FP-XRF measurements revealed that the chemical compositions of post-annealed films prepared under optimum conditions were in significant agreement with those of the target. Sr2+ ion doping in the films was performed by adding SrS in the targets, which enhanced the p-type electrical conductivity from 6.4x10(-5) (x=0) to 20 S cm(-1) (x=0.03). Optical transmission spectra of the films showed high optical transmission (>60%) in the visible-near IR region. (C) 2002 Elsevier Science B.V. All rights reserved.
  • K Nomura, H Ohta, K Ueda, M Orita, M Hirano, H Hosono
    Thin Solid Films 411 1 147 - 151 2002年05月 [査読無し][通常論文]
     
    Single crystalline thin films of ln(2)O(3)(ZnO)(5) were grown by a novel solid state diffusion technique using a ZnO epitaxial layer as solid template. Crystal quality of the film was evaluated by high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). Multilayered films composed of amorphous In2O3(ZnO)(4) and epitaxial ZnO were grown on (111) yttrium-stabilized-zirconia (YSZ) by a pulsed-laser-deposition method, followed by annealing the films fully covered by YSZ plate at 1450 degreesC in air. HR-XRD measurement revealed In2O3(ZnO)(5) to be a single crystalline film. Sharp diffraction peaks of (0001) due to superlattices were seen in the out-of-plane HR-XRD pattern. Out-of-plane and in-plane orientations of the film and the substrate were In2O3(ZnO)(5) (0001)//YSZ (111) and In2O3(ZnO)(5) (11 (2) over bar0)//YSZ(1 (1) over bar0), respectively Step and terrace structure was clearly observed in an AFM image of the In2O3(ZnO)(5) film. (C) 2002 Elsevier Science B.V. All rights reserved.
  • M Orita, H Hiramatsu, H Ohta, M Hirano, H Hosono
    Thin Solid Films 411 1 134 - 139 2002年05月 [査読無し][通常論文]
     
    beta-Ga2O3 is a unique oxide that exhibits deep ultraviolet transparency as well as good electric conductivity when dopants are introduced. With the purpose of realizing highly conductive films using this material, tin doped Ga2O3 films were deposited on Al2O3 (0001) substrates by the pulsed laser deposition method. ((2) over bar 01) oriented beta-phase films were grown at substrate temperatures of between 380 degreesC and 435 degreesC, and deep ultraviolet transparent films with a conductivity as high as 8.2 S cm(-1) were obtained at 380 degreesC under laser ablation with a low repetition rate of 1 Hz. With a further increase in the deposition temperature, the films underwent a crystalline phase transition, accompanied by an abrupt decrease in conductivity. (C) 2002 Elsevier Science B.V All rights reserved.
  • H Hosono, H Ohta, K Hayashi, M Orita, M Hirano
    JOURNAL OF CRYSTAL GROWTH 237 496 - 502 2002年04月 [査読無し][通常論文]
     
    A near-UV emitting diode was fabricated by successive heteroepitaxial growth of In2O3:Sn(ITO), n-electrode, n-ZnO, and p-SrCu2O2 (SCO) on an extremely flat surface of yttria-stabilized ZrO2 single crystalline substrate using a pulsed-laser deposition technique. Upon current injection at room temperature, the resulting diode emitted near-UV light with a peak at 382 nm. The threshold voltage for emission was similar to 3V, which is in agreement with the energy difference between the ZnO donor level and the SCO acceptor level. The epitaxial relationship between SCO and ZnO depends on the deposition temperature of SCO on ZnO. The inhomogeneity of the SCO crystal was improved by raising the deposition temperature from 350degreesC to 600degreesC. The threshold Current for near-UV emission from the diode fabricated by deposition of the SCO film at 600degreesC was reduced to similar to 15 of that at 350degreesC. A reason why this heteroepitaxial growth occurs for oxides with large lattice misfit is explained in terms of the high ionicity of chemical bonds in oxides. (C) 2002 Elsevier Science B.V. All rights reserved.
  • H Hosono, H Ohta, K Hayashi, M Orita, M Hirano
    J. Crystal Growth 237 496 - 502 2002年04月 [査読無し][通常論文]
     
    A near-UV emitting diode was fabricated by successive heteroepitaxial growth of In2O3:Sn(ITO), n-electrode, n-ZnO, and p-SrCu2O2 (SCO) on an extremely flat surface of yttria-stabilized ZrO2 single crystalline substrate using a pulsed-laser deposition technique. Upon current injection at room temperature, the resulting diode emitted near-UV light with a peak at 382 nm. The threshold voltage for emission was similar to 3V, which is in agreement with the energy difference between the ZnO donor level and the SCO acceptor level. The epitaxial relationship between SCO and ZnO depends on the deposition temperature of SCO on ZnO. The inhomogeneity of the SCO crystal was improved by raising the deposition temperature from 350degreesC to 600degreesC. The threshold Current for near-UV emission from the diode fabricated by deposition of the SCO film at 600degreesC was reduced to similar to 15 of that at 350degreesC. A reason why this heteroepitaxial growth occurs for oxides with large lattice misfit is explained in terms of the high ionicity of chemical bonds in oxides. (C) 2002 Elsevier Science B.V. All rights reserved.
  • H Ohta, M Orita, M Hirano, H Hosono
    JOURNAL OF APPLIED PHYSICS 91 6 3547 - 3550 2002年03月 [査読無し][通常論文]
     
    Highly electrically conductive (resistivity: 1x10(-4) Omega cm) indium tin oxide (ITO) thin films were grown heteroepitaxially on (111) and (100) surfaces of yttria-stabilized zirconia (YSZ) at a growth temperature of 900 degreesC using a pulsed-laser deposition technique, and their crystal quality and surface morphology were evaluated by using high-resolution x-ray diffraction and an atomic force microscope. Higher growth temperature, the use of a cleaved surface as a substrate, and an increase in oxygen pressure are essential to fabricate an ITO thin film with an atomically flat surface. The full width at half maximum of out-of-plane x-ray rocking curves of ITO (222) grown on YSZ (111) and of ITO (400) on YSZ (100) were 54 and 720 arcseconds, respectively, which indicates that (111)-oriented ITO films exhibited higher crystal quality than (100)-oriented ITO films. The ITO film was grown on YSZ (111) by the three-dimensional spiral growth mode, with flat terraces and steps corresponding to (222) plane spacing of 0.293 nm, and the grain size was reduced with increasing oxygen pressure during ITO film growth, to improve overall surface flatness. On the other hand, only a columnar structure was observed in the (100)-oriented ITO film surface. (C) 2002 American Institute of Physics.
  • H Ohta, M Orita, M Hirano, H Hosono
    JOURNAL OF APPLIED PHYSICS 91 6 3547 - 3550 2002年03月 [査読無し][通常論文]
     
    Highly electrically conductive (resistivity: 1x10(-4) Omega cm) indium tin oxide (ITO) thin films were grown heteroepitaxially on (111) and (100) surfaces of yttria-stabilized zirconia (YSZ) at a growth temperature of 900 degreesC using a pulsed-laser deposition technique, and their crystal quality and surface morphology were evaluated by using high-resolution x-ray diffraction and an atomic force microscope. Higher growth temperature, the use of a cleaved surface as a substrate, and an increase in oxygen pressure are essential to fabricate an ITO thin film with an atomically flat surface. The full width at half maximum of out-of-plane x-ray rocking curves of ITO (222) grown on YSZ (111) and of ITO (400) on YSZ (100) were 54 and 720 arcseconds, respectively, which indicates that (111)-oriented ITO films exhibited higher crystal quality than (100)-oriented ITO films. The ITO film was grown on YSZ (111) by the three-dimensional spiral growth mode, with flat terraces and steps corresponding to (222) plane spacing of 0.293 nm, and the grain size was reduced with increasing oxygen pressure during ITO film growth, to improve overall surface flatness. On the other hand, only a columnar structure was observed in the (100)-oriented ITO film surface. (C) 2002 American Institute of Physics.
  • M Miyakawa, R Noshiro, T Ogawa, K Ueda, H Kawazoe, H Ohta, M Orita, M Hirano, H Hosono
    JOURNAL OF APPLIED PHYSICS 91 4 2112 - 2117 2002年02月 [査読有り][通常論文]
     
    Epitaxial thin films of MgIn2O4 were grown on MgO(100) single-crystal substrates through the pulsed laser deposition technique. X-ray diffraction measurements revealed film orientations MgIn2O4(h00)//MgO(h00) and MgIn2O4(0k0)//MgO(0k0), respectively. Proton implantation was applied to generate carrier electrons in the films. The electrical conductivity of the as-deposited films is below similar to10(-7) S cm(-1) at room temperature. The maximum conductivity of similar to70 S cm(-1) was obtained by the implantation. Hall voltage measurements revealed that H+ implantation causes carrier generation in proportion to H+ fluence without reduction of electron mobility. Following the post-annealing process resulted in further enhancement of the conductivity in each H+-implanted film, as conductivity and generation efficiency were found to increase up to similar to2x10(2) S cm(-1) and similar to95% at the maximum, respectively. This differs from the behavior of polycrystalline films in which conductivity decreased by post-annealing due to a decrease in the Hall mobility of electrons. Thus it is concluded that crystal quality is crucial for heavy carrier doping by ion implantation, especially when utilizing post-annealing treatments to enhance the carrier generation efficiency without reduction of the Hall mobility. (C) 2002 American Institute of Physics.
  • H Hiramatsu, H Ohta, M Hirano, H Hosono
    SOLID STATE COMMUNICATIONS 124 10-11 411 - 415 2002年 [査読無し][通常論文]
     
    Zinc sulfide (ZnS) films were grown on transparent yittria-stabilized-zirconia (YSZ) substrates by pulsed laser deposition. Deposition at 500 degreesC in a H2S/Ar mixed gas atmosphere of 10 Pa on YSZ(111) substrate resulted in epitaxial growth of a single-phase cubic transparent ZnS film. X-ray reciprocal space mappings and optical spectroscopy clearly indicate that the film is composed of single-phase zinc blende. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Carrier doping into MgIn2O4 epitaxial thin films by proton implantation(共著)
    J. Appl. Phys. 90 3074  2002年 [査読無し][通常論文]
  • H Ohta, M Orita, M Hirano, K Ueda, H Hosono
    INTERNATIONAL JOURNAL OF MODERN PHYSICS B 16 1-2 173 - 180 2002年01月 [査読無し][通常論文]
     
    Transparent conductive oxides of ITO, ZnO, beta-Ga2O3, CuGaO2 and SrCu2O2 were grown on single crystal substrates of alpha-Al2O3 and YSZ by pulsed-laser deposition, and their crystallinity was evaluated by using high-resolution X-ray diffraction and electron microscope. Heteroepitaxial growth was observed in spite of relatively large lattice mismatches between film and substrate. A few disordered layers were generated automatically at film/substrate boundary, played buffer layer to suppress propagation of edge dislocations.
  • H Yanagi, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    SOLID STATE COMMUNICATIONS 121 1 15 - 17 2002年 [査読無し][通常論文]
     
    A transparent homojunction was fabricated with a successive deposition of p- and n-type electronic conductive CuInO2 delafossite-type oxide films by a pulsed laser deposition technique. Each conductivity was realized by doping of Sn4+ ions as donor or Ca2+ ions as acceptor ions. The diode, composed of the p-n homojunction sandwiched by ITO films as anode and cathode, exhibits rectifying characteristics with a turn-on voltage of similar to1.8 V, keeping an optical transmission of 60%-80% in the visible region (its total thickness: 1.8 mum). (C) 2001 Elsevier Science Ltd. All rights reserved.
  • H Hiramatsu, H Ohta, M Hirano, H Hosono
    Solid State Comm. 124 10-11 411 - 415 2002年 [査読無し][通常論文]
     
    Zinc sulfide (ZnS) films were grown on transparent yittria-stabilized-zirconia (YSZ) substrates by pulsed laser deposition. Deposition at 500 degreesC in a H2S/Ar mixed gas atmosphere of 10 Pa on YSZ(111) substrate resulted in epitaxial growth of a single-phase cubic transparent ZnS film. X-ray reciprocal space mappings and optical spectroscopy clearly indicate that the film is composed of single-phase zinc blende. (C) 2002 Elsevier Science Ltd. All rights reserved.
  • Carrier doping into MgIn2O4 epitaxial thin films by proton implantation(共著)
    J. Appl. Phys. 90 3074  2002年 [査読無し][通常論文]
  • H Ohta, M Orita, M Hirano, K Ueda, H Hosono
    INTERNATIONAL JOURNAL OF MODERN PHYSICS B 16 1-2 173 - 180 2002年01月 [査読無し][通常論文]
     
    Transparent conductive oxides of ITO, ZnO, beta-Ga2O3, CuGaO2 and SrCu2O2 were grown on single crystal substrates of alpha-Al2O3 and YSZ by pulsed-laser deposition, and their crystallinity was evaluated by using high-resolution X-ray diffraction and electron microscope. Heteroepitaxial growth was observed in spite of relatively large lattice mismatches between film and substrate. A few disordered layers were generated automatically at film/substrate boundary, played buffer layer to suppress propagation of edge dislocations.
  • H Yanagi, K Ueda, H Ohta, M Orita, M Hirano, H Hosono
    Solid State Comm. 121 1 15 - 17 2002年 [査読無し][通常論文]
     
    A transparent homojunction was fabricated with a successive deposition of p- and n-type electronic conductive CuInO2 delafossite-type oxide films by a pulsed laser deposition technique. Each conductivity was realized by doping of Sn4+ ions as donor or Ca2+ ions as acceptor ions. The diode, composed of the p-n homojunction sandwiched by ITO films as anode and cathode, exhibits rectifying characteristics with a turn-on voltage of similar to1.8 V, keeping an optical transmission of 60%-80% in the visible region (its total thickness: 1.8 mum). (C) 2001 Elsevier Science Ltd. All rights reserved.
  • H Ohta, M Orita, M Hirano, H Hosono
    ASIAN CERAMIC SCIENCE FOR ELECTRONICS I 214-2 75 - 80 2002年 [査読有り][通常論文]
     
    UV light-emitting diode is realized for the first time using a hetero p-n junction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Ni/SrCu2O2/ZnO/ITO multi-layered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n heterojunction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 3V relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO.
  • M. Orita, H. Ohta, H. Hiramatsu, M. Hirano, S. Den, M. Sasaki, T. Katagiri, H. Mimura, H. Hosono
    Review of Scientific Instruments 72 8 3340 - 3343 2001年08月 [査読無し][通常論文]
     
    A pulsed laser deposition system capable of raising substrate temperatures up to 1500°C in oxygen atmosphere has been developed to grow oxide thin films with excellent crystalline quality. The high temperature is achieved by condensation onto a substrate of infrared light emitted from a halogen lamp through a SiO2 rod serving as an optical guide. Deposition temperatures are regulated ranging from 1500 to 150°C, under a partial oxygen pressure from 100 to 5 × 10-6Pa. An appropriate thermal insulation between the substrate holder and the surrounding components suppresses the heat flow inbetween, leading to a smooth rotation of the substrate holder and preventing the mechanical components from being oxidized. A high-precision five-axis alignment mechanism equipped with a substrate manipulator allows for the development of an application in coaxial impact-collision ion scattering spectroscopy. As a demonstrative experiment, highly oriented Ga2O3 films, which are useful as deep ultraviolet transparent semiconductors, have been deposited on an Al2O3 (0001) single crystal substrate at 1000°C under an oxygen pressure of 5 × 10-5Pa. © 2001 American Institute of Physics.
  • M Orita, H Ohta, H Hiramatsu, M Hirano, S Den, M Sasaki, T Katagiri, H Mimura, H Hosono
    Rev. Sci. Inst. 72 8 3340 - 3343 2001年08月 [査読無し][通常論文]
     
    A pulsed laser deposition system capable of raising substrate temperatures up to 1500 degreesC in oxygen atmosphere has been developed to grow oxide thin films with excellent crystalline quality. The high temperature is achieved by condensation onto a substrate of infrared light emitted from a halogen lamp through a SiO2 rod serving as an optical guide. Deposition temperatures are regulated ranging from 1500 to 150 degreesC, under a partial oxygen pressure from 100 to 5x10(-6) Pa. An appropriate thermal insulation between the substrate holder and the surrounding components suppresses the heat flow inbetween, leading to a smooth rotation of the substrate holder and preventing the mechanical components from being oxidized. A high-precision five-axis alignment mechanism equipped with a substrate manipulator allows for the development of an application in coaxial impact-collision ion scattering spectroscopy. As a demonstrative experiment, highly oriented Ga2O3 films, which are useful as deep ultraviolet transparent semiconductors, have been deposited on an Al2O3 (0001) single crystal substrate at 1000 degreesC under an oxygen pressure of 5x10(-5) Pa. (C) 2001 American Institute of Physics.
  • M Orita, H Ohta, M Hirano, S Narushima, H Hosono
    PHILOSOPHICAL MAGAZINE B-PHYSICS OF CONDENSED MATTER STATISTICAL MECHANICS ELECTRONIC OPTICAL AND MAGNETIC PROPERTIES 81 5 501 - 515 2001年05月 [査読無し][通常論文]
     
    With the purpose of creating ZnO-based amorphous transparent conductors, a range of amorphous films InGaO3(ZnO)(m) (where m less than or equal to 4) was prepared using a pulsed-laser deposition method. The resulting films exhibited an optical bandgap of 2.8-3.0eV, and an n-type electric conductivity of 170-400S cm(-1) at room temperature, displaying a slight dependence on the value of m, in which the carrier density was 10(19)-10(20) cm(-3) the electron mobility was 12-20 cm(2) V-1 s(-1) showing no p-n anomaly between Hall and Seebeck coefficients. The conductivity; displayed no significant dependence on the temperature ranging from 10 to 300 K. X-ray diffraction, transmission electron microscopy and extended X-ray absorption fine structure measurements confirmed that the films were amorphous phases. A combined use of bremsstrahlung isochromat spectroscopy and ultraviolet photoelectron spectroscopy revealed that the conduction band tail had a large dispersion and that the Fermi level was located at the conduction band edge. The percolation theory and overlap integral calculations suggest that the extended conduction band of the amorphous metal oxides are formed when the (n - 1)d(10) ns(0) metal ions (n being the principal quantum number) occupy more than 20% of the atoms and the overlap integral between the vacant ns orbitals exceeds a threshold value of approximately 0.4. The present system is the first amorphous oxide semiconductor in which Zn 4s orbitals form the extended conduction band.
  • H Ohta, M Orita, M Hirano, H Hosono
    JOURNAL OF APPLIED PHYSICS 89 10 5720 - 5725 2001年05月 [査読無し][通常論文]
     
    An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350 degreesC, while the preferential plane was changed into the (100) when the temperature was increased to 600 degreesC. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficiency improved with higher SrCu2O2 deposition temperatures. On the other hand, increased laser power density at 600 degreesC during deposition raised the incidence of insulating layer formation between the p and n layers, probably due to migration of K+ ions doped as an acceptor impurity. The resulting p-i-n diode emits broad luminescence centered at 500 nm for forward voltage greater than 14 V. (C) 2001 American Institute of Physics.
  • M Orita, H Ohta, M Hirano, S Narushima, H Hosono
    Phil. Mag. 81 5 501 - 515 2001年05月 [査読無し][通常論文]
     
    With the purpose of creating ZnO-based amorphous transparent conductors, a range of amorphous films InGaO3(ZnO)(m) (where m less than or equal to 4) was prepared using a pulsed-laser deposition method. The resulting films exhibited an optical bandgap of 2.8-3.0eV, and an n-type electric conductivity of 170-400S cm(-1) at room temperature, displaying a slight dependence on the value of m, in which the carrier density was 10(19)-10(20) cm(-3) the electron mobility was 12-20 cm(2) V-1 s(-1) showing no p-n anomaly between Hall and Seebeck coefficients. The conductivity; displayed no significant dependence on the temperature ranging from 10 to 300 K. X-ray diffraction, transmission electron microscopy and extended X-ray absorption fine structure measurements confirmed that the films were amorphous phases. A combined use of bremsstrahlung isochromat spectroscopy and ultraviolet photoelectron spectroscopy revealed that the conduction band tail had a large dispersion and that the Fermi level was located at the conduction band edge. The percolation theory and overlap integral calculations suggest that the extended conduction band of the amorphous metal oxides are formed when the (n - 1)d(10) ns(0) metal ions (n being the principal quantum number) occupy more than 20% of the atoms and the overlap integral between the vacant ns orbitals exceeds a threshold value of approximately 0.4. The present system is the first amorphous oxide semiconductor in which Zn 4s orbitals form the extended conduction band.
  • H Ohta, M Orita, M Hirano, H Hosono
    JOURNAL OF APPLIED PHYSICS 89 10 5720 - 5725 2001年05月 [査読無し][通常論文]
     
    An ultraviolet light-emitting diode (UV-LED) was realized using a p-n heterojunction composed of the transparent oxide semiconductors p-SrCu2O2 and n-ZnO. A Ni/SrCu2O2/ZnO/ITO multilayered film was epitaxially grown on an extremely flat YSZ (111) surface by a pulsed-laser deposition technique. SrCu2O2 (112) was preferentially grown on ZnO (0001) at 350 degreesC, while the preferential plane was changed into the (100) when the temperature was increased to 600 degreesC. The grown films were processed by conventional photolithography followed by reactive ion etching to fabricate heterojunction diodes. The resulting devices exhibited rectifying I-V characteristics inherent to p-n junctions. A relatively sharp electroluminescence band centered at 382 nm, attributed to transitions associated with exciton-exciton collision or electron-hole plasma in ZnO, was generated by applying a forward bias voltage greater than the turn-on voltage of 3 V. UV-LED performance characteristics such as threshold current and conversion efficiency improved with higher SrCu2O2 deposition temperatures. On the other hand, increased laser power density at 600 degreesC during deposition raised the incidence of insulating layer formation between the p and n layers, probably due to migration of K+ ions doped as an acceptor impurity. The resulting p-i-n diode emits broad luminescence centered at 500 nm for forward voltage greater than 14 V. (C) 2001 American Institute of Physics.
  • M. Orita, H. Ohta, M. Hirano, S. Narushima, H. Hosono
    Philosophical Magazine B: Physics of Condensed Matter; Statistical Mechanics, Electronic, Optical and Magnetic Properties 81 5 501 - 515 2001年05月 [査読有り][通常論文]
     
    With the purpose of creating ZnO-based amorphous transparent conductors, a range of amorphous films InGaO3(ZnO)m (where m ≤ 4) was prepared using a pulsed-laser deposition method. The resulting films exhibited an optical bandgap of 2.8-3.0 eV, and an n-type electric conductivity of 170-400 S cm-1 at room temperature, displaying a slight dependence on the value of m, in which the carrier density was 1019-1020 cm-3 the electron mobility was 12-20 cm2V-1 s-1, showing no p-n anomaly between Hall and Seebeck coefficients. The conductivity displayed no significant dependence on the temperature ranging from 10 to 300 K. X-ray diffraction, transmission electron microscopy and extended X-ray absorption fine structure measurements confirmed that the films were amorphous phases. A combined use of bremsstrahlung isochromat spectroscopy and ultraviolet photoelectron spectroscopy revealed that the conduction band tail had a large dispersion and that the Fermi level was located at the conduction band edge. The percolation theory and overlap integral calculations suggest that the extended conduction band of the amorphous metal oxides are formed when the (n - 1)d10ns0 metal ions (n being the principal quantum number) occupy more than 20% of the atoms and the overlap integral between the vacant ns orbitals exceeds a threshold value of approximately 0.4. The present system is the first amorphous oxide semiconductor in which Zn4s orbitals form the extended conduction band.
  • K Ueda, T Hase, H Yanagi, H Kawazoe, H Hosono, H Ohta, M Orita, M Hirano
    JOURNAL OF APPLIED PHYSICS 89 3 1790 - 1793 2001年02月 [査読無し][通常論文]
     
    Transparent p-type conducting CuGaO2 thin films were prepared on alpha -Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60 degrees with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (similar to 80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3 x 10(-2) S cm(-1), 1.7 x 10(18) cm(-3), and 0.23 cm(2)V(-1) s(-1), respectively. (C) 2001 American Institute of Physics.
  • K Ueda, T Hase, H Yanagi, H Kawazoe, H Hosono, H Ohta, M Orita, M Hirano
    J. Appl. Phys. 89 3 1790 - 1793 2001年02月 [査読無し][通常論文]
     
    Transparent p-type conducting CuGaO2 thin films were prepared on alpha -Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60 degrees with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (similar to 80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3 x 10(-2) S cm(-1), 1.7 x 10(18) cm(-3), and 0.23 cm(2)V(-1) s(-1), respectively. (C) 2001 American Institute of Physics.
  • UV-LED composed of p-SrCu2O2 / n-ZnO heterojunction realized for the first time
    J. Sur. Sci. Jpn. 22 419  2001年 [査読無し][通常論文]
  • UV-LED composed of p-SrCu2O2 / n-ZnO heterojunction realized for the first time
    J. Sur. Sci. Jpn. 22 419  2001年 [査読無し][通常論文]
  • 透明酸化物半導体を用いた近紫外発光ダイオードの開発
    セラミックス 36 4 285  2001年 [査読無し][通常論文]
  • M Orita, H Ohta, M Hirano, H Hosono
    APPLIED PHYSICS LETTERS 77 25 4166 - 4168 2000年12月 [査読無し][通常論文]
     
    Thin films of beta -Ga2O3 with an energy band gap of 4.9 eV were prepared on silica glass substrates by a pulsed-laser deposition method. N-type conductivity up to similar to1 S cm(-1) was obtained by Sn-ion doping and deposition under low O-2 partial pressure (similar to 10(-5) Pa) at substrate temperatures above 800 degreesC. The resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film, making this the most ultraviolet-transparent conductive oxide thin film to date and opening up prospects for applications such as ultraviolet transparent antistatic electric films in ultraviolet lithography. (C) 2000 American Institute of Physics. [S0003-6951(00)04249-2].
  • M Orita, H Ohta, M Hirano, H Hosono
    Appl. Phys. Lett. 77 25 4166 - 4168 2000年12月 [査読無し][通常論文]
     
    Thin films of beta -Ga2O3 with an energy band gap of 4.9 eV were prepared on silica glass substrates by a pulsed-laser deposition method. N-type conductivity up to similar to1 S cm(-1) was obtained by Sn-ion doping and deposition under low O-2 partial pressure (similar to 10(-5) Pa) at substrate temperatures above 800 degreesC. The resulting internal transmittance at the wavelength (248 nm) of the KrF excimer laser exceeded 50% for the 100-nm-thick film, making this the most ultraviolet-transparent conductive oxide thin film to date and opening up prospects for applications such as ultraviolet transparent antistatic electric films in ultraviolet lithography. (C) 2000 American Institute of Physics. [S0003-6951(00)04249-2].
  • Hiromichi Ohta, Ken-Ichi Kawamura, Masahiro Orita, Masahiro Hirano, Nobuhiko Sarukura, Hideo Hosono
    Applied Physics Letters 77 4 475 - 477 2000年07月24日 [査読無し][通常論文]
     
    An ultraviolet light-emitting diode (LED) operating at room temperature was realized using a p-n heterojunction composed of transparent conductive oxides. p-SrCu2O2 and n-ZnO. Multilayered films prepared by a pulsed-laser deposition technique were processed by conventional photolithography with the aid of reactive ion etching to fabricate the LED device. A rather sharp emission band centered at 382 nm was generated when a forward bias voltage exceeding the turn-on voltage of 3 V was applied to the junction. The emission may be attributed to a transition associated with the electron-hole plasma of ZnO. © 2000 American Institute of Physics.
  • Hiromichi Ohta, Ken-Ichi Kawamura, Masahiro Orita, Masahiro Hirano, Nobuhiko Sarukura, Hideo Hosono
    Applied Physics Letters 77 4 475 - 477 2000年07月24日 [査読無し][通常論文]
     
    An ultraviolet light-emitting diode (LED) operating at room temperature was realized using a p-n heterojunction composed of transparent conductive oxides. p-SrCu2O2 and n-ZnO. Multilayered films prepared by a pulsed-laser deposition technique were processed by conventional photolithography with the aid of reactive ion etching to fabricate the LED device. A rather sharp emission band centered at 382 nm was generated when a forward bias voltage exceeding the turn-on voltage of 3 V was applied to the junction. The emission may be attributed to a transition associated with the electron-hole plasma of ZnO. © 2000 American Institute of Physics.
  • H Ohta, M Orita, M Hirano, H Tanji, H Kawazoe, H Hosono
    APPLIED PHYSICS LETTERS 76 19 2740 - 2742 2000年05月 [査読無し][通常論文]
     
    Highly electrically conductive indium-tin-oxide thin films were epitaxially grown on an extremely flat (100) surface of yttria-stabilized zirconia single-crystal substrates at a substrate temperature of 600 degrees C by a pulsed-laser deposition technique. A resistivity down to 7.7x10(-5) Omega cm was reproducibly obtained, maintaining optical transmission exceeding 85% at wavelengths from 340 to 780 nm. The carrier densities of the films were enhanced up to 1.9x10(21) cm(-3), while the Hall mobility showed a slight, almost linear, decrease from 55 to 40 cm(2) V-1 s(-1) with increasing SnO2 concentration. The low resistivity is most likely the result of the good crystal quality of the films. (C) 2000 American Institute of Physics. [S0003-6951(00)01319-X].
  • H Ohta, K Kawamura, M Orita, N Sarukura, M Hirano, H Hosono
    ELECTRONICS LETTERS 36 11 984 - 985 2000年05月 [査読無し][通常論文]
     
    A UV-emitting diode composed of a hetero-SrCu2O2/ZnO pit junction was fabricated by pulsed laser deposition (PLD). On injecting an electrical current through a pn heterojunction of p-SrCu2O2/n-ZnO, an emission peak centred at 387 nm was observed, originating from excitons or the electron hole plasma in ZnO.
  • H Ohta, M Orita, M Hirano, H Tanji, H Kawazoe, H Hosono
    APPLIED PHYSICS LETTERS 76 19 2740 - 2742 2000年05月 [査読無し][通常論文]
     
    Highly electrically conductive indium-tin-oxide thin films were epitaxially grown on an extremely flat (100) surface of yttria-stabilized zirconia single-crystal substrates at a substrate temperature of 600 degrees C by a pulsed-laser deposition technique. A resistivity down to 7.7x10(-5) Omega cm was reproducibly obtained, maintaining optical transmission exceeding 85% at wavelengths from 340 to 780 nm. The carrier densities of the films were enhanced up to 1.9x10(21) cm(-3), while the Hall mobility showed a slight, almost linear, decrease from 55 to 40 cm(2) V-1 s(-1) with increasing SnO2 concentration. The low resistivity is most likely the result of the good crystal quality of the films. (C) 2000 American Institute of Physics. [S0003-6951(00)01319-X].
  • H Ohta, K Kawamura, M Orita, N Sarukura, M Hirano, H Hosono
    ELECTRONICS LETTERS 36 11 984 - 985 2000年05月 [査読無し][通常論文]
     
    A UV-emitting diode composed of a hetero-SrCu2O2/ZnO pit junction was fabricated by pulsed laser deposition (PLD). On injecting an electrical current through a pn heterojunction of p-SrCu2O2/n-ZnO, an emission peak centred at 387 nm was observed, originating from excitons or the electron hole plasma in ZnO.
  • 透明酸化物半導体の新しい展開
    表面 38 7 28  2000年 [査読無し][通常論文]
  • M Orita, H Ohta, M Hirano, H Hosono, K Morita, H Tanji, H Kawazoe
    MATERIALS SCIENCE OF NOVEL OXIDE-BASED ELECTRONICS 623 291 - 296 2000年 [査読有り][通常論文]
     
    Novel amorphous transparent conductive oxides, InGaO3(ZnO)(m), where m is an integer less than four, was developed. Optical transmittance in the visible region exceeded over 80 % and the electric conductivity at 300 K was as large as 400 S/cm. Both Seebeck and Hall coefficients exhibited negative values, indicating the conduction was n-type. It was suggested that 4s orbital of Zn2+ played a significant role for the formation of the extended state responsible for the conduction, while In3+ acted as a modifier for the stabilization of amorphous state.
  • H Ohta, M Orita, M Hirano, H Hosono, H Kawazoe, H Tanji
    MATERIALS SCIENCE OF NOVEL OXIDE-BASED ELECTRONICS 623 253 - 258 2000年 [査読有り][通常論文]
     
    High quality ITO thin films were grown hetero-epitaxially on extremely flat substrate of (001) YSZ by a pulsed laser deposition technique at a substrate temperature of 600 degreesC. The crystal orientation relationship between the film and YSZ were confirmed as ITO (001) // YSZ (001) and ITO (010) // YSZ (010), respectively, by HR-XRD and HR-TEM. The carrier densities of the films were almost equal to SnO2 concentration in the films. That is, almost all the doped Sn4+ ions were activated to release electrons to the conduction band. The carrier densities of the films were enhanced up to 1.9x10(21)cm(-3), while the Hall mobility showed a slight, almost linear, decrease from 55 to 40cm(2)V(-1)s(-1) with increasing SnO2 concentration. The low resistivity is due to larger electron mobility, which most likely resulted from good crystal quality of the films. The optical transmissivity of the film exceeded 85% at wavelengths from 340 to 780nm.
  • H Ohta, K Kawamura, M Orita, N Sarukura, M Hirano, H Hosono
    MATERIALS SCIENCE OF NOVEL OXIDE-BASED ELECTRONICS 623 283 - 288 2000年 [査読無し][通常論文]
     
    Room temperature operation of UV LED is realized for the first time using a hetero p-n junction composed of transparent conductive oxides, p-SrCu2O2 and n-ZnO. Ni/SrCu2O2/ZnO/ITO multi-layered film was epitaxially grown on an. extremely flat YSZ (111) surface by a PLD. The grown films were processed by a conventional photolithography, followed by reactive ion etching to fabricate p-n junction diode. The resultant device exhibited rectifying I-V characteristics inherent to p-n junction whose turn-on voltage was about 1.5V. A relatively sharp electro-luminescence band centered at 382nm was generated when applying the forward bias voltage larger than the turn-on voltage of 3V. The red shift in the EL peak was noticed from that of photo-luminescence (377nm), which was most likely due to the difference in the excited state density between the emission processes. The EL band is attributed to transition in ZnO, probably to that associated with electron-hole plasma. The photo-voltage was also generated when the p-n junction was irradiated with UV light of which energy coincided with both exciton and band-to-band transitions in ZnO.
  • H Ohta, H Tanji, M Orita, H Hosono, H Kawazoe
    EPITAXIAL GROWTH-PRINCIPLES AND APPLICATIONS 570 309 - 313 1999年 [査読有り][通常論文]
     
    Heteroepitaxial ZnO films were grown on (111) surface of yttria stabilized zirconia (YSZ) and (0001) surface of sapphire by PLD method, using KrF eximer laser (248nm) in an ultra-high-vacuum chamber. ZnO grown on YSZ (111) at the substrate temperature of 800 degrees C had an epitaxial relationship at the ZnO/YSZ interface of ZnO [11 (2) over bar 0]//YSZ [1 (1) over bar 0]. Hexagonal-shaped grains were observed whose surfaces were atomically flat. The grain size of ZnO increased and the Hall mobility rose toward 1400nm and 75cm(2)/Vs, respectively as film thickness increased from 10 nln to 800 nm.
  • Hidenori Hiramatsu, Hiromichi Ohta, Won-Seon Seo, Kunihito Koumoto
    Funtai Oyobi Fummatsu Yakin/Journal of the Japan Society of Powder and Powder Metallurgy 44 1 44 - 49 1997年 [査読無し][通常論文]
     
    We have reported that the thermoelectric figures of merit (Z) of the sintered bodies of layered structured (ZnO)mIn2O3, (m=5, 7, and 9) are comparatively large among the semiconducting oxides. In this work, (ZnO)5In2O3 thin films were prepared by r.f. (radio frequency) sputtering method to clarify their anisotropic nature. Under optimum sputtering conditions, (ZnO)5In2O3 thin films developed either (0021) or (110) crystallographic preferred orientation and had dense columnar structures. For c-axis and ab-plane oriented (ZnO)5In2O3 thin films, Seebeck coefficient (α) and electrical conductivity (σ) along the sheet direction were measured at 573-973K. Electrical conductivity of a c-axis oriented thin film was about an order of magnitude higher than that of an ab-plane oriented thin film, while their Seebeck coefficients were substantially the same. This observation suggests that the carrier mobility of (ZnO)5In2O3 along the c-plane is larger than along the ab-plane.
  • Hidenori Hiramatsu, Hiromichi Ohta, Won-Seon Seo, Kunihito Koumoto
    J. Jpn. Soc. Powder and Powder Metal. 44 1 44 - 49 1997年 [査読無し][通常論文]
     
    We have reported that the thermoelectric figures of merit (Z) of the sintered bodies of layered structured (ZnO)mIn2O3, (m=5, 7, and 9) are comparatively large among the semiconducting oxides. In this work, (ZnO)5In2O3 thin films were prepared by r.f. (radio frequency) sputtering method to clarify their anisotropic nature. Under optimum sputtering conditions, (ZnO)5In2O3 thin films developed either (0021) or (110) crystallographic preferred orientation and had dense columnar structures. For c-axis and ab-plane oriented (ZnO)5In2O3 thin films, Seebeck coefficient (α) and electrical conductivity (σ) along the sheet direction were measured at 573-973K. Electrical conductivity of a c-axis oriented thin film was about an order of magnitude higher than that of an ab-plane oriented thin film, while their Seebeck coefficients were substantially the same. This observation suggests that the carrier mobility of (ZnO)5In2O3 along the c-plane is larger than along the ab-plane.
  • H Ohta, WS Seo, K Koumoto
    JOURNAL OF THE AMERICAN CERAMIC SOCIETY 79 8 2193 - 2196 1996年08月 [査読無し][通常論文]
     
    Homologous compounds of (ZnO)(m)In2O3 (m = integer) with layered structures were synthesized by reaction-sintering a mixed powder of ZnO and In2O3 at 1823 K for 2 h in air, and their thermoelectric properties, i.e., electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured at 500 to 1100 K, Their thermoelectric figure of merit depended on the composition, and an optimum value of m apparently existed giving the largest figure of merit.
  • Hiromichi Ohta, Won-Seon Seo, Kunihito Koumoto
    Journal of the American Ceramic Society 79 8 2193 - 2196 1996年 [査読無し][通常論文]
     
    Homologous compounds of (ZnO)mIn2O3 (m = integer) with layered structures were synthesized by reaction-sintering a mixed powder of ZnO and In2O3 at 1823 K for 2 h in air, and their thermoelectric properties, i.e., electrical conductivity, Seebeck coefficient, and thermal conductivity, were measured at 500 to 1100 K. Their thermoelectric figure of merit depended on the composition, and an optimum value of m apparently existed giving the largest figure of merit.
  • K Koumoto, H Ohta, WS Seo
    PROCEEDINGS ICT '96 - FIFTEENTH INTERNATIONAL CONFERENCE ON THERMOELECTRICS 172 - 175 1996年 [査読有り][通常論文]

書籍

  • 2D Nanomaterials for Energy Applications (1st Edition)
    Yu-qiao Zhang, Hiromichi Ohta (担当:分担執筆範囲:2D thermoelectrics)
    Elsevier 2019年11月
  • Hiromichi Ohta (担当:分担執筆範囲:Electric Field Thermopower Modulation of 2D Electron Systems)
    Springer 2019年07月 (ISBN: 9783030200428) xv, 211 p.
  • エレクトロクロミックデバイスの開発最前線
    太田裕道, 小野里尚記 (担当:共著範囲:第2編 エレクトロクロミック材料の応用 酸化物エレクトロクロミック材料を用いたメモリデバイスの開発)
    シーエムシー出版 2019年07月
  • Nanoinformatics
    Hiromichi Ohta, Hidenori Hiramatsu (担当:共著範囲:10.Fabrication, characterization, and modulation of functional nanolayers)
    Springer 2018年02月
  • Metal Oxide-Based Thin Film Structures (1st Edition) Formation, Characterization and Application of Interface-based Phenomena
    太田 裕道 (担当:共著範囲:Chapter 4.3 Thermoelectrics)
    ELSEVIER 2017年09月
  • 透明導電膜の技術 改訂3版
    太田 裕道 (担当:共著範囲:6・5 パルスレーザ堆積法)
    オーム社 2014年04月
  • Multifunctional Oxide Heterostructures(Eds.) E. Y. Tsymbal, E. R. A. Dagotto, C-B. Eom, and R. Ramesh
    H. Ohta, K. Koumoto (担当:共著範囲:Chapter 10 Thermoelectric oxides: films and heterostructures)
    Oxford 2012年
  • セラミック機能化ハンドブック
    太田 裕道 (担当:共著範囲:第3章 電子・磁性材料 第2節 熱電材料)
    エヌ・ティー・エス 2011年
  • Handbook of Transparent Conductors (Editors: David Ginley, Hideo Hosono, and David Paine)
    Hiromichi Ohta (担当:共著範囲:Chapter 14 Junctions)
    Springer 2010年
  • Handbook of Transparent Conductors (Editors: David Ginley, Hideo Hosono, and David Paine), Chapter 14 Junctions
    Springer 2010年
  • 無機材料必須300-原理・物性・応用(守吉佑介 編)
    太田裕道 (担当:共著範囲:核生成、結晶成長、スピノーダル分解、エピタキシャル成長、導電性材料、固体撮像材料)
    三共出版 2008年
  • 熱電変換技術ハンドブック(梶川武信 監修 )
    太田裕道 (担当:共著範囲:第2章第4節3 酸化物超格子)
    エヌ・ティ・エス 2008年
  • 透明導電膜の技術(改訂2版)
    太田裕道 (担当:共著範囲:6章5節 「パルスレーザー堆積法」)
    オーム社 2007年
  • 名古屋大学21世紀COE「自然に学ぶ材料プロセッシング」
    太田裕道 (担当:共著範囲:2.3.1 核生成・成長)
    三共出版 2007年
  • 透明酸化物機能材料とその応用
    太田裕道 (担当:共著範囲:第2章 「透明p型導電性酸化物とpn接合デバイス」)
    シーエムシー出版 2006年
  • 環境調和型新材料シリーズ「熱電変換材料の技術戦略」
    太田裕道 (担当:共著範囲:3章3節 構造・形態制御 「エピタキシャル薄膜」)
    日本セラミックス協会・日本熱電学会編 2005年
  • 新訂版・薄膜作製応用ハンドブック
    太田裕道, 細野秀雄 (担当:共著範囲:「透明導電膜~ITOを中心に~」)
    エヌ・ティ・エス 2002年
  • エレクトロニクス材料・技術シリーズ「透明導電膜の新展開II」
    太田裕道, 細野秀雄 (担当:共著範囲:第3章 p型透明酸化物半導体とpnヘテロ接合発光ダイオード)
    シーエムシー出版 2002年

講演・口頭発表等

  • Electron transport properties of wide bandgap transparent oxide semiconductor, BaSnO3-SrSnO3  [招待講演]
    Hiromichi Ohta
    The 12th International Workshop on Oxide Surfaces: IWOX-XII 2020年01月 口頭発表(招待・特別)
  • Single Crystalline Film Growth of Layer Structured Oxides and Their Phonon Transports  [招待講演]
    Hiromichi Ohta, Hai Jun Cho, Yuzhang Wu, Bin Feng, Masashi Mikami, Woosuck Shin, Yuichi Ikuhara, and Keiji Saito
    2019 MRS Fall Meeting 2019年12月 口頭発表(招待・特別)
  • 酸化物を使った電界(電解)効果トランジスタの開発  [招待講演]
    太田裕道
    日本セラミックス協会東北北海道支部 第27回北海道地区セミナー2019 2019年11月 口頭発表(招待・特別)
  • 透明酸化物半導体BaSnO3–SrSnO3の電子輸送―エピタキシャル薄膜と薄膜トランジスタ―  [招待講演]
    太田裕道
    日本金属学会 2019年秋期(第165回)講演大会 2019年09月 口頭発表(招待・特別)
  • Double enhancement of thermoelectric power factor in SrTiO3 based electron sandwitch  [招待講演]
    Hiromichi Ohta, Yuqiao Zhang
    4th Functional Oxide Thin Films for Advanced Energy and Information Technology Conference 2019年07月 口頭発表(招待・特別)
  • Thermoelectric properties of two-dimensional electron system -superlattices and TFT channel-  [招待講演]
    太田 裕道
    2019 Asia-Pacific Workshop on Fundamentals and Applications of Advanced Semiconductor Devices (AWAD 2019) 2019年07月 口頭発表(招待・特別)
  • Electron transport properties of transparent oxide semiconductor, BaSnO3-SrSnO3: Epitaxial films and thin film transistors  [招待講演]
    Hiromichi Ohta, Anup V. Sanchela
    EM-NANO 2019 2019年06月 口頭発表(招待・特別)
  • 熱電変換材料としての金属酸化物の可能性  [招待講演]
    太田 裕道
    第5回大型実験施設とスーパーコンピュータとの連携利用シンポジウム 2019年03月 口頭発表(招待・特別)
  • 熱電材料の二次元薄膜化と特性向上  [招待講演]
    太田 裕道
    日本学術振興会161委員会 研究会 2019年01月 口頭発表(招待・特別)
  • Electric field thermopower modulation measurements of two-dimensional electron gas  [招待講演]
    太田 裕道
    International mini-workshop on Nonequlibrium transport and phase transition in novel materials 2018年11月 口頭発表(招待・特別)
  • Double thermoelectric powerfactor of a 2D electron system  [招待講演]
    太田 裕道
    the International Conference on Electronic Materials and Nanotechnology for Green Environment (ENGE 2018) 2018年11月 口頭発表(招待・特別)
  • Development of three-terminal electrochromic device using water electrolysis  [招待講演]
    太田 裕道
    IUMRS-ICEM 2018 2018年08月 口頭発表(招待・特別)
  • Thermopower of 2D electron systems  [招待講演]
    太田 裕道
    The 2018 International Symposium for Advanced Materials Research (ISAMR 2018) 2018年08月 口頭発表(招待・特別)
  • 捨てられている熱を電気に変える熱電変換材料  [招待講演]
    太田 裕道
    国立大学共同利用・共同研究拠点協議会 知の拠点セミナー 2018年07月 口頭発表(招待・特別)
  • Double enhancement of thermoelectric power factor in two-dimensional electron system  [招待講演]
    太田 裕道
    the 16th International Nanotech Symposium & Nano-Convergence Exhibition, NANO KOREA 2018 2018年07月 口頭発表(招待・特別)
  • Electric field thermopower modulation of two-dimensional electron gas  [招待講演]
    太田 裕道
    The 25th International Workshop on Active-Matrix Flatpanel Displays and Devices – TFT Technologies and FPD Materials – (AM-FPD 18) 2018年07月 口頭発表(招待・特別)
  • 二次元電子層のSeebeck効果-人工超格子と電界誘起二次元電子ガス-  [招待講演]
    太田 裕道
    物性談話会 2018年06月 口頭発表(招待・特別)
  • Thermoelectric Seebeck effect of 2D electron systems  [招待講演]
    太田 裕道
    The first International Joint Symposium of CEFMS-NCTU, RCAS-AS (Taiwan) and 5-Star Alliance (Japan) 2018年05月 口頭発表(招待・特別)
  • Thermopower of two-dimensional electron system  [招待講演]
    太田 裕道
    Physics Seminar in Pusan National University 2018年04月 口頭発表(招待・特別)
  • 二次元電子系の熱電ゼーベック効果  [招待講演]
    太田 裕道
    第65回 応用物理学会春季学術講演会 2018年03月 口頭発表(招待・特別)
  • 水を使った機能性酸化物の光・電気・磁気特性切替え  [招待講演]
    太田 裕道
    平成29年度日本表面科学会東北・北海道支部講演会 2018年03月 口頭発表(招待・特別)
  • 半導体薄膜の熱電特性  [招待講演]
    太田 裕道
    電子情報通信学会シリコン材料デバイス・電子デバイス合同研究会 2018年03月 口頭発表(招待・特別)
  • Thermopower of oxide heterostructure  [招待講演]
    H. Ohta, A. Sanchela
    ICAMD2017 (The 11th International Conference on Advanced Materials and Devices) 2017年12月 口頭発表(招待・特別)
  • Double enhancement of thermoelectric power factor in oxide two-dimensional electron system via precise dimensionality control  [招待講演]
    Hiromichi Ohta, Yu-Qiao Zhang
    2017 Fall Korean Physical Society (KPS) Meeting 2017年10月 口頭発表(招待・特別)
  • 薄膜トランジスタ構造を用いた熱電能の計測  [招待講演]
    太田 裕道
    第14回 薄膜材料デバイス研究会 2017年10月 口頭発表(招待・特別)
  • Electric field thermopower modulation measurement  [招待講演]
    太田 裕道
    Seoul National University 2017年10月 公開講演,セミナー,チュートリアル,講習,講義等
  • Epitaxial film growth and some applications of functional oxides  [招待講演]
    太田 裕道
    Distinguished Lecture Series at Materials Science & Engineering 2017年10月 公開講演,セミナー,チュートリアル,講習,講義等
  • Electrochemical modulation of functional oxides using three-terminal thin film transistor structure with water infiltrated gate dielectrics  [招待講演]
    太田 裕道
    Seminar at Tshinghua University (hosted by Prof. Yu Pu) 2017年09月 公開講演,セミナー,チュートリアル,講習,講義等
  • 水を使った機能性酸化物薄膜の光・電気・磁気物性切替え手法  [招待講演]
    太田 裕道
    産業技術総合研究所 中部センター 講演会 2017年09月 公開講演,セミナー,チュートリアル,講習,講義等
  • 固相エピタキシャル成長法を駆使した機能性酸化物ナノ層の創製  [招待講演]
    太田 裕道
    2017年09月 口頭発表(基調)
  • Electric field modulation of thermopower in two-dimensional electron gas  [招待講演]
    H. Ohta
    IUMRS-ICAM (The 15th International Conference on Advanced Materials) 2017年08月 口頭発表(招待・特別)
  • Electrochemical function modulation of oxides using three-terminal thin film transistor structure with water infiltrated insulator  [招待講演]
    太田 裕道
    The 3rd Functional Oxide Thin Films for Advanced Energy and Information Technology 2017年07月 口頭発表(招待・特別)
  • 導電性酸化物薄膜の物性改質方法  [招待講演]
    太田 裕道
    第64回応用物理学会春季学術講演会(合同セッションK, シンポジウム「金属酸化物の結晶物性に迫る」) 2017年03月 口頭発表(招待・特別)
  • Thermoelectric Seebeck effect of two dimensional electron gas in SrTiO3  [招待講演]
    太田 裕道
    International conference on Advances in Functional Materials (ICAFM2017) 2017年01月 口頭発表(招待・特別)
  • Unusually large thermopower of nanostructured oxides  [招待講演]
    太田 裕道
    ENGE 2016 (International Conference on Electronic Materials and Nanotechnology for Green Environment) 2016年11月 口頭発表(招待・特別)
  • 熱電能電界変調法:AlGaN/GaN-MOSHEMT  [招待講演]
    太田裕道, 金木奨太, 橋詰 保
    2016年 第77回応用物理学会秋季学術講演会 2016年09月 口頭発表(招待・特別)
  • Electro-chemical redox switching of functional oxide thin films using water-infiltrated nanoporous glass  [招待講演]
    H. Ohta, T. Katase
    International Workshop on Oxide Surfaces (IWOX-X) 2016年01月 口頭発表(招待・特別)
  • Water electrolysis induced modification of functional oxides−Thermoelectric properties−  [招待講演]
    H. Ohta, T. Katase
    IUMRS-ICAM 2015 2015年10月 口頭発表(招待・特別)
  • 熱電変換材料って何?  [招待講演]
    太田 裕道
    日本化学会秋季事業 第5回CSJ化学フェスタ2015 2015年10月 口頭発表(招待・特別)
  • 水の電気分解を利用した機能性酸化物ナノ層創製  [招待講演]
    太田 裕道
    日本金属学会 2015年秋期講演大会 2015年09月 口頭発表(基調)
  • Development of oxide-based nanostructured thermoelectric materials  [招待講演]
    H. Ohta
    4th International Symposium on Energy Challenges and Mechanics -working on small scales 2015年08月 口頭発表(基調)
  • Two-dimensional giant thermopower –SrTiO3-based superlattices and transistors-  [招待講演]
    H. Ohta
    The American Ceramic Society’s 11th International Conference on Ceramic Materials and Components for Energy and Environmental Applications (CMCEE-11) 2015年06月 口頭発表(招待・特別)
  • Thermoelectric effect of extremely thin electron doped SrTiO3  [招待講演]
    H. Ohta
    The 1st IOP-RIES Joint Workshop 2015年03月 口頭発表(招待・特別)
  • Thermopower enhancement of two-dimensional electron gas in oxide semiconductors  [招待講演]
    H. Ohta
    The American Ceramic Society’s Electronic Materials and Applications 2015 (EMA2015) 2015年01月 口頭発表(招待・特別)
  • 酸化物薄膜作製におけるパルスレーザー堆積法とその応用  [招待講演]
    太田 裕道
    3次元造形&薄膜実践セミナー 2014年09月 口頭発表(招待・特別)
  • 水電気分解を利用した酸化物の熱電能変調  [招待講演]
    太田 裕道
    第75回 応用物理学会秋季学術講演会(シンポジウム:固液界面を使った新しい酸化物エレクトロニクス:化学とデバイスの融合) 2014年09月 口頭発表(招待・特別)
  • Epitaxial Film Growth and Application of Functional Oxides  [招待講演]
    太田 裕道
    HOKUDAI-NCTU Joint Symposium on Nano, Photo and Bio Sciences, RIES 2014年09月 口頭発表(招待・特別)
  • 酸化物半導体に蓄積された二次元電子ガスー熱電能を中心にー  [招待講演]
    太田 裕道
    統合物質創製化学推進事業 第5回若手研究会 2014年06月 口頭発表(招待・特別)
  • Electric Field Modulation of a Thermoelectric Material  [招待講演]
    太田 裕道
    Thermec 2013 2013年12月 口頭発表(招待・特別)
  • 酸化物半導体のエピタキシャル薄膜成長  [招待講演]
    太田 裕道
    酸化物アライアンス第11回研究会(第5回公開講演会)「透明導電膜のサイエンス」 2013年05月 公開講演,セミナー,チュートリアル,講習,講義等
  • Two dimensional thermoelectric effect  [招待講演]
    太田 裕道
    Distinguished Lecture Series for 2012 fall semester in Sungkyunkwan University 2012年12月 公開講演,セミナー,チュートリアル,講習,講義等
  • Electric field thermopower modulation of 2DEG in oxide semiconductor based field effect transistors  [通常講演]
    太田 裕道
    MRS 2012 Fall Meeting 2012年11月 口頭発表(招待・特別)
  • Unusually large thermopower enhancement in an electric field induced two-dimensional electron gas  [招待講演]
    太田 裕道
    E-MRS 2012 Spring Meeting 2012年05月 口頭発表(招待・特別)
  • Electric-Field Thermopower Modulation Method  [招待講演]
    太田 裕道
    Japan-Finland March Meeting for the future in thermoelectrics 2012年03月 口頭発表(招待・特別)
  • 高分解能X線回折法による金属酸化物薄膜の分析  [通常講演]
    太田裕道
    第240回X線分析研究懇談会 2011年12月 名古屋大学
  • ナノ孔に閉じ込められた水を利用した二次元電子ガス生成と巨大熱電能変調  [通常講演]
    太田裕道, 水野 拓, S. Zheng, 加藤丈晴, 幾原雄一, 安部勝美, 雲見日出也, 野村研二, 細野秀雄
    日本金属学会2011年秋期大会 2011年11月 沖縄コンベンションセンターおよびカルチャーリゾートフェストーネ(沖縄)
  • 電界誘起二次元電子ガスの巨大熱電能変調  [通常講演]
    太田裕道, 水野 拓, Shijian Zheng, 加藤丈晴, 幾原雄一, 安部勝美, 雲見日出也, 野村研二, 細野秀雄
    第72回応用物理学会学術講演会 2011年08月 山形大学小石川キャンパス(山形)
  • 温度差で発電する熱電材料-酸化物の挑戦  [通常講演]
    太田裕道
    超精密加工専門委員会第62回研究会(高度エネルギー変換材料) 2011年07月 大阪ガーデンパレス(大阪)
  • チタン酸ストロンチウムの熱電現象  [通常講演]
    太田裕道
    新化学発展協会 電子情報技術部会講演会 2011年03月 新化学発展協会会議室(東京)
  • 水の電気分解を利用したSrTiO3の金属化と熱電能  [通常講演]
    太田裕道
    第55回固体イオニクス研究会「遷移金属複合酸化物:新しい合成法、物質、物性」 2011年01月 京都国際会館(京都)

作品等

  • 科学研究費補助金 新学術領域研究(研究領域提案型)領域番号6103 「機能コアの材料科学」(領域代表者:松永克志 教授・名古屋大学)19H05791 界面制御による高機能薄膜材料創製(研究代表者)
    2019年 -2023年
  • 科学研究費補助金 基盤研究(A) 17H01314 熱電材料の高ZT化に向けたナノ周期平行平板構造の熱伝導率解明(研究代表者)
    太田 裕道  2017年04月
  • 科学研究費補助金 基盤研究(A) 25246023 酸化物半導体一次元電子ガスの電界誘起と超巨大熱電能変調(研究代表者)
    太田 裕道  2013年04月
  • 科学研究費補助金 新学術領域研究(研究領域提案型)領域番号2505 「ナノ構造情報のフロンティア開拓-材料科学の新展開」(領域代表者:田中 功 教授・京都大学)25106007 原子層制御による新しい材料機能探索(研究代表者)
    太田 裕道  2013年
  • 科研費基盤研究(B)「酸化物薄膜トランジスタにおける巨大熱電能の電界変調と赤外線センサー応用」
    2010年 -2012年
  • 科研費特定領域「電界誘起二次元電子層の可視化」
    2010年 -2011年
  • 科学技術振興機構さきがけ「電界誘起二次元伝導層の熱起電力と制御」
    2008年 -2011年
  • 科研費特定領域「誘電体へテロ界面に蓄積された二次元電子ガスの電子状態と量子サイズ効果の起源」
    2008年 -2010年
  • 科研費若手研究(A)「二次元電子ガスを有する酸化物超格子の作製と熱電変換物性」
    2006年 -2008年
  • NEDO産業技術研究助成事業「局在化した二次元電子ガスを有する誘電体人工超格子の作製と熱電変換材料への応用」
    2005年 -2008年

その他活動・業績

  • Jiang Pu, Kaito Kanahashi, Nguyen Thanh Cuong, Chang-Hsiao Chen, Lain-Jong Li, Susumu Okada, Hiromichi Ohta, Taishi Takenobu Physical Review B 94 (1) 014312 2016年07月27日 [査読有り][通常論文]
     
    The carrier-density-dependent conductance and thermoelectric properties of large-area MoS2 and WSe2 monolayers are simultaneously investigated using the electrolyte gating method. The sign of the thermoelectric power changes across the transistor off-state in the ambipolar WSe2 transistor as the majority carrier density switches from electron to hole. The thermopower and thermoelectric power factor of monolayer samples are one order of magnitude larger than that of bulk materials, and their carrier-density dependences exhibit a quantitative agreement with the semiclassical Mott relation based on the two-dimensional energy band structure, concluding the thermoelectric properties are enhanced by the low-dimensional effect.
  • Yi Feng Wang, Kyu Hyoung Lee, Hiromichi Ohta, Kunihito Koumoto CERAMICS INTERNATIONAL 35 (1) 537 -537 2009年01月 [査読無し][通常論文]
  • 酸化物半導体の薄膜トランジスタへの応用
    野村研二, 神谷利夫, 太田裕道, 平野正浩, 細野秀雄 機能材料 28 (3月号) 42 -53 2008年 [査読無し][通常論文]
  • 中野 智仁, 小林 航, 舟橋 良次, 杉浦 健二, 太田 裕道, 河本 邦仁, 寺崎 一郎 日本物理学会講演概要集 62 (0) 607 -607 2007年 [査読無し][通常論文]
  • Hidenori Hiramatsu, Hayato Kamioka, Kazushige Ueda, Hiromichi Ohta, Toshio Kamiya, Masahiro Hirano, Hideo Hosono phys. stat. sol. (a) 203 (11) 2800 -2811 2006年 [査読無し][通常論文]
  • 透明酸化物半導体を用いた透明電界効果トランジスタ
    野村研二, 太田裕道, 神谷利夫, 平野正浩, 細野秀雄 マテリアルインテグレーション 2005年 [査読無し][通常論文]
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, F Hosono INTEGRATION OF ADVANCED MICRO-AND NANOELECTRONIC DEVICES-CRITICAL ISSUES AND SOLUTIONS 811 391 -396 2004年 [査読無し][通常論文]
     
    Carrier transport properties and electronic structure of an n-type transparent oxide semiconductor, InGaO3(ZnO)(5), were investigated using single-crystalline thin films. Room-temperature Hall mobility strongly depends on carrier concentration, and rapidly increased from similar to2 cm(2) (Vs)(-1) to > 10 cm(2) (Vs)(-1) around the carrier concentration (N-th) similar to3 x 10(18) cm(-3). This change is associated with insulator-metal transition. These results are explained by a model similar to Anderson localization, in which shallow semi-localized states are formed originating from random distribution of Ga3+ and Zn2+ ions in the intrinsic crystal structure of InGaO3(ZnO)(5). The present conclusion suggests that electron densities larger than N-th are necessary to attain high performances in drift carrier devices fabricated using InGaO3(ZnO)(5). It was demonstrated that transparent filed-effect transistors exhibited good performances such as a "normally-off characteristics", an on/off current ratios as large as 10(5), and a field-effect mobility similar to80 cm(2) (Vs)(-1) when high-k material, amorphous HfOx, was used as a gate insulator.
  • H Ohta, K Nomura, H Hiramatsu, K Ueda, T Kamiya, M Hirano, H Hosono SOLID-STATE ELECTRONICS 47 (12) 2261 -2267 2003年12月 [査読無し][通常論文]
     
    Recent advancements of transparent oxide semiconductors (TOS) toward new frontiers of "oxide electronics" are reviewed based on our efforts, categorized as "novel functional materials", "heteroepitaxial growth techniques", and "device fabrications". Topics focused in this paper are: (1) highly conductive ITO thin film with atomically flat surface, (2) p-type TOS material ZnRh2O4, (3) deep-ultraviolet (DUV) transparent conductive oxide beta-Ga2O3 thin film, (4) electrochromic oxyfuolide NbO2F, (5) single-crystalline films of InGaO3(ZnO)(m) grown by reactive solid-phase epitaxy, (6) p-type semiconductor LaCuOS/Se epitaxial films capable of emitting UV- and purple-light, (7) p-n homojunction based on bipolar CuInO2, (8) transparent FET based on single-crystalline InGaO3(ZnO)(5) films, and (9) UV-light emitting diode based on p-n heterojunction. (C) 2003 Elsevier Ltd. All rights reserved.
  • Fabrication of transparent MISFET using InGaO3(ZnO)5 single crystalline thin film with normally insulating nature
    Kenji Nomura, Hiromichi Ohta, Kazushige Ueda, Toshio Kamiya, Masahiro Hirano, Hideo Hosono Mater. Res. Soc. Symp. Proc. 747 267 2003年 [査読無し][通常論文]
  • Transparent MISFETs using homologous compounds, RMO3(ZnO)m (R=In, Lu; M= In, Ga; m=integer) single crystalline thin films
    K. Nomura, H. Ohta, K, Ueda, T. Kamiya, M. Hirano, H. Hosono Thin Solid Films 445 322 -326 2003年 [査読無し][通常論文]
  • 上部電極にITO薄膜を用いたニオブ酸イットリウム薄膜のEL特性
    山口誉滋, 掛本博文, 和田智志, 鶴見敬章, 太田裕道, 細野秀雄 日本セラミックス協会2003年年会、3/22/03-3/24/03、東京 2003年 [査読無し][通常論文]
  • H. Ohta, K. Nomura, H. Hiramatsu, T, Suzuki K. Ueda, M. Orita, M. Hirano, Y. Ikuhara, Hideo Hosono MRS Symposium proceedings 747 257 -265 2003年 [査読無し][通常論文]
     
    We have developed a novel growth method for single-crystalline film of natural superlattice oxides and named the method "Reactive Solid-Phase Epitaxy (R-SPE)." Single-crystalline thin films of homologous series InGaO3(ZnO)(m) (m=integer) are fabricated by the R-SPE method and its growth mechanism, especially a role of ZnO epitaxial layer, is clarified. High-temperature annealing of bi-layer films consisting of an amorphous InGaO3(ZnO)(5) layer deposited at room temperature and an epitaxial ZnO layer on YSZ substrate allows for the growth of single-crystalline film with a controlled chemical composition. The ZnO layer plays an essential role in determining the crystallographic orientation, while the thickness ratio between the two layers controls the film composition.
  • K Nomura, H Ohta, K Ueda, T Kamiya, M Hirano, H Hosono CRYSTALLINE OXIDE-SILICON HETEROSTRUCTURES AND OXIDE OPTOELECTRONICS 747 267 -272 2003年 [査読無し][通常論文]
     
    Transparent metal-insulator-semiconductor field-effect transistors (MISFETs) were fabricated using a single-crystalline thin film of an n-type transparent oxide semiconductor, a homologous compound InGaO3(ZnO)(5), grown by a reactive solid phase epitaxy method. The transparent MISFET exhibited good performances with "nofmally-off characteristics", "an on/off current ratio as large as 10(5)" and "insensitivity to visible light". Field-effect mobility was about 2 cm(2)(VS)(-1)' which is larger than those reported previously for MISFETs fabricated in transparent oxide semiconductors. These improved performance is thought to result from the low defect density and intrinsic-level carrier concentration of the single-crystalline InGaO3(ZnO)(5) film.
  • 佐々木健夫, 松永克志, 太田裕道, 細野秀雄, 山本剛久, 幾原雄一 材料 52 (6) 555 -559 2003年 [査読無し][通常論文]
     
    Thin Cu film was deposited on the (0001) surface of α-Al2O3 by a pulsed-laser deposition technique at substrate temperature of 800°C. The atomic structure of Cu/Al2O3 (0001) interface was characterized by high-resolution transmission electron microscopy (HRTEM). It was found that the interface was atomically sharp and the following orientation relationship (OR) was existed (111)Cu//(0001)Al(2)O(3), [11̄0]Cu//[11̄00]Al(2)O(3). In contrast, the OR with geometrically high coherency across the interface was evaluated by the coincidence of reciprocal lattice points method, and the result showed that the preferred OR was (111)Cu//(0001)Al(2)O(3), [112̄]Cu//[11̄00]Al(2)O(3). This OR is not consistent with the experimental results, and suggests that interfacial chemical bonding plays an important role to form the actual OR. In order to understand the nature of the chemical bonding at the interface, HRTEM image simulations were performed and their results were compared with the experimental images. It was found that the Cu(111)/Al2O3(0001) interface terminated at oxygen-layer, which indicates that Cu-O bonds determine the stability of the interface.
  • Hiromichi Ohta, Masahiro Orita, Masahiro Hirano, Iwao Yagi, Kazushige Ueda, Hideo Hosono Journal of Applied Physics 91 (5) 3074 -3078 2002年02月15日 [査読無し][通常論文]
     
    The electronic structure of SrCu2O2, a wide gap (∼3.3 eV) p-type oxide semiconductor, was examined by photoelectron and optical spectroscopy. The spectroscopy results were compared with the energy band structure calculated by the local density approximation method to clarify the origins of p-type conductivity in this material. Despite the significant difference in band gap energy, the basic electronic structure around the band gap region was found to be quite similar to that of Cu2O. Thus, the 3d-4sp orbital of Cu+ ion is hybridized with the 2p orbital of ligand O2- ions due to the covalency of Cu-O bonds, to form states near the valence band maximum the conduction band minimum is predominantly composed of the hybridized orbital of Cu 4sp and O 2p, forming the direct band gap at Γ point. A sharp absorption band observed near the fundamental absorption edge is likely attributable to an exciton. Although the corresponding exciton emission was not observed near the absorption edge, a blue-green emission band (Stokes shift of ∼1 eV) was observed at ∼2.47 eV. The emission is presumably attributable to intra-atomic transitions of Cu+, partially allowed by p-orbital mixing into s and d orbitals. © 2002 American Institute of Physics.
  • HRTEM and Orientation Relationship of Cu/Al2O3 Interface
    T. Sasaki, K. Matsunaga, T. Yamamoto, H. Ohta, H. Hosono, Y. Ikuhara Proc. of the International Conference on Designing of Interfacial Structures in Advanced Materials and their Joints 237 2002年 [査読無し][通常論文]
  • 透明導電膜の新展開II「第3章 p型透明酸化物半導体とpnヘテロ接合発光ダイオード」
    太田裕道, 細野秀雄 エレクトロニクス材料・技術シリーズ, シーエムシー出版 41 2002年 [査読無し][通常論文]
  • P型アモルファス酸化物の発見とアモルファス酸化物PN接合の形成
    鳴島暁, 溝口拓, 折田政寛, 太田裕道, 平野正浩, 神谷利夫, 清水健一, 細野秀雄 第35回アモルファス物質に関するセミナーテキスト 63 2002年 [査読無し][通常論文]
  • 太田裕道, 折田政寛, 河村賢一, 猿倉信彦, 平野正浩, 細野秀雄 応用物理学関係連合講演会講演予稿集 47th (2) 578 2000年03月28日 [査読無し][通常論文]
  • Fabrication of UV-Light Emitting Diode Using Transparent Conductive Oxides
    H. Ohta, K. Kawamura, N. Sarukura, M. Hirano, H. Hosono Appl. Phys. Lett. 77 (4) 475 2000年 [査読無し][通常論文]
  • 初めて実現した酸化物紫外発光ダイオード:p-SrCu_2_O_2_/n-ZmO
    太田裕道, 折田政寛, 平野正浩, 細野秀雄 第285回蛍光体同学会講演資料集 17 -22 2000年 [査読無し][通常論文]
  • M Orita, H Ohta, H Tanji, H Hosono, H Kawazoe FLAT-PANEL DISPLAYS AND SENSORS: PRINCIPLES, MATERIALS AND PROCESSES 558 399 -402 2000年 [査読無し][通常論文]
     
    In2O3 films were deposited on YSZ (001) single crystal surface at 800 degrees C at an oxygen pressure of 14 Pa by pulsed laser deposition (PLD) method. A heteroepitaxial relationship between the film and the substrate was seen in TEM photographs and X-ray diffraction measurements. The omega locking curve full width of half maximum (FWHM) of the In2O3 (004) x-ray diffraction was 0.06 degrees. Film conductivities were similar to 10 S/cm or less, while carriers on the order of 10(18)/cm(3) were generated.
  • 透明酸化物導電体の新しい展開
    折田政寛, 太田裕道, 細野秀雄 表面 38 344 2000年 [査読無し][通常論文]

特許

  • 酸化物半導体PN接合デバイス
    特許4164563
  • ホモロガス薄膜を活性層として用いる透明薄膜電界効果型トランジスタ
    特許4164562
  • 超平坦p型酸化物半導体NiO単結晶薄膜
    特許4014473
  • LnCuO(S,Se,Te)単結晶薄膜とその製造方法、及び該単結晶薄膜を用いた光デバイス又は電子デバイス
    特許4083486
  • 半導体発光素子
    特開2003-273400
  • 自然超格子ホモロガス単結晶薄膜とその製造方法
    特許4298194
  • 基板回転・加熱装置並びにこれを用いた成膜装置及び分析装置
    特開2002-280306
  • 透明酸化物p-n接合ダイオード
    特許3969959
  • 紫外透明導電膜とその製造方法
    特許4083396
  • 超平坦透明導電膜およびその製造方法
    特許3531865
  • 発光ダイオードおよび半導体レーザー
    特許3398638
  • 低抵抗ITO薄膜及びその製造方法
    特許4397511
  • 酸化インジウム薄膜及びその製造方法
    特許4480809
  • 透明導電性薄膜及びその製造方法
    特許4397451
  • 高単結晶性酸化亜鉛薄膜及び製造方法
    特許4237861
  • 透明導電性薄膜
    特許3824289
  • 透明導電性酸化物薄膜を有する物品及びその製造方法
    特許4170454
  • 透明導電性酸化物薄膜を有する物品及びその製造方法
    特開2000-26119
  • 電池の製造方法
    特許3717632
  • 薄型密閉電池
    特許3588225
  • 固体電解質電池及びその製造方法
    特許3354430
  • ポリマー電解質電池
    特許3378761
  • 薄型電池
    特開平10-284021
  • 非水電解液二次電池
    特開平10-199528
  • 非水電解液二次電池
    特開平10-162826
  • 熱電変換材料、赤外線センサ及び画像作製装置
    (PCT) WO 2007/132782 A1
  • 熱電変換材料及び熱電変換材料の製造方法
    WO 2006/054550 A1
  • 熱電素子
    特開2009-117430
  • アモルファス酸化物及び薄膜トランジスタ
    WO2005/088726
  • アモルファス酸化物薄膜
    特開2010-219538
  • アモルファス酸化物薄膜の気相成膜方法
    特開2010-212696
  • 熱電変換材料及び熱電変換材料の製造方法
    WO 2007/132782 A1
  • Thermoelectric Material, Infared Sensor and Image Forming Device
    (PCT) WO 2007/132782 A1
  • Thermoelectric Conversion Material and Production Method for Thermoelectric Conversion Material
    WO 2006/054550 A1
  • Thermoelectric Material, Infared Sensor and Image Forming Device
    WO 2007/132782 A1

受賞

  • 2015年03月 北海道大学 平成26年度北海道大学研究総長賞 優秀賞
     
    受賞者: 太田 裕道
  • 2010年 第32回(2010年度)応用物理学会優秀論文賞
  • 2005年 平成16年度日本セラミックス協会進歩賞
  • 2000年 第8回(2000年春季)応用物理学会講演奨励賞

共同研究・競争的資金等の研究課題

  • 界面制御による高機能薄膜材料創製
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2019年06月 -2024年03月 
    代表者 : 太田 裕道, 寺崎 一郎, 齊藤 圭司
  • 熱電材料の高ZT化に向けたナノ周期平行平板構造の熱伝導率解明
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2017年04月 -2021年03月 
    代表者 : 太田 裕道
     
    本研究では、パルスレーザー堆積法や反応性固相エピタキシャル成長法を駆使してナノ周期平行平板構造薄膜を作製し、その物質、組成、角度と熱伝導率の関係をTDTR法によって精密に計測することで、「ナノ周期平行平板構造が熱伝導率に及ぼす影響を実験的に解明する」ことを目標している。2018年度は、計画通り、反応性固相エピタキシャル成長法によって自然超格子InGaO3(ZnO)m単結晶薄膜を作製し、超格子に垂直方向の熱伝導率を計測した。m値を大きく変化させた単結晶薄膜を用いて計測することで、単結晶であるにも関わらず、層に直交方向の熱伝導率はアモルファス状態よりも低いことが明らかになった。これは、結晶格子を持たないアモルファスよりも、InO2層/GaO(ZnO)m層の界面熱抵抗が熱伝導率を下げるのに有用であることを示す結果である。現在は、超格子が傾斜した自然超格子単結晶薄膜を用いて、層に平行方向の熱伝導率を計測している。また、層状構造を有するAxCoO2系熱電材料や、温度やイオンの脱挿入によって金属-絶縁体転移を起こす遷移金属酸化物の薄膜作製と、その熱伝導率計測を行った。AxCoO2では、Aイオンの原子量の増加に伴い、熱伝導率の異方性が減少し、層に平行な方向の熱伝導率が下げられることが分かってきた。また、NbO2薄膜やWOx薄膜などの熱伝導率計測を行い、いくつかは既に論文としてまとめることができた。論文、招待講演などの実績(数)は、原著論文:12報、招待講演:12件、著書:0(執筆済2)、新聞等報道(海外含):39件、など。
  • エネルギー応用に向けた層状遷移金属酸化物の機能化
    日本学術振興会:二国間交流事業(韓国との共同研究)
    研究期間 : 2018年04月 -2020年03月 
    代表者 : 太田 裕道
  • 半導体界面に蓄積された二次元電子ガスの熱電能電界変調
    旭硝子財団:2017年度 ステップアップ助成
    研究期間 : 2017年04月 -2020年03月 
    代表者 : 太田 裕道
  • 色・導電率を切替え可能な三端子型WO3薄膜トランジスタの低電圧・高速動作に向けた材料改質
    三菱財団:平成29年度 自然科学部門 助成金
    研究期間 : 2017年10月 -2018年09月 
    代表者 : 太田 裕道
  • 光ドープされた伝導体の電子輸送特性
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2014年04月 -2018年03月 
    代表者 : 寺崎 一郎, 太田 裕道, 河野 浩, 岡崎 竜二, 田辺 賢士, 藤田 優, 白石 祐芽
     
    本研究の目的は、酸化物絶縁体に光を照射することによって伝導体を創りだし、その伝導パラメタを精密に計測し、光ドープされたキャリアの輸送特性と微視的機構を明らかにすることである。 具体的には、我々はZnS、ZnO、SrTiO3、CuO、La2CuO4、CuFeS2など様々な絶縁体に可視から紫外領域の光を照射し、光伝導率、光ホール係数、光ゼーベック係数などを定量的に計測し、光ドープされたキャリアの輸送特性を明らかにした。特にSrTiO3では量子常誘電性により熱起電力が遮蔽されることを明らかにした。ZnOにおいては、薄膜、単結晶、多結晶で光ゼーベック係数は同じ値で飽和することを見出した。
  • 原子層制御による新しい材料機能探索
    日本学術振興会:科学研究費助成事業 新学術領域研究(研究領域提案型)
    研究期間 : 2013年06月 -2018年03月 
    代表者 : 太田 裕道, 平松 秀典, 片瀬 貴義, 山ノ内 路彦
     
    原子レベルで厚さ制御された薄膜「ナノ層」を作製し、新しい材料機能の探索を行うとともに、作製した試料を他班に提供することでナノ構造情報の開拓に取り組んだ。主な結果を列挙する。1. 酸化物人工超格子で熱を電気に変換する性能をバルク比2倍の増強に成功した、2. 透明酸化物半導体薄膜の電子移動度抑制の起源を解明し、高電子移動度を実現した、3. 半導体二次元電子ガスが高い熱電変換出力因子を示すことを見出した、4. 光透過率・電気抵抗率・磁気的性質を同時に変えられる薄膜デバイスを実現した、5. 金属テープ基板上で、実用レベルに匹敵する臨界電流密度を示す鉄系超伝導薄膜を実現した。
  • 酸化物半導体一次元電子ガスの電界誘起と超巨大熱電能変調
    日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2013年04月 -2017年03月 
    代表者 : 太田 裕道, 片瀬 貴義
     
    ナノ多孔性ガラス中のH+/OH-イオンと酸化物半導体の酸化・還元反応を利用したAFMリソグラフィーによる極細一次元電子ガス(1DEG)の電界誘起と超巨大熱電能の観測を目指した。形状的特徴から、SrTiO3-1DEGのAFMリソグラフィーに成功したが、誘起された1DEGの安定性が極めて低く、熱電能計測には至らなかった。安定に酸化・還元反応が起こる物質の探索を行った結果、絶縁体VO2⇔金属HxVO2、絶縁体SrCoO2.5⇔金属SrCoO3、絶縁体WO3⇔金属HWO3の可逆変化に成功し、論文・新聞・TV上で公表した。上記物質を用いてAFMリソグラフィーを行うことで1DEGが作製できると考えている。
  • 超微細熱電材料用汎用熱電能計測装置の開発
    旭硝子財団:平成26年度研究助成金
    研究期間 : 2014年04月 -2016年03月 
    代表者 : 太田 裕道
  • 二次元電子層の電界制御と熱起電力に関する研究
    科学技術振興機構:さきがけ
    研究期間 : 2008年10月 -2012年03月 
    代表者 : 太田裕道
  • 酸化物薄膜トランジスタにおける巨大熱電能の電界変調と赤外線センサー応用
    日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2010年 -2012年 
    代表者 : 太田 裕道
     
    酸化物半導体SrTiO3単結晶上にトップゲート型薄膜トランジスタ構造を作製し、次にゲート電圧印加によって形成される極薄2DEG層の電子輸送及び熱電能Sを詳細に測定・解析した。その結果、(1)用いるゲート絶縁体として含水ナノ多孔質ガラス(CAN:H.Ohtaetal.,NatureCommun.1:118(2010);Adv.Mater.24,740(2012))が極めて有用であり、(2)含水ナノ多孔質ガラスにより極薄2DEGが誘起できるのはECBM>EH2の酸化物半導体であることを明らかにした。また、赤外線センサー特性を決定づける熱電特性についてバルク-人工超格子-電界誘起極薄2DEGの比較を行ったところ、体積キャリア濃度<3×1020cm-3では電界誘起2DEGとバルクの熱電特性は変わらないが、4×1020cm-3以上のキャリア濃度において極薄2DEGは人工超格子と同様にバルクの5倍の熱電能(=25倍の性能指数ZT~2)を示し、赤外線センサーとして有用であることが分かった。
  • 電界誘起二次元電子層の可視化
    日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2010年 -2011年 
    代表者 : 太田 裕道
     
    SrTiO_3人工超格子中に閉じ込められた極薄二次元電子層はバルクの数倍の巨大熱電能を示す[H.Ohta et al., Nature Mater, 6,129(2007)]ことから、高効率熱電材料の設計指針を与えるモデルとして有望である。本研究では、srTiO_3単結晶電界効果トランジスタへのゲート電圧印加によりゲート絶縁体/SrTiO_3単結晶界面に高密度で蓄積される二次元電子層を可視化し、極薄二次元電子層の巨大熱電能の起源を明らかにすることを目的とした。具体的には高分解能電子顕微鏡観察ならびにデバイスシミュレーションにより、ゲート絶縁体/SrTiO_3界面に蓄積された電界誘起二次元電子層を可視化し、熱電能との関係を調査した。本特定領域内AO1ナノ計測(ア)班との連携研究により,(1)ゲート絶縁体薄膜が水で満たされた直径約10nmのナノボアを30体積パーセント含み[H.Ohta et al., Nature Communications 1:118(2010).]、(2)ゲート電圧印加によってSrTiO_3表面に押し付けられたナノボア中のH_3O^+イオンが、(3)SrTiO_3中の残留キャリア電子を強力に引き寄せることで、バルクの5倍の巨大熱電能を示す極薄二次元電子層(究極の機能元素)が生成することが明らかになった[H.Ohta et al., Adv. Mater. 24,740-744(2012).]。以上の研究から、機能元素「極薄二次元電子層」を熱電材料の表面に誘起し、巨大熱電能という有用な機能を発現させるための「水電気分解による二次元電子層の電界誘起の学理」を構築することに成功した。また、本研究で得られた水電気分解による二次元電子層の作製手法を応用し、一次元AFMリソグラフィーを行うことで更に大きな熱電能を示す材料が創製できると考えられる。
  • 誘電体ヘテロ界面に蓄積された二次元電子ガスの電子状態と量子サイズ効果の起源
    日本学術振興会:科学研究費助成事業 特定領域研究
    研究期間 : 2008年 -2009年 
    代表者 : 太田 裕道
     
    本公募研究の目的は、(1)誘電体人工超格子やヘテロ酸化物界面近傍に誘起される二次元電子ガスの電子状態と量子サイズ効果の相関を明らかにすること、(2)誘電体中の二次元電子ガスを積極的に活用した機能材料設計指針を提案することである。二次元電子ガス近傍の原子・電子状態を高いエネルギー分解能で捉えることにより、巨大Seebeck係数のオリジンを解き明かし、誘電体中の二次元電子ガスを積極的に活用した材料設計指針を提案することが可能と考えている。SrTiO_3/SrTi_<0.8>Nb_<0.2>O_3人工超格子やTiO_2/SrTiO_3などの誘電体ヘテロ界面のSrTiO_31単位格子層(0.3905nm)に局在化した高濃度の電子ガスは、バルクの5倍に相当する巨大熱起電力(Seebeck係数)を示すことから、未だ実現していない酸化物熱電変換材料のひとつの開発指針として注目されている。Seebeck係数の大きさは、フェルミエネルギーにおける伝導帯状態密度(DOS(E))のエネルギー微分(∂DOS(E)/∂E)に依存するため、量子サイズ効果によるDOS(E)の増大が巨大熱起電力の起源と考えられている。また、Seebeck係数は伝導電子濃度の関数であるため、伝導電子濃度を連続的に変化させながらSeebeck係数を計測することでDOS(E)の情報を得ることができる。平成21年度は、誘電体ヘテロ界面の電子濃度を連続的に変化させるため誘電体をチャネルとした電界効果トランジスタを作製し、絶縁体/SrTiO_3および絶縁体/KTaO_3ヘテロ界面のSeebeck係数の電界変調に成功した。
  • 局在化した二次元電子ガスを有する誘電体人工超格子の作製と熱電変換材料への応用
    NEDO:産業技術研究助成事業費助成金
    研究期間 : 2006年01月 -2008年12月 
    代表者 : 太田 裕道
  • 二次元電子ガスを有する酸化物超格子の作製と熱電変換物性
    日本学術振興会:科学研究費助成事業 若手研究(A)
    研究期間 : 2006年 -2008年 
    代表者 : 太田 裕道
     
    人工宝石として知られ、毒性がなく、融点が高いSrTiO_3を用いた熱電変換材料を創製した。精密な超薄膜技術により、電子を生成させた厚さ0.4nmの金属SrTiO_3:Nb層と、絶縁性SrTiO_3からなるサンドイッチ構造を作製することで、二次元平面に高濃度の伝導電子(1021cm^<-3>)を溜め込んだ結果、熱起電力が同濃度の電子を生成させたバルクSrTiO_3の約5倍と大きく、熱電変換性能指数ZT(=[熱起電力]×[熱起電力]×[導電率]×[温度]÷[熱伝導率])が既存重金属材料の約2倍になることを見出した。

教育活動情報

主要な担当授業

  • 大学院共通授業科目(一般科目):自然科学・応用科学
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 大学院共通科目
    キーワード : ナノ構造、微細加工、光機能性材料、光化学、液晶、分子性物質、導電性、磁性、薄膜、電子顕微鏡、走査型プローブ顕微鏡、ナノワイヤー、二次元電子ガス、物性理論
  • 光電子物性学特論
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 情報科学研究科
    キーワード : 半導体と量子構造、半導体電子デバイス、フォトニックデバイス、磁性、スピントロニクスデバイス
  • 光電子物性学特論
    開講年度 : 2018年
    課程区分 : 博士後期課程
    開講学部 : 情報科学研究科
    キーワード : 半導体と量子構造、半導体電子デバイス、フォトニックデバイス、磁性、スピントロニクスデバイス
  • 電子デバイス工学
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 半導体、電子と正孔(ホール)、ダイオード、トランジスタ、集積回路、受光素子、発光素子、表示素子

大学運営

委員歴

  • 2017年04月 - 現在   日本セラミックス協会東北・北海道支部   幹事
  • 2008年04月 - 現在   日本熱電学会   評議員
  • 2004年04月 - 2006年03月   日本セラミックス協会   学術論文誌編集委員


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