研究者データベース

橋詰 保(ハシヅメ タモツ)
量子集積エレクトロニクス研究センター
特任教授

基本情報

所属

  • 量子集積エレクトロニクス研究センター

職名

  • 特任教授

学位

  • 工学博士(北海道大学)

ホームページURL

科研費研究者番号

  • 80149898

J-Global ID

研究キーワード

  • 半導体デバイス   Semiconductor device   Semiconductor nanostructure   

研究分野

  • ナノテク・材料 / 薄膜、表面界面物性
  • ものづくり技術(機械・電気電子・化学工学) / 電気電子材料工学

職歴

  • 2016年04月 - 現在 名古屋大学 未来材料・システム研究所 教授(クロスアポイントメント制度)
  • 2016年 - 現在 Professor, IMaSS, Nagoya Univ.
  • 2004年04月 - 現在 北海道大学 量子集積エレクトロニクス研究センター 教授
  • 2001年 - 現在 Professor, RCIQE, Hokkaido University
  • 2014年04月 - 2020年03月 北海道大学 量子集積エレクトロニクス研究センター センター長
  • 2014年 - 2020年 Director, RCIQE, Hokkaido Univ.
  • 2001年 - 2004年 北海道大学助教授(量子集積エレクトロニクス研究センター・組織換) 助教授
  • 1998年 - 2004年 Associate Professor, Research Center for Integrated Quantum Electronics (RCIQE), Hokkaido Univ.
  • 1998年 - 2001年 北海道大学助教授(量子界面エレクトロニクス研究センター・配置換) 助教授
  • 1994年 - 1998年 北海道大学助教授(工学部電気工学科) 助教授
  • 1994年 - 1998年 Associate Professor, Faculty of Eng., Hokkaido Univ.
  • 1987年 - 1994年 北海道職業訓練短期大学校講師(電気科) 講師
  • 1987年 - 1994年 Lecturer, Hokkaido Polytechnic College
  • 1981年 - 1987年 釧路工業高等専門学校助手(電子工学科) 助手
  • 1981年 - 1987年 Research Associate, Kushiro National College of Technology

所属学協会

  • 電子情報通信学会   応用物理学会   

研究活動情報

論文

  • Joel T. Asubar, Zenji Yatabe, Dagmar Gregusova, Tamotsu Hashizume
    Journal of Applied Physics 129 12 121102 - 121102 2021年03月28日 [査読有り][招待有り]
  • Shota Kaneki, Tamotsu Hashizume
    AIP Advances 11 1 015301 - 015301 2021年01月01日 [査読有り][通常論文]
  • Ryota Ochi, Erika Maeda, Toshihide Nabatame, Koji Shiozaki, Taketomo Sato, Tamotsu Hashizume
    AIP Advances 10 6 065215 - 065215 2020年06月01日 [査読有り][通常論文]
  • S Ozaki, K Makiyama, T Ohki, N Okamoto, Y Kumazaki, J Kotani, S Kaneki, K Nishiguchi, N Nakamura, N Hara, T Hashizume
    Semiconductor Science and Technology 35 3 035027 - 035027 2020年03月01日 [査読有り][通常論文]
  • D. Gregušová, L. Tóth, O. Pohorelec, S. Hasenöhrl, Š. Haščík, I. Cora, Z. Fogarassy, R. Stoklas, A. Seifertová, M. Blaho, A. Laurenčíková, T. Oyobiki, B. Pécz, T. Hashizume, J. Kuzmík
    Jpn. J. Appl. Phys. 58 SCCD21-1 - 5 2019年05月 [査読有り][通常論文]
  • K. Fukuda, H. Asai, J. Hattori, M. Shimizu, T. Hashizume
    Jpn. J. Appl. Phys. 58 SBBD06-1 - 6 2019年02月 [査読有り][通常論文]
  • S. Nakazawa, H.-A. Shih, N. Tsurumi, Y. Anda, T. Hatsuda, T. Ueda, T. Kimoto, T. Hashizume
    Jpn. J. Appl. Phys. 58 030902-1 - 4 2019年02月 [査読有り][通常論文]
  • Y. Ando, S. Kaneki, T. Hashizume
    Appl. Phys. Express 12 2 024002  2019年02月 [査読有り][通常論文]
  • M. Ťapajna, J. Drobný, F. Gucmann, K. Hušeková, D. Gregušová, T. Hashizume, J. Kuzmík
    Mat. Sci. Semicond. Process. 91 356 - 361 2019年01月 [査読有り][通常論文]
  • Joel Tacla Asubar, Hirokuni Tokuda, Masaaki Kuzuhara, Zenji Yatabe, Kenya Nishiguchi, Tamotsu Hashizume
    PISIKA 1 1 11-1 - 6 2018年12月 [査読有り][通常論文]
  • M. Matys, K. Nishiguchi, B. Adamowicz, J. Kuzmik, T. Hashizume
    J. Appl. Phys. 124 224502  2018年12月 [査読有り][通常論文]
  • T. Hashizume, S. Kaneki, T. Oyobiki, Y. Ando, S. Sasaki, K. Nishiguchi
    Appl. Phys. Express 11 124102  2018年12月 [査読有り][通常論文]
  • GaN-based Metal-Insulator-Semiconductor Transistors on Si for Power Switching Applications
    Satoshi Nakazawa, Hong-An Shih, Naohiro Tsurumi, Yoshiharu Anda, Tsuguyasu Hatsuda, Tetsuzo Ueda, Mikito Nozaki, Takahiro Yamada, Takuji Hosoi, Takayoshi Shimura, Heiji Watanabe, Tamotsu Hashizume
    2018年06月 [査読有り][招待有り]
  • Tamotsu Hashizume, Kenya Nishiguchi, Shota Kaneki, Jan Kuzmik, Zenji Yatabe
    Materials Science in Semiconductor Processing 78 85 - 95 2018年05月01日 [査読有り][招待有り]
     
    In this article, we review recent progress on AlGaN/GaN and InAlN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) using Al-based oxides, nitride dielectrics, SiO2 and high-k dielectrics. Although GaN MIS-HEMTs have been suffering from the instability of threshold voltage (VTH), recent interface technologies using in-situ SiNx and surface oxidation of (Al)GaN achieved excellent DC and dynamic performances of GaN MIS-HEMTs with stable VTH. Furthermore, a new design of the gate dielectric such as a nanolaminate structure has been applied to GaN HEMTs. GaN-based MIS-HEMTs sometimes showed sudden current saturation at forward gate bias, and we discuss effects of electronic states at insulator-semiconductor interfaces on current linearity of GaN MIS-HEMTs. Finally, we present effective surface passivation schemes in conjunction with field-plate structures and emerging device structures utilizing multi nanochannels under the gate region.
  • K. Fukuda, H. Asai, J. Hattori, M. Shimizu, T. Hashizume
    Jpn. J. Appl. Phys. 57 4 04FG04  2018年04月01日 [査読有り][通常論文]
     
    In this study, GaN MOS capacitance-voltage device simulations considering various interface and bulk traps are performed in the transient mode. The simulations explain various features of capacitance-voltage curves, such as plateau, hysteresis, and frequency dispersions, which are commonly observed in measurements of GaN MOS capacitors and arise from complicated combinations of interface and bulk deep-level traps. The objective of the present study is to provide a good theoretical tool to understand the physics of various nonideal measured curves.
  • S. Nakazawa, H. A. Shih, N. Tsurumi, Y. Anda, T. Hatsuda, T. Ueda, M. Nozaki, T. Yamada, T. Hosoi, T. Shimura, H. Watanabe, T. Hashizume
    Technical Digest - International Electron Devices Meeting, IEDM 25.1.1 - 25.1.4 2018年01月23日 [査読有り][通常論文]
     
    In this paper, high current and high voltage AlGaN/GaN metal-insulator-semiconductor (MIS) heterojunction field-effect transistors (HFETs) on Si are demonstrated. The devices exhibit a drain current of 20 A as well as a breakdown voltage of 730 V, serving normally-off operations. Stable interfacial characteristics free from the hysteresis in the transfer characteristics are enabled by the introduction of AlON gate insulator. A recessed gate structure formed by epitaxial regrowth of AlGaN over the grooved AlGaN/GaN heterojunction successfully reduces the on-state resistance and eliminates the processing damage on the surface of the grooved structure. Note that an oxygen annealing followed by the deposition of AlON shifts the threshold voltage Vth to positive side. The resultant switching performance by the 20 A/730 V AlGaN/GaN MIS-HFET is very fast with dV/dt of 78 V/ns and 169 V/ns for turn-on and turn-off transitions, respectively, indicating that the proposed MIS-HFETs are very promising for practical power switching applications.
  • Hiromichi Ohta, Sung Wng Kim, Shota Kaneki, Atsushi Yamamoto, Tamotsu Hashizume
    Advanced Science 5 1 1700696  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.
  • M. Matys, S. Kaneki, K. Nishiguchi, B. Adamowicz, T. Hashizume
    JOURNAL OF APPLIED PHYSICS 122 22 224504  2017年12月 [査読有り][通常論文]
     
    We proposed that the disorder induced gap states (DIGS) can be responsible for the threshold voltage (V-th) instability in Al2O3/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors. In order to verify this hypothesis, we performed the theoretical calculations of the capacitance voltage (C-V) curves for the Al2O3/AlGaN/GaN structures using the DIGS model and compared them with measured ones. We found that the experimental C-V curves with a complex hysteresis behavior varied with the maximum forward bias and the sweeping rate can be well reproduced theoretically by assuming a particular distribution in energy and space of the DIGS continuum near the Al2O3/AlGaN interface, i.e., a U-shaped energy density distribution and exponential depth decay from the interface into Al2O3 layer (up to 4 nm), as well as suitable DIGS capture cross sections (the order of magnitude of 10(-15) cm(2)). Finally, we showed that the DIGS model can also explain the negative bias induced threshold voltage instability. We believe that these results should be critical for the successful development of the passivation techniques, which allows to minimize the V-th instability related effects. Published by AIP Publishing.
  • Koichi Fukuda, Junichi Hattori, Hidehiro Asai, Mitsuaki Shimizu, Tamotsu Hashizume
    International Conference on Simulation of Semiconductor Processes and Devices, SISPAD 2017- 233 - 236 2017年10月25日 [査読有り][通常論文]
     
    C-F curves of GaN MOS capacitors are analyzed by transient-mode semiconductor device simulation. Problems of numerical convergence in the device simulation caused by the wide range time scales of the deep level trap physics are solved by a new discretization scheme of the trap formula that takes into account the dynamic change of their ionization rates. Consequently, frequency dispersions, hysteresis characteristics, and other non-ideal characteristics of C-F curves are stably obtained for various conditions. The present method could be a strong analysis tool, providing detail trap information when compared to a wide range of C-F measurements of GaN MOS capacitors. The present method is not restricted to GaN, and is applicable to MOS capacitors or other characteristics of other semiconductor materials suffering influences of the deep level traps.
  • Kenya Nishiguchi, Syota Kaneki, Shiro Ozaki, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 56 10 101001  2017年10月 [査読有り][通常論文]
     
    To investigate current linearity and operation stability of metal-oxide-semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al2O3-gate MOS-HEMTs without and with a bias annealing in air at 300 degrees C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of I-D-V-G curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance-voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al2O3/AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMTshowed small threshold voltage shift after applying forward bias stress and stable operation even at high temperatures. (C) 2017 The Japan Society of Applied Physics
  • Hidekazu Yamamoto, Kazuya Agui, Yuhki Uchida, Shota Mochizuki, Takeshi Uruma, Nobuo Satoh, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 56 8 08LB07-1 - 5 2017年08月 [査読有り][通常論文]
     
    The carrier concentration in a gallium nitride (GaN) substrate of a GaN-on-GaN wafer grown by hydride vapor phase epitaxy (HVPE) was evaluated by Raman spectroscopy and Kelvin probe force microscopy (KFM). On the basis of the longitudinal optical phonon-plasmon coupled (LOPC) mode of Raman spectra and surface potential measurements by KFM, the carrier concentration at the periphery of the HVPE-GaN substrate was found to be about one order of magnitude lower than that at the center. The decrease in carrier concentration is considered to be due to the out-diffusion of dopants during the metal organic chemical vapor deposition (MOCVD) of the epitaxial layer. In silicon (Si) epitaxial wafers, the autodoping of out-diffused dopants introduces nonuniform device characteristics. This undesirable effect needs to be suppressed to successfully move from prototype GaN-on-GaN power devices to commercial products. (C) 2017 The Japan Society of Applied Physics
  • Yusuke Kumazaki, Keisuke Uemura, Taketomo Sato, Tamotsu Hashizume
    JOURNAL OF APPLIED PHYSICS 121 18 184501-1 - 6 2017年05月 [査読有り][通常論文]
     
    The photocarrier-regulated electrochemical (PREC) process was developed for fabricating recessed-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) for normally off operation. The PREC process is based on photo-assisted electrochemical etching using low-energy chemical reactions. The fundamental photo-electrochemical measurements on AlGaN/GaN heterostructures revealed that the photo-carriers generated in the top AlGaN layer caused homogeneous etching of AlGaN with a smooth surface, but those generated in the GaN layer underneath caused inhomogeneous etching that roughens the surface. The concept of the PREC process is to supply the photo-carriers generated only in the AlGaN layer by selecting proper conditions on light wavelength and voltage. The phenomenon of self-termination etching has been observed during the PREC process, where the etching depth was controlled by light intensity. The recessed-gate AlGaN/GaN HEMT fabricated with the PREC process showed positive threshold voltage and improvement in transconductance compared to planar-gate AlGaN/GaN HEMTs. Published by AIP Publishing.
  • S. Ozaki, K, Makiyama, T, Ohki, N, Okamoto, S, Kaneki, K, Nishiguchi, N, Hara and T, Hashizume
    Appl. Phys. Express 10 061001-1 - 4 2017年05月 [査読有り][通常論文]
  • R. Stoklas, D. Gregusova, M. Blaho, K. Froehlich, J. Novak, M. Matys, Z. Yatabe, P. Kordos, T. Hashizume
    SEMICONDUCTOR SCIENCE AND TECHNOLOGY 32 4 045018-1 - 8 2017年04月 [査読有り][通常論文]
     
    The electrical properties of AlGaN/GaN MOSHFETs with HfO2 prepared by atomic layer deposition with and w/o oxygen-plasma treatment (further referred to as PHf-MOS and Hf-MOS) were investigated. The sub-threshold slope of the MOSHFETs (350 and 150 mV dec(-1) for Hf-MOS and PHf-MOS, respectively) were lower than that for HFET (450 mV dec-1), which also correspond with lower leakage current of the MOSHFETs (similar to 10(-8)A mm(-1) at -9 V for PHf-MOS). In addition, the density of the interface states at the oxide/GaN-cap layer near the conduction band edge and mid-gap (similar to 5 x 10(12) and 2 x 10(11) cm(-2) eV(-1), respectively) after PHf-MOS was lower than that for Hf-MOS (similar to 3 x 10(13) and 2 x 10(12) cm(-2) eV(-1), respectively). From the x-ray photoemission spectroscopy analysis we observed a shift in the Auger Ga LMM peaks (0.6 eV) and an increase of the intensity area of the Ga-O bond in the Ga2p3 spectrum after the oxygen-plasma treatment, mainly because the GaN-cap layer was oxidized and Ga2O3 was formed.
  • M. Matys, B. Adamowicz, A. Domanowska, A. Michalewicz, R. Stoklas, M. Akazawa, Z. Yatabe, T. Hashizume
    JOURNAL OF APPLIED PHYSICS 120 22 225305-1 - 12 2016年12月 [査読有り][通常論文]
     
    The energy spectrum of interface state density, D-it(E), was determined at oxide/III-N heterojunction interfaces in the entire band gap, using two complementary photo-electric methods: (i) photo-assisted capacitance-voltage technique for the states distributed near the midgap and the conduction band (CB) and (ii) light intensity dependent photo-capacitance method for the states close to the valence band (VB). In addition, the Auger electron spectroscopy profiling was applied for the characterization of chemical composition of the interface region with the emphasis on carbon impurities, which can be responsible for the interface state creation. The studies were performed for the AlGaN/GaN metal-insulator-semiconductor heterostructures (MISH) with Al2O3 and SiO2 dielectric films and AlxGa1-x layers with x varying from 0.15 to 0.4 as well as for an Al2O3/InAlN/GaN MISH structure. For all structures, it was found that: (i) D-it(E) is an U-shaped continuum increasing from the midgap towards the CB and VB edges and (ii) interface states near the VB exhibit donor-like character. Furthermore, D-it(E) for SiO2/AlxGa1-x/GaN structures increased with rising x. It was also revealed that carbon impurities are not present in the oxide/III-N interface region, which indicates that probably the interface states are not related to carbon, as previously reported. Finally, it was proven that the obtained D-it(E) spectrum can be well fitted using a formula predicted by the disorder induced gap state model. This is an indication that the interface states at oxide/III-N interfaces can originate from the structural disorder of the interfacial region. Furthermore, at the oxide/barrier interface we revealed the presence of the positive fixed charge (Q(F)) which is not related to D-it(E) and which almost compensates the negative polarization charge (Q(pol)(-)). Published by AIP Publishing.
  • Shota Kaneki, Joji Ohira, Shota Toiya, Zenji Yatabe, Joel T. Asubar, Tamotsu Hashizume
    APPLIED PHYSICS LETTERS 109 16 162104-1 - 5 2016年10月 [査読有り][通常論文]
     
    Interface characterization was carried out on Al2O3/GaN structures using epitaxial n-GaN layers grown on free-standing GaN substrates with relatively low dislocation density (<3 x 10(6) cm(-2)). The Al2O3 layer was prepared by atomic layer deposition. The as-deposited metal-oxide-semiconductor (MOS) sample showed a significant frequency dispersion and a bump-like feature in capacitance-voltage (C-V) curves at reverse bias, showing high-density interface states in the range of 10(12) cm(-1) eV(-1). On the other hand, excellent C-V characteristics with negligible frequency dispersion were observed from the MOS sample after annealing under a reverse bias at 300 degrees C in air for 3 h. The reverse-bias-annealed sample showed state densities less than 1 x 10(11) cm(-1) eV(-1) and small shifts of flat-band voltage. In addition, the C-V curve measured at 200 degrees C remained essentially similar compared with the room-temperature C-V curves. These results indicate that the present process realizes a stable Al2O3/GaN interface with low interface state densities. Published by AIP Publishing.
  • Zenji Yatabe, Joel T. Asubar, Tamotsu Hashizume
    JOURNAL OF PHYSICS D-APPLIED PHYSICS 49 39 3933001-1 - 19 2016年10月 [査読有り][招待有り]
     
    Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Si-and GaAs-based devices counterparts. Metal-insulator-semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance-voltage (C-V) characteristics. A HEMT MIS structure typically shows a 2-step C-V behavior. However, several groups reported C-V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-kappa dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C-V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology.
  • M. Matys, R. Stoklas, J. Kuzmik, B. Adamowicz, Z. Yatabe, T. Hashizume
    JOURNAL OF APPLIED PHYSICS 119 20 205304-1 - 7 2016年05月 [査読有り][通常論文]
     
    We performed, for the first time, quantitative characterization of electron capture cross sections sigma of the interface states at dielectric/III-N heterojunction interfaces. We developed a new method, which is based on the photo-assisted capacitance-voltage measurements using photon energies below the semiconductor band gap. The analysis was carried out for AlGaN/GaN metal-insulatorsemiconductor heterojunction (MISH) structures with Al2O3, SiO2, or SiN films as insulator deposited on the AlGaN layers with Al content (x) varying over a wide range of values. Additionally, we also investigated an Al2O3/InAlN/GaN MISH structure. Prior to insulator deposition, the AlGaN and InAlN surfaces were subjected to different treatments. We found that sigma for all these structures lies in the range between 5 x 10(-19) and 10(-16) cm(2). Furthermore, we revealed that sigma for dielectric/AlxGa1-xN interfaces increases with increasing x. We showed that both the multiphonon-emission and cascade processes can explain the obtained results. Published by AIP Publishing.
  • Shunsuke Okada, Hideto Miyake, Kazumasa Hiramatsu, Reina Miyagawa, Osamu Eryu, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 55 1 2016年01月 [査読有り][通常論文]
     
    The thermal stability of GaN surfaces was investigated with respect to homo-epitaxy on free-standing GaN substrates. Morphologies and etching rates of the GaN surfaces for free-standing polar (0001), nonpolar (10 (1) over bar0), and semipolar (20 (2) over bar1) and (20 (2) over bar(1) over bar) planes were studied before and after thermal cleaning. In the case of the polar (0001) plane, polishing scratches disappeared after thermal cleaning at temperatures above 1000 degrees C. The surface morphology depended on not only the cleaning temperature, but also the substrate off-angle. The surface after thermal cleaning became rough for the substrate with off-angle less than 0.05 degrees. In the case of nonpolar and semipolar planes after thermal cleaning, surface morphologies and etching rates were strongly dependent on the planes. A flat surface was maintained at cleaning temperatures up to 1100 degrees C for the (10 (1) over bar0) plane, but the surface of the (20 (2) over bar1) plane became rough with increasing cleaning temperature. (C) 2016 The Japan Society of Applied Physics
  • M. Tapajna, L. Valik, D. Gregusova, K. Frohlich, F. Gucmann, T. Hashizume, J. Kuzmik
    2016 11TH INTERNATIONAL CONFERENCE ON ADVANCED SEMICONDUCTOR DEVICES & MICROSYSTEMS (ASDAM) 1 - 4 2016年 [査読有り][通常論文]
     
    Metal-oxide-semiconductor (MOS) structure represents an important gate technology in GaN HEMTs. As oxide/semiconductor interface quality is remaining reliability concern, several techniques for determination of interface state density (D-it) has been proposed. In the literature, the hysteresis in C-V Weeps (or V-th Skit, Delta V-th) is often interpreted as D-it in particular energy range in the semiconductor band-gap. In this work, we critically assessed a relevancy of relation between Delta V-th, (measured at 25 and 125 degrees C) and experimentally determined D-it distribution, to point out possible pitfalls in the data interpretation. D-it, distributions were measured by combination of complementary techniques and 1D simulations applied to state-of-the-art MOS-HEMT structures with Al2O3 films grown by ALD on AlGaN/GaN heterostructures. It is demonstrated that, apart from interface traps, also other parasitic effects related to border traps and oxide bulk traps can have dominant impact on Delta V-th. This means that Delta V-th could not be solely related to D-it, unless negligibility of other relevant effects is confirmed.
  • Interface control technologies for high-power GaN transistors: Self-stopping etching of p-GaN layers utilizing electrochemical reactions
    Taketomo Sato, Yusuke Kumazaki, Masaaki Edamoto, Masamichi Akazawa, Tamotsu Hashizume
    Proceeding of SPIE 9748, Gallium Nitride Materials and Devices XI 9748 97480Y  2016年 [査読有り][招待有り]
  • Joel T. Asubar, Yoshiki Sakaida, Satoshi Yoshida, Zenji Yatabe, Hirokuni Tokuda, Tamotsu Hashizume, Masaaki Kuzuhara
    Applied Physics Express 8 11 111001-1 - 4 2015年11月01日 [査読有り][通常論文]
     
    We studied the effects of pre-passivation oxygen plasma treatment of the AlGaN surface on the current collapse of AlGaN/GaN high-electronmobility transistors (HEMTs). Oxygen plasma-treated devices generally exhibited significantly less dynamic on-resistance (Ron) compared with untreated control devices. We also extended our investigation to HEMTs with a GaN cap layer. Interestingly, after oxygen plasma treatment, we found that GaN-capped HEMTs showed a dynamic Ron behavior that was essentially similar to that of oxygen plasma-treated uncapped HEMTs, suggesting that the GaN cap layer plays an inconsequential role in current collapse mitigation when employed in conjunction with oxygen plasma treatment.
  • Joel T. Asubar, Yohei Kobayashi, Koji Yoshitsugu, Zenji Yatabe, Hirokuni Tokuda, Masahiro Horita, Yukiharu Uraoka, Tamotsu Hashizume, Masaaki Kuzuhara
    IEEE Transactions on Electron Devices 62 8 2423 - 2428 2015年08月01日 [査読有り][通常論文]
     
    We have demonstrated for the first time a remarkable reduction of current collapse in AlGaN/GaN high-electron-mobility transistors (HEMTs) by high-pressure water vapor annealing (HPWVA). The device subjected to HPWVA exhibited considerably low dynamic ON-resistance ( R-{\\mathrm{{\\scriptscriptstyle ON}}} ), suggesting highly improved performance of these devices. Analyses of the results on normalized dynamic R-{\\mathrm{{\\scriptscriptstyle ON}}} experiments have shown the elimination of deeper traps by HPWVA, leading to the substantially reduced current collapse. X-ray photoelectron spectroscopy (XPS) studies revealed a significant increase in the oxygen core-level O 1s peak. Moreover, angle-resolved XPS suggested the formation of surface oxide layer. These results indicate that the effective reduction of current collapse in the HPWVA-processed samples is likely due to the incorporation of active oxygen species generated by the HPWV into the AlGaN surface. These oxygen atoms eventually fill up near-surface nitrogen vacancies and promote the formation of Ga< inf> 2< /inf> O< inf> 3< /inf> native oxide and possibly Ga< inf> 2< /inf> O suboxide, which is known to be an excellent III-V surface passivant. HPWVA is a relatively simple, low-damage, and lowerature process, and hence, it is found to be a highly feasible and promising alternative for realizing AlGaN/GaN HEMTs with improved performance.
  • Takaharu Ishibashi, Masayuki Okamoto, Eiji Hiraki, Toshihiko Tanaka, Tamotsu Hashizume, Daigo Kikuta, Tetsu Kachi
    IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS 51 3 2415 - 2422 2015年05月 [査読有り][通常論文]
     
    Wide-bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are promising materials for next-generation power devices. We have fabricated a normally on GaN-based high-electron-mobility transistor (GaN HEMT) for power electronic converters. In this paper, the current collapse phenomena, which are distinctive characteristics of GaN devices, are evaluated in detail for several voltages with two switching frequencies. We also evaluate a gate drive circuit that we previously proposed for the normally on GaN HEMT with a single positive voltage source. We construct and test prototype circuits for a boost-type dc-dc converter and a single-phase full-bridge inverter with a gate drive circuit. The problems to be solved for the normally on GaN HEMT, which has a (static) voltage rating of over 600 V, are clarified on the basis of the experimental results.
  • Zenji Yatabe, Joel T. Asubar, Taketomo Sato, Tamotsu Hashizume
    PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 212 5 1075 - 1080 2015年05月 [査読有り][通常論文]
     
    We have investigated the effects of the inductively coupled plasma (ICP) etching of AlGaN surface on the resulting interface properties of the Al2O3/AlGaN/GaN structures. The experimentally measured capacitance-voltage (C-V) characteristics were compared with those calculated taking into account the interface states density at the Al2O3/AlGaN interface. As a complementary method, photoassisted C-V method utilizing photons with energies less than the bandgap of GaN was also used to probe the interface state density located near AlGaN midgap. It was found that the ICP etching of the AlGaN surface significantly increased the interface state density at the Al2O3/AlGaN interface. It is likely that ICP etching induced the interface roughness, disorder of chemical bonds and formation of various type of defect complexes including nitrogen-vacancy-related defects at the AlGaN surface, leading to poor C-V curve due to higher interface state density at the Al2O3/AlGaN interface.
  • Zenji Yatabe, Yujin Hori, Wan-Cheng Ma, Joel T. Asubar, Masamichi Akazawa, Taketomo Sato, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 53 10 100213-1 - 10 2014年10月 [査読有り][通常論文]
     
    This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal-insulator-semiconductor (MIS) structures. SiO2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiNx is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al2O3/GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance-voltage (C-V) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/AlGaN interfaces, a photo-assisted C-V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C-V technique, we estimate interface state density distributions at the Al2O3/AlGaN interfaces. (C) 2014 The Japan Society of Applied Physics
  • M. Tapajna, M. Jurkovic, L. Valik, S. Hascik, D. Gregusova, F. Brunner, E. -M. Cho, T. Hashizume, J. Kuzmik
    JOURNAL OF APPLIED PHYSICS 116 10 104501-1 - 7 2014年09月 [査読有り][通常論文]
     
    Oxide/semiconductor interface trap density (D-it) and net charge of Al2O3/(GaN)/AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistor (MOS-HEMT) structures with and without GaN cap were comparatively analyzed using comprehensive capacitance measurements and simulations. D-it distribution was determined in full band gap of the barrier using combination of three complementary capacitance techniques. A remarkably higher D-it (similar to 5-8 X 10(12) eV(-1) cm(-2)) was found at trap energies ranging from E-C-0.5 to 1 eV for structure with GaN cap compared to that (D-it similar to 2-3 X 10(12) eV(-1) cm(-2)) where the GaN cap was selectively etched away. D-it distributions were then used for simulation of capacitance-voltage characteristics. A good agreement between experimental and simulated capacitance-voltage characteristics affected by interface traps suggests (i) that very high D-it (>10(13) eV(-1) cm(-2)) close to the barrier conduction band edge hampers accumulation of free electron in the barrier layer and (ii) the higher D-it centered about E-C-0.6 eV can solely account for the increased C-V hysteresis observed for MOS-HEMT structure with GaN cap. Analysis of the threshold voltage dependence on Al2O3 thickness for both MOS-HEMT structures suggests that (i) positive charge, which compensates the surface polarization, is not necessarily formed during the growth of III-N heterostructure, and (ii) its density is similar to the total surface polarization charge of the GaN/AlGaN barrier, rather than surface polarization of the top GaN layer only. Some constraints for the positive surface compensating charge are discussed. (C) 2014 AIP Publishing LLC.
  • Joel T. Asubar, Zenji Yatabe, Tamotsu Hashizume
    APPLIED PHYSICS LETTERS 105 5 053510-1 - 5 2014年08月 [査読有り][通常論文]
     
    Dramatic reduction of thermal resistance was achieved in AlGaN/GaN Multi-Mesa-Channel (MMC) high electron mobility transistors (HEMTs) on sapphire substrates. Compared with the conventional planar device, the MMC HEMT exhibits much less negative slope of the I-D-V-DS curves at high V-DS regime, indicating less self-heating. Using a method proposed by Menozzi and co-workers, we obtained a thermal resistance of 4.8 K-mm/W at ambient temperature of similar to 350K and power dissipation of similar to 9W/mm. This value compares well to 4.1 K-mm/W, which is the thermal resistance of AlGaN/GaN HEMTs on expensive single crystal diamond substrates and the lowest reported value in literature. (C) 2014 AIP Publishing LLC.
  • Kenya Nishiguchi, Tamotsu Hashizume
    IMFEDK 2014 - 2014 International Meeting for Future of Electron Devices, Kansai 2014年07月28日 [査読有り][通常論文]
     
    Using a dual-gate transistor structure, we have evaluated the effect of off-bias-stress spatially induced surface charging at the AlGaN/GaN surface. Pulsed voltage stress on the drain access region led only to increase in the on-resistance, whereas the stress on the source region additionally decreased the saturation drain current. In addition, significant surface charging in the access region resulted in a threshold voltage shift. The 2D potential simulation indicated that surface negative charges modulate the potential distribution near the gate edge. It was also found that such surface charging can extend as far as 0.5 μm from the stressing gate edge.
  • Kenya Nishiguchi, Joel T. Asubar, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 53 7 070301-1 - 4 2014年07月 [査読有り][通常論文]
     
    Using a dual-gate transistor structure, we have evaluated the effect of off-bias-stress spatially induced surface charging at the AlGaN/GaN surface. Pulsed voltage stress on the drain access region led only to increase in the on-resistance, whereas the stress on the source region additionally decreased the saturation drain current. In addition, significant surface charging in the access region resulted in a threshold voltage shift. The two-dimensional (20) potential simulation indicated that surface negative charges modulate the potential distribution near the gate edge. It was also found that such surface charging can extend as far as 0.5 mu m from the stressing gate edge. (C) 2014 The Japan Society of Applied Physics
  • 橋詰 保, 谷田部 然治, 佐藤 威友
    表面科学 35 2 96 - 101 2014年02月 [査読有り][通常論文]
     
    Interface properties of GaN-based heterostructures have been characterized. Schottky contacts on dry-etched n-GaN layers showed leaky I-V characteristics. An anneal process at 400 ºC was effective in recovering the rectifying characteristics. To characterize interface properties of Al<sub>2</sub>O<sub>3</sub> insulated gates on AlGaN/GaN structures with and without the inductively coupled plasma (ICP) etching of AlGaN, we have developed a C-V calculation method taking into account electronic state charges at the Al<sub>2</sub>O<sub>3</sub>/AlGaN interface and a photoassisted C-V technique utilizing photons with energies less than the bandgap of AlGaN. It was found that the ICP etching caused the monolayer-level interface roughness, disorder of the chemical bonds and formation of various types of defect complexes at the AlGaN surface, resulting in poor C-V characteristics due to high-density interface states at the Al<sub>2</sub>O<sub>3</sub>/AlGaN interface.
  • Masayuki Okamoto, Koyo Matuzaki, Hiroaki Yamada, Toshihiko Tanaka, Tamotsu Hashizume
    IECON 2014 - 40TH ANNUAL CONFERENCE OF THE IEEE INDUSTRIAL ELECTRONICS SOCIETY 982 - 987 2014年 [査読有り][通常論文]
     
    Gallium nitride high-electron mobility transistors (GaN HEMTs) have attractive properties, low on-resistances and fast switching speeds. This paper presents the characteristics of a normally-on GaN HEMT that we fabricated. Further, the circuit operation of a Class-E amplifier is analyzed. Experimental results demonstrate the excellent performance of the gate drive circuit for the normally-on GaN HEMT and the 13.56MHz radio frequency (RF) power amplifier.
  • Joel Asubar, Kota Ohi, Kenya Nishiguchi, Tamotsu Hashizume
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 11, NO 3-4 11 3-4 857 - 861 2014年 [査読有り][通常論文]
     
    We present further experimental results that demonstrate the superior current stability of multi-mesa-channel (MMC) AlGaN/GaN HEMTs over their conventional planar counterparts. Pulsed ID-VDS measurements revealed almost overlapping ID-VDS curves for the MMC device even with increasing drain-to-source quiescent bias VDSq with gate-to-source quiescent bias V-GSq kept constant. With all other device dimensions equal, the MMC device exhibited less on-resistance RON change with varying gate-to-drain spacing LGD. This could be the key to understanding why the MMC device operation is relatively immune to current collapse, a stability issue widely believed to be connected to surface charging at the gate-drain access region. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Y. Hori, Z. Yatabe, T. Hashizume
    Journal of Applied Physics 114 24 244503-1 - 8 2013年12月28日 [査読有り][通常論文]
     
    We have investigated the relationship between improved electrical properties of Al2O3/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al2O3/AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al2O3/AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N2O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al2O3 insulator. As compared to the sample without the treatment, the N2O-radical treated Al 2O3/AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al2O3/AlGaN interface were estimated to be 1 × 1012 cm-2 eV-1 or less around the midgap and 8 × 1012 cm-2 eV-1 near the conduction-band edge. In addition, we observed higher maximum drain current at the positive gate bias and suppressed threshold voltage instability under the negative gate bias stress even at 150 °C. Results presented in this paper indicated that the N2O-radical treatment is effective both in reducing the interface states and improving the electrical properties of the Al2O3/AlGaN/GaN MOS-HEMTs. © 2013 AIP Publishing LLC.
  • Kota Ohi, Joel Tacla Asubar, Kenya Nishiguchi, Tamotsu Hashizume
    IEEE TRANSACTIONS ON ELECTRON DEVICES 60 10 2997 - 3004 2013年10月 [査読有り][招待有り]
     
    We develop and characterize multi-mesa-channel (MMC) AlGaN/GaN high-electron mobility transistors (HEMTs), in which a periodic trench structure is fabricated only under the gate electrode. A surrounding-field effect in the MMC structure results in a shallower threshold voltage and a smaller subthreshold slope than those of the standard planar-type HEMT. In addition, the MMC HEMT shows a low knee voltage and a weak dependence of on-resistance (R-ON) on the gate-drain distance. Following identical off-state bias stress, the MMC HEMT exhibits low current collapse. The relative decrease in access resistance of the MMC device compared with the planar device can reduce the effects of access resistance on the drain current. It is likely that a high impedance of each nanochannel also contributes to the current stability of the MMC HEMT.
  • M. Matys, B. Adamowicz, Y. Hori, T. Hashizume
    APPLIED PHYSICS LETTERS 103 2 021603-1 - 4 2013年07月 [査読有り][通常論文]
     
    We determined the energy distribution of donor-like interface state density D-itD(E) at the Al2O3/AlGaN interface in a metal/Al2O3/AlGaN/GaN heterostructure (MISH) capacitor. In this order, we developed a point-by-point graphical method based on the measurement and simulations of the MISH photocapacitance versus ultraviolet light intensity. We found a tail-like shaped D-itD(E) strongly decreasing from the value of 5 x 10(13) to 4 x 10(12) eV(-1) cm(-2) in the energy range between 0.12 eV and 0.45 eV from the AlGaN valence band edge. (C) 2013 AIP Publishing LLC.
  • Zenji Yatabe, Yujin Hori, Sungsik Kim, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 6 1 016502.1-016502.4  2013年01月 [査読無し][通常論文]
     
    The effects of the inductively coupled plasma (ICP) etching of AlGaN on the interface properties of the Al2O3/AlGaN/GaN structures prepared by atomic layer deposition were investigated. It was found from the photoassisted capacitance-voltage (C-V) results that the ICP etching of the AlGaN surface significantly increased the interface state density up to 8 x 10(12) cm(-2) eV(-1) at the Al2O3/AlGaN interface. The transmission electron microscopy and X-ray photoelectron spectroscopy analyses indicated that the monolayer-level roughness, disorder of the chemical bonds at the AlGaN surface caused poor C-V characteristics due to high-density interface states at the Al2O3/ICP-etched AGaN interface. (C) 2013 The Japan Society of Applied Physics
  • MacIej Matys, Boguslawa Adamowicz, Tamotsu Hashizume
    AIP Conference Proceedings 1566 57 - 58 2013年 [査読有り][通常論文]
     
    A novel method was developed to determine the interface state density distribution Dit(E) in the whole wide bandgap of metal-insulator- semiconductor structures from the measurements of capacitance-voltage, C(V), and photocapacitance - UV light intensity, CL(Φ). The C(V) characteristics are governed mainly by the interface states in the upper part of the bandgap, whereas CL(Φ) ones under UV illumination and negative gate bias depend on the deeper states in the lower part, which become charged by excited holes. From the comparison of theoretical and experimental C(V) and CL(Φ) curves we obtained the full Dit(E) in a metal/Al2O3/n-GaN structure. © 2013 AIP Publishing LLC.
  • Yujin Hori, Taketomo Sato, Tamotsu Hashizume
    1st IEEE Workshop on Wide Bandgap Power Devices and Applications, WiPDA 2013 - Proceedings 143 - 146 2013年 [査読有り][通常論文]
     
    Interface properties of Al2O3 insulated gates on AlGaN/GaN structures prepared by atomic layer deposition have been characterized, focusing on the interface state density distribution at the Al2O3/AlGaN interface. We have developed a C-V calculation method taking into account electronic states charges at the Al 2O3/AlGaN interface and a photoassisted C-V technique utilizing photons with energies less than the bandgap of AlGaN. These techniques were then applied to investigate the effect of the ICP etching of AlGaN on the interface properties. © 2013 IEEE.
  • Takaharu Ishibashi, Masayuki Okamoto, Eiji Hiraki, Toshihiko Tanaka, Tamotsu Hashizume, Daigo Kikuta, Tetsu Kachi
    2013 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE) 1584 - 1591 2013年 [査読有り][通常論文]
     
    Wide bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are the promising materials for next-generation power devices. A normally-on GaN-based high-electron-mobility transistor (GaN HEMT) has been fabricated for power electronic converters. In this study, the current collapse phenomena, which are distinctive characteristics of GaN devices, are evaluated in detail under several voltages with two switching frequencies. Prototype circuits for a boost-type dc-dc converter and a single-phase full-bridge inverter, which use the newly fabricated GaN HEMT, are constructed and tested. The experimental results demonstrate that the newly fabricated normally-on GaN HEMT with the previously proposed gate drive circuit is suitable for use as a next-generation power device.
  • Takaharu Ishibashi, Masayuki Okamoto, Eiji Hiraki, Toshihiko Tanaka, Tamotsu Hashizume, Tetsu Kachi
    2013 IEEE ECCE ASIA DOWNUNDER (ECCE ASIA) 365 - 371 2013年 [査読有り][通常論文]
     
    Wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), are promised materials for next-generation power devices. The authors have recently fabricated a GaN-based high-electron-mobility transistor (HEMT), which is a normally-on device, for power electronics application. However, the power consumption in the constructed gate drive circuit increases when the GaN HEMT is used under higher-frequency operation. A new gate drive circuit with lower power consumption for the normally-on GaN HEMT is strongly required. In this paper, a new resonant gate drive circuit, which is most suitable for the newly fabricated GaN HEMT, is proposed. The validity and high practicability of the proposed resonant gate drive circuit are demonstrated by simulation and experimental results.
  • M. Matys, B. Adamowicz, T. Hashizume
    Applied Physics Letters 101 23 2012年12月03日 [査読有り][通常論文]
     
    We developed a method for determining of the deep donor-like interface state density distribution D i t (E) at the insulator/wide bandgap semiconductor interface in metal/insulator/semiconductor structures from the measurements of photocapacitance vs. ultraviolet light intensity C L (Φ). From the comparison of theoretical and experimental C L (Φ) curves we obtained the continuous donor D i t (E) in the energy range between 0.15 eV and 1 eV from the valence band top for a metal/ Al2 O3/n - GaN device. In addition, the acceptor-like interface state D i t (E) in the upper part of the bandgap was determined from the capacitance-voltage method. © 2012 American Institute of Physics.
  • Toshiyuki Yoshida, Tamotsu Hashizume
    APPLIED PHYSICS LETTERS 101 12 122102-122102-4  2012年09月 [査読無し][通常論文]
     
    The air-gap capacitance-voltage characteristics of InGaAs surfaces were measured after 1-, 2-, 6-, 9-, and 17-cycle atomic layer deposition (ALD) Al2O3 processing. A high density of mid-gap states was found to be generated and increased during these ALD process steps, while the native oxide component was reduced. On the other hand, the mid-gap state density was drastically reduced after the usual annealing process. The generation of the mid-gap states seemed to be relevant to a non-stoichiometric Al-oxide component associated with a deficit in oxygen atoms, which became re-oxidized during the annealing process. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4753927]
  • 橋詰 保, 堀 祐臣, 赤澤 正道
    電子情報通信学会技術研究報告 : 信学技報 112 154 17 - 20 電子情報通信学会 2012年07月 [査読無し][招待有り]
  • Sungsik Kim, Yujin Hori, Wang-Cheng Ma, Daigo Kikuta, Tetsuo Narita, Hiroko Iguchi, Tsutomu Uesugi, Tetsu Kachi, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 51 6 060201.1-060201.3  2012年06月 [査読無し][通常論文]
     
    The effects of the Cl-2-based inductively coupled plasma (ICP) etching of GaN on the interface properties of Al2O3/GaN structures prepared by atomic layer deposition (ALD) were investigated. We used n-GaN layers grown on freestanding n(+)-GaN substrates with low dislocation density. The ICP etching caused slight disorder of the chemical bonds at the GaN surface and monolayer-level interface roughness at the Al2O3/GaN interface, resulting in poor capacitance-voltage (C-V) characteristics due to high-density interface states including nitrogen-vacancy (V-N) related levels. The postannealing process in N-2 at 400 degrees C drastically improved the C-V characteristics, probably owing to the partial recovery of the V-N-related defects and the increased ordering of chemical bonds in the GaN surface region. (c) 2012 The Japan Society of Applied Physics
  • Cheng-Yu Hu, Tamotsu Hashizume
    JOURNAL OF APPLIED PHYSICS 111 8 084504-084504-9  2012年04月 [査読無し][通常論文]
     
    For AlGaN/GaN heterojunction field-effect transistors, on-state-bias-stress (on-stress)-induced trapping effects were observed across the entire drain access region, not only at the gate edge. However, during the application of on-stress, the highest electric field was only localized at the drain side of the gate edge. Using the location of the highest electric field as a reference, the trapping effects at the gate edge and at the more distant access region were referred to as localized and non-localized trapping effect, respectively. Using two-dimensional-electron-gas sensing-bar (2DEG-sensing-bar) and dual-gate structures, the non-localized trapping effects were investigated and the trap density was measured to be similar to 1.3 x 10(12) cm(-2). The effect of passivation was also discussed. It was found that both surface leakage currents and hot electrons are responsible for the non-localized trapping effects with hot electrons having the dominant effect. Since hot electrons are generated from the 2DEG channel, it is highly likely that the involved traps are mainly in the GaN buffer layer. Using monochromatic irradiation (1.24-2.81 eV), the trap levels responsible for the non-localized trapping effects were found to be located at 0.6-1.6 eV from the valence band of GaN. Both trap-assisted impact ionization and direct channel electron injection are proposed as the possible mechanisms of the hot-electron-related non-localized trapping effect. Finally, using the 2DEG-sensing-bar structure, we directly confirmed that blocking gate injected electrons is an important mechanism of Al2O3 passivation. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704393]
  • M. Akazawa, B. Gao, T. Hashizume, M. Hiroki, S. Yamahata, N. Shigekawa
    Physica Status Solidi (C) Current Topics in Solid State Physics 9 3-4 592 - 595 2012年03月 [査読有り][通常論文]
     
    AlGaN-based spacer layers for lattice-matched and nearly lattice matched InAlN/GaN interfaces were examined in Al 2O 3/InAlN/AlGaN/AlN/GaN structures. An Al 2O 3 overlayer was deposited to investigate the characteristics under positive bias by capacitance-voltage (C-V) measurement. The C-V characteristic for a sample with an Al 0.38Ga 0.62N/AlN double spacer layer indicated unfavorable electron accumulation at the InAlN/AlGaN interface inside the barrier under positive bias. To suppress the unfavorable accumulation, attempts were made to increase the Al molar fraction of the AlGaN layer to reduce the conduction band discontinuity and interface charge at InAlN/AlGaN interface. An Al 0.44Ga 0.56N/AlN double spacer layer and an Al 0.44Ga 0.56N single spacer layer of almost the same total thickness were investigated. Although both spacer layers result in normal C-V characteristics without the indication of unfavorable electron accumulation, the InAlN layer on a 1.5-nm-thick Al 0.44Ga 0.56N single spacer layer exhibited superior surface morphology without deteriorating the mobility of the two-dimensional electron gas despite the absence of the AlN layer. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • Cheng-Yu Hu, Tamotsu Hashizume
    Device Research Conference - Conference Digest, DRC 137 - 138 2012年 [査読有り][通常論文]
     
    Recently, O 2 or N 2O plasma treatment has been extensively investigated about the passivation effect of AlGaN/GaN HFETs [1-3]. However, the effect was not consistent. Due to ion bombardment on the sample surface, the devices might be significantly degraded [2,3], suggesting that it might be difficult to suppress the plasma damage. On the other hand, Tapajna et al. [4] have reported the reliability issue of such oxidation process. In this work, a method called N 2O radical treatment will be introduced. With this method, we suppressed the current collapse of AlGaN/GaN HFETs by ∼40%. And the reliability was also improved by ∼60%. © 2012 IEEE.
  • Kota Ohi, Tamotsu Hashizume
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 3-4 9 3-4 898 - 902 2012年 [査読有り][通常論文]
     
    Transport properties and off-stress-induced current collapse characteristics of the multi-mesa-channel (MMC) AlGaN/GaN HEMTs were investigated. The surrounding field effect in the MMC structure led to a shallower threshold voltage and a smaller subthreshold slope compared to those of the conventional planar HEMT. After applying off-state bias stress, the planar HEMT showed remarkable current collapse, including significant increase in on-resistance increasing the drain stress bias. On the other hand, for the MMC HEMT, we observed less change in on-resistance (R-ON) even after the application of high drain bias stress. This shows that the MMC structure has resistance to the off-stress-induced current collapse. In the case of a mesa-top width (W-top) of less than 100 nm, the channel resistance of the MMC HEMT is estimated to be about one order of magnitude higher than the increase in access resistance induced by the off-state stress. It is thus likely that the operation of the MMC HEMT is rather insensitive to change in access resistance. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Tamotsu Hashizume
    NINTH INTERNATIONAL CONFERENCE ON ADVANCED SEMICONDUCTOR DEVICES AND MICROSYSTEMS 31 - 36 2012年 [査読有り][通常論文]
     
    We observed the peculiar capacitance-voltage (C-V) characteristics with two capacitance steps in the Al2O3/AlGaN/GaN samples prepared by atomic layer deposition where the interface states near the midgap or deeper in energies act as fixed charges. From the voltage shift at the reverse bias in the photo-assisted C-V curve, we estimated the interface state density distribution at the Al2O3/AlGaN interface for the first time. The effects of the inductively coupled plasma (ICP) etching of AlGaN on the interface properties of the Al2O3/AlGaN/GaN structures were investigated. The transmission electron microscopy and X-ray photoelectron spectroscopy analyses indicated that monolayer-level roughness and disorder of the chemical bonds at the AlGaN surface caused poor C-V characteristics due to high-density interface states at the Al2O3/ICP-etched AGaN interface.
  • Yujin Hori, Chihoko Mizue, Tamotsu Hashizume
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 6 9 6 1356 - 1360 2012年 [査読有り][通常論文]
     
    We have characterized the interface states of Al2O3/GaN and Al2O3/AlGaN/GaN structures prepared by atomic layer deposition using the conventional and photo-assisted capacitance-voltage (C-V) measurements. In order to control the interface states, an N2O-radical treatment was applied to the GaN and AlGaN surfaces prior to the deposition of the Al2O3 layer. We observed good C-V behavior and a relatively low density of interface states for the N2O-radical treated Al2O3/GaN structure. To estimate the state density distributions at the Al2O3/AlGaN interface, we applied the photo-assisted C-V measurement to the Al2O3/AlGaN/GaN heterostructures. The present Al2O3/AlGaN structre showed higher interface state densities than the Al2O3/GaN structure. However, we found that the N2O-radical treatment is effective in reducing the density of interface states at the Al2O3/GaN and Al2O3/AlGaN systems.
  • R. Stoklas, D. Gregusova, J. Novak, P. Kordos, M. Tajima, T. Hashizume
    NINTH INTERNATIONAL CONFERENCE ON ADVANCED SEMICONDUCTOR DEVICES AND MICROSYSTEMS 37 - 40 2012年 [査読有り][通常論文]
     
    An impact of the different voltage ramps (from 25mV/s to 20V/s) in off-state regime for HFET and MOS-HFET was experimentally investigated. Because of the temperature in the channel drops at low voltage steps, density of temperatureactive interface states and states in the GaN buffer near the channel is reduced, and therefore the I-DS,I-off-state decreases. A positive temperature coeficient is attended. An another behaviour for MOS-HFET for fast traps in comparison to HFET was observed. A smaller temperature-dependence of fast traps in comparison to slow traps can be responsible for this effect. The same tendency of I-DS,I-off. state for all devices were found. The MOSHFET exhibited no change of drain current I-DS,I- off-state for 5 hours, but for the HFET the value was more than 500 times higher.
  • M. Matys, M. Miczek, B. Adamowicz, Z.R, Zytkiewicz, E. Kaminska, A. Piotrowska, T. Hashizume
    ACTA PHYSICA POLONICA A 120 6-A A-73  2011年11月 [査読有り][通常論文]
  • Toshiyuki Yoshida, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 7 070209.1-070209.3  2011年07月 [査読無し][通常論文]
     
    Surface state density distributions of p-InP surfaces before and after nitrogen (N(2))-radical exposure were characterized using an air-gap capacitance-voltage (C-V) technique. Before N(2)-radical exposure, a discrete level with a high density and extremely slow electron capture rate existed, causing strong Fermi level pinning. The peak density and full width at half maximum of this level were estimated. After N(2)-radical exposure, the density of the discrete level decreased to (5-6) x 10(12) cm(-2) eV(-1). On the other hand, the continuous level became to be observed. Furthermore, the electron capture rate of the discrete level became larger after the N(2)-radical exposure. (C) 2011 The Japan Society of Applied Physics
  • Marcin Miczek, Piotr Bidzinski, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume
    SOLID STATE COMMUNICATIONS 151 11 830 - 833 2011年06月 [査読無し][通常論文]
     
    An influence of electronic states at an insulator/GaN interface on the behavior of excess holes in an ultraviolet-illuminated metal/SiO2/n-GaN structure has been studied by numerical simulations for weak (gate bias of -0.1 V) and strong (-1 V) depletion, in a wide range of excitation light intensities (from 10(10) to 10(20) photons cm(-2) s(-1)) and for various bulk carrier lifetimes (from 1 to 100 ns). It has been found that the interface states with densities of 10(12) eV(-1) cm(-2) dramatically reduce the total (integrated in the whole GaN layer) density of photogenerated holes and thus degrade the sensitivity of the metal/insulator/GaN-based photodetector. (C) 2011 Elsevier Ltd. All rights reserved.
  • Masafumi Tajima, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 6 061001.1-061001.7  2011年06月 [査読無し][通常論文]
     
    Using a dual-gate structure, we have investigated the impact of gate-stress position on the current collapse behavior of AlGaN/GaN high-electron-mobility transistors (HEMTs) without surface passivation. When the gate-bias stress under the off state was applied to the additional gate between the main gate and the drain electrode, we observed a marked increase in on-resistance (RON). On the other hand, the off-state stress on the main gate itself caused a decrease in drain saturation current as well as an increase in RON. The calculation of electric field at the AlGaN surface showed that the field peaks existed at the gate edges on both the drain and source sides, probably causing electron charging at the AlGaN surface near both gate-edge areas. These results indicated that the off-state gate stress induces "virtual gates'' in the gate edges expanding in both the drain and source directions. The impacts of device structures on the current collapse have been characterized, using Schottky-gate HEMTs with and without surface passivation and metal-oxide-semiconductor (MOS) gate HEMTs. The surface passivation and MOS-gate structure was effective in mitigating current collapse, which was explained in terms of surface state density, electric field strength, and gate leakage current. (C) 2011 The Japan Society of Applied Physics
  • Chen Chen, Rui Jia, Haofeng Li, Yanlong Meng, Xinyu Liu, Tianchun Ye, Seiya Kasai, Hashizume Tamotsu, Nanjian Wu, Shanli Wang, Junhao Chu
    APPLIED PHYSICS LETTERS 98 14 143108  2011年04月 [査読無し][通常論文]
     
    In the case of the silicon (Si) nanowire (NW)-array-textured solar cells, the electrode-contact enhancement has been achieved using a simple and convenient double-step diffusion process to form a highly doped N+ region at the tips of a Si-NW array. The series resistance can be effectively reduced, leading to an increase in the short-circuit current density in the cell. We have studied the physical mechanism of the impact of an increase in doping level at the tips of a Si-NW array on the electrode-contact property, which would benefit in realizing an improvement in cell performance in such a nanostructure solar cell. (C) 2011 American Institute of Physics. [doi:10.1063/1.3576924]
  • Piotr Bidzinski, Marcin Miczek, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 4 04DF08.1-04DF08.8  2011年04月 [査読無し][通常論文]
     
    The influence of interface state density and bulk carrier lifetime on the dependencies of photocapacitance versus wide range of gate bias (-0.1 to -3 V) and light intensity (10(9) to 10(20) photon cm(-2) s(-1)) was studied for metal/insulator/n-GaN UV light photodetector by means of numerical simulations. The light detection limit and photocapacitance saturation were analyzed in terms of the interface charge and interface Fermi level for electrons and holes and effective interface recombination velocity. It was proven that the excess carrier recombination through interface states is the main reason of photocapacitance signal quenching. It was found that the photodetector can work in various modes (on-off or quantitative light measurement) adjusted by the gate bias. A comparison between experimental data and theoretical capacitance-light intensity characteristics was made. A new method for the determination of the interface state density distribution from capacitance-voltage-light intensity measurements was also proposed. (C) 2011 The Japan Society of Applied Physics
  • LI Weilong, JIA Rui, CHEN Chen, LI Haofeng, LIU Xinyu, YUE Huihui, DING Wuchang, YE Tianchun, KASAI Seiya, HASHIZUME Tamotsu, WU Nanjian, XU Bingshe
    J Vac Sci Technol B Nanotechnol Microelectron Mater Process Meas Phenom 29 2 021018  2011年03月 [査読無し][通常論文]
  • CHEN Chen, JIA Rui, YUE Huihui, LI Haofeng, LIU Xinyu, YE Tianchun, KASAI Seiya, TAMOTSU Hashizume, WU Nanjian, WANG Shanli, CHU Junhao, XU Bingshe
    J Vac Sci Technol B Nanotechnol Microelectron Mater Process Meas Phenom 29 2 021014  2011年03月 [査読無し][通常論文]
  • Eri Ogawa, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 2 021002.1-021002.6  2011年02月 [査読無し][通常論文]
     
    We characterize the chemical and photoluminescence (PL) properties of GaN with Mg doping densities of 7 x 10(17) and 2 x 10(19) cm(-3), particularly focusing on their variations caused by the high-temperature process. Before the thermal process, band-edge emission dominated the PL spectrum in the lightly Mg-doped GaN, whereas luminescence at 3.2 eV with a high intensity similar to that of the band-edge line was observed in the highly Mg-doped GaN. For the lightly Mg-doped GaN, the thermal process at 900 degrees C enhanced the intensity of the broad band in the energy range from 1.5 to 2.5 eV. Pronounced Ga outdiffusion caused by the high-temperature process was also confirmed by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy, indicating that these emission lines originate from the transitions between the conduction band and deep levels related to a defect complex involving a Ga vacancy (V-Ga + X). For the highly Mg-doped GaN, the peak energy of the dominant PL line abruptly changed from 3.2 to 2.8 eV after the process at 1000 degrees C. Simultaneously, a non uniform profile of Mg density was found near the GaN surface, arising from the pronounced surface segregation of Mg. It is thus likely that a defect complex involving a Mg interstitial and a Ga vacancy (Mg-I + V-Ga) is responsible for the 2.8 eV band in the highly Mg-doped and high-temperature-processed GaN. (C) 2011 The Japan Society of Applied Physics
  • Chihoko Mizue, Yujin Hori, Marcin Miczek, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 50 2 021001.1-021001.7  2011年02月 [査読無し][通常論文]
     
    The potential modulation and interface states of Al2O3/Al0.25Ga0.75N/GaN structures prepared by atomic layer deposition (ALD) were characterized by capacitance-voltage (C-V) measurements. We observed the peculiar C-V characteristics with two capacitance steps in forward and reverse bias regions, corresponding to the electron accumulation or depletion behavior at the Al2O3/AlGaN and AlGaN/GaN interfaces. From the experimental and calculated C-V characteristics, it was found that the charging and discharging of interface states near the AlGaN conduction-band edge mainly caused the stretch-out and hysteresis of the C-V curve at the forward bias. On the other hand, it is likely that the interface states near the midgap or deeper in energies act as fixed charges. From the bias-dependent hysteresis voltage in the forward bias region and the photo-induced voltage shift at the reverse bias, we estimated the interface state density distribution at the Al2O3/AlGaN interface for the first time. The present ALD-Al2O3/AlGaN/GaN structure showed relatively high interface state densities with a minimum density of 1 x 10(12) cm(-2) eV(-1) or higher. (C) 2011 The Japan Society of Applied Physics
  • Naohisa Harada, Yujin Hori, Naoki Azumaishi, Kota Ohi, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 4 2 021002.1-021002.3  2011年02月 [査読無し][通常論文]
     
    A selective electrochemical oxidation has been applied to the AlGaN surface to fabricate a recessed-oxide-gate structure for normally-off AlGaN/GaN high-electron-mobility transistors (HEMTs). We observed bias-dependent oxidation current characteristics peculiar to the AlGaN/GaN heterostructure. A flat interface between the oxide and AlGaN was confirmed by cross-sectional transmission electron microscopy. The selective formation of the recessed oxide allowed the local depletion of two-dimensional electron gas at the AlGaN/GaN interface and thus the achievement of normally-off operation. The recessed-oxide-gate HEMT with the oxide thickness of 20nm showed good gate control of drain current with the threshold voltage of +1.2 V. (C) 2011 The Japan Society of Applied Physics
  • Masayuki Okamoto, Genki Toyoda, Eiji Hiraki, Toshihiko Tanaka, Tamotsu Hashizume, Tetsu Kachi
    2011 IEEE ENERGY CONVERSION CONGRESS AND EXPOSITION (ECCE) 2011 Vol.3 1795 - 1800 2011年 [査読無し][通常論文]
     
    Wide-band-gap semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), are expected to be used as materials for new switching devices with low loss and high switching speed. The authors have recently developed a GaN-based high-electron-mobility-transistor (HEMT) for application to power electronics. In this paper we present models of a GaN-based transistor and Schottky barrier diode (SBD) for a simulation program with integrated circuit emphasis (SPICE). The results of the SPICE simulation show that the static and dynamic characteristics of GaN- based devices are precisely simulated by the SPICE models. The application of a GaN-based device to a single-phase AC-AC direct converter results in a reduction of power loss of over 30% compared with a silicon-based one.
  • Chen Chen, Rui Jia, Huihui Yue, Haofeng Li, Xinyu Liu, Tianchun Ye, Seiya Kasai, Hashizume Tamotsu, Nanjian Wu, Shanli Wang, Junhao Chu, Bingshe Xu
    Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics 29 2 2011年 [査読有り][通常論文]
     
    Silicon (Si) nanostructure solar cells have been synthesized using a nanowire (NW) array as the surface texturing. Optical-reflection measurement exhibits an excellent photon-harvesting property for the Si-NW-array texturization. Less than 2% reflection ratio at an 800 nm wavelength was achieved. Results show that an optimized 125×125 mm2 Si nanostructure solar cell with an excellent photon-harvesting property has a 35.4% higher energy-conversion efficiency than the c -Si solar cell due to its enhanced optical-absorption characteristics. However, for the nanostructured solar cells, the decrease in external quantum efficiencies in the short-wavelength region proves that the surface recombination plays a critical role in determining the final quantum-efficiency performance, indicating that optimum surface passivation was a prerequisite in high-efficiency Si nanostructure solar cells. © 2011 American Vacuum Society.
  • Weilong Li, Rui Jia, Chen Chen, Haofeng Li, Xinyu Liu, Huihui Yue, Wuchang Ding, Tianchun Ye, Seiya Kasai, Tamotsu Hashizume, Nanjian Wu, Bingshe Xu
    Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics 29 2 2011年 [査読有り][通常論文]
     
    Annealing thin films of silicon containing HfO2 films deposited by an electron-beam coevaporation produces silicon nanocrystals embedded in high- κ dielectric HfO2. Such films can be used to fabricate nonvolatile memory devices. By changing the Si content in the precursor HfSix O2 (x=1, 2, 3, or 4) film, the size and density of silicon nanocrystal could be controlled and high-density of silicon nanocrystals could be obtained. Transmission electron microscopy observations showed that the maximum density of silicon nanocrystals was as high as 1.3× 1013 cm-2 for HfSi4 O2 and the average nanocrystal diameter was 4.3 nm. The metal-oxide semiconductor capacitor memory structure with embedded silicon nanocrystals in HfSi4 O2 exhibited the largest memory window, 3.94 V under ±5 V sweep voltage. © 2011 American Vacuum Society.
  • Hidekazu Yamamoto, Tamotsu Hashizume
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 3 8 3 2011年 [査読有り][通常論文]
     
    Silicon wafers used as starting materials are significantly different in MOS-LSIs and power devices, because each device requires a specific structure. For power devices, FZ wafers or epitaxial wafers with a thick epitaxial layer are widely used. Epitaxial wafers are advantageous with respect to low breakdown voltage devices, and FZ wafers are advantageous for high breakdown voltage devices. The influence of COPs formed during CZ crystal growth on highly miniaturized MOS-LSIs is significant; however, COPs do not pose a problem to power devices prepared from FZ or epitaxial wafers. BMDs and slip dislocations have caused device failures on power devices where the device operation layer is deep and the process temperature is high. On the other hand, misfit dislocations which are generated with high concentration do not introduce device failures. Failure due to physical contamination is observed when Fe has introduced a failure to the power device. Cu/pit failures occur due to chemical contamination by Cu only for more highly miniaturized MOS-LSIs, but are not a problem for power devices whose miniaturization level is not required as that for MOS-LSIs. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • M. Akazawa, B. Gao, T. Hashizume, M. Hiroki, S. Yamahata, N. Shigekawa
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 7-8 8 7-8 2139 - 2141 2011年 [査読有り][通常論文]
     
    The polarization-induced electric field in ultrathin InxAl(1-x)N (0.17 <= x <= 0.30) layers on GaN was investigated by using X-ray photoelectron spectroscopy (XPS). The core-level energy position, E-CL, and the full width at half maximum (FWHM) of the Al2p, In4d, and In3d spectra from 2.5-nm-thick InAlN layers increased with the increase in the photoelectron exit angle (elevation angle). These increases were well reproduced with numerical calculations assuming polarization-induced internal fields combined with surface Fermi level pinning. The magnitudes of the internal field decreased as the In molar fraction increased. The Ga3d spectra from the host GaN layers markedly shifted by 530 meV depending on the molar fraction of InAlN layers, which was independent of the exit angle. This indicated that the Fermi level was unpinned at the interfaces, or GaN surfaces, and shifted due to the potential drop in the InAlN layers. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Chen Chen, Rui Jia, Huihui Yue, Haofeng Li, Xinyu Liu, Deqi Wu, Wuchang Ding, Tianchun Ye, Seiya Kasai, Hashizume Tamotsu, Junhao Chu, Shanli Wang
    JOURNAL OF APPLIED PHYSICS 108 9 094318  2010年11月 [査読無し][通常論文]
     
    In this paper, a vertical-aligned silicon nanowires (Si NWs) array has been synthesized and implemented to the Si NW-array-textured solar cells for photovoltaic application. The optical properties of a Si NWs array on both the plane and pyramid-array-textured substrates were examined in terms of optical reflection property. Less than 2% reflection ratio at 800 nm wavelength was achieved. Using leftover monocrystalline Si (c-Si) wafer (125 x 125 mm(2)), a 16.5% energy conversion efficiency, with 35.4% enhancement compared to the pyramid-array-textured c-Si solar cells, was made by the Si NW-array-textured solar cells due to their enhanced optical absorption characteristics. However, without SiNx passivation, the short circuit current reduced due to the increased surface recombination when using Si NWs array as surface texturing, indicating that an optimum surface passivation was prerequisite in high-efficiency Si NW-array-textured solar cells. (C) 2010 American Institute of Physics. [doi:10.1063/1.3493733]
  • Cheng-Yu Hu, Tamotsu Hashizume, Kota Ohi, Masafumi Tajima
    APPLIED PHYSICS LETTERS 97 22 222103  2010年11月 [査読無し][通常論文]
     
    An electric-field-dependent trapping effect was observed in our AlGaN/GaN transmission-line-model (TLM) structure. Therefore, we adopted a "gradual voltage stress" scheme to induce a similar trapping effect, that is, a similar sheet resistance variation (Delta R-SH) for all intervals of our AlGaN/GaN TLM structure. By measuring the TLM structure under "gradual voltage stress" (stressed TLM measurement), we investigated the trapping effects in gateless AlGaN/GaN heterojunction field-effect transistors. 10 nm of Al2O3 passivation film substantially decreased the voltage-stress-induced Delta R-SH, suggesting surface traps were involved. The feasibility and reproducibility of the stressed TLM measurement method was confirmed with repeated passivation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3506583]
  • Yujin Hori, Chihoko Mizue, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 49 8 080201.1-080201.3  2010年08月 [査読無し][通常論文]
     
    The effects of fabrication processes on the electrical properties of Al2O3/GaN structures prepared by atomic layer deposition were investigated "The annealing process at 800 degrees C for the formation of ohmic electrodes brought a large number of microcrystallization regions into the Al2O3 layer, causing a marked leakage in the current-voltage characteristics of the Al2O3/GaN structure The "ohmic-first" process with a SiN protection layer was thus applied to the GaN surface In this process, the amorphous phase in the atomic configuration of Al2O3 was maintained, leading to the I sufficient suppression of leakage current at the Al2O3/GaN interface In addition, the Al2O3/GaN structures showed good capacitance-voltage characteristics, resulting in low interface state densities of less than 1 x 10(12) cm(-2) eV(-1) (C) 2010 The Japan Society of Applied Physics
  • Taketomo Sato, Naoki Yoshizawa, Tamotsu Hashizume
    THIN SOLID FILMS 518 15 4399 - 4402 2010年05月 [査読無し][通常論文]
     
    Extremely low reflectance was obtained from InP porous nanostructures in UV, visible, and near-infrared ranges. Porous samples were electrochemically prepared on which 130-nm-diameter nanopores were formed in a straight, vertical direction and were laterally separated by 50-nm-thick InP nanowalls. The reflectance strongly depended on the surface morphology. The lowest reflectance of 0.1% in the visible light range was obtained after the irregular top layer had been completely removed. Superior photoelectrochemical properties were obtained on the InP porous structures clue to two unique features: the large surface area inside pores, and the large photon absorption enhanced on the low reflectance surface. (C) 2010 Elsevier B.V. All rights reserved.
  • Taketomo Sato, Akinori Mizohata, Tamotsu Hashizume
    JOURNAL OF THE ELECTROCHEMICAL SOCIETY 157 2 H165 - H169 2010年 [査読無し][通常論文]
     
    The electrochemical functionalization of n-type InP porous nanostructures and their feasibility for biochemical sensor applications were investigated. The porous structures have extremely large surface areas, i.e., over 10 m(2)/cm(3), and superior electrical properties with conductive semiconductor substrates. As a first attempt at electrochemical functionalization, we successfully deposited a glucose oxidase (GOD) membrane onto an InP surface under an applied anodic bias of 1.2 V. With the addition of glucose, the response currents on the porous electrodes increased compared to those on planar InP electrodes due to their enlarged surface area. The sensitivity curves of the porous electrodes we used showed good linearity between the response currents and concentrations in a range from 0 to 5 mM.
  • Toshiharu Kubo, Hiroyuki Taketomi, Hideto Miyake, Kazumasa Hiramatsu, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 3 2 021004.1-021004.3  2010年 [査読無し][通常論文]
     
    Chemical and electronic properties of Si-doped AlGaN surfaces with a wide range of Al compositions were investigated. The surface potential of AlGaN increased systematically from 0.9 (for GaN) to 2.6 eV (for Al0.87Ga0.13N), corresponding to the fact that the position of the surface Fermi level (E-Fsurf) approaches the midgap as the Al composition increases. In addition, the plots of EFsurf position closely follows the Fermi level stabilization energy reported by Walukiewicz et al. [J. Cryst. Growth 269 (2004) 119], indicating that the Fermi level position at the AlGaN surface can be governed by some kinds of surface defect transformations. (C) 2010 The Japan Society of Applied Physics DOI. 10.1143/APEX.3.021004
  • Kimihito Ooyama, Katsuya Sugawara, Shinya Okuzaki, Hiroyuki Taketomi, Hideto Miyake, Kazumasa Hiramatsu, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 49 10 101001.1-101001.5  2010年 [査読無し][通常論文]
     
    Deep electronic levels of AlxGa1-xN (0: 25 < x < 0: 60) were investigated by deep level transient spectroscopy (DLTS) and photocapacitance methods. Si-doped AlGaN layers were grown on an AlN/sapphire template by metal-organic vapor phase epitaxy. DLTS analysis using a sampling time window of up to 100 s showed two dominant deep levels with activation energies (Delta E) higher than 1.0 eV in AlxGa1-xN with x = 0.5 and 0.37. The densities of those levels were higher than 1 x 10(16) cm(-3). For the Al0.60Ga0.40N sample, the deeper levels (Delta E > 1. eV) were detected by photocapacitance measurement. It was found that the energy position of the dominant deep level closely followed the Fermi level stabilization energy reported by Walukiewicz et al. [J. Cryst. Growth 269 (2004) 119], indicating that the origin of the dominant deep level in AlGaN is related to a defect complex including anti-site defects and divacancies. (C) 2010 The Japan Society of Applied Physics
  • Toshiyuki Yoshida, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 3 11 116601.1-116601.3  2010年 [査読無し][通常論文]
     
    Air-gap capacitance-voltage (C-V) characteristics of p-InP surfaces covered with natural oxide and treated with hydrogen-fluoride (HF) acid are reported for the first time. These surfaces show deep-depletion C-V behavior with large and distinctive hysteresis. To interpret the obtained C-V curves, a model was used that introduced a donor-like discrete level in the band-gap. Another assumption in this model is that the capture velocity of the discrete level and/or the generation-recombination rate of the minority carrier of the substrate is very low. By use of this model, the measured air-gap C-V curve was successfully reproduced. (c) 2010 The Japan Society of Applied Physics
  • T. Sato, N. Yoshizawa, H. Okazaki, T. Hashizume
    ECS Transactions 25 42 83 - 88 2010年 [査読有り][通常論文]
     
    Extremely low reflectance was obtained from InP porous nanostructures in UV, visible, and near-infrared light ranges. The reflectance strongly depended on the surface morphology of the porous structures prepared by the electrochemical process, and the lowest reflectance of 0.1% in the visible light range was obtained from a sample after the irregular top layer was completely removed. Large anodic photocurrents were obtained on the InP porous structures that had low reflectance surfaces with deeper pores. ©The Electrochemical Society.
  • R. Stoklas, D. Gregušová, M. Blaho, P. Kordoš, M. Tajima, T. Hashizume
    Conference Proceedings - The 8th International Conference on Advanced Semiconductor Devices and Microsystems, ASDAM 2010 155 - 158 2010年 [査読有り][通常論文]
     
    Atomic layer deposition (ALD) technique at 300 °C was used to prepare an Al2O3 dielectric layer, to form MOS-HFETs. The static (output and transfer) and dynamic (Capacitance-Voltage) characteristics were used for evaluation of investigated devices. From the static characteristic, an increase of the saturation drain current (up to 35%) and extrinsic transconductance (up to 10%) of the MOS-HFETs were observed. Higher nS on the MOS-structure, evaluated from the C-V measurement, can be responsible for these effects. The gate leakage current was also reduced about four orders of magnitude in comparison to the HFET. ©2010 IEEE.
  • Toshiyuki Yoshida, Tamotsu Hashizume
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 366 - 368 2010年 [査読有り][通常論文]
     
    A novel air-gap capacitance-Voltage (C-V) method is introduced. This method is powerful tool for characterizing electrical properties of "free" or ultrathin-insulator-covered surfaces for III-V semiconductors. Air-gap C-V characteristics of p-InP surfaces with and without natural oxide are reported for the first time. Unexpectedly, the surface with natural oxide gives relatively low-density surface states. Surfaces covered with ultrathin Al 2O3 film were also measured, indicating the importance for further investigation and optimization of surface process.
  • Hiroyuki Okazaki, Taketomo Sato, Naoki Yoshizawa, Tamotsu Hashizume
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 77 - 80 2010年 [査読有り][通常論文]
     
    We demonstrated to form InP porous structures on n-type epitaxial layers grown on p-type (001) substrates. The high-density array of straight pores with 150nm diameter and 5000nm depth was formed by the electrochemical anodization process, where the pore depth could be controlled by the anodization time in the n-type layer. The present p-n InP porous structures show the low optical reflectance in UV-, visible- and near-infrared region. The current transport properties clearly show the rectifying behavior. These features are very promising for practical application to high-efficiency photo-sensitive devices.
  • Takaaki Takahara, Masayuki Okamoto, Eiji Hiraki, Toshihiko Tanaka, Tamotsu Hashizume, Tetsu Kachi
    PROCEEDINGS OF 14TH INTERNATIONAL POWER ELECTRONICS AND MOTION CONTROL CONFERENCE (EPE-PEMC 2010) 2010年 [査読有り][通常論文]
     
    This paper presents performance characteristics of a novel soft switching three-phase utility frequency AC (UFAC) to high frequency AC (HFAC) direct power converter with PFC function for industrial induction heating (IH) applications. The proposed "simple three-phase system" consists of a single-phase UFAC to HFAC direct converter unit in each phase, three-phase low pass filter and three-phase IH load. This power converter can regulate the output power under a condition of the constant frequency zero voltage soft switching (ZVS) commutation principle on the basis of unique asymmetrical PWM control scheme. In this paper, high frequency IH output and utility AC current characteristics of three-phase direct converter supposed 5kW class IH system are demonstrated and evaluated from computer aided simulation with PSIM software. Furthermore, basic performance analysis of the single-phase converter unit is evaluated from an experimental point of view.
  • Chen Chen, Rui Jia, Weilong Li, Haofeng Li, Tianchun Ye, Xinyu Liu, Ming Liu, Seiya Kasai, Hashizume Tamotsu, Nanjian Wu
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 27 6 2462 - 2467 2009年11月 [査読無し][通常論文]
     
    In this article, a simple and flexible electron-beam coevaporation (EBCE) technique has been reported of fabrication of the silicon nanocrystals (Si NCs) and their application to the nonvolatile memory. For EBCE, the Si and SiOx(x=1 or 2) were used as source materials. Transmission electron microscopy images and Raman spectra measurement verified the formation of the Si NCs. The average size and area density of the Si NCs can be adjusted by increasing the Si:O weight ratio in source material, which has a great impact on the crystalline volume fraction of the deposited film and on the charge storage characteristics of the Si NCs. A memory window as large as 6.6 V under +/- 8 V sweep voltage was observed for the metal-oxide-semiconductor capacitor structure with the embedded Si NCs.
  • Naoki Yoshizawa, Taketomo Sato, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 48 9 091102.1-091102.4  2009年09月 [査読無し][通常論文]
     
    We prove with this paper that InP-based open-gate field-effect transistors (FETs) work well as liquid-phase chemical sensors. The open-gate FET clearly exhibited current saturation and a pinch-off behavior in the electrolyte, resulting in a rapid response to the gate bias applied via the electrolyte. A series of sensing measurements showed that the surface potential of the InP linearly changed with the pH values of the electrolytes in a pH range from 3.0 to 12.0. The pH sensitivity of the open-gate FETs depended on the ion species contained in the electrolyte. A Si3N4 layer was useful as an ion selective membrane for the InP open-gate FETs to improve the selectivity of H+ ions. (C) 2009 The Japan Society of Applied Physics
  • Kota Ohi, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 48 8 081002.1-081002.5  2009年08月 [査読無し][通常論文]
     
    The surrounding-field effect in a multi-mesa-channel (MMC) with an AlGaN/GaN structure, in which a periodic trench structure is fabricated directly under a gate electrode, was successfully observed. This effect resulted in a shallower threshold voltage, a smaller subthreshold slope, and a higher current drivability of a high electron mobility transistor (HEMT) than those of a standard planar-type HEMT. In addition, the MMC HEMT showed a low knee voltage, even with a wide spacing between the gate and drain electrodes. Excellent current, stability in the saturation region of the MMC HEMT, probably due to the effective radiation of heat from both mesa sides of the channel, was also observed. Both planar and MMC HEMTs showed similar breakdown voltages under off-state operation, indicating no significant degradation in the breakdown characteristics of AlGaN/GaN HEMTs with a periodic trench structure in the gate region. (C) 2009 The Japan Society of Applied Physics
  • Katsuya Sugawara, Junji Kotani, Tamotsu Hashizume
    APPLIED PHYSICS LETTERS 94 15 152106  2009年04月 [査読無し][通常論文]
     
    A deep level with an activation energy of 1.0 eV in n-type Al0.26Ga0.74N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy (DLTS) with a sampling time window of several seconds. The deep-level density was 6x10(15) cm(-3). At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from E-C-1.5 to E-C-2.3 eV.
  • Marcin Miczek, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 48 4 04C092.1-04C092.6  2009年04月 [査読無し][通常論文]
     
    The effect of electronic states at insulator/AlGaN and AlGaN/GaN interfaces on capacitance-voltage (C-V) characteristics of a metal/insulator/semiconductor (MIS) capacitor with an AlGaN layer and a metal/insulator/semiconductor heterostructure (MISH) with an AlGaN/GaN heterojunction was studied theoretically taking into account extremely slow electron emission from the deep interface levels. The states at the insulator/AlGaN interface in MIS and at the AlGaN/GaN interface in MISH stretch out the C-V curves as usual, whereas the insulator/AlGaN interface states in MISH only shift the characteristics at room temperature and 300 degrees C (fixed-charge-like behavior). These effects were explained by the different positions of the Fermi level at the studied interfaces. Theoretical C-V curves were compared with experimental characteristics, which are available in the literature, measured at room and higher temperatures by the authors and others. (c) 2009 The Japan Society of Applied Physics
  • Nanako Shiozaki, Tamotsu Hashizume
    JOURNAL OF APPLIED PHYSICS 105 6 064912  2009年03月 [査読無し][通常論文]
     
    Surface control of n-GaN was performed by applying a photoelectrochemical oxidation method in a glycol solution to improve the optical and electronic characteristics. The fundamental properties of the oxidation were investigated. The oxidation, chemical composition, and bonding states were analyzed by x-ray photoelectron spectroscopy and micro-Auger electron spectroscopy, in which confirmed the formation of gallium oxide on the surface. The oxide formation rate was about 8 nm/min under UV illumination of 4 mW/cm(2). After establishing the basic properties for control of n-GaN oxidation, the surface control technique was applied to achieve low-damage etching, enhancement of the photoluminescence intensity, and selective passivation of the air-exposed sidewalls in an AlGaN/GaN high electron mobility transistor wire structure. The capacitance-voltage measurement revealed the minimum interface-state density between GaN and anodic oxide to be about 5x10(11) cm(-2) eV(-1), which is rather low value for compound semiconductors.
  • Masafumi Tajima, Junji Kotani, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 48 2 020203.1-020203.3  2009年02月 [査読無し][通常論文]
     
    We have investigated the effects of thin native oxide layers on the AlGaN surface on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). After HEIVIT fabrication, the AlGaN surface between the electrodes was intentionally oxidized using either O(2) or N(2)O plasma. X-ray photoelectron spectroscopy (XPS) analysis showed that both methods produced a native oxide of AlGaN with a thickness of about 1 nm and that an N-O chemical bond was present in the N(2)O plasma oxide. We observed pronounced degradation in the DC characteristics and current collapse in the HEMT with O(2) plasma oxidation. In contrast, the formation of native oxide by the N(2)O plasma had no effect on the DC characteristics or current stability of the AIGaN/GaN HEMT. Possible mechanisms for device degradation were discussed in terms of stress and deep levels in the AlGaN layer induced by oxide formation or oxygen incorporation. (C) 2009 The Japan Society of Applied Physics
  • Chihoko Mizue, Marcin Miczek, Junji Kotani, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 48 2 020201.1-020201.3  2009年02月 [査読無し][通常論文]
     
    We performed a feasibility study of UV detection using an AlOx/n-GaN structure. The AlOx layer was deposited on n-GaN using diethyl aluminum ethoxide as a precursor. We obtained a refractive index of 1.59, a bandgap of 7.0eV, and an oxygen composition of 1.48 for the AlOx layer. Even at higher temperatures, no significant change appeared in the capacitance-voltage curves, which were close to the ideal curve, indicating well-controlled and stable interface properties. From the photo response measurements, pronounced variation of the interface potential at the AlOx/GaN interface was observed under UV illumination, which was independent of temperature. The gate bias systematically modulated the depth of the interface potential well and thereby the density of accumulated holes resulting from UV illumination. The AlOx/GaN structure is sensitive to flame with a low power density. (C) 2009 The Japan Society of Applied Physics
  • Tamotsu Hashizume, Nanako Shiozaki, Kota Ohi
    GALLIUM NITRIDE MATERIALS AND DEVICES IV 7216 72160U.1-72160U.8  2009年 [査読無し][通常論文]
     
    Surface characterization and control technologies were applied to GaN and AlGaN surfaces. It was found that a unique "air-gap CV" technique is effective in evaluating surface state density on free AlGaN surfaces. A photoelectrochemical process, utilizing a mixed solution of propylene glycol and tartaric acid, was employed to form a thin oxide layer on GaN and AlGaN. We observed an enhancement of drain current in the AlGaN/GaN HEMT having a narrow channel width of 500 nm after the oxidation of the channel walls by the electrochemical process. To improve the uniformity of the effective electric field in the channel, a multi-mesa-channel (MMC) AlGaN/GaN HEMT has been proposed and developed. With forming a periodic trench just under the gate region by an ECR-plasma assisted dry etching, the MMC HEMT has parallel mesa-shaped channels with 2-dimensional electron gas (2DEG) surrounded by the top-and side-gate electrodes.
  • Takeshi Kimura, Tamotsu Hashizume
    JOURNAL OF APPLIED PHYSICS 105 1 014503  2009年01月 [査読無し][通常論文]
     
    We intentionally incorporated carbon into n-GaN by high-temperature annealing of a SiN(x)/CN(x)/GaN structure to study the effect of unintentional carbon incorporation on the electrical properties of n-type GaN surfaces. X-ray photoelectron spectroscopy results showed outdiffusion of Ga atoms from the GaN surface during high-temperature annealing even when the SiN(x) layer was present. The current-voltage characteristics showed a drastic increase in current in the forward and reverse directions of the Schottky diode in the carbon-incorporated sample. They also showed no temperature dependence from 150 to 300 K. The current-voltage curves of the carbon- incorporated samples in the forward and reverse directions could be almost completely reproduced by assuming an exponentially decaying distribution from the surface for shallow donors. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056395]
  • Abdul Manaf Hashim, Zon Fazlila Mohd Ahir, Seiya Kasai, Tamotsu Hashizume, Hideki Hasegawa
    AIP Conference Proceedings 1150 311 - 323 2009年 [査読有り][通常論文]
     
    A theoretical three-dimensional transverse magnetic (TM) mode analysis to describe the presence of interactions between surface plasma waves of carriers in a two-dimensional electron gas (2DEG) at AlGaAs/GaAs heterostructure and electromagnetic space harmonics slow waves using the so-called interdigital-gated high-electron-mobility-transistor (HEMT) plasma wave devices is presented. First, the device structure and the outline of theoretical formulation to determine the admittance of the interdigital structure are described. Then, the analysis of the space harmonics propagating through the interdigital slow-wave structures is performed. Next, the numerical procedures to solve the integral equations which are used in determining the admittance are explained. Finally, we point out and discuss the main results of the theoretical analysis where an appearance of negative conductance is obtained. © 2009 American Institute of Physics.
  • Wan-Cheng Zhang, Nan-Jian Wu, Tamotsu Hashizume, Seiya Kasai
    Proceedings of The International Symposium on Multiple-Valued Logic 337 - 342 2009年 [査読有り][通常論文]
     
    This paper proposes novel multiple-valued (MV)logic gates by using asymmetric single-electron transistors (SETs). Asymmetric single-electron transistors have two tunneling junctions with largely different resistances and capacitances. We fully exploited the unique Coulomb staircase characteristic of asymmetric SETs to compactly finish logic operations. We build MV literal gates with wide range of radixes by using a pair of asymmetric SETs. We showed that, arbitrary radix-4 literal gate can be realized using a pair of asymmetric SETs. We also proposed MV analog-digital conversion circuits. The MV logic gates have very compact structures and low power dissipation. © 2009 IEEE.
  • Hong-Quan Zhao, Seiya Kasai, Yuta Shiratori, Tamotsu Hashizume
    Nanotechnology 20 24 2009年 [査読有り][通常論文]
     
    A two-bit arithmetic logic unit (ALU) was successfully fabricated on a GaAs-based regular nanowire network with hexagonal topology. This fundamental building block of central processing units can be implemented on a regular nanowire network structure with simple circuit architecture based on graphical representation of logic functions using a binary decision diagram and topology control of the graph. The four-instruction ALU was designed by integrating subgraphs representing each instruction, and the circuitry was implemented by transferring the logical graph structure to a GaAs-based nanowire network formed by electron beam lithography and wet chemical etching. A path switching function was implemented in nodes by Schottky wrap gate control of nanowires. The fabricated circuit integrating 32node devices exhibits the correct output waveforms at room temperature allowing for threshold voltage variation. © 2009 IOP Publishing Ltd.
  • Tsutomu Uesugi, Tetsu Kachi, Masahiro Sugimoto, Tetsuya Matsuyama, Chihoko Mizue, Tamotsu Hashizume
    JOURNAL OF APPLIED PHYSICS 104 1 016103  2008年07月 [査読無し][通常論文]
     
    We performed a feasibility study on the deposition of aluminum oxides (AlOx) on GaN using diethyl aluminum ethoxide [(C2H5)(2)AlOC2H5] as a precursor by electron-cyclotron-resonance-assisted chemical vapor deposition. We determined the refractive index and permittivity of the deposited AlOx layer to be 1.59 and 8.3, respectively. An x-ray photoelectron spectroscopy (XPS) study showed that the energy positions of the Al- and O-core levels were very close to those of the reference crystalline Al2O3 and that there was no significant peak related to carbon in the AlOx film. XPS also indicated a bandgap of 7.0 eV and oxygen composition of 1.48 for the AlOx layer deposited on GaN. We found good capacitance-voltage (C-V) behavior for the Ni/AlOx/n-GaN diode including accumulation and depletion behavior at room temperature. Even at high temperatures, the C-V slope remained unchanged indicating relatively low interface state densities near the midgap. (c) 2008 American Institute of Physics.
  • Kimihito Ooyama, Hiroki Kato, Marcin Miczek, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS 47 7 5426 - 5428 2008年07月 [査読無し][通常論文]
     
    With a combination of the static capacitance-voltage (C-V) and the capacitance transient (C-t) methods. the interface-state response in in Al2O3/n-GaN structure was investigated at temperatures ranging from 23 to 300 degrees C. We observed pronounced degradation of the static C-V curves measured at high temperatures. arising front the enhancement of charging/discharging rates of interface states, at deeper energies within the bandgap of GaN. Faster responses with larger magnitudes also appeared in the time-dependent capacitance at high temperatures. From a simple analysis of the C-t results, we estimated the capture cross section of the states to be on the order 10(-19) cm(2).
  • Marcin Miczek, Chihoko Mizue, Tamotsu Hashizume, Boguslawa Adamowicz
    JOURNAL OF APPLIED PHYSICS 103 10 104510  2008年05月 [査読無し][通常論文]
     
    The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density D-it(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300 degrees C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure measured at room temperature and 300 degrees C, and extracted a part of D-it(E). The relatively low D-it (similar to 10(11) eV(-1) cm(-2)) in the upper bandgap indicates that the SiNx/Al2O3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices. (C) 2008 American Institute of Physics.
  • Taketomo Sato, Akinori Mizohata, Naoki Yoshizawa, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 1 5 051202.1-051202.3  2008年05月 [査読無し][通常論文]
     
    The electrocatalytic activity of n-type InP porous nanostructures was investigated in terms of their application to amperometric biochemical sensors. The current sensitivities for H(2)O(2) detection were strongly dependent on the structural properties of these porous nanostructures. A sample with deeper pores responsed more sensitively because of the enlarged surface area inside the nanopores. The removal of an irregular top layer also effectively improved the current sensitivity. The conductive porous nanostructures presented here were very promising for the direct amperometric detection of H(2)O(2). (c) 2008 The Japan Society of Applied Physics.
  • Takahiro Tamura, Junji Kotani, Seiya Kasai, Tamotsu Hashizume
    APPLIED PHYSICS EXPRESS 1 2 023001.1-023001.3  2008年02月 [査読無し][通常論文]
     
    We fabricated a multi-mesa-channel (MMC) structure by forming a periodic trench just under a gate electrode to improve the uniformity of effective electric field in the channel in an AlGaN/GaN high electron mobility transistor (HEMT). A unique performance, i.e., a nearly temperature-independent saturation drain current, was observed in the MMC device in a wide temperature range. A two-dimensional (2D) potential calculation indicates that the mesa-side gate effectively modulates the potential, resulting in a field surrounding 2D electron gas. Such a surrounding-field effect and a relatively lower source access resistance may be related to a unique current behavior in the MMC HEMT. (C) 2008 The Japan Society of Applied Physics.
  • Hong-Quan Zhao, Seiya Kasai, Tamotsu Hashizume, Nan-Jian Wu
    IEICE Transactions on Electronics E91-C 7 1063 - 1069 2008年 [査読有り][通常論文]
     
    For realization of hexagonal BDD-based digital systems, active and sequential circuits including inverters, flip flops and ring oscillators are designed and fabricated on GaAs-based hexagonal nanowire networks controlled by Schottky wrap gates (WPGs), and their operations are characterized. Fabricated inverters show comparatively high transfer gain of more than 10. Clear and correct operation of hexagonal set-reset flip flops (SR-FFs) is obtained at room temperature. Fabricated hexagonal Dtype flip flop (D-FF) circuits integrating twelve WPG field effect transistors (FETs) show capturing input signal by triggering although the output swing is small. Oscillatory output is successfully obtained in a fabricated 7-stage hexagonal ring oscillator. Obtained results confirm that a good possibility to realize practical digital systems can be implemented by the present circuit approach. Copyright © 2008 The Institute of Electronics, Information and Communication Engineers.
  • Tamotsu Hashizume
    Device Research Conference - Conference Digest, DRC 283 - 284 2008年 [査読有り][通常論文]
  • Taketomo Sato, Akinori Mizohata, Naoki Yoshizawa, Tamotsu Hashizume
    ECS Transactions 16 3 405 - 410 2008年 [査読有り][通常論文]
     
    The photo-electrochemical (PEC) process was developed to remove the irregular top layer of InP porous nanostructures for the application to amperometric chemical sensors. The PEC etching rate of the pore surface was associated with their structural properties, resulting in the complete removal of the irregular layer by monitoring and controlling the anodic photo-currents. The porous electrode prepared by PEC process gave higher sensitivity for detecting H2O2 due to the enlarged openings of all pores with complete removal of irregular top layer. ©The Electrochemical Society.
  • Tamotsu Hashizume
    ASDAM 2008, CONFERENCE PROCEEDINGS 17 - 22 2008年 [査読有り][通常論文]
  • Taketomo Sato, Akinori Mizohata, Toshiyuki Fujino, Tamotsu Hashizume
    Physica Status Solidi (C) Current Topics in Solid State Physics 5 11 3475 - 3478 2008年 [査読有り][通常論文]
     
    In this paper, we report the electrochemical formation of the InP porous nanostructures and their feasibility for the application to the amperometric chemical sensors. Our two step electrochemical process consists of the pore formation on a (001) n-type InP substrate and the subsequent etching of pore walls caused by changing the polarity of the InP electrode in a HCl-based electrolyte. By applying the anodic bias to the InP electrode, the high-density array of uniform nanopores was formed on the surface. Next, the cathodic bias was applied to the porous sample to reduce the wall thickness by cathodic decomposition of InP, where the thickness of InP nanowall decreased uniformly along the entire depth of the porous layer. From the amperometric measurements of the porous electrode, it was found that the electrocatalytic activity was much higher than that of the planar electrode. Furthermore, the current sensitivity for the H2O2 detection was much enhanced after the cathodic decomposition process. The InP porous nanostructure formed by the present process is one of the promising structures for the application to the semiconductor-based bio/chemical sensors. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
  • Abdul Manaf Hashim, Seiya Kasai, Tamotsu Hashizume, Hideki Hasegawa
    Microelectronics Journal 38 12 1263 - 1267 2007年12月 [査読有り][通常論文]
     
    Interdigital-gated AlGaAs/GaAs high-electron-mobility transistor (HEMT) structure was used to investigate the interaction between the drifting carrier plasma waves and electromagnetic (EM) waves. It was shown theoretically that the interaction in the range from microwave to terahertz (THz) at room temperature should produce negative conductance characteristics when the carrier drift velocity slightly exceeds the phase velocity of EM waves. S-parameter reflection measurements were carried out at room temperature for a frequency range from 1 to 20 GHz and a drastic change in conductance was observed at 5 and 10 GHz with the increase of drain-source voltage. Large conductance change over 1000 mS/mm was obtained and it showed a peak at a certain frequency. The peak position could be controlled by changing the pitch size of the interdigital gates. These characteristics can be used for high-frequency applications such as high-speed switching devices although a feature size of our interdigital-gated HEMT device is much larger than conventional HEMT device. © 2007 Elsevier Ltd. All rights reserved.
  • Abdul Manaf Hashim, Seiya Kasai, Kouichi Iizuka, Tamotsu Hashizume, Hideki Hasegawa
    Microelectronics Journal 38 12 1268 - 1272 2007年12月 [査読有り][通常論文]
     
    Theoretical analysis of potential distribution in the interdigital-gated high electron mobility transistor (HEMT) plasma wave device was carried out. The dc I-V characteristics of capacitively coupled interdigital structure showed that uniformity of electric field under the interdigital gates was improved compared to the dc-connected interdigital gate structure. Admittance measurements of capacitively coupled interdigital gate structure in the microwave region of 10-40 GHz showed the conductance modulation by drain-source voltage. These results indicate the existence of plasma wave interactions. © 2007 Elsevier Ltd. All rights reserved.
  • Junji Kotani, Masafumi Tajima, Seiya Kasai, Tamotsu Hashizume
    APPLIED PHYSICS LETTERS 91 9 093501-1-093501-3  2007年08月 [査読無し][通常論文]
     
    Lateral surface leakage current (I-S) on an AlGaN/GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (L-GG) of 200 nm-5 mu m. The surface current I-s systematically increased as LGG decreased. A simple resistive layer conduction that should show 1/L-GG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1 mu m. However, no dependence on L-GG was seen in vertical current that flows in the Schottky interface. The I-s showed a clear temperature dependence proportional to exp(-T (-1/ 3)), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation. (c) 2007 American Institute of Physics.
  • Eri Ogawa, Tamotsu Hashizume, Satoshi Nakazawa, Tetsuzo Ueda, Tsuyoshi Tanaka
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 20-24 L590 - L592 2007年06月 [査読無し][通常論文]
     
    We investigate the chemical and potential-bending characteristics of in situ SiNx/AlGaN interfaces prepared by metal-organic chemical vapor deposition. X-ray photoelectron spectroscopy showed that the in situ SiNx layer had typical chemical binding energies corresponding to the Si-N bonds. The in situ SiNx deposition brought no chemical degradation on the AlGaN surface at the SiNx/AlGaN interface, whereas the ex situ deposition of SiNx by a plasma process induced chemical disorder on the AlGaN surface including a composition change and the formation of interfacial oxides. A significant reduction in the surface band bending was observed on the AlGaN surface after the in situ SiNx passivation, probably due to a decrease in the surface state density.
  • Nanako Shiozaki, Taketomo Sato, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 46 4A 1471 - 1473 2007年04月 [査読無し][通常論文]
     
    In this paper, we report on the feasibility of oxidizing n-GaN by an electrochemical process in a mixture containing an aqueous solution of tartaric a id and propylene glycol. Photons generated by UV illumination were supplied at the electrolyte-GaN interface during the oxidation process. In the constant-voltage mode, X-ray photoelectron spectroscopy analysis revealed that relatively thick Ga oxide layer formed on the n-GaN surface. However, the oxide surface was very rough. In addition, we found metallic Ga components in the oxide layer or at the oxide-GaN interface for longer oxidation times. On the other hand, a thin Ga2O3 layer with a smooth surface was grown by a constant-current process.
  • Yuki Hayakawa, Yoshiyuki Suda, Tamotsu Hashizume, Hirotake Sugawara, Yosuke Sakai
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 12-16 L362 - L364 2007年04月 [査読無し][通常論文]
     
    Hydrogen gas sensors based on single-walled carbon nanotube (SWCNT) films were fabricated with Pd comb-like electrodes. The SWCNT films were deposited by Mo/Co catalyst-assisted chemical vapor deposition using alcohol. The film included a high-density network of SWCNTs. The SWCNT film sensor showed a large conductance change and a fast response (20s) upon exposure to 1% H-2 gas in air at room temperature, while the conductance change was negligible for devices using Au electrodes.
  • ZHANG Wancheng, WU Nan‐Jian, HASHIZUME Tamotsu, KASAI Seiya
    IEEE Trans Nanotechnol 6 2 146 - 157 2007年03月 [査読無し][通常論文]
  • Takeshi Kimura, Shinya Ootomo, Takehiko Nomura, Seikoh Yoshida, Tamotsu Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 8-11 L224 - L226 2007年03月 [査読無し][通常論文]
     
    We performed a feasibility study on the solid-phase diffusion of carbon into GaN using a SiNx/CNx/GaN structure prepared by electron-cyclotron-resonance-assisted chemical vapor deposition. An X-ray photoelectron spectroscopy study on the CNx layer deposited on GaN showed that its energy positions and spectrum features are very close to those of a C-N bond, and the N composition was estimated to be 24%, indicating a highly C-rich layer. No degradation in the chemical properties of the GaN surface was found after the diffusion process at 1000 degrees C. A secondary ion mass spectrometry result clearly showed a diffusion of carbon into GaN. We also observed an increase in resistivity for the C-diffused GaN layer.
  • Abdul Manaf Hashim, Seiya Kasai, Kouichi Iizuka, Tamotsu Hashizume, Hideki Hasegawa
    AIP Conference Proceedings 909 180 - 185 2007年 [査読有り][通常論文]
     
    Theoretical analysis of potential distribution in the interdigital-gated high electron mobility transistor (HEMT) plasma wave device was carried out. The dc I-V characteristics of capacitively coupled interdigital structure showed that uniformity of electric field under the interdigital gates was improved compared to the dc connected interdigital gate structure. Admittance measurements of capacitively coupled interdigital gate structure in the microwave region of 10-40 GHz showed the conductance modulation by drain-source voltage. These results indicate the existence of plasma wave interactions. © 2007 American Institute of Physics.
  • Kokawa Takuya, Sato Taketomo, Hasegawa Hideki, Hashizume Tamotsu
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 24 4 1972 - 1976 2006年07月25日 [査読有り][通常論文]
     
    Liquid-phase sensing characteristics of open-gate AlGaN/GaN high electron mobility transistor (HEMT) structures were investigated in aqueous solutions and polar liquids. In de-ionized water, the open-gate HEMT clearly showed good drain I-V characteristics with current saturation and pinch-off behavior, very similar to I-V characteristics of typical Schottky-gate HEMTs. We observed a fine parallel shift in the transfer curves according to change in the pH value in a solution, indicating the corresponding potential change at the AlGaN surface. The sensitivity for the potential change was 57.5...
  • Taketomo Sato, Takeshi Oikawa, Hideki Hasegawa, Tamotsu Hashizume
    Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 24 4 2087 - 2092 2006年07月 [査読有り][通常論文]
     
    Fundamental growth properties were investigated for the size-controlled selective MBE growth of AlGaN/GaN nanowires on the GaN (0001) prepatterned substrates both experimentally and theoretically. The lateral size of the present GaN nanowire was determined by two facet boundaries formed within AlGaN barrier layers. From the series of wire growth experiments, the growth selectivity and the measured angle of the facet boundary strongly depended on the Al composition and the initial crystalline facets of the mesa patterned templates. The experimental evolution of the cross-sectional structures was well reproduced by a computer simulation based on the phenomenological growth model where the slope angle dependence of lifetime of adatoms was taken into account. The lateral width of present nanowires could be kinetically controlled by the growth conditions and the supply thickness of AlGaN layers. © 2006 American Vacuum Society.
  • Takuya Kokawa, Taketomo Sato, Hideki Hasegawa, Tamotsu Hashizume
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 24 4 1972 - 1976 2006年07月 [査読有り][通常論文]
     
    Liquid-phase sensing characteristics of open-gate AlGaN/GaN high electron mobility transistor (HEMT) structures were investigated in aqueous solutions and polar liquids. In de-ionized water, the open-gate HEMT clearly showed good drain I-V characteristics with current saturation and pinch-off behavior, very similar to I-V characteristics of typical Schottky-gate HEMTs. We observed a fine parallel shift in the transfer curves according to change in the pH value in a solution, indicating the corresponding potential change at the AlGaN surface. The sensitivity for the potential change was 57.5 mV/pH, very close to the theoretical value of 58.9 mV/pH at 24 degrees C for the Nernstian response to, H+ ions. In the low drain bias region, the drain current linearly decreased with the pH value. This also indicated a systematic potential change at the AlGaN surface due to pH change. The present open-gate device showed a fast response to the pH change and a stable operation at fixed pH values. A possible mechanism for the pH response of the AlGaN surface is discussed in terms of equilibrium reactions of hydroxyls at the AlGaN surface with H+ in a solution. It was also found that the device was quite sensitive to changes in the electrostatic boundary conditions of the open-gate area by exposure to polar liquids. The drain current linearly decreased with increasing normalized liquid dipole moment. (c) 2006 American Vacuum Society.
  • Hideki Hasegawa, Taketomo Sato, Seiya Kasai, Boguslawa Adamowicz, Tamotsu Hashizume
    Solar Energy 80 6 629 - 644 2006年06月 [査読有り][通常論文]
     
    Characterization methods and fundamental aspects of surface/interface states and recombination process in Si and III-V materials are reviewed. Various measurement considerations are pointed out for the conventional metal-insulator-semiconductor (MIS) capacitance-voltage (C-V) method, a contactless C-V method, and the microscopic scanning tunneling spectroscopy (STS) method, and general features of surface states are discussed. Surface states are shown to have U-shaped distributions of donor-acceptor continuum with a characteristic charge neutrality level, EHO. Rigorous simulation of dynamics of surface recombination process has shown that the effective surface recombination velocity, Seff, is not a constant of the surface, but its value changes by many orders of magnitude with the incident light intensity and the polarity and amount of fixed charge. From this, new methods of surface state characterization based on photoluminescence and cathodoluminescence are derived. Attempts to control surface states and Fermi level pinning at metal semiconductor interface and free surfaces of nano-structures are presented as efforts toward "nano-photovoltaics". © 2006.
  • Tatsuya Nakamura, Yuji Abe, Seiya Kasai, Hideki Hasegawa, Tamotsu Hashizume
    Journal of Physics: Conference Series 38 1 104 - 107 2006年05月10日 [査読有り][通常論文]
     
    A new single electron (SE) binary-decision diagram (BDD) node device having a single quantum dot connected to three nanowire branches through tunnel barriers was fabricated using etched AlGaAs/GaAs nanowires and nanometer-sized Schottky wrap gates (WPGs), and their operation was characterized experimentally, for the hexagonal BDD quantum circuit. Fabricated devices showed clear and steep single electron pass switching by applying only an input voltage signal, which was completely different from switching properties in the previous SE BDD node devices composed of two single electron switches. As the possible switching mechanism, the correlation between the probabilities of tunnelling thorough a single quantum dot in exit branches was discussed. © 2006 IOP Publishing Ltd.
  • Takahiro Tamura, Isao Tamai, Seiya Kasai, Taketomo Sato, Hideki Hasegawa, Tamotsu Hashizume
    Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 45 4 B 3614 - 3620 2006年04月25日 [査読有り][通常論文]
     
    The basic feasibility of constructing hexagonal binary decision diagram (BDD) quantum circuits on GaAs-based selectively grown (SO) nanowires was investigated from viewpoints of electrical connections through embedded nanowires and electrical uniformity of devices formed on nanowires. For this, (110)- and (510)-oriented nanowires and hexagonal network structures combining these nanowires were formed on (001) GaAs substrates by selective molecular beam epitaxy (MBE) growth. The width and vertical position of the nanowires could be controlled by growth conditions for both (110)- and (510)-directions. By current-voltage (I-V) measurements, good electrical connection was confirmed at the node point where vertical alignment of embedded GaAs nanowire pieces was found to be important. SG quantum wire (QWR) switches formed on the nanowires showed good gate control over a wide temperature range with clear conductance quantization at low temperatures. Good device uniformities were obtained on the test chips, providing a good prospect for future integration. BDD node devices using SG QWR switches showed clear path switching characteristics. Estimated power-delay product values were very small, confirming the feasibility of ultra low-power operation of future circuits. © 2006 The Japan Society of Applied Physics.
  • T Kimura, H Hasegawa, T Sato, T Hashizume
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 45 4B 3414 - 3422 2006年04月 [査読無し][通常論文]
     
    Hydrogen sensing characteristics of Pt/InP Schottky diodes fabricated by electro-deposition were investigated. Such diodes gave high Schottky barrier heights (SBHs) of 650-810 meV. Upon exposure to hydrogen in air, the diodes exhibited a remarkable increase in forward and reverse currents. The saturation current on a log scale and the transient speed changed in proportion to the square root of the hydrogen pressure, P-H2. Upon exposure to hydrogen in vacuum or in nitrogen, a much larger and much faster increase in current took place. However, the saturation Current was almost independent Of P-H2, and the current increase remained almost the same after the hydrogen was removed. The sensing mechanism is explained in terms of changes in SBH caused by interface dipoles formed at Pt/InP interfaces due to adsorbed atomic hydrogen. Transient waveforms and dependence of saturation current and transient time constant on P-H2 were explained quantitatively by a simple theory where processes including atomic hydrogen generation, transport; storage, adsorption, desorption, and reaction with oxygen are effectively included.
  • S Kasai, J Kotani, T Hashizume, H Hasegawa
    JOURNAL OF ELECTRONIC MATERIALS 35 4 568 - 575 2006年04月 [査読無し][通常論文]
     
    Gate control properties together with gate leakage currents in AlGaN/GaN heterostructure field effect transistors (HFETs) with nanometer-scale Schottky gates were investigated, focusing on the effects of AlGaN surfaces at the gate periphery. Fabricated AlGaN/GaN HFETs showed unexpectedly small gate length (L-G) dependence of transconductance, g(m). Comparing the transfer characteristics from theory and experiment, effective L-G values in the fabricated devices were found to be much longer than the geometrical size on the order of 100 nm, indicating the formation of virtual gates. Detailed analysis of the gate leakage current behaviors based on a thin surface barrier model showed the presence of a strong electric field at the gate periphery. The mechanism of the virtual gate formation was discussed based on the obtained nanometer-scale Schottky gate behaviors.
  • Toshiyuki Fujino, Takeshi Kimura, Taketomo Sato, Hideki Hasegawa, Tamotsu Hashizume
    e-Journal of Surface Science and Nanotechnology 4 184 - 191 2006年02月10日 [査読有り][通常論文]
     
    Attempts were made to form various nanostructures on InP substrates using anodization process. Surprisingly wide varieties of nanostructures were obtained. They included zigzag pore arrays and chaotic porous structures on (111)B substrate, and quasi-periodic straight nanopore arrays on (001) substrate. Straight pore samples showed blue-shifted peaks due to quantum confinement whereas no clear energy shifts were observed in (111)B samples. Zigzag and straight pore samples also showed red-shifted PL emissions probably due to emission from surface state continuum. © 2006 The Surface Science Society of Japan.
  • T Hashizume, J Kotani, A Basile, M Kaneko
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 45 4-7 L111 - L113 2006年02月 [査読無し][通常論文]
     
    We proposed a surface control process for suppressing the tunneling leakage of Schottky gates on AlGaN/GaN heterostructures. For the recovery of nitrogen-vacancy-related defects and reduction in the amount of oxygen impurities at the AlGaN surface, the process consisted of nitrogen radical treatment, the deposition of an ultrathin Al layer, UHV annealing and finally the removal of the Al layer. Ni/Au Schottky gates fabricated on processed AlGaN surfaces showed pronounced reduction in leakage current and a clear temperature dependence of I-V characteristics, indicating the effective suppression of tunneling leakage in current transport through AlGaN Schottky interfaces.
  • Isao Tamai, Taketomo Sato, Hideki Hasegawa, Tamotsu Hashizume
    e-Journal of Surface Science and Nanotechnology 4 19 - 24 2006年01月12日 [査読有り][通常論文]
     
    Attempts were made to further elaborate our experimental growth method and the theoretical growth simulation method for formation of AlGaAs/GaAs QWRs on the (111)B substrates, paying attention to Al composition dependence of growth. A series of repeated growth experiments were carried out on simple one-sided mesa patterns, and from their analysis of the results led to determination of parameter values needed for computer simulation based on the continuum model. The experimental evolution of the cross-sectional structures was well reproduced by simulation, not only on one-side mesa, but also on mesa stripes actually used for wire growth. Finally, an optimum growth design was derived for growth of an array of GaAs triangular QWRs with 40 nm base width on GaAs (111)B substrate by the simulation, and the actual growth experiment confirmed its realization. © 2006 The Surface Science Society of Japan.
  • Abdul Manaf Hashim, Seiya Kasai, Tamotsu Hashizume, Hideki Hasegawa
    2006 International RF and Microwave Conference, (RFM) Proceedings 262 - 266 2006年 [査読有り][通常論文]
     
    Interdigital-gated AlGaAs/GaAs high electron mobility transistor (HEMT) structure was used to investigate the interaction between the drifting carrier plasma waves and electromagnetic (EM) waves. It was shown theoretically that the interaction in the range from microwave to terahertz (THz) at room temperature should produce negative conductance characteristics when the carrier drift velocity slightly exceeds the phase velocity of EM waves. S-parameter reflection measurements were carried out at room temperature for a frequency range from 1 to 20 GHz and a drastic change in conductance was observed at 5GHz and 10GHz with the increase of drain-source voltage. Large conductance change over 1000 mS/mm was obtained and it showed a peak at a certain frequency. The peak position could be controlled by changing the pitch size of the interdigital gates. These characteristics can be used for high frequency applications such as high-speed switching devices although a feature size of our interdigital-gated HEMT device is much larger than conventional HEMT device. ©2006 IEEE.
  • Hideki Hasegawa, Seiya Kasai, Taketomo Sato, Tamotsu Hashizume
    IEICE Transactions on Electronics E89-C 7 874 - 882 2006年 [査読有り][通常論文]
     
    With advent of the ubiquitous network era and due to recent progress of III-V nanotechnology, the present III-V heterostructure microelectronics will turn into what one might call III-V heterostructure nanoelectronics, and may open up a new future in much wider application areas than today, combining information technology, nanotechnology and biotechnology. Instead of the traditional top-down approach, new III-V heterostructure nanoelectronics will be formed on nanostructure networks formed by combination of top-down and bottom-up approaches. In addition to communication devices, emerging devices include high speed digital LSIs, various sensors, various smart-chips, quantum LSIs and quantum computation devices covering varieties of application areas. Ultra-low power quantum LSIs may become brains of smart chips and other nano-space systems. Achievements of new functions and higher performances and their on chip integration are key issues. Key processing issue remains to be understanding and control of nanostructure surfaces and interfaces in atomic scale. Copyright © 2006 The Institute of Electronics, Information and Communication Engineers.
  • Junji Kotani, Masamitsu Kaneko, Kazushi Matsuo, Tamotsu Hashizume
    ASDAM '06: SIXTH INTERNATIONAL CONFERENCE ON ADVANCED SEMICONDUCTOR DEVICES AND MICROSYSTEMS, CONFERENCE PROCEEDINGS 221 - 228 2006年 [査読有り][通常論文]
     
    We proposed a surface control process using ultrathin Al layer for suppressing surface donor states at AlGaN. The Ni/Au Schottky diodes fabricated on the processed AlGaN surfaces showed pronounced reduction of leakage current and clear temperature dependence of I-V curves. The AlGaN/GaN HFETs fabricated with surface control showed remarkable reduction of gate leakage currents and the improvement of stability of drain currents against high-temperature/current stress. The surface process also brought a significant decrease in dark current in the Pd/AlGaN/GaN H-2 sensor. The Pd/AlGaN/GaN diode showed the systematic shift in the C-V curve for different partial pressure of H-2. The maximum shift in the threshold voltage obtained was about 1200 m V which is much larger than 500 m V reported for the Pd-based Si MOS sensors.
  • Junji Kotani, Seiya Kasai, Hideki Hasegawa, Tamotsu Hashizume
    e-Journal of Surface Science and Nanotechnology 3 433 - 438 2005年12月09日 [査読無し][通常論文]
     
    Gate leakage currents in AlGaN/GaN HFETs were investigated by comparing experiments with computer simulations based on the thin surface barrier (TSB) model involving unintentional surface donors. Leakage currents in large area Schottky diodes were explained by the TSB model involving nitrogen vacancy related deep donors and oxygen shallow donors. On the other hand, in AlGaN/GaN HFETs with nanometer scale Schottky gates, gate leakage currents include an additional leakage component due to lateral electron injection through tunneling at the gate edge where the barrier thinning is mainly controlled by oxygen donors. By combining vertical and lateral tunneling components, experiments could be reproduced on computer. Lateral components may be responsible for current collapse. © 2005 The Surface Science Society of Japan.
  • Kazushi Matsuo, Takeshi Kimura, Hideki Hasegawa, Tamotsu Hashizume
    e-Journal of Surface Science and Nanotechnology 3 314 - 318 2005年11月14日 [査読無し][通常論文]
     
    Basic H2 gas-sensing characteristics of Pd Schottky diodes formed on the AlGaN/GaN heterostructure were investigated in vacuum. By introducing a novel surface control process to reduce reverse leakage currents, an unprecedented high H2 sensitivity was achieved where 1 Torr hydrogen caused five orders of magnitude change of current. Surprisingly, the surface control process did not change the C - V characteristics which showed an unexpectedly large shift 1000 mV on H2 exposure. Transient waveforms were almost exponential for the logarithm of current, and response speed increased with increase of H2 pressure and temperature. These results were explained in terms of Schottky barrier height due to adsorption of atomic hydrogen generated by Pd together with a due consideration of the current transport mechanism under reverse bias. © 2005 The Surface Science Society of Japan.
  • Nanako Shiozaki, Sanguan Anantathanasarn, Taketomo Sato, Tamotsu Hashizume, Hideki Hasegawa
    Applied Surface Science 244 1-4 71 - 74 2005年05月15日 [査読有り][通常論文]
     
    Etched GaAs quantum wires (QWRs) and selectively grown (SG) QWRs were fabricated, and dependence of their photoluminescence (PL) properties on QWR width (W) and QWR distance to surface (d) were investigated. PL intensity greatly reduced with reduction of W and d, due to non-radiative recombination through surface states. Surface passivation by growing a Si interface control layer (Si-ICL) on group III-terminated surfaces greatly improved PL properties. © 2004 Elsevier B.V. All rights reserved.
  • Abdul Manaf Hashim, Seiya Kasai, Tamotsu Hashizume, Hideki Hasegawa
    Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 44 4 B 2729 - 2734 2005年04月 [査読無し][通常論文]
     
    To investigate the presence of interactions between surface plasma waves of carriers in a two-dimensional electron gas (2DEG) at AlGaAs/GaAs heterostructure and electromagnetic space harmonic slow waves, interdigital-gated high-electron-mobility transistor (HEMT) devices were fabricated, and their input admittances were measured in the microwave region of 1-15 GHz. A large modulation of conductance, more than 1000mS/mm, was observed. The conductance modulation was controlled by a drain-source voltage and showed a peak at a certain frequency whose position could be controlled by changing the pitch of the interdigital gates. The observed conductance and capacitance characteristics were in good agreement with the transverse magnetic (TM) mode analysis taking into account a nonuniform field distribution along the 2DEG channel. The result seems to prove the existence of surface plasma wave interactions even under the strongly collision-dominant situation in the microwave region and provides great hope for increased interactions at THz frequencies under nearly collision-free conditions. © 2005 The Japan Society of Applied Physics.
  • Seiichi Shibata, Mayumi B. Takeyama, Atsushi Noya, Tamotsu Hashizume, Hideki Hasegawa
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 2005 21 - 24 2005年 [査読有り][通常論文]
     
    The solid-phase reaction and/or diffusion at the contact interface of a Cu-Zr amorphous alloy film on InP varying the alloy composition around Cu 60Zr40 were investigated so as to examine the correlation with the ohmic behavior of the contacts. The Cu60Zr 40/n-InP contact showed a linear I-V characteristic with extremely low contact resistance (≤ 1×10-6Ωcm2) after rapid thermal annealing (RTA) at 500°C. It was revealed that the solid-phase reaction at the Cu-Zr/n-InP interface is sensitive to the alloy composition, and the well-controlled Cu diffusion at contact interface is essential to obtain ohmic behavior in the Cu-Zr metallized InP contact. © 2005 IEEE.
  • Takeshi Kimura, Taketomo Sato, Tamotsu Hashizume, Hideki Hasegawa
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 2005 533 - 536 2005年 [査読有り][通常論文]
     
    Pt Schottky diodes were fabricated on n-InP by pulsed electrodeposition, and their H2 sensing properties were investigated. Deposition resulted in formation of Pt nanoparticles. As compared with diodes by vacuum deposition, electrodeposited diodes gave much higher Schottky barrier heights (SBHs) of 810 meV. They showed marked increase of the reverse current on exposure to H2 gas in air. The magnitudes of current change and on-off transients depended on the partial pressure of H2. The detection mechanism is explained in terms of SBH change caused by adsorption/desorption of atomic hydrogen at the Pt/semiconductor interface. Its behavior was strongly affected by the presence of oxygen in the atmosphere. Integration of the present sensor with a hexagonal quantum wire network is proposed for realization of a wireless hydrogen sensor chip. © 2005 IEEE.
  • S Ootomo, H Hasegawa, T Hashizume
    IEICE TRANSACTIONS ON ELECTRONICS E86C 10 2043 - 2050 2003年10月 [査読有り][通常論文]
     
    In order to clarify the mechanism of gate leakage in AlGaN/GaN heterostructure field effect transistors (HFETs), temperature (T)-dependent current-voltage (I-V) characteristics of Ni/n-AlGaN Schottky contact were measured in detail. Large deviations from the thermionic emission transport were observed in I-V-T behavior with anomalously large reverse leakage currents. An analysis based on the thin surface barrier (TSB) model showed that the nitrogen-vacancy-related near-surface donors play a dominant role in the leakage through the AlGaN Schottky interface. As a practical scheme for suppressing the leakage currents, use of an insulated gate (IG) structure was investigated. As the insulator, Al2O3 was selected, and an Al2O3 IG structure was formed on the AlGaN/GaN heterostructure surface after an EGR-N-2 plasma treatment. An in-situ XPS analysis exhibited successful formation of an ultrathin stoichiometric Al2O3 laver which has a large conduction band offset of 2.1 eV at the Al2O3/Al0.3Ga0.7N interface. The fabricated Al2O3 IG HFET achieved pronounced reduction of gate leakage, resulting in the good gate control of drain currents up to V-GS = +3V. The maximum drain saturation current and transconductance were 0.8A/mm and 120 mS/mm, respectively. No current collapse was observed in the Al2O3 IG-HFETs, indicating a remarkable advantage of the present Al2O3-based insulated gate and passivation structure.
  • T Hashizume, S Ootomo, T Inagaki, H Hasegawa
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 21 4 1828 - 1838 2003年07月 [査読有り][通常論文]
     
    We have systematically investigated effects of plasma processing, formation of Si-based dielectrics, and formation of a thin Al2O3 film on the chemical and electronic properties of GaN and GaN/AlGaN heterostructure surfaces. The surface treatment in H-2-plasma excited by electron-cyclotron-resonance (ECR) source, produced nitrogen-vacancy-related defect levels at GaN and AlGaN surfaces, while the ECR-N-2-plasma treatment improved electronic properties of the surfaces. The deposition of a SiO2 film on GaN and AlGaN surfaces was found to induce high-density interface states, due to unexpected and uncontrollable oxidation reactions on the surfaces during the deposition process. In comparison, the SiNx/GaN passivation structure prepared by ECR-plasma assisted chemical vapor deposition showed good interface properties with the minimum D-it value of I X 10(11) cm(-2) eV(-1). However, excess leakage currents governed by Fowler-Nordheim tunneling were observed in the SiNx/Al0.3Ga0.7N structure, due to a relatively small conduction band offset of 0.7 eV between SiNx, and Al0.3Ga0.7N. A novel Al2O3-based passivation structure was proposed and fabricated by molecular beam deposition of Al and subsequent ECR O-2-plasma oxidation. In situ x-ray photoelectron spectroscopy showed successful formation of the Al2O3 layer with a thickness of 3.5 nm and a large conduction band offset of 2.1 eV between Al2O3 and A(0.3)Ga(0.7)N. The GaN/AlGaN insulated-gate heterostructure field-effect transistors (IG HFETs) having the Al2O3-based passivation structure showed a good gate control of drain currents up to V-GS = + 3 V and achieved drain saturation current of 0.8 A/mm. The observed maximum g value is 120 mS/mm. No current collapse was observed in the Al2O3 IG HFETs, indicating a remarkable advantage of the present Al2O3-based passivation structure. (C) 2003 American Vacuum Society.
  • H Hasegawa, T Inagaki, S Ootomo, T Hashizume
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 21 4 1844 - 1855 2003年07月 [査読有り][通常論文]
     
    In order to clarify the mechanisms of drain current collapse and gate leakage currents in the AlGaN/GaN heterostructure field effect transistor (HFET), detailed electrical properties of the ungated portion and Schottky-gated portion of the device were investigated separately, using a gateless HFET structure and an AlGaN Schottky diode structure. The gateless device was subjected to plasma treatments and surface passivation processes including our novel Al2O3-based surface passivation. dc I-V curves of gateless HFETs were highly nonlinear due to virtual gating by surface states. After drain stress, air-exposed, H-2 plasma-treated and SiO2-deposited gateless HFETs showed an initial large-amplitude exponential current transient followed by a subsequent smaller, slow, and highly nonexponential response. The former was explained by emission from deep donors at E-c-0.37eV, and the latter by emission from surface states. Capture transients with stress-dependent capture barriers were also observed. An x-ray photoelectron spectroscopy (XPS) study indicated that 0.37 eV-deep donors are N-vacancy related. On the other hand, no current transients took place in N-2 plasma treated and Al2O3-passivated samples. Temperature dependences of I-V curves of Schottky diodes were extremely small and reverse currents were anomalously large. They were explained by the "thin surface barrier" (TSB) model where thermionic field emission and field emission through the TSB region formed by deep donors produce leakage current paths. By combining the results on gateless HFETs and Schottky diodes, a new unified model of near-surface electronic states for the free surface and Schottky interface of AlGaN is proposed. It consists of a U-shaped surface state continuum and N-vacancy related near-surface discrete deep donors. The model can explain the observed large gate leakage and drain current collapse in AlGaN/GaN HFETs in a unified way. It is also shown that our novel Al2O3 passivation, when also used as a gate insulator, can completely suppress current collapse and gate leakage. (C) 2003 American Vacuum Society.
  • Takanori Inagaki, Tamotsu Hashizume, Hideki Hasegawa
    Applied Surface Science 216 1-4 519 - 525 2003年06月30日 [査読有り][通常論文]
     
    Effects of hydrogen and nitrogen electron cyclotron resonance (ECR) plasma surface treatments on the current transport of two-dimensional electron gas (2DEG) at AlGaN/GaN interface were investigated by using a gateless heterostructure field-effect transistors (HFETs) test device. Current-voltage (I-V) characteristics of the gateless device exhibited linear and saturation current behavior similar to that of a gated device, indicating presence of strong Fermi level pinning. As compared with the air-exposed surface, the current reduced significantly with appearance of hysteresis after H2-plasma treatment, whereas it slightly increased after N2-plasma treatment. After experiencing a positive pulse voltage, current in the H2-plasma-treated device dropped and then showed a fast exponential recovery followed by highly non-exponential slow recovery. Temperature dependence of initial recovery gave an activation energy of 0.37 eV. Observed current transients could be reproduced by numerical simulation assuming presence of dominant discrete near-surface deep donors and a U-shaped surface state continuum. X-ray photoelectron spectroscopy (XPS) analysis indicated near-surface reduction of N atoms after the H2-plasma treatment, indicating that the discrete states are related to N-vacancies. Since generation of N-vacancies is likely to occur in growth and processing of GaN-related materials, the present observation explains surface-related instabilities such as current collapse in AlGaN/GaN HFETs. For oxide removal, the N2-plasma treatment is much better than H2-plasma treatment, effectively removing oxides without producing defects. © 2003 Elsevier Science B.V. All rights reserved.
  • Tamotsu Hashizume, Shinya Ootomo, Hideki Hasegawa
    Physica Status Solidi C: Conferences 7 2380 - 2384 2003年 [査読有り][通常論文]
     
    A novel Al2O3-based passivation and insulated gate (IG) structure has successfully applied to AlGaN/GaN heterostructure field-effect transistors (HFETs). Al2O3 was formed by a molecular beam deposition of Al followed by its oxidation using the electron-cyclotron-resonance (ECR) excited O2 plasma in an ultrahigh vacuum environment. A large conduction-band offset of 2.1 eV was obtained at the Al2O3/Al0.3Ga0.7N interface, leading to the pronounced reduction of gate leakage currents. The fabricated IG-type HFETs exhibited good gate control of drain currents up to VGS = +3 V. The maximum transconductance was 130 mS/mm. No current collapse was observed in the Al2O3 IG HFETs under the pulse-mode gate stress, indicating remarkable passivation effects of the present Al 2O3-based insulated gate and passivation structure. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA.
  • K. Kumakura, T. Makimoto, N. Kobayashi, T. Hashizume, T. Fukui, H. Hasegawa
    IEEE International Symposium on Compound Semiconductors, Proceedings 2003- 49 - 50 2003年 [査読有り][通常論文]
     
    In this paper, we have investigated the doping concentration and dislocation density dependence of minority carrier diffusion length parallel to c-axis in Si-doped and Mg-doped GaN by electron beam induced current (EBIC) measurements to optimize the base structure of nitride heterojunction bipolar transistors.
  • Tamotsu Hashizume, Ryusuke Nakasaki
    Applied Physics Letters 80 24 4564 - 4566 2002年06月17日 [査読有り][通常論文]
     
    Detailed studies on the defect-related surface states of plasma-exposed n-GaN surfaces were carried out. An anomalous flat portion appeared in the metal-insulator-semiconductor capacitance-voltage characteristics for the sample exposed to H2 plasma, corresponding to a localized peak at E C-0.5eV in the surface state density distribution. Atomic-force microscope and x-ray photoemission studies revealed the formation of Ga droplets on H2-plasma-treated GaN surfaces, caused by the desorption of nitrogen atoms in the form of NHx. These results suggested that a nitrogen-vacancy-related state near the conduction-band edge was introduced on the H2-plasma-treated GaN surface. No such effects took place on the N2-plasma-treated GaN surfaces. © 2002 American Institute of Physics.
  • S Oyama, T Hashizume, H Hasegawa
    APPLIED SURFACE SCIENCE 190 1-4 322 - 325 2002年05月 [査読有り][通常論文]
     
    Detailed current-voltage-temperature (I-V-T) measurements were performed on the Schottky diodes fabricated on MOVPE-grown n-GaN layers. A large deviation from the thermionic emission (TE) transport was observed in the reverse I-Vcurves with a large excess leakage. From the calculation based on the thermionic-field emission (TFE) model, it was found that the tunneling plays an important role in the carrier transport across the GaN Schottky barrier even for doping densities as low as 1 x 10(17) cm(-3). A novel barrier-modified TFE model based on presence of near-surface fixed charges or surface states is proposed to explain the observed large reverse leakage currents. (C) 2002 Elsevier Science B.V. All rights reserved.
  • Z Jin, T Hashizume, H Hasegawa
    APPLIED SURFACE SCIENCE 190 1-4 361 - 365 2002年05月 [査読有り][通常論文]
     
    The effects of nitrogen addition on methane-based ECR plasma etching of GaN were studied. The etch rate 30 nm/min and r.m.s.. roughness 2.6 nm were obtained when the GaN sample was etched by a methane-based gas mixture without N-2. The addition of N-2 gas resulted in a decrease of etch rate and a smoother etched surface. The r.m.s. roughness became less than 0.4 nm even only 1.5 seem N-2 gas was added to the mixture. In situ XPS measurements showed that, without N-2, heavy N-depletion took place on the etched surface, resulting in appearance of Ga metal on the surface. In contrast, the loss of N was compensated when the N-2 gas was added, and the etched surface approached the stoichiometric one with the increase of N-2 gas flow. This suppression of preferential loss of N was considered to be the main reason that improved the etched surface morphology. (C) 2002 Elsevier Science B.V. All rights reserved.
  • S Ootomo, T Hashizume, H Hasegawa
    PHYSICA STATUS SOLIDI A-APPLIED RESEARCH 188 1 371 - 374 2001年11月 [査読有り][通常論文]
     
    We demonstrate a novel surface passivation process for AlGaN/GaN heterostructures utilizing an ultrathin Al2O3 layer ( approximate to1 nm). The Al deposition and thermal annealing processes in UHV were found to form the ultrathin Al2O3 layer on the surface of the AlGaN/GaN heterostructure, which was:confirmed by in-situ X-ray photoelectron spectroscopy (XPS) analysis. The reverse leakage current for the Schottky gate contact on the Al2O3-passivated heterostructure surface was reduced by three orders of magnitude compared to that for the conventional Schottky gate structure. C-V results showed good gate controllability of the two-dimensional electron gas (2DEG) by the novel gate structure.
  • T Hashizume, R Nakasaki, S Ootomo, S Oyama, H Hasegawa
    IEICE TRANSACTIONS ON ELECTRONICS E84C 10 1455 - 1461 2001年10月 [査読有り][通常論文]
     
    Surface passivation process of GaN utilizing electron-cyclotron-resonance (ECR) excited plasma has been characterized and optimized for realization of stable operation in GaN-based high-power/high-frequency electronic devices. From XPS analysis, the NH4OH treatment as well as the ECR-N-2 and ECR-H-2 plasma treatments were found to be effective in removing natural oxide and contaminants from the GaN surface. The SiNx/GaN structure prepared by the ECR excited plasma chemical vapor deposition (ECR-CVD) process showed better C-V behavior compared to the SiO2/GaN structure. Surface treatment process using the ECR plasma improved interface properties and achieved the Dit value of 2 x 10(11) cm(-2) eV(-1) or less. An estimate of the valence band offset by XPS showed that the present SiNx/n-GaN structure has a type-I band lineup, suitable for the surface passivation of GaN-based devices. No pronounced stress remained at the SiNx/GaN interface, which was confirmed by Raman spectroscopy.
  • T Hashizume, S Ootomo, S Oyama, M Konishi, H Hasegawa
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 19 4 1675 - 1681 2001年07月 [査読有り][通常論文]
     
    Chemical and electrical properties of the surfaces of GaN and GaN/AlGaN heterostructures were systematically investigated by x-ray photoelectron spectroscopy (XPS), capacitance-voltage, and current-voltage measurements. From in situ XPS study, relatively smaller band bending of 0.6 eV was seen at the GaN (2 X 2) surface grown by radio frequency-assisted molecular beam epitaxy on the metalorganic vapor phase epitaxy GaN template. After exposing the sample surface to air, strong band bending took place at the surface. The surface treatment in NH4OH solution and N-2 plasma was found to reduce the surface Fermi level pinning. Surface passivation process of GaN utilizing SiNx film by electron-cyclotron-resonance assisted plasma chemical vapor deposition (ECR-CVD) achieved low interface state density, 2 X 10(11)cm(-2) eV(-1). No pronounced stress remained at the SiNx/GaN interface, which was confirmed by Raman spectroscopy. The present NH4OH/ECR-N-2 plasma treatment was also found to be effective in realizing well-ordered and nearly oxide-free surface of a GaN/AlGaN heterostructure. The subsequent passivation process using the ECR-CVD SiNx, film enhanced the drain current in the gateless GaN/AlGaN high electron mobility transistor. A surface passivation process utilizing an ultrathin Al-oxide layer reduced leakage current and improved gate controllability of two-dimensional electron gas in the Schottky gate contact fabricated on the GaN/AlGaN heterostructures. (C) 2001 American Vacuum Society.
  • Z Jin, T Hashizume, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 40 4B 2757 - 2761 2001年04月 [査読有り][通常論文]
     
    The effects of nitrogen gas on the methane-based reactive ion beam etching (RIBE) of InP were studied in detail. The surface was very rough and covered with In droplets and heavy polymer when it was etched by the mixture Of CH4/H-2 without nitrogen gas. In contrast, the surface smoothness was improved markedly when nitrogen gas was added, realizing a root mean square (RMS) roughness comparable to that of the initial InP surface before etching. From the detailed X-ray photoelectron spectroscopy (XPS) analysis, it was found that the formation of P-N bonds on the surface could suppress the preferential evaporation of P atoms in the PH3 form and balance the In- and P-reactions during the etching process. This resulted in the decrease of In droplets and a smoother etched InP surface. It was also found that the rate limiting process for etching was the In-related reaction. The study of Raman scattering showed that the crystal quality of the etched surface was improved when NZ gas was added to the CH4/H-2 mixture.
  • T Hashizume, R Nakasaki, S Ootomo, S Oyama, H Hasegawa
    MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY 80 1-3 309 - 312 2001年03月 [査読有り][通常論文]
     
    Surface properties of GaN and Al0.17Ga0.83N materials grown by metal organic vapor phase epitaxy (MOVPE) were systematically investigated by X-ray photoelectron spectroscopy (XPS). Air-exposed samples showed highly non-stoichiometric surfaces, which included a large amount of natural oxides. Deposition of Al on the air-exposed GaN surface caused interfacial reactions, resulting in the formation of oxide lavers including: Al2O3 and Ga oxide at the interface. A natural oxide layer of AlGaN surface possessed a complicated composition distribution in depth where the Al-oxide component was dominant on the topmost layer. Such natural oxide layers were found to be removed from GaN and AlGaN surfaces after the treatment in an NH4OH solution at 50 degreesC for 10 min, resulting in oxide-free and well-ordered surfaces. (C) 2001 Elsevier Science B.V. All rights reserved.
  • Zhengwen Fu, Hiroshi Takahashi, Tamotsu Hashizume, Seiya Kasai, Hideki Hasegawa
    Conference Proceedings - International Conference on Indium Phosphide and Related Materials 413 - 416 2001年 [査読有り][通常論文]
     
    Attempts were made to clarify the cause for the poor reproducibility of successful processing accompanied with a low the transconductance in the previous oxide-free InP MISFET having an ultrathin Si interface control layer(Si ICL). A detailed in-situ XPS study made for each step of processing indicated that deficiency of P on the InP surface took place by the irradiation of high energy Si beam during the growth of Si ICL. Then, a modified passivation structure having an In0.53Ga0.47As cap layer was proposed and investigated. In-situ XPS study indicated that the novel gate structure prevents desorption of P from the InP surface. In-situ contactless C-V method showed a low and wide interface state density distribution with a minimum of 2×1011 cm-2eV-1. A long-gate InP MISFET test device with a gate length of 2μm exhibited a maximum gm of 123 mS/mm and a high drain current of 389 mA/mm.
  • S Anantathanasarn, S Ootomo, T Hashizume, H Hasegawa
    APPLIED SURFACE SCIENCE 159 456 - 461 2000年06月 [査読有り][通常論文]
     
    Attempts were made to passivate the GaAs (001) surface by a pseudomorphic ultra-thin cubic GaN layer formed by a nitrogen radical (N-radical) or nitrogen plasma irradiation technique. Reflection high-energy electron diffraction (RHEED) pattern observations and detailed X-ray photoelectron spectroscopy (XPS) analysis have shown that ultra-thin cubic GaN layer on GaAs (001) surface with desirable surface stoichiometry can be realized with the optimization of surface nitridation process parameters. The passivation effects, characterized by ultra-high vacuum photoluminescence (UHV FL) analysis, revealed strong enhancement in band-edge PL intensity of GaAs after passivation as large as a factor of 10 when compared with the as-gown clean molecular beam epitaxy (MBE) GaAs surface. (C) 2000 Elsevier Science B.V. All rights reserved.
  • R Nakasaki, T Hashizume, H Hasegawa
    PHYSICA E 7 3-4 953 - 957 2000年05月 [査読有り][通常論文]
     
    Interface properties of Si3N4/GaN and SiO2/GaN structures produced by CVD processes were investigated by X-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) methods, paying attention to surface treatments prior to insulator deposition. Electron cyclotron resonance assisted plasma chemical vapor deposition (ECR-PCVD) and radio frequency-assisted PCVD (RF-PCVD) were used. For the interfaces having natural oxides of GaN, the surface Fermi level was strongly pinned due to the existence of high density of interface states. A surface treatment in NH4OH solution was found to be effective in reducing natural oxides and interface states. Further reduction of interface states was realized by the NZ plasma treatment of GaN surfaces just prior to the deposition of insulating films. A minimum value of interface state density of 5 x 10(10) cm-l eV(-1) was obtained for the Si3N4/n-GaN structures formed by ECR-PCVD with a combination of NH4OH treatment and N-2 plasma treatment. (C) 2000 Elsevier Science B.V. All rights reserved.
  • SY Ootomo, T Hashizume, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 39 4B 2407 - 2413 2000年04月 [査読有り][通常論文]
     
    The nitridation process of a Gap (100) surface using rf nitrogen radicals and electron cyclotron resonance (ECR) Nz plasma at 400-500 degrees C has been investigated by reflection high-energy electron diffraction (RHEED), ii? situ X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM) and cathodoluminescence (CL) methods. The P-stabilized (2 x 4) surface prepared by ultra high-vacuum (UHV) thermal cleaning was used as an initial surface. At the initial stage of nitridation, both the P-N and Ga-N components were formed on the GaP surface. The subsequent radical process led to the dominant formation of strained small granular grains of cubic GaN with diameters of 5-10 nm. Further nitridation produced much larger hemispherical GaN grains. The surface exposed to ECR Nz plasma for 60 min exhibited an anisotropic surface morphology with rectangular structures which aligned to the [011] direction. Clear CL peaks originating from the cubic GaN structure were observed on these nitrided surfaces.
  • Tatsuo Shiozawa, Toshiyuki Yoshida, Tamotsu Hashizume, Hideki Hasegawa
    Applied Surface Science 159 98 - 103 2000年 [査読有り][通常論文]
     
    Interface state properties and tunnel transport properties of ultrathin insulators formed on Si surfaces at low temperatures (LT) were characterized by XPS, contactless UHV C-V and I-V methods. Electron cyclotron resonance (ECR)-assisted N2O plasma process realized the formation of good interface with low interface state density. On the other hand, strong Fermi level pinning was observed at the interfaces formed by chemical oxidation and LT thermal oxidation, due to high density of interface states. In these interfaces, there was a large discrepancy between the measured tunnel I-V curves and the calculated ones using the direct tunnel theory. This discrepancy was explained in terms of the large band bending due to the high density of interface states, which causes depletion of electrons at the interface.
  • S Chakraborty, T Yoshida, T Hashizume, H Hasegawa, T Saki
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 16 4 2159 - 2164 1998年07月 [査読有り][通常論文]
     
    We have undertaken the characterization of the low-temperature direct electron cyclotron resonance (ECR) N2O plasma process on the technically important Si(100) surface by the combined use of the newly developed ultrahigh vacuum (UHV)-based contactless capacitance-voltage (C-V) and x-ray photoelectron spectroscopy (XPS) techniques. An UHV-based contactless C-V measurement system allowed for the first time a direct in,situ assessment of electronic properties of "free'' Si surfaces and the surfaces covered with an ultrathin insulator layer. The UHV contactless C-V analysis indicated that a oxynitride/Si interface with a minimum interface state density of 1 X 10(11) eV(-1) cm(-2) could be realized by the ECR N2O oxynitridation process at 400 degrees C. The angle-resolved XPS study showed that the formation of a phase-separated Si3N4/SiO2 structure is important to realize such a interface at low temperatures. The electronic and the chemical properties of the interfaces were found to be strongly correlated. (C) 1998 American Vacuum Society.
  • K Iizuka, T Hashizume, H Hasegawa
    SOLID-STATE ELECTRONICS 41 10 1463 - 1468 1997年10月 [査読有り][通常論文]
     
    Small-signal response of interface states at passivated InGaAs surfaces was investigated from 1 Hz to 1 GHz at room temperature. A response of interface state admittance was observed over a surprisingly wide frequency range from 1 Hz to microwave frequencies. The complicated frequency dependence of the MIS admittance can be well fitted into an equivalent circuit based on the disorder-induced gap state (DIGS) model. (C) 1997 Elsevier Science Ltd.
  • SATO Taketomo, UNO Shouichi, HASHIZUME Tamotsu, HASEGAWA Hideki
    Japanese journal of applied physics. Pt. 1, Regular papers & short notes 36 3B 1811 - 1817 1997年03月30日 [査読有り][通常論文]
     
    Pt Schottky barriers were formed on InP-based materials by a novel in-situ electrochemical process. The electrical characteristics, surfaces and interfaces of the Schottky diodes were investigated by current-voltage (I-V), capacitance-voltage (C-V), deep-level transient spectroscopy (DLTS) and atomic force microscopy (AFM) measurements. The mechanism for increasing the Schottky barrier heights (SBH) was explained in terms of possible ordered interface formation from the viewpoint of the disorder induced gap state (DIGS) model.
  • T Hashizume, S Shiobara, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 36 3B 1775 - 1780 1997年03月 [査読有り][通常論文]
     
    Photocapacitance transient behavior of the dominant S1 electron trap with an activation energy of 0.64 eV in low-temperature GaAs (LT-GaAs) grown by molecular beam epitaxy (MBE), which we recently detected by deep-level transient spectroscopy, was investigated in detail. It was found that the S1 trap has metastable properties causing a marked photoquenching behavior. Transition processes between the ground state and the metastable state were found to be very different from those of EL2, indicating that the dominant S1 trap is not EL2, but a new deep level peculiar to the LT-GaAs layer.
  • K Ikeya, T Hashizume, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 36 3B 1756 - 1762 1997年03月 [査読有り][通常論文]
     
    In order to make the silicon interface control layer (Si ICL)-based surface passivation technique applicable to air-exposed AlGaAs surfaces, various surface treatments were systematically studied. The treatments investigated include UHV thermal cleaning, (NH4)(2)S-x treatment, HCl treatment and H-2 plasma treatment. Chemical status and quality of the treated surfaces were evaluated by in-situ and ex-situ X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. A strong correlation was found between the band-edge PL intensity and the amount of surface oxide components, in particular Al-oxides. Marked reduction of the amount of oxide components and enhancement of PL intensity were realizable by a combination of HCl surface treatment and Si ICL formation.
  • H Fujikura, S Kodama, T Hashizume, H Hasegawa
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 14 4 2888 - 2894 1996年07月 [査読有り][通常論文]
     
    Surface state effects in In0.53Ga0.47As one-dimensional quantum wires and the effectiveness of the Si interface control layer (Si ICL)-based passivation technique are investigated using photoluminescence (PL) as the probe. Scanning electron microscope and x-ray photoelectron spectroscopy measurements were made to characterize the structure and the interface properties. The In0.53Ga0.47As quantum wires embedded in In(0.5)2Al(0.48)As barrier material were fabricated by selective molecular beam epitaxy growth on patterned InP substrates. Unpassivated near-surface quantum wires showed an exponential decrease of PL intensity with reduction of surface-to-well distance, t(ws), similarly to the near-surface quantum wells. By applying the Si ICL-based ws, passivation, a nearly complete recovery of PL intensity was achieved with an observed maximum recovery factor of 250 for the InGaAs quantum wire directly passivated with SiO2/Si ICL (t(ws)=0). The mechanism for the PL recovery is explained in terms of suppression of surface states by passivation. (C) 1996 American Vacuum Society.
  • S Shiobara, T Hashizume, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 35 2B 1159 - 1164 1996年02月 [査読有り][通常論文]
     
    Deep levels in low-temperature (LT) GaAs layers grown by molecular beam epitaxy were investigated by deep level transient spectroscopy (DLTS), photocapacitance and photoluminescence (PL) techniques. Five electron traps were detected, for the first time, in Si-doped conductive LT-GaAs layers grown at 300-400 degrees C. The dominant trap was found to be the S1 level with an activation energy of 0.64 eV and an electron capture cross section of sigma(n) = 2.4 x 10(-14) cm(2). The S1 level showed remarkable photoquenching behavior. No PL was observed from LT-layers. The mechanism responsible for the semi-insulating property of LT-GaRs layers is probably due to compensation by the S1 trap, which is not EL2 but a new electron trap peculiar to LT-GaAs layers.
  • K Jinushi, H Okada, T Hashizume, H Hasegawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 35 2B 1132 - 1139 1996年02月 [査読有り][通常論文]
     
    In an attempt to enhance the operation temperature of GaAs-based single-electron transistors (SETs), a novel Schottky in-plane gate (IPG) SET based on voltage-controlled depletion of two-dimensional electron gas (2DEG) was proposed, fabricated and characterized. For device fabrication, EB lithography and the in-situ electrochemical process were applied to a MBE-grown AlGaAs/GaAs 2DEG wafer, and narrow Pt IPGs with 200 nm length were realized on the side walls of 2DEG. The fabricated SETs clearly showed Coulomb oscillations with a Coulomb gap and Coulomb staircase at low temperatures. Coulomb oscillation was observed up to 20 K which is much higher than the operation temperature of split gate devices having similar dimensions of several hundred nm. The small. capacitance values as well as the observed strong bias dependences of device characteristics are explained in terms of the depletion characteristics of 2DEG with the Schottky IPG. The current leakage within the Coulomb gap is compatible with the cotunneling theory.
  • S KODAMA, T HASHIZUME, H HASEGAWA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 34 8B 4540 - 4543 1995年08月 [査読有り][通常論文]
     
    Photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) study is performed on unpassivated and passivated AlGaAs/GaAs near-surface quantum wells (QWs) in order to clarify the mechanism of the recently found large PL intensity increase which was achieved by a novel interface control technique utilizing a SiO2/Si3N4/Si structure including an ultrathin silicon interface control layer (Si ICL). It is shown that the novel Si ICL technique produces a coherent interface structure free of oxides and nitrides, and this removes surface states without introducing additional confined electronic states that interfere with the fundamental e1-hh1 transition of the near-surface QW. The present technique seems to be applicable to passivation of various kinds of compound semiconductor quantum structures.
  • Hideki Hasegawa, Tamotsu Hashizume, Hiroshi Okada, Keiichiro Jinushi
    Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures 13 4 1744 - 1750 1995年07月 [査読有り][通常論文]
     
    Novel Schottky in-plane gate (IPG) quantum wire transistors were fabricated for the first time, and their transport properties were investigated. For fabrication of transistors, an AlGaAs/GaAs quantum well wire (QWW) was produced by etching, and platinum IPG electrodes were directly formed on both edges of the QWW by a new in situ electrochemical process. The current-voltage (I-V) characteristics of the fabricated long-channel and short-channel devices exhibited good field effect transistor operation at 3-300 K. Simple theoretical models assuming either a constant mobility or a constant velocity were developed. They provide a reasonably good phenomenological description of the observed I-V characteristics. Limitations of the models are also discussed. At low temperatures, the short-channel device exhibited sharp quantized conductance steps in the units of 2e2/h near pinch-off, indicating one-dimensional ballistic quantum transport. The first plateau of the conductance step remained visible up to 40 K, which is the highest reported so far for the AlGaAs/GaAs system.
  • H HASEGAWA, T HASHIZUME, H OKADA, K JINUSHI
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 13 4 1744 - 1750 1995年07月 [査読有り][通常論文]
     
    Novel Schottky in-plane gate (IPG) quantum wire transistors were fabricated for the first time, and their transport properties were investigated. For fabrication of transistors, an AlGaAs/GaAs quantum well wire (QWW) was produced by etching, and platinum LPG electrodes were directly formed on both edges of the QWW by a new in situ electrochemical process. The current-voltage (I-V) characteristics of the fabricated long-channel and short-channel devices exhibited good field effect transistor operation at 3-300 K. Simple theoretical models assuming either a constant mobility or a constant velocity were developed. They provide a reasonably good phenomenological description of the observed I-V characteristics. Limitations of the models are also discussed. At low temperatures, the short-channel device exhibited sharp quantized conductance steps in the units of 2e(2)/h near pinch-off, indicating one-dimensional ballistic quantum transport. The first plateau of the conductance step remained visible up to 40 K, which is the highest reported so far for the AlGaAs/GaAs system. (C) 1995 American Vacuum Society.
  • S KODAMA, S KOYANAGI, T HASHIZUME, H HASEGAWA
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 13 4 1794 - 1800 1995年07月 [査読有り][通常論文]
     
    In view of the urgent necessity to establish a suitable passivation technology applicable to compound semiconductor quantum structure surfaces, the latest version of the silicon interlayer based passivation process was applied to passivation of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As near-surface quantum wells (QWs). The process utilizes an ultrathin molecular beam epitaxy silicon/ultrathin photo-enhanced chemical vapor deposition (photo-CVD) silicon nitride double layer as the interface control layer together with a main passivation dielectric of thick photo-CVD SiO2 layer. The effectiveness of passivation was studied by comparing the photoluminescence (PL) intensities of passivated samples with those of unpassivated QWs that showed exponential decrease with reduction of surface-to-well distance. A complete recovery of PL intensity was achieved by passivation with a maximum recovery factor larger than 10(3), consistent with reduced interface state densities in low 10(10) cm(-2) eV(-1) range recently realized on In0.53Ga0.47As metal-insulator-semiconductor capacitors using the same technique. (C) 1995 American Vacuum Society.
  • S KODAMA, S KOYANAGI, T HASHIZUME, H HASEGAWA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 34 2B 1143 - 1148 1995年02月 [査読有り][通常論文]
     
    A novel passivation scheme using an ultrathin molecular beam epitaxy (MBE) silicon interface control layer (ICL) and an ultrathin silicon nitride layer is investigated for use in the passivation of compound semiconductor quantum structures. Process optimization was performed by in-situ. X-ray photoelectron spectroscopy (XPS) and electrical characterization of the insulator-semiconductor interface formed on In0.53Ga0.47As. A drastic reduction of interface state density into the 10(10) cm(-2) . eV(-1) range was obtained under optimum condition. The passivation scheme was then applied to passivation of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As near-surface quantum wells. Remarkable recovery of photoluminescence (PL) intensity was observed upon application of the present passivation scheme to the barrier surface, which was consistent with the reduction of surface states.
  • A MALININ, H TOMOZAWA, T HASHIZUME, H HASEGAWA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 34 2B 1138 - 1142 1995年02月 [査読有り][通常論文]
     
    Deep levels in Si-doped InxAl1-xAs layers grown by molecular beam epitaxy were systematically investigated, changing the alloy composition for the first time. Deep level traps were characterized by deep level transient spectroscopy (DLTS) measurements and photoluminescence (PL) measurements. Two kinds of deep electron traps, E1 and E2 were observed. Least squares fitting curves of all data on the E1 and E2 traps were almost parallel to the Gamma-band edge. From the observed dependence of trap level position on the alloy composition as well as from the comparison between DLTS and PL results, observed traps seem to be not DX center like donors but normal deep donors associated with the Gamma-band.
  • NJ WU, T HASHIZUME, H HASEGAWA, Y AMEMIYA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 34 2B 1162 - 1167 1995年02月 [査読有り][通常論文]
     
    Schottky contacts on n-InP were fabricated by a, novel in situ electrochemical process. The characteristics: of the Schottky contacts were investigated using atomic force microscopy (AFM). X-ray photoemission spectroscopy (XPS), current-voltage (I-V) and capacitance-voltage (C-V) techniques. The results of AFM and XPS measurements indicate that the novel in situ electrochemical process produces smooth and oxide-free interfaces. The Schottky contacts show nearly ideal thermionic emission characteristics. The novel electrochemical process was found to reduce Fermi-level pinning at the Schottky contact interfaces. The Schottky barrier height was found to change over a wide range from 0.35 eV to 0.86 eV, depending on the work function of the contact metals. The Pt/InP contact gave the highest barrier height of 0.86 eV.
  • T HASHIZUME, G SCHWEEGER, NJ WU, H HASEGAWA
    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 12 4 2660 - 2666 1994年07月 [査読有り][通常論文]
     
    The Pt/GaAs Schottky interfaces with excellent qualities were fabricated by a novel in situ electrochemical etching and plating technology using the electrical and photo pulses. The etched GaAs surface was very smooth, and no oxidized and disordered layer was produced at the interface. This technology also prevents formation of process-induced near-surface defect levels of GaAs. The Pt/GaAs diodes exhibited nearly ideal thermionic emission characteristics with a barrier height larger than 1.0 eV and an ideality factor lower than 1.05. Selective deposition of Schottky barriers to the edge of a two-dimensional electron gas in a GaAs/AlGaAs quantum well structure was truly achieved by this process, which is confirmed by the agreement of the experimental C-V results with theoretical predictions, as well as the results of the electron beam-induced current measurements.
  • Tamotsu Hashizume, Masaki Yoshino, Mitsuo Shimozuma
    Journal of Applied Physics 76 10 5955 - 5960 1994年 [査読有り][通常論文]
     
    Chemical properties and thermal stability of the SiO2/amorphous As/GaAs passivation systems were investigated for the postannealing of GaAs. The 50 Hz plasma-enhanced chemical-vapor-deposition technique allowed the deposition of high-quality SiO2 films onto GaAs at room temperature without destroying the surface structure of amorphous-As-covered GaAs. No interfacial reactions took place in the present passivation system during the rapid thermal annealing (RTA) process. In addition, this system gave abrupt carrier profiles without extended tailing for the Si-implanted and RTA-processed GaAs.
  • NJ WU, T HASHIZUME, H HASEGAWA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 33 1B 936 - 941 1994年01月 [査読有り][通常論文]
     
    An oxide-free Pt/GaAs Schottky barrier was fabricated by a novel in situ photopulse-assisted electrochemical process. Nearly ideal thermionic emission characteristics of a high barrier height phi(Bn) of 1.07 eV and an ideality factor of n=1.05, were observed over a range of 7 orders of magnitude of electric current. The results of atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS) and deep level transient spectroscopy (DLTS) measurements indicated that the novel electrochemical process produces a smooth and oxide-free interface and prevents formation of process-induced damage. It produces firm Fermi level pinning which was previously possible only by ultrahigh-vacuum (UHV) processes.
  • T HASHIZUME, H HASEGAWA, R RIEMENSCHNEIDER, HL HARTNAGEL
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 33 1B 727 - 733 1994年01月 [査読有り][通常論文]
     
    Near-surface defects of InP produced during three different chemical vapor deposition (CVD) processes were systematically characterized by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) techniques. Deposition of plasma-enhanced CVD (PECVD) phosphosilicate glass (PSG) and SiO2 films produced the same bulk level, lying at 0.35 eV below the conduction band edge,near the surface region of InP. Such a level was absent in the samples prepared by the photo CVD process. In addition, the photo CVD process gave a lower density of interface states than the PECVD process. The origin of the bulk trap and the energy- and space-distributed nature of interface states are discussed.
  • T HASHIZUME, H HASEGAWA, T SAWADA, A GRUB, HL HARTNAGEL
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 32 1B 486 - 490 1993年01月 [査読有り][通常論文]
     
    Various preparation methods used for the fabrication of submillimeter-wave Pt/GaAs Schottky diodes are compared with respect to deep levels present in the diode surface region. In the samples prepared by the standard electron-beam evaporation process, three process-induced deep levels were detected with the concentrations in the range of 10(13)-10(14) cm-3. On the other hand, these levels were very much reduced in the diodes produced by a new in-situ electrochemical technique. This appears to be related to the reported very low noise property of the latter diodes.
  • T HASHIZUME, H HASEGAWA
    JOURNAL OF APPLIED PHYSICS 68 9 4598 - 4603 1990年11月 [査読有り][通常論文]
  • T HASHIZUME, H HASEGAWA, H OHNO
    JOURNAL OF APPLIED PHYSICS 68 7 3394 - 3400 1990年10月 [査読有り][通常論文]
  • Y AKATSU, H OHNO, H HASEGAWA, T HASHIZUME
    JOURNAL OF CRYSTAL GROWTH 81 1-4 319 - 325 1987年02月 [査読有り][通常論文]
  • T HASHIZUME, E IKEDA, Y AKATSU, H OHNO, H HASEGAWA
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS 23 5 L296 - L298 1984年 [査読有り][通常論文]

書籍

  • 高周波半導体材料・デバイスの新展開
    シーエムシー出版 2006年

その他活動・業績

  • Joel T. Asubar, Hirokuni Tokuda, Masaaki Kuzuhara, Zenji Yatabe, Kenya Nishiguchi, Tamotsu Hashizume Proceedings of the 34th Samahang Pisika ng Pilipinas Physics Congress 34 INV-1B-01-1 -4 2016年08月 [査読無し][招待有り]
  • 橋詰 保, 谷田部 然治, 佐藤 威友 電気学会研究会資料. EDD, 電子デバイス研究会 EDD-14 (39-49) 13 -16 2014年03月 [査読無し][招待有り]
  • 谷田部 然治, 堀 祐臣, 金 聖植, 橋詰 保 電子情報通信学会技術研究報告. ED, 電子デバイス 112 (32) 49 -52 2012年05月 [査読無し][通常論文]
  • 金 聖植, 堀 祐臣, 谷田部 然治, 橋詰 保 電子情報通信学会技術研究報告. ED, 電子デバイス 111 (290) 25 -28 2011年11月 [査読無し][通常論文]
  • Marcin Miczek, Piotr Bidzinski, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume SOLID STATE COMMUNICATIONS 151 (11) 830 -833 2011年06月 [査読無し][通常論文]
     
    An influence of electronic states at an insulator/GaN interface on the behavior of excess holes in an ultraviolet-illuminated metal/SiO2/n-GaN structure has been studied by numerical simulations for weak (gate bias of -0.1 V) and strong (-1 V) depletion, in a wide range of excitation light intensities (from 10(10) to 10(20) photons cm(-2) s(-1)) and for various bulk carrier lifetimes (from 1 to 100 ns). It has been found that the interface states with densities of 10(12) eV(-1) cm(-2) dramatically reduce the total (integrated in the whole GaN layer) density of photogenerated holes and thus degrade the sensitivity of the metal/insulator/GaN-based photodetector. (C) 2011 Elsevier Ltd. All rights reserved.
  • Masafumi Tajima, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 50 (6) 061001-1-061001-7 2011年06月 [査読無し][通常論文]
     
    Using a dual-gate structure, we have investigated the impact of gate-stress position on the current collapse behavior of AlGaN/GaN high-electron-mobility transistors (HEMTs) without surface passivation. When the gate-bias stress under the off state was applied to the additional gate between the main gate and the drain electrode, we observed a marked increase in on-resistance (RON). On the other hand, the off-state stress on the main gate itself caused a decrease in drain saturation current as well as an increase in RON. The calculation of electric field at the AlGaN surface showed that the field peaks existed at the gate edges on both the drain and source sides, probably causing electron charging at the AlGaN surface near both gate-edge areas. These results indicated that the off-state gate stress induces "virtual gates'' in the gate edges expanding in both the drain and source directions. The impacts of device structures on the current collapse have been characterized, using Schottky-gate HEMTs with and without surface passivation and metal-oxide-semiconductor (MOS) gate HEMTs. The surface passivation and MOS-gate structure was effective in mitigating current collapse, which was explained in terms of surface state density, electric field strength, and gate leakage current. (C) 2011 The Japan Society of Applied Physics
  • Piotr Bidzinski, Marcin Miczek, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 50 (4) 04DF08-1-04DF08-5 2011年04月 [査読無し][通常論文]
     
    The influence of interface state density and bulk carrier lifetime on the dependencies of photocapacitance versus wide range of gate bias (-0.1 to -3 V) and light intensity (10(9) to 10(20) photon cm(-2) s(-1)) was studied for metal/insulator/n-GaN UV light photodetector by means of numerical simulations. The light detection limit and photocapacitance saturation were analyzed in terms of the interface charge and interface Fermi level for electrons and holes and effective interface recombination velocity. It was proven that the excess carrier recombination through interface states is the main reason of photocapacitance signal quenching. It was found that the photodetector can work in various modes (on-off or quantitative light measurement) adjusted by the gate bias. A comparison between experimental data and theoretical capacitance-light intensity characteristics was made. A new method for the determination of the interface state density distribution from capacitance-voltage-light intensity measurements was also proposed. (C) 2011 The Japan Society of Applied Physics
  • M. Akazawa, B. Gao, T. Hashizume, M. Hiroki, S. Yamahata, N. Shigekawa APPLIED PHYSICS LETTERS 98 (14) 142117-1-142117-3 2011年04月 [査読無し][通常論文]
     
    The barrier structure in lattice-matched InAlN/GaN heterostructures with AlGaN-based spacer layers grown by metal organic vapor phase epitaxy was studied by the capacitance-voltage (C-V) method. To investigate the characteristics under positive bias, an Al2O3 overlayer was added. The C-V characteristic of a sample with an Al0.38Ga0.62N (5 nm)/AlN (0.75 nm) double spacer layer exhibited an anomalous saturation at a value far below the insulator capacitance under positive bias, which indicated electron accumulation at the InAlN/AlGaN interface. The C-V characteristic of an alternative sample with a single Al0.44Ga0.56N (1.5 nm) spacer layer did not exhibit the anomalous saturation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3578449]
  • Naohisa Harada, Yujin Hori, Naoki Azumaishi, Kota Ohi, Tamotsu Hashizume APPLIED PHYSICS EXPRESS 4 (2) 021002 2011年02月 [査読無し][通常論文]
     
    A selective electrochemical oxidation has been applied to the AlGaN surface to fabricate a recessed-oxide-gate structure for normally-off AlGaN/GaN high-electron-mobility transistors (HEMTs). We observed bias-dependent oxidation current characteristics peculiar to the AlGaN/GaN heterostructure. A flat interface between the oxide and AlGaN was confirmed by cross-sectional transmission electron microscopy. The selective formation of the recessed oxide allowed the local depletion of two-dimensional electron gas at the AlGaN/GaN interface and thus the achievement of normally-off operation. The recessed-oxide-gate HEMT with the oxide thickness of 20nm showed good gate control of drain current with the threshold voltage of +1.2 V. (C) 2011 The Japan Society of Applied Physics
  • Chihoko Mizue, Yujin Hori, Marcin Miczek, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 50 (2) 021001-1-021001-7 2011年02月 [査読無し][通常論文]
     
    The potential modulation and interface states of Al2O3/Al0.25Ga0.75N/GaN structures prepared by atomic layer deposition (ALD) were characterized by capacitance-voltage (C-V) measurements. We observed the peculiar C-V characteristics with two capacitance steps in forward and reverse bias regions, corresponding to the electron accumulation or depletion behavior at the Al2O3/AlGaN and AlGaN/GaN interfaces. From the experimental and calculated C-V characteristics, it was found that the charging and discharging of interface states near the AlGaN conduction-band edge mainly caused the stretch-out and hysteresis of the C-V curve at the forward bias. On the other hand, it is likely that the interface states near the midgap or deeper in energies act as fixed charges. From the bias-dependent hysteresis voltage in the forward bias region and the photo-induced voltage shift at the reverse bias, we estimated the interface state density distribution at the Al2O3/AlGaN interface for the first time. The present ALD-Al2O3/AlGaN/GaN structure showed relatively high interface state densities with a minimum density of 1 x 10(12) cm(-2) eV(-1) or higher. (C) 2011 The Japan Society of Applied Physics
  • Eri Ogawa, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 50 (2) 021002-1-021002-6 2011年02月 [査読無し][通常論文]
     
    We characterize the chemical and photoluminescence (PL) properties of GaN with Mg doping densities of 7 x 10(17) and 2 x 10(19) cm(-3), particularly focusing on their variations caused by the high-temperature process. Before the thermal process, band-edge emission dominated the PL spectrum in the lightly Mg-doped GaN, whereas luminescence at 3.2 eV with a high intensity similar to that of the band-edge line was observed in the highly Mg-doped GaN. For the lightly Mg-doped GaN, the thermal process at 900 degrees C enhanced the intensity of the broad band in the energy range from 1.5 to 2.5 eV. Pronounced Ga outdiffusion caused by the high-temperature process was also confirmed by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy, indicating that these emission lines originate from the transitions between the conduction band and deep levels related to a defect complex involving a Ga vacancy (V-Ga + X). For the highly Mg-doped GaN, the peak energy of the dominant PL line abruptly changed from 3.2 to 2.8 eV after the process at 1000 degrees C. Simultaneously, a non uniform profile of Mg density was found near the GaN surface, arising from the pronounced surface segregation of Mg. It is thus likely that a defect complex involving a Mg interstitial and a Ga vacancy (Mg-I + V-Ga) is responsible for the 2.8 eV band in the highly Mg-doped and high-temperature-processed GaN. (C) 2011 The Japan Society of Applied Physics
  • Jpn. J. Appl. Phys. 50 070209-1-070209-3 2011年 [査読無し][通常論文]
  • M. Akazawa, B. Gao, T. Hashizume, M. Hiroki, S. Yamahata, N. Shigekawa JOURNAL OF APPLIED PHYSICS 109 (1) 013703 2011年01月 [査読無し][通常論文]
     
    The valence band offsets, Delta E-V, of In0.17Al0.83N/GaN, In0.25Al0.75N/GaN, and In0.30Al0.70N/GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy (XPS). The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The Delta E-V values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated Delta E-V values were 0.2 +/- 0.2 eV for In0.17Al0.83N/GaN, 0.1 +/- 0.2 eV for In0.25Al0.75N/GaN, and 0.0 +/- 0.2 eV for In0.30Al0.70N/GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in Delta E-V was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3527058]
  • Cheng-Yu Hu, Tamotsu Hashizume, Kota Ohi, Masafumi Tajima APPLIED PHYSICS LETTERS 97 (22) 222103 2010年11月 [査読無し][通常論文]
     
    An electric-field-dependent trapping effect was observed in our AlGaN/GaN transmission-line-model (TLM) structure. Therefore, we adopted a "gradual voltage stress" scheme to induce a similar trapping effect, that is, a similar sheet resistance variation (Delta R-SH) for all intervals of our AlGaN/GaN TLM structure. By measuring the TLM structure under "gradual voltage stress" (stressed TLM measurement), we investigated the trapping effects in gateless AlGaN/GaN heterojunction field-effect transistors. 10 nm of Al2O3 passivation film substantially decreased the voltage-stress-induced Delta R-SH, suggesting surface traps were involved. The feasibility and reproducibility of the stressed TLM measurement method was confirmed with repeated passivation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3506583]
  • Toshiyuki Yoshida, Hashizume T. Tamotsu Applied Physics Express 3 (11) 116601 2010年11月 [査読無し][通常論文]
     
    Air-gap capacitance-voltage (C-V) characteristics of p-InP surfaces covered with natural oxide and treated with hydrogen-fluoride (HF) acid are reported for the first time. These surfaces show deep-depletion C-V behavior with large and distinctive hysteresis. To interpret the obtained C-V curves, a model was used that introduced a donor-like discrete level in the band-gap. Another assumption in this model is that the capture velocity of the discrete level and/or the generation-recombination rate of the minority carrier of the substrate is very low. By use of this model, the measured air-gap C-V curve was successfully reproduced. © 2010 The Japan Society of Applied Physics.
  • Yujin Hori, Chihoko Mizue, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 49 (8) 080201 2010年08月 [査読無し][通常論文]
     
    The effects of fabrication processes on the electrical properties of Al2O3/GaN structures prepared by atomic layer deposition were investigated "The annealing process at 800 degrees C for the formation of ohmic electrodes brought a large number of microcrystallization regions into the Al2O3 layer, causing a marked leakage in the current-voltage characteristics of the Al2O3/GaN structure The "ohmic-first" process with a SiN protection layer was thus applied to the GaN surface In this process, the amorphous phase in the atomic configuration of Al2O3 was maintained, leading to the I sufficient suppression of leakage current at the Al2O3/GaN interface In addition, the Al2O3/GaN structures showed good capacitance-voltage characteristics, resulting in low interface state densities of less than 1 x 10(12) cm(-2) eV(-1) (C) 2010 The Japan Society of Applied Physics
  • D. Gregusova, R. Stoklas, Ch Mizue, Y. Hori, J. Novak, T. Hashizume, P. Kordos JOURNAL OF APPLIED PHYSICS 107 (10) 106104 2010年05月 [査読無し][通常論文]
     
    Atomic layer deposition (ALD) of Al2O3 was used to prepare metal-oxide-semiconductor (MOS) devices on two different_AlGaN/GaN heterostructures, with and without a thin GaN cap layer. Their trapping effects were evaluated by the frequency dependent conductance measurement. The trap state density decreased sharply from similar to 1 x 10(12) cm(-2) eV(-1) at the energy of 0.27 eV to similar to 3 x 10(10) cm(-2) eV(-1) at 0.45 eV. The low trap state density and exactly exponential dependence of the trap state time constant on the gate voltage show a good quality of the gate oxide. The trap state density in the structure with a GaN cap is about 2-3 times lower than that in the structure without a cap, which might be due to the different Al2O3/GaN and Al2O3/AlGaN interface properties. The trap state density in the structures investigated is lower than those reported for the devices with the metal-organic chemical vapor deposition and Al-oxidized Al2O3 gate oxide. This shows an importance of the ALD technique for the preparation of high-performance AlGaN/GaN MOS transistors. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3428492]
  • M. Akazawa, T. Matsuyama, T. Hashizume, M. Hiroki, S. Yamahata, N. Shigekawa APPLIED PHYSICS LETTERS 96 (13) 132104 2010年03月 [査読無し][通常論文]
     
    The valence-band offset of a lattice-matched In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy (MOVPE) was investigated by x-ray photoelectron spectroscopy (XPS). Atomic force microscopy and angle-resolved XPS indicated that a thin In0.17Al0.83N (2.5 nm) layer was successfully grown by MOVPE on GaN. The XPS result showed that the valence band offset was 0.2 +/- 0.3 eV. This result indicates that the conduction-band offset at the In0.17Al0.83N/GaN interface is large, i.e., 0.9 to 1.0 eV, and occupies a large part of the entire band discontinuity.
  • Kimihito Ooyama, Katsuya Sugawara, Shinya Okuzaki, Hiroyuki Taketomi, Hideto Miyake, Kazumasa Hiramatsu, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 49 (10) 101001 2010年 [査読無し][通常論文]
     
    Deep electronic levels of AlxGa1-xN (0: 25 < x < 0: 60) were investigated by deep level transient spectroscopy (DLTS) and photocapacitance methods. Si-doped AlGaN layers were grown on an AlN/sapphire template by metal-organic vapor phase epitaxy. DLTS analysis using a sampling time window of up to 100 s showed two dominant deep levels with activation energies (Delta E) higher than 1.0 eV in AlxGa1-xN with x = 0.5 and 0.37. The densities of those levels were higher than 1 x 10(16) cm(-3). For the Al0.60Ga0.40N sample, the deeper levels (Delta E > 1. eV) were detected by photocapacitance measurement. It was found that the energy position of the dominant deep level closely followed the Fermi level stabilization energy reported by Walukiewicz et al. [J. Cryst. Growth 269 (2004) 119], indicating that the origin of the dominant deep level in AlGaN is related to a defect complex including anti-site defects and divacancies. (C) 2010 The Japan Society of Applied Physics
  • Toshiharu Kubo, Hiroyuki Taketomi, Hideto Miyake, Kazumasa Hiramatsu, Tamotsu Hashizume APPLIED PHYSICS EXPRESS 3 (2) 021004 2010年 [査読無し][通常論文]
     
    Chemical and electronic properties of Si-doped AlGaN surfaces with a wide range of Al compositions were investigated. The surface potential of AlGaN increased systematically from 0.9 (for GaN) to 2.6 eV (for Al0.87Ga0.13N), corresponding to the fact that the position of the surface Fermi level (E-Fsurf) approaches the midgap as the Al composition increases. In addition, the plots of EFsurf position closely follows the Fermi level stabilization energy reported by Walukiewicz et al. [J. Cryst. Growth 269 (2004) 119], indicating that the Fermi level position at the AlGaN surface can be governed by some kinds of surface defect transformations. (C) 2010 The Japan Society of Applied Physics DOI. 10.1143/APEX.3.021004
  • Kota Ohi, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 48 (8) Art. No. 081002 2009年08月 [査読無し][通常論文]
     
    The surrounding-field effect in a multi-mesa-channel (MMC) with an AlGaN/GaN structure, in which a periodic trench structure is fabricated directly under a gate electrode, was successfully observed. This effect resulted in a shallower threshold voltage, a smaller subthreshold slope, and a higher current drivability of a high electron mobility transistor (HEMT) than those of a standard planar-type HEMT. In addition, the MMC HEMT showed a low knee voltage, even with a wide spacing between the gate and drain electrodes. Excellent current, stability in the saturation region of the MMC HEMT, probably due to the effective radiation of heat from both mesa sides of the channel, was also observed. Both planar and MMC HEMTs showed similar breakdown voltages under off-state operation, indicating no significant degradation in the breakdown characteristics of AlGaN/GaN HEMTs with a periodic trench structure in the gate region. (C) 2009 The Japan Society of Applied Physics
  • Katsuya Sugawara, Junji Kotani, Tamotsu Hashizume APPLIED PHYSICS LETTERS 94 (15) 152106 2009年04月 [査読無し][通常論文]
     
    A deep level with an activation energy of 1.0 eV in n-type Al0.26Ga0.74N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy (DLTS) with a sampling time window of several seconds. The deep-level density was 6x10(15) cm(-3). At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from E-C-1.5 to E-C-2.3 eV.
  • Marcin Miczek, Boguslawa Adamowicz, Chihoko Mizue, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 48 (4) Art. No. 04C092 2009年04月 [査読無し][通常論文]
     
    The effect of electronic states at insulator/AlGaN and AlGaN/GaN interfaces on capacitance-voltage (C-V) characteristics of a metal/insulator/semiconductor (MIS) capacitor with an AlGaN layer and a metal/insulator/semiconductor heterostructure (MISH) with an AlGaN/GaN heterojunction was studied theoretically taking into account extremely slow electron emission from the deep interface levels. The states at the insulator/AlGaN interface in MIS and at the AlGaN/GaN interface in MISH stretch out the C-V curves as usual, whereas the insulator/AlGaN interface states in MISH only shift the characteristics at room temperature and 300 degrees C (fixed-charge-like behavior). These effects were explained by the different positions of the Fermi level at the studied interfaces. Theoretical C-V curves were compared with experimental characteristics, which are available in the literature, measured at room and higher temperatures by the authors and others. (c) 2009 The Japan Society of Applied Physics
  • Nanako Shiozaki, Tamotsu Hashizume JOURNAL OF APPLIED PHYSICS 105 (6) 064912 2009年03月 [査読無し][通常論文]
     
    Surface control of n-GaN was performed by applying a photoelectrochemical oxidation method in a glycol solution to improve the optical and electronic characteristics. The fundamental properties of the oxidation were investigated. The oxidation, chemical composition, and bonding states were analyzed by x-ray photoelectron spectroscopy and micro-Auger electron spectroscopy, in which confirmed the formation of gallium oxide on the surface. The oxide formation rate was about 8 nm/min under UV illumination of 4 mW/cm(2). After establishing the basic properties for control of n-GaN oxidation, the surface control technique was applied to achieve low-damage etching, enhancement of the photoluminescence intensity, and selective passivation of the air-exposed sidewalls in an AlGaN/GaN high electron mobility transistor wire structure. The capacitance-voltage measurement revealed the minimum interface-state density between GaN and anodic oxide to be about 5x10(11) cm(-2) eV(-1), which is rather low value for compound semiconductors.
  • Masafumi Tajima, Junji Kotani, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 48 (2) Art. No. 020203 2009年02月 [査読無し][通常論文]
     
    We have investigated the effects of thin native oxide layers on the AlGaN surface on the DC characteristics of AlGaN/GaN high electron mobility transistors (HEMTs). After HEIVIT fabrication, the AlGaN surface between the electrodes was intentionally oxidized using either O(2) or N(2)O plasma. X-ray photoelectron spectroscopy (XPS) analysis showed that both methods produced a native oxide of AlGaN with a thickness of about 1 nm and that an N-O chemical bond was present in the N(2)O plasma oxide. We observed pronounced degradation in the DC characteristics and current collapse in the HEMT with O(2) plasma oxidation. In contrast, the formation of native oxide by the N(2)O plasma had no effect on the DC characteristics or current stability of the AIGaN/GaN HEMT. Possible mechanisms for device degradation were discussed in terms of stress and deep levels in the AlGaN layer induced by oxide formation or oxygen incorporation. (C) 2009 The Japan Society of Applied Physics
  • Chihoko Mizue, Marcin Miczek, Junji Kotani, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 48 (2) Art. No. 020201 2009年02月 [査読無し][通常論文]
     
    We performed a feasibility study of UV detection using an AlOx/n-GaN structure. The AlOx layer was deposited on n-GaN using diethyl aluminum ethoxide as a precursor. We obtained a refractive index of 1.59, a bandgap of 7.0eV, and an oxygen composition of 1.48 for the AlOx layer. Even at higher temperatures, no significant change appeared in the capacitance-voltage curves, which were close to the ideal curve, indicating well-controlled and stable interface properties. From the photo response measurements, pronounced variation of the interface potential at the AlOx/GaN interface was observed under UV illumination, which was independent of temperature. The gate bias systematically modulated the depth of the interface potential well and thereby the density of accumulated holes resulting from UV illumination. The AlOx/GaN structure is sensitive to flame with a low power density. (C) 2009 The Japan Society of Applied Physics
  • 日本結晶成長学会誌 36 205 -213 2009年 [査読無し][通常論文]
  • Tamotsu Hashizume, Nanako Shiozaki, Kota Ohi GALLIUM NITRIDE MATERIALS AND DEVICES IV 7216 7216-0U-1 2009年 [査読無し][通常論文]
     
    Surface characterization and control technologies were applied to GaN and AlGaN surfaces. It was found that a unique "air-gap CV" technique is effective in evaluating surface state density on free AlGaN surfaces. A photoelectrochemical process, utilizing a mixed solution of propylene glycol and tartaric acid, was employed to form a thin oxide layer on GaN and AlGaN. We observed an enhancement of drain current in the AlGaN/GaN HEMT having a narrow channel width of 500 nm after the oxidation of the channel walls by the electrochemical process. To improve the uniformity of the effective electric field in the channel, a multi-mesa-channel (MMC) AlGaN/GaN HEMT has been proposed and developed. With forming a periodic trench just under the gate region by an ECR-plasma assisted dry etching, the MMC HEMT has parallel mesa-shaped channels with 2-dimensional electron gas (2DEG) surrounded by the top-and side-gate electrodes.
  • Takeshi Kimura, Tamotsu Hashizume JOURNAL OF APPLIED PHYSICS 105 (1) 014503-1-014503-5 2009年01月 [査読無し][通常論文]
     
    We intentionally incorporated carbon into n-GaN by high-temperature annealing of a SiN(x)/CN(x)/GaN structure to study the effect of unintentional carbon incorporation on the electrical properties of n-type GaN surfaces. X-ray photoelectron spectroscopy results showed outdiffusion of Ga atoms from the GaN surface during high-temperature annealing even when the SiN(x) layer was present. The current-voltage characteristics showed a drastic increase in current in the forward and reverse directions of the Schottky diode in the carbon-incorporated sample. They also showed no temperature dependence from 150 to 300 K. The current-voltage curves of the carbon- incorporated samples in the forward and reverse directions could be almost completely reproduced by assuming an exponentially decaying distribution from the surface for shallow donors. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056395]
  • Tsutomu Uesugi, Tetsu Kachi, Masahiro Sugimoto, Tetsuya Matsuyama, Chihoko Mizue, Tamotsu Hashizume JOURNAL OF APPLIED PHYSICS 104 (1) 016103-1-016103-3 2008年07月 [査読無し][通常論文]
     
    We performed a feasibility study on the deposition of aluminum oxides (AlOx) on GaN using diethyl aluminum ethoxide [(C2H5)(2)AlOC2H5] as a precursor by electron-cyclotron-resonance-assisted chemical vapor deposition. We determined the refractive index and permittivity of the deposited AlOx layer to be 1.59 and 8.3, respectively. An x-ray photoelectron spectroscopy (XPS) study showed that the energy positions of the Al- and O-core levels were very close to those of the reference crystalline Al2O3 and that there was no significant peak related to carbon in the AlOx film. XPS also indicated a bandgap of 7.0 eV and oxygen composition of 1.48 for the AlOx layer deposited on GaN. We found good capacitance-voltage (C-V) behavior for the Ni/AlOx/n-GaN diode including accumulation and depletion behavior at room temperature. Even at high temperatures, the C-V slope remained unchanged indicating relatively low interface state densities near the midgap. (c) 2008 American Institute of Physics.
  • Kimihito Ooyama, Hiroki Kato, Marcin Miczek, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS 47 (7) 5426 -5428 2008年07月 [査読無し][通常論文]
     
    With a combination of the static capacitance-voltage (C-V) and the capacitance transient (C-t) methods. the interface-state response in in Al2O3/n-GaN structure was investigated at temperatures ranging from 23 to 300 degrees C. We observed pronounced degradation of the static C-V curves measured at high temperatures. arising front the enhancement of charging/discharging rates of interface states, at deeper energies within the bandgap of GaN. Faster responses with larger magnitudes also appeared in the time-dependent capacitance at high temperatures. From a simple analysis of the C-t results, we estimated the capture cross section of the states to be on the order 10(-19) cm(2).
  • Marcin Miczek, Chihoko Mizue, Tamotsu Hashizume, Boguslawa Adamowicz JOURNAL OF APPLIED PHYSICS 103 (10) 104510-1-104510-11 2008年05月 [査読無し][通常論文]
     
    The impact of states at the insulator/AlGaN interface on the capacitance-voltage (C-V) characteristics of a metal/insulator/AlGaN/GaN heterostructure (MISH) capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiNx/AlGaN interface state density D-it(E) was increased. We attribute this behavior to the position of the Fermi level at the SiNx/AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300 degrees C. We analyzed the experimental C-V characteristics from a SiNx/Al2O3/AlGaN/GaN structure measured at room temperature and 300 degrees C, and extracted a part of D-it(E). The relatively low D-it (similar to 10(11) eV(-1) cm(-2)) in the upper bandgap indicates that the SiNx/Al2O3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices. (C) 2008 American Institute of Physics.
  • Taketomo Sato, Akinori Mizohata, Naoki Yoshizawa, Tamotsu Hashizume APPLIED PHYSICS EXPRESS 1 (5) 051202 2008年05月 [査読無し][通常論文]
     
    The electrocatalytic activity of n-type InP porous nanostructures was investigated in terms of their application to amperometric biochemical sensors. The current sensitivities for H(2)O(2) detection were strongly dependent on the structural properties of these porous nanostructures. A sample with deeper pores responsed more sensitively because of the enlarged surface area inside the nanopores. The removal of an irregular top layer also effectively improved the current sensitivity. The conductive porous nanostructures presented here were very promising for the direct amperometric detection of H(2)O(2). (c) 2008 The Japan Society of Applied Physics.
  • Takahiro Tamura, Junji Kotani, Seiya Kasai, Tamotsu Hashizume APPLIED PHYSICS EXPRESS 1 (2) 023001-1-023001-3 2008年02月 [査読無し][通常論文]
     
    We fabricated a multi-mesa-channel (MMC) structure by forming a periodic trench just under a gate electrode to improve the uniformity of effective electric field in the channel in an AlGaN/GaN high electron mobility transistor (HEMT). A unique performance, i.e., a nearly temperature-independent saturation drain current, was observed in the MMC device in a wide temperature range. A two-dimensional (2D) potential calculation indicates that the mesa-side gate effectively modulates the potential, resulting in a field surrounding 2D electron gas. Such a surrounding-field effect and a relatively lower source access resistance may be related to a unique current behavior in the MMC HEMT. (C) 2008 The Japan Society of Applied Physics.
  • Junji Kotani, Masafumi Tajima, Seiya Kasai, Tamotsu Hashizume APPLIED PHYSICS LETTERS 91 (9) 093501-1-093501-3 2007年08月 [査読無し][通常論文]
     
    Lateral surface leakage current (I-S) on an AlGaN/GaN heterostructure was systematically investigated by using a two-parallel gate structure with a gap distance (L-GG) of 200 nm-5 mu m. The surface current I-s systematically increased as LGG decreased. A simple resistive layer conduction that should show 1/L-GG dependence failed to account for the drastic increase in Is when LGG was reduced to less than 1 mu m. However, no dependence on L-GG was seen in vertical current that flows in the Schottky interface. The I-s showed a clear temperature dependence proportional to exp(-T (-1/ 3)), indicating two-dimensional variable-range hopping through high-density surface electronic states in AlGaN. A pronounced reduction in surface current of almost four orders of magnitude was observed in a sample with SiNx passivation. (c) 2007 American Institute of Physics.
  • Toshiyuki Fujino, Taketomo Sato, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 46 (7A) 4375 -4380 2007年07月 [査読無し][通常論文]
     
    A high-densjty array of size-controlled porous nanostructures was formed on InP(001) substrates by a two-step electrochemical process using anodic and cathodic reactions. After the formation of InP porous structures by anodization, we subsequently applied cathodic decomposition to the reduction of the lateral thickness of the InP pore wall. The etching rate of the pore wall was extremely low and strongly dependent on the cathodic bias and crystal orientations of the wall surface. The thickness of InP walls could be precisely controlled in the range of 15-30nm by changing cathodic bias and cathodic decomposition time. From the PL measurement at room temperature, the porous structures showed strong blue-shifted emissions due to quantum confinement in thinner InP nanowalls.
  • Eri Ogawa, Tamotsu Hashizume, Satoshi Nakazawa, Tetsuzo Ueda, Tsuyoshi Tanaka JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 (20-24) L590 -L592 2007年06月 [査読無し][通常論文]
     
    We investigate the chemical and potential-bending characteristics of in situ SiNx/AlGaN interfaces prepared by metal-organic chemical vapor deposition. X-ray photoelectron spectroscopy showed that the in situ SiNx layer had typical chemical binding energies corresponding to the Si-N bonds. The in situ SiNx deposition brought no chemical degradation on the AlGaN surface at the SiNx/AlGaN interface, whereas the ex situ deposition of SiNx by a plasma process induced chemical disorder on the AlGaN surface including a composition change and the formation of interfacial oxides. A significant reduction in the surface band bending was observed on the AlGaN surface after the in situ SiNx passivation, probably due to a decrease in the surface state density.
  • M. Kaneko, T. Hashizume, V. A. Odnoblyudov, C. W. Tu JOURNAL OF APPLIED PHYSICS 101 (10) 103707-1-103707-5 2007年05月 [査読無し][通常論文]
     
    We have investigated electrical properties and deep levels of n-GaP1-xNx (x=0%-0.62%) grown on (100) n-GaP substrates by gas source molecular beam epitaxy. The x-ray photoelectron spectroscopy results showed no significant effects on the chemical bonding status of the host Ga-P matrix by the incorporation of small amounts of N atoms. In the Raman spectra, the zone-edge GaP-like vibration was observed at 387 cm(-1), originating from alloy disorder or local distortion of the GaP1-xNx lattice. The electrical properties of the GaP1-xNx surfaces were characterized using a Schottky contact structure. An ideality factor of 1.10-1.15 and a Schottky barrier height of 1.1 eV were obtained from the current-voltage characteristics of Ni/GaP1-xNx diodes, indicating good interface quality. The thermal admittance spectroscopy clearly detected the Si donor level with an activation energy of 84 +/- 4 meV in GaP and GaP1-xNx. For the GaP1-xNx samples, we observed deep levels probably associated with N-induced defects such as N-N pairs, N clusters, and an N-containing complexes. (c) 2007 American Institute of Physics.
  • Yuki Hayakawa, Yoshiyuki Suda, Tamotsu Hashizume, Hirotake Sugawara, Yosuke Sakai JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 (12-16) L362 -L364 2007年04月 [査読無し][通常論文]
     
    Hydrogen gas sensors based on single-walled carbon nanotube (SWCNT) films were fabricated with Pd comb-like electrodes. The SWCNT films were deposited by Mo/Co catalyst-assisted chemical vapor deposition using alcohol. The film included a high-density network of SWCNTs. The SWCNT film sensor showed a large conductance change and a fast response (20s) upon exposure to 1% H-2 gas in air at room temperature, while the conductance change was negligible for devices using Au electrodes.
  • Nanako Shiozaki, Taketomo Sato, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 46 (4A) 1471 -1473 2007年04月 [査読無し][通常論文]
     
    In this paper, we report on the feasibility of oxidizing n-GaN by an electrochemical process in a mixture containing an aqueous solution of tartaric a id and propylene glycol. Photons generated by UV illumination were supplied at the electrolyte-GaN interface during the oxidation process. In the constant-voltage mode, X-ray photoelectron spectroscopy analysis revealed that relatively thick Ga oxide layer formed on the n-GaN surface. However, the oxide surface was very rough. In addition, we found metallic Ga components in the oxide layer or at the oxide-GaN interface for longer oxidation times. On the other hand, a thin Ga2O3 layer with a smooth surface was grown by a constant-current process.
  • Takeshi Kimura, Shinya Ootomo, Takehiko Nomura, Seikoh Yoshida, Tamotsu Hashizume JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 46 (8-11) L224 -L226 2007年03月 [査読無し][通常論文]
     
    We performed a feasibility study on the solid-phase diffusion of carbon into GaN using a SiNx/CNx/GaN structure prepared by electron-cyclotron-resonance-assisted chemical vapor deposition. An X-ray photoelectron spectroscopy study on the CNx layer deposited on GaN showed that its energy positions and spectrum features are very close to those of a C-N bond, and the N composition was estimated to be 24%, indicating a highly C-rich layer. No degradation in the chemical properties of the GaN surface was found after the diffusion process at 1000 degrees C. A secondary ion mass spectrometry result clearly showed a diffusion of carbon into GaN. We also observed an increase in resistivity for the C-diffused GaN layer.
  • Wancheng Zhang, Nan-Jian Wu, Tamotsu Hashizume, Seiya Kasai IEEE TRANSACTIONS ON NANOTECHNOLOGY 6 (2) 146 -157 2007年03月 [査読無し][通常論文]
     
    This paper proposes two kinds of novel hybrid voltage controlled ring oscillators (VCO) using a single electron transistor (SET) and metal-oxide-semiconductor (MOS) transistor. The novel SET/MOS hybrid VCO circuits possess the merits of both the SET circuit and the MOS circuit. The novel VCO circuits have several advantages: wide frequency tuning range, low power dissipation, and large load capability. We use the SPICE compact macro model to describe the SET and simulate the performances of the SET/MOS hybrid VCO circuits by HSPICE simulator. Simulation results demonstrate that the hybrid circuits can operate well as a VCO at room temperature. The oscillation frequency of the VCO circuits could be as high as 1 GHz, with a -71 dBc/Hz phase noise at 1 MHz offset frequency. The power dissipations are lower than 2 uW. We studied the effect of fabrication tolerance, background charge, and operating temperature on the performances of the circuits.
  • Tatsuya Nakamura, Seiya Kasai, Yuta Shiratori, Tamotsu Hashizume APPLIED PHYSICS LETTERS 90 (10) 102104 2007年03月 [査読無し][通常論文]
     
    A three-terminal nanowire junction device controlled by double nanometer-sized Schottky wrap gates (WPGs), which control left and right branches independently, are fabricated utilizing AlGaAs/GaAs etched nanowires and characterized experimentally. Fabricated device exhibits clear nonlinear characteristics of output voltage at the center terminal by applying voltages to left and right terminals in push-pull fashion. Applying asymmetric gate voltages to left and right WPGs provides clear asymmetry in the output voltage. The nonlinearity in the low voltage regions is greatly enhanced by squeezing both left and right branches using WPGs. (c) 2007 American Institute of Physics.
  • Boguslawa Adamowicz, Marcin Miczek, Tamotsu Hashizume, Andrzej Klimasek, Piotr Robek, Janusz Zywicki OPTICA APPLICATA 37 (4) 327 -334 2007年 [査読無し][通常論文]
     
    AlGaN/GaN heterostructures passivated by SiO2/Si3N4 and SiNx/Si3N4 bilayers were characterized, electrically by capacitance-voltage measurements and chemically by Auger microscopy chemical in-depth profiling. The 2-dimensional electron gas density was estimated from C-V curves and the electronic quality of the bilayers was evaluated from C-V hysteresis. Detailed variations of Auger peaks, in particular for oxygen, silicon, nitrogen, and carbon, versus argon ion sputtering time were registered. The electronic properties of these two structures were compared with each other and to their chemistry.
  • K. Kumakura, T. Makimoto, N. Kobayashi, T. Hashizume, T. Fukui, H. Hasegawa JOURNAL OF CRYSTAL GROWTH 298 787 -790 2007年01月 [査読無し][通常論文]
     
    We fabricated InGaN p-n junction diode structures on SiC substrates by metalorganic vapor phase epitaxy, and investigated the minority carrier diffusion length in n- and p-InGaN layers by electron beam induced current measurements. The minority electron diffusion length in p-InGaN was little affected by the In content in InGaN. The diffusion length decreased with increasing Mg-doping concentration. The minority hole diffusion length in n-InGaN was little affected by Si-doping concentration but slightly decreased with increasing In content in InGaN. We also fabricated pup AlGaN/InGaN/GaN double heterojunction bipolar transistors and investigated their common-emitter current-voltage characteristics. The Si-doping concentration in the base was 4 x 10(19) cm(-3). The maximum current gain was 21 at a collector current of -10 mA for an emitter size of 30 mu m x 50 mu m. This good performance is ascribed to the large conduction band discontinuity between the AlGaN emitter and InGaN base. (c) 2006 Elsevier B.V. All rights reserved.
  • Taketomo Sato, Toshiyuki Fujino, Tamotsu Hashizume ELECTROCHEMICAL AND SOLID STATE LETTERS 10 (5) H153 -H155 2007年 [査読無し][通常論文]
     
    A two-step electrochemical process using anodic and cathodic reactions was developed to form size-controlled nanostructures on InP(001) substrates. After anodic formation of a nanopore array, the cathodic decomposition process was applied to reduce the thickness of InP nanowalls. The etching rate of the nanowalls was extremely small and strongly dependent on the cathodic bias and crystal orientations of the wall surface. Wall thickness could be controlled in the range of 10-30 nm by changing the cathodic bias and processing time. (c) 2007 The Electrochemical Society.
  • Junji Kotani, Masamitsu Kaneko, Hideki Hasegawa, Tamotsu Hashizume JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 24 (4) 2148 -2155 2006年07月 [査読無し][通常論文]
     
    Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area AlGaN Schottky diodes after the application of the surface control process. This process also reduced leakage currents of the gate of the heterostructure field effect transistor device by more than one order of magnitude and increased temperature dependences of current. (c) 2006 American Vacuum Society.
  • T Hashizume, J Kotani, A Basile, M Kaneko JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 45 (4-7) L111 -L113 2006年02月 [査読無し][通常論文]
     
    We proposed a surface control process for suppressing the tunneling leakage of Schottky gates on AlGaN/GaN heterostructures. For the recovery of nitrogen-vacancy-related defects and reduction in the amount of oxygen impurities at the AlGaN surface, the process consisted of nitrogen radical treatment, the deposition of an ultrathin Al layer, UHV annealing and finally the removal of the Al layer. Ni/Au Schottky gates fabricated on processed AlGaN surfaces showed pronounced reduction in leakage current and a clear temperature dependence of I-V characteristics, indicating the effective suppression of tunneling leakage in current transport through AlGaN Schottky interfaces.
  • Influence of surface states and bulk traps on non-equilibrium phenomena at GaAs and GaN surfaces
    OPTICA APPLICATA 35 355 -361 2006年 [査読無し][通常論文]
  • Masamitsu Kaneko, Tamotsu Hashizume PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6 3 (6) 1758 -1761 2006年 [査読無し][通常論文]
     
    A surface control process which suppresses leakage currents in Ni Schottky interfaces fabricated on AlGaN/GaN heterostructures has been suggested. XPS results showed that oxygen impurities which were preferentially incorporated in AlGaN, could be removed by formation of ultrathin aluminum oxide through aluminum deposition and in-situ annealing at 700 degrees C. From the detailed analysis of current-voltage characteristics, it is found that the leakage currents depend on the tunneling transport through Ni/AlGaN Schottky interfaces. Ni/AlGaN Schottky contacts with surface control process exhibit the suppression of reverse leakage current by three orders of magnitude as compared to those without surface control process. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • Junji Kotani, Seiya Kasai, Hideki Hasegawa, Tamotsu Hashizume e-Journal of Surface Science and Nanotechnology 3 433 -438 2005年12月09日 [査読無し][通常論文]
     
    Gate leakage currents in AlGaN/GaN HFETs were investigated by comparing experiments with computer simulations based on the thin surface barrier (TSB) model involving unintentional surface donors. Leakage currents in large area Schottky diodes were explained by the TSB model involving nitrogen vacancy related deep donors and oxygen shallow donors. On the other hand, in AlGaN/GaN HFETs with nanometer scale Schottky gates, gate leakage currents include an additional leakage component due to lateral electron injection through tunneling at the gate edge where the barrier thinning is mainly controlled by oxygen donors. By combining vertical and lateral tunneling components, experiments could be reproduced on computer. Lateral components may be responsible for current collapse. © 2005 The Surface Science Society of Japan.
  • Kazushi Matsuo, Takeshi Kimura, Hideki Hasegawa, Tamotsu Hashizume e-Journal of Surface Science and Nanotechnology 3 314 -318 2005年11月14日 [査読無し][通常論文]
     
    Basic H2 gas-sensing characteristics of Pd Schottky diodes formed on the AlGaN/GaN heterostructure were investigated in vacuum. By introducing a novel surface control process to reduce reverse leakage currents, an unprecedented high H2 sensitivity was achieved where 1 Torr hydrogen caused five orders of magnitude change of current. Surprisingly, the surface control process did not change the C - V characteristics which showed an unexpectedly large shift 1000 mV on H2 exposure. Transient waveforms were almost exponential for the logarithm of current, and response speed increased with increase of H2 pressure and temperature. These results were explained in terms of Schottky barrier height due to adsorption of atomic hydrogen generated by Pd together with a due consideration of the current transport mechanism under reverse bias. © 2005 The Surface Science Society of Japan.
  • J Kotani, S Kasai, T Hashizume, H Hasegawa JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 23 (4) 1799 -1807 2005年07月 [査読無し][通常論文]
     
    The gate leakage and gate control characteristics of AlGaN/GaN heterostructure field effect transistors (HFETs) were systematically investigated in an attempt to clarify possible effects of surface states. The experiments were compared to rigorous computer simulations. We observed large amounts of leakage currents in the Schottky diodes fabricated on the AlGaN epitaxial layers. By the calculation based on a thin surface barrier model in which the effects of surface defect donor were taken into account, this large leakage was well explained by enhancement of tunneling transport processes due to the barrier thinning associated with ionization of surface-defect donor. On the other hand, the analysis on the current-voltage characteristics for the nanometer-scale Schottky contacts on AlGaN/GaN HFETs, indicated additional lateral leakage components. The comparison of the gate control characteristics between experiment and calculation clearly showed that the effective lateral expansion of gate length significantly impeded the g,, enhancement by the reduction of geometrical gate length. This can be explained by the lateral electron tunneling process at the AlGaN surface stimulated by the pronounced gate leakage currents. Due to frequent tunneling transfer at the gate periphery, surface state occupancy near the gate becomes governed by the metal Fermi level, causing the dynamic surface state charging effects. This resulted in effective widening of the gate length, leading to degradation of gate control performance in AlGaN/GaN HFETs. (c) 2005 American Vacuum Society.
  • K Matsuo, N Negoro, J Kotani, T Hashizume, H Hasegawa APPLIED SURFACE SCIENCE 244 (1-4) 273 -276 2005年05月 [査読無し][通常論文]
     
    Exposure of Pt/GaN and Pt/AlGaN/GaN Schottky diodes to H-2 gas at moderately high temperatures around 100 degrees C resulted in marked increase of forward and reverse currents. Increase was much larger in the Pt/AlGaN/GaN diode than in the Pt/GaN diode. Rapid turn-on responses and somewhat slower turn-off responses were observed with reproducible response magnitudes. A rigorous computer simulation of I-V curves indicated that current changes are due to changes in the Schottky barrier height caused either by H-induced formation of interfacial dipole or by hydrogen passivation of interface states. (c) 2005 Published by Elsevier B.V.
  • T Oikawa, F Ishikawa, T Sato, T Hashizume, H Hasegawa APPLIED SURFACE SCIENCE 244 (1-4) 84 -87 2005年05月 [査読無し][通常論文]
     
    This paper attempts to form AlGaN/GaN quantum wire (QWR) network structures on patterned GaN (0 0 0 1) substrates by selective molecular beam epitaxy (MBE) growth. Substrate patterns were prepared along < 1 1 2 0 > - and < 1 1 0 0 >-directions by electron cyclotron resonance assisted reactive-ion beam etching (ECR-RIBE) process. Selective growth was possible for both directions in the case of GaN growth, but only in the < 1 1 2 0 >-direction in the case of AlGaN growth. A hexagonal QWR network was successfully grown on a hexagonal mesa pattern by combining the < 1 1 2 0 >-direction and two other equivalent directions. AFM observation confirmed excellent surface morphology of the grown network. A clear cathodoluminescence (CL) peak coming from the embedded AlGaN/GaN QWR structure was clearly identified. (c) 2004 Published by Elsevier B.V.
  • K Kumakura, T Makimoto, N Kobayashi, T Hashizume, T Fukui, H Hasegawa APPLIED PHYSICS LETTERS 86 (5) 052105-1-052105-3 2005年01月 [査読無し][通常論文]
     
    We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p-n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108 cm(-2). It increased from 220 to 950 nm with decreasing Mg doping concentration from 3 X 10(19) to 4 X 10(18) cm(-3). For relatively high dislocation density above 109 cm(-2), it was less than 300 nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250 nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length. (C) 2005 American Institute of Physics.
  • "GaN系ショットキー接合のリーク電流"<解説論文>
    電子情報通信学会論文誌(c) J88-C巻 (8号) 621 -629 2005年 [査読無し][通常論文]
  • J Kotani, H Hasegawa, T Hashizume APPLIED SURFACE SCIENCE 237 (1-4) 213 -218 2004年10月 [査読無し][通常論文]
     
    This paper attempts a rigorous computer simulation of the current transport in GaN and AlGaN Schottky diodes on the basis of the thin surface barrier (TSB) model recently proposed by the authors' group. First, a computer program was developed which can calculate current transport through an arbitrary potential profile of Schottky barrier by a combined mechanism of thermionic emission (TE), thermionic-field emission (TFE) and field emission (FE). Then, from the view point of the TSB model, attempts were made to fit the theoretical temperature dependent current voltage (I-V-T) curves to the measured I-V-T data taken on Ni/nGaN and Ni/n-AlGaN Schottky diodes changing the barrier profiles and the energy depth of the surface donor. As compared with the previous poor fitting using approximate analytic formulas, excellent fitting was obtained for both forward and reverse current, confirming the validity of the TSB model as the mechanism for anomalously large leakage currents in GaN and AlGaN Schottky diodes. Best fittings for GaN and Al0.26Ga0.74N were obtained for exponentially decaying distributions of surface defect donors with the peak density of 5 x 10(18) cm(-3) and 1 x 10(19) cm(-3), the characteristic decay depth of 11 nm and 11.5 nm and the energy depth of 0.25 eV and 0.37 eV, respectively. (C) 2004 Elsevier B.V. All rights reserved.
  • J Kotani, T Hashizume, H Hasegawa JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 22 (4) 2179 -2189 2004年07月 [査読無し][通常論文]
     
    Using a rigorous computer simulation program for current transport through a Schottky barrier with an arbitrary potential profile, the leakage current mechanism in GaN and AlGaN Schottky diodes was investigated on the basis of the thin surface barrier (TSB) model recently proposed by the authors' group. Computer simulation assuming various possible defect density distributions was carried out to reproduce the measured temperature dependent current voltage. (I-V)-temperature characteristics of the GaN and AlGaN Schottky diodes which showed excessive reverse leakage. By assuming exponentially decaying distributions from surface for defect donors with energy depth of 0.25 eV for GaN and 0.37 eV for Al0.15Ga0.85N, I-V curves measured by our group as well as reported in the literatures were almost completely reproduced both in forward and reverse direction over a wide temperature range. The defect donors are proposed to be N vacancies or their related complexes that are formed during metal deposition. The result confirms the validity of the TSB model. From the viewpoint of the TSB model, attempts were also made to suppress leakage currents. It was found that a low-energy electrochemical metal deposition process and a metal-insulator-semiconductor Schottky structure using an ultrathin Al2O3 film by electron cyclotron resonance oxidation of Al film were remarkably effective in reducing excess leakage currents due to reduction of defect deep donors.
  • T Hashizume, H Hasegawa APPLIED SURFACE SCIENCE 234 (1-4) 387 -394 2004年07月 [査読無し][通常論文]
     
    Effects of device processing on chemical and electronic properties of AlGaN surfaces were investigated. The X-ray photoelectron spectroscopy analysis showed serious deterioration such as stoichiometry disorder and nitrogen deficiency (N deficiency) at the AlGaN surfaces processed by high-temperature annealing, H-2-plasma cleaning, dry etching in CH4/H-2/Ar plasma and deposition Of SiO2. This resulted in high density of surface states at the processed AlGaN surface. Furthermore, the N deficiency introduced a localized deep donor level related to N vacancy (V-N) at AlGaN surfaces. Such electronic states governed by a V-N-related deep donor and surface state continuum can cause strong Fermi level pinning at the AlGaN surface, reduction of the barrier height and excess leakage currents at the AlGaN Schottky interface and serious drain current collapse in AlGaN/GaN heterostructure field effect transistors. The SiNx- or Al2O3-based passivation scheme with a combination of a remote N-2-plasma treatment was found to be effective in suppressing formation Of VN-related surface defects at AlGaN surfaces. (C) 2004 Elsevier B.V. All rights reserved.
  • T Hashizume, J Kotani, H Hasegawa APPLIED PHYSICS LETTERS 84 (24) 4884 -4886 2004年06月 [査読無し][通常論文]
     
    Based on detailed temperature-dependent current-voltage (I-V-T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I-V-T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I-V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces. (C) 2004 American Institute of Physics.
  • T Hashizume, S Anantathanasarn, N Negoro, E Sano, H Hasegawa, K Kumakura, T Makimoto JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 43 (6B) L777 -L779 2004年06月 [査読無し][通常論文]
     
    An Al2O3 insulated-gate (IG) structure was utilized for controlling the surface potential and suppressing the gate leakage in Al0.2Ga0.8N/GaN heterostructure field effect transistors (HFETs) having thin AlGaN barrier layers (less than 10 nm). In comparison with the Schottky-gate devices, the Al2O3 IG device showed successful gate control of drain current up to V-GS = +4V without leakage problems. The threshold voltage in the Al2O3 IG HFET was about -0.3V, resulting in the quasi-normally-off mode operation.
  • "窒化物半導体電子デバイスにおける表面制御(解説)"
    応用物理、解説 73巻 (3号) 333 -338 2004年 [査読無し][通常論文]
  • T Hashizume, S Ootomo, H Hasegawa APPLIED PHYSICS LETTERS 83 (14) 2952 -2954 2003年10月 [査読無し][通常論文]
     
    We investigated effects of electronic states at free surfaces of AlGaN/GaN heterostructure field-effect transistors (HFETs) on the inner current transport at the heterointerfaces. The analysis on transient currents for the air-exposed and H-2-plasma-treated devices showed that N-vacancy-related near-surface traps play an important role in current collapse in AlGaN/GaN HFETs. An Al2O3-based surface passivation scheme including an N-2-plasma surface treatment was proposed and applied to an insulated-gate HFET. A large conduction-band offset of 2.1 eV was achieved at the Al2O3/Al0.3Ga0.7N interface. No current collapse was observed in the fabricated Al2O3 insulated-gate HFETs under both drain stress and gate stress. (C) 2003 American Institute of Physics.
  • T Hashizume JOURNAL OF APPLIED PHYSICS 94 (1) 431 -436 2003年07月 [査読無し][通常論文]
     
    Chemical and electronic properties of Mg-doped p-GaN surfaces were systematically investigated by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The doping density of Mg ranged from 3x10(19) to 9x10(19) cm(-3). The XPS and AES analyses revealed the accumulation of Mg for all the air-exposed and chemically treated p-GaN surfaces. The apparent density of Mg calculated from the XPS integrated intensity and the AES intensity was more than one order higher than the value in bulk determined by secondary ion mass spectroscopy. Mg accumulation as well as large amounts of oxides made up the disordered region on the p-GaN:Mg surfaces. Large surface band bending of 1.2-1.6 eV was found at the p-GaN surfaces even after treatment in KOH and NH4OH solutions, due to the existence of high-density surface states. It was found that electron cyclotron resonance assisted N-2 -plasma treatment at 300 degreesC for 1 min is very effective in removing such surface disordered regions and reducing surface band bending. (C) 2003 American Institute of Physics.
  • Hasegawa,H., Inagaki,T., Ootomo,S. and Hashizume,T.:"Mechanisms of Current Collapse and Gate Leakage Currents in AlGaN/GaN Heterostructure Field Effect Transistors", J. Vac. Sci. Technol B 21:1844-1855(2003).*
    2003年 [査読無し][通常論文]
  • Hashizume,T., Ootomo,S., Inagaki,T. and Hasegawa,H.:"Surface passivation of GaN and GaN/AlGaN heterostructures by dielectric films and its application to insulated-gate heterostructure transistors", J. Vac. Sci. Technol B 21:1828-1838(2003).*
    2003年 [査読無し][通常論文]
  • Ootomo,S., Hasegawa,H. and Hashizume,T.:"Gate Leakage in AlGaN/GaN Heterostructure Field Effect Transistors and Its Suppression by Novel Al2O3 Insulated Gate", IEICE Trans. Electron. E86-C:2043-2050(2003).*
    2003年 [査読無し][通常論文]
  • Hashizume,T., Ootomo,S. and Hasegawa,H:"Al2O3-based surface passivation and insulated gate structure for AlGaN/GaN HFETs", phys. stat. sol. (c) 0:2380-2384(2003).*
    2003年 [査読無し][通常論文]
  • Inagaki,T., Hashizume, T. and Hasegawa, H.:"Effects of Surface Processing on 2DEG Current Transport at AlGaN/GaN Interface Studied by Gateless HFET Structure", Appl. Sur. Sci. 216:519-525(2003).*
    2003年 [査読無し][通常論文]
  • Ootomo,S., Hashizume,T. and Hasegawa,H.:" A Novel Thin Al2O3 Gate Dielectric by ECR-plasma Oxidation of Al for AlGaN/GaN Insulated Gate Heterostructure Field-Effect Transistors", phys. stat. sol. (c), 0, 90-94(2002).*
    2002年 [査読無し][通常論文]
  • Jin,Z., Hashizume,T. and Hasegawa,H.:"Effects of nitrogen addition on methan-based ECR plasma etching of gallium nitride", Appl. Sur. Sci. 190, 361-365(2002).*
    2002年 [査読無し][通常論文]
  • Hashizume,T.:"Surface Fermi level position and gap state distribution of InGaP surface grown by metal-organic vapor phase epitaxy", Appl. Phys. Lett. 81, 2382-2384 (2002).*
    2002年 [査読無し][通常論文]
  • Oyama,S., Hashizume,T. and Hasegawa,H.:"Mechanism of Current Leakage through Metal/n-GaN Interfaces", Appl. Sur. Sci. 190, 322-325(2002).*
    2002年 [査読無し][通常論文]
  • Hashizume,T. and Nakasaki,R.: "Discrete surface state related to nitrogen-vacancy defect on plasma treated GaN surfaces", Appl. Phys. Lett. 80, 4564-4566 (2002).*
    2002年 [査読無し][通常論文]
  • Konishi,M., Hashizume,T. and Hasegawa,H.:"In-situ XPS and Photoluminescence Characterization of GaN Surfaces Grown by MBE on MOVPE GaN Templates", Inst. Phys. Conf. Ser. 170, 837-842 (2002).*
    2002年 [査読無し][通常論文]
  • Ootomo,S., Hashizume,T. and Hasegawa,H. :"Surface passivation of AlGaN/GaN heterostructures using an ultrathin Al2O3 layer", phys. stat. sol. A, 188, 371-374 (2001).*
    2001年 [査読無し][通常論文]
  • Hashizume, T. and Saitoh,T. :"Natural oxides on air-exposed and chemically-treated InGaP surfaces grown by metal-organic vapor phase epitaxy", Appl. Phys. Lett., 78, 2318-2320 (2001).*
    2001年 [査読無し][通常論文]
  • Hashizume,T., Ootomo,S., Oyama,S., Konishi,M. and Hasegawa,H. :"Chemistry and Electrical Properties of Surfaces of GaN and AlGaN/GaN Heterostructures", J. Vac. Sci. Technol. B 19, 1675-1681 (2001).*
    2001年 [査読無し][通常論文]
  • Hashizume,T., Nakasaki,R., Ootomo,S., Oyama,S. and Hasegawa,H.:"Surface Characterization of GaN and AlGaN Layers Grown by MOVPE", Mater. Sci. Eng. B, 80, 309-312(2001).*
    2001年 [査読無し][通常論文]
  • Hashizume,T., Nakasaki,R., Ootomo,S., Oyama,S. and Hasegawa,H. :"Surface Passivation Process for GaN-Based Electronic Devices Utilizing ECR-CVD SiNx Film", IEICE Trans. Electron., E84-C, 1455-1461 (2001).*
    2001年 [査読無し][通常論文]
  • Jin,Z., Hashizume,T. and Hasegawa,H.:"In-situ x-ray photoelectron spectroscopy study of etch chemistry of methane-based reactive ion-beam etching of InP using N2", Jpn. J. Appl. Phys, 40, 2757-2761(2001).*
    2001年 [査読無し][通常論文]
  • T Hashizume, E Alekseev, D Pavlidis, KS Boutros, J Redwing JOURNAL OF APPLIED PHYSICS 88 (4) 1983 -1986 2000年08月 [査読無し][通常論文]
     
    Hashizume,T., Aleksee,E., Pavlidis,D., Boutros,K. S. and Redwing,J.: "Capacitance-voltage characterization of AlN/GaN metal-insulator-semiconductor structures grown on sapphire substrate by metal organic chemical vapor deposition", J. Appl. Phys., 88:1983-1986(2000)*
  • Hashizume,T., Ootomo,S., Nakasaki,R., Oyama,S. and Kihara,M.:"X-ray photoelectron spectroscopy characterization of AlGaN surfaces exposed to air and treated in NH4OH solution", Appl. Phys. Lett., 76:2880-2882(2000)*
    2000年 [査読無し][通常論文]
  • Shiozawa,T., Yoshida,T., Hashizume,H. and Hasegawa,H.:"Correlation between interface state properties and electron transport at ultrathin insulator/Si interfaces", Appl. Sur. Sci, 159-160:98-103(2000)*
    2000年 [査読無し][通常論文]
  • Yamada,M., Takahashi,H., Hashizume,T. and Hasegawa,H.:"Fabrication and characterization of novel oxide free InP MISFETs having an ultra-narrow Si surface quantum well", Jpn. J. Appl. Phys, 39:2439-2443(2000)*
    2000年 [査読無し][通常論文]
  • Ootomo,S., HashizumeT. and Hasegawa,H.:"Properties of as-grown, chemically treated and thermally oxidized surfaces of AlGaN/GaN heterostructure", The Institute of Pure and Applied Physics (IPAP) Conference Series 1, pp.934-937(2000)*
    2000年 [査読無し][通常論文]
  • Anantathanasarn,S., Hashizume,T. and Hasegawa,H.:"Passivation effects of nitrided GaAs surafce using nitrogen radical irradiation", Appl. Sur. Sci,159-160:456-461(2000)*
    2000年 [査読無し][通常論文]
  • Ootomo,S., Hashizume,T. and Hasegawa,H.:"Characterization of nitrided GaP (100) surfaces prepared by nitrogen plasma and radicals", Jpn. J. Appl. Phys, 39:2407-2413(2000)*
    2000年 [査読無し][通常論文]
  • Nakasaki,R., Hahsizume,T. and Hasegawa,H.:"Insulator-GaN Interface Structures Formed by Plasma-Assisted Chemical Vapor Deposition", Physica E, 7:953-957(2000)*
    2000年 [査読無し][通常論文]
  • Y Koyama, T Hashizume, H Hasegawa SOLID-STATE ELECTRONICS 43 (8) 1483 -1488 1999年08月 [査読無し][通常論文]
     
    In view of applications to FETs, an attempt was made to characterize and optimize vacuum deposition-based formation processes of Schottky and ohmic contacts on it-type GaN. Detailed I-V and XPS measurements were made, changing surface treatments and metal species. Pre-deposition surface treatments were found to be crucial to obtain good electrical characteristics. Au Schottky contacts formed on warm NH4OH treated surfaces showed nearly ideal thermionic-emission I-V characteristics with a highest Schottky barrier height of 1.03 eV. For ohmic contacts, Ti/Al contacts were investigated, using the circular transmission line model (TLM) method. Excellent ohmic characteristics with a minimum contact resistance of 5-8 x 10(-5) Omega cm(-2) were obtained by annealing at 600 degrees C for 1 min in N-2 atmosphere. Finally, narrow ring Au Schottky gates with Ti/Al ohmic electrodes were successfully fabricated, demonstrating applicability of the present contact formation processes for FET applications. (C) 1999 Elsevier Science Ltd. All rights reserved.
  • T Hashizume APPLIED PHYSICS LETTERS 75 (5) 615 -617 1999年08月 [査読無し][通常論文]
     
    Nitridation of GaP(001) surface was achieved at 400 degrees C by exposure to electron cyclotron resonance assisted N-2 plasma. Detailed x-ray photoelectron spectroscopy analysis showed that after the initial stage of nitridation, the GaP surface included both of the Ga-N and P-N bonding components, while a near-stoichiometric GaN layer was formed after a 60 min nitridation process. A weak cathodoluminescence peak was detected at around 3.3 eV for the nitrided surface after 60 min of nitridation. (C) 1999 American Institute of Physics. [S0003-6951(99)00231-4].
  • M.B. Takeyama, A. Noya, T. Hashizume and H. Hasegawa: "Interfacial Reaction and Electrical properties in the sputter-deposited Al/Ti ohmic contact to n-InP", Jpn. J. Appl. Phys., 38:253-257 (1999)*
    1999年 [査読無し][通常論文]
  • H. Hasegawa, Y. Koyama, T. Hashizume: "Properties of Metal-Semiconductor Interfaces Formed on n-type GaN", Jpn. J. Appl. Phys., 38:2634-9 (1999)*
    1999年 [査読無し][通常論文]
  • H. Takahashi, T. Hashizume and H. Hasegawa: "XPS and UHV contactless characterization of novel oxide-free InP passivation process using silicon surface quantum well", Jpn. J. Appl. Phys., 38:1128-1132 (1999)*
    1999年 [査読無し][通常論文]
  • 岡田 浩, 佐藤 好弘, 藤倉 序章, 橋詰 保, 長谷川 英機 電子情報通信学会技術研究報告. ED, 電子デバイス 98 (184) 15 -20 1998年07月21日 [査読無し][通常論文]
     
    新しいショットキーラップゲート(WPG)構造を有する、化合物半導体単電子デバイスを提案し、その特性を評価した。AlGaAs/GaAsの擬1次元細線にWPGを適用した単電子トランジスタ(SET)は、明確なクーロンブロッケード振動、ならびにクーロンブロッケード特性を示し、同時に、1を越える電圧利得が得られた。さらに、分子線エピタキシーの選択成長法によるInAlAs/InGaAsリッジ量子細線にWPG構造を適用した新しいSETを実現した。このSETは、最高30Kまでクーロンブロッケード振動を示し、10mVのクーロンギャップを示した。これらの結果は、提案したショットキーWPGゲート構造が、化合物半導体単電子デバイスの実現に有用であることを示唆している。
  • H Okada, H Fujikura, T Hashizume, H Hasegawa SOLID-STATE ELECTRONICS 42 (7-8) 1419 -1423 1998年07月 [査読無し][通常論文]
     
    An attempt was made, for the first time, to fabricate a novel Schottky wrap gate (WPG) controlled single electron transistor (SET) directly on to an InGaAs stereographic ridge quantum wire grown by molecular beam epitaxy (MBE). To analyze the controllability of the quantum dot by Schottky WPG, a computer simulation of potential distribution was carried out. The fabricated SET showed clear conductance oscillation with an effective Coulomb gap of 10 meV. Comparison of the observed experimental behavior with simulation indicates that the SET is in the quantum confinement regime with resonant tunneling. (C) 1998 Elsevier Science Ltd. All rights reserved.
  • T Hashizume, Y Ishikawa, T Yoshida, H Hasegawa JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 37 (3B) 1626 -1630 1998年03月 [査読無し][通常論文]
     
    Correlation between macroscopic electronic properties of the molecular beam epitaxy (MBE)-grown Si-doped GaAs (001) (2 x 4) surfaces with microscopic atomic structures was in situ investigated using ultrahigh vacuum scanning tunneling microscope (UHV STM), UHV contactless capacitance-voltage (C-V), UHV photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) techniques. Surface defects including kinks, steps, holes, islands and missing As atoms in As dimer rows were observed at the Si-doped (2 x 4) GaAs surfaces. Contactless C-V results directly showed the surface Fermi level pinning. The observed macroscopic C-V and PL behavior cannot be explained by the previous kink-acceptor model, assuming that each kink forms a discrete acceptor level, but it indicates Fermi level pinning by U-shaped continuous surface states.
  • CHAKRABORTY S, YOSHIDA T, HASHIZUME T, HASEGAWA H, SAKAI T J. Vac. Sci. Technol. B 16 (4) 2159 -2164 1998年 [査読無し][通常論文]
  • T Hashizume, K Ikeya, M Mutoh, H Hasegawa APPLIED SURFACE SCIENCE 123 599 -602 1998年01月 [査読無し][通常論文]
     
    GaAs surfaces were successfully passivated by utilizing an ultrathin Si3N4/Si interface control layer (ICL) formed by molecular beam epitaxy (MBE) and in situ electron-cyclotron-resonance (ECR) plasma-assisted nitridation. Detailed X-ray photoelectron spectroscopy (XPS) analysis showed that optimization of the ECR plasma process led to the realization of well-defined Si3N4/Si double-layer structure without change in chemical status of GaAs surface. As compared with the clean MBE surface, surface Fermi level pinning became weaker and the band-edge photoluminescence (PL) intensity became larger after passivation. (C) 1998 Elsevier Science B.V.
  • Hiroshi Takahashi, Tamotsu Hashizume, Hideki Hasegawa Applied Surface Science 123-124 615 -618 1998年 [査読無し][通常論文]
     
    A novel oxide-free InP MIS structure, having an ultrathin Si3N4/Si layer produced by ECR partial nitridation of MBE Si layer, is proposed on the basis of the modified silicon interface control layer (Si ICL) concept, and is successfully realized. Use of ECR plasma allowed monolayer level control of Si ICL thickness so that the previous problem that a large portion of the conduction band and the valence band states of the Si ICL fall in the energy gap of InP was avoided by making the Si ICL so thin that all the states in the valence band surface quantum well were pushed away due to the quantum effect. In-situ XPS measurements confirmed formation of a well-defined Si3N4/Si ICL interface structure. Capacitance-voltage measurements showed a broad U-shaped interface state density (Nss) distribution with a minimum of 2 × 1010 cm-2 eV-1. © 1998 Elsevier Science B.V.
  • H Okada, S Kasai, H Fujikura, T Hashizume, H Hasegawa JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 36 (6B) 4156 -4160 1997年06月 [査読無し][通常論文]
     
    Okada, H., Kasai, S., Fujikura, H., Hashizume, T. and Hasegawa, H. : "Basic Control Characteristics of Novel Schottky In-Plane and Wrap Gate Structures Studied by Simulation and Transport Measurements in GaAs and InGaAs Quantum Wires", Japanese Journal of Applied Physics, 36, 4156-4160 (1997)*
  • T Yoshida, T Hashizume, H Hasegawa JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 36 (3B) 1453 -1459 1997年03月 [査読無し][通常論文]
     
    Yoshida, T., Hashizume, T. and Hasegawa, H. : "Characterization of Interface Electronic Properties of Low-Temperature Ultrathin Oxides and Oxynitrides Formed on Si(111) Surfaces by Contactless Capacitance-voltage and Photoluminescence Methods", Japanese Journal of Applied Physics, 36 : 1453-1459 (1997)*
  • Hiroshi Okada, Hajime Fujikura, Tamotsu Hashizume, Hideki Hasegawa Japanese Journal of Applied Physics 36 (3B) 1672 -1677 1997年 [査読無し][通常論文]
  • Dohmae, Y., Suzuki, S., Hashizume, T. and Hasegawa, H. : "Capacitance-voltage Behavior of Insulated Gate InGaAs HEMT Capacitors Having Silicon Interface Control Layer", Japanese Journal of Applied Physics, 36 : 1834-1840 (1997)*
    1997年 [査読無し][通常論文]
  • Ikeya, K., Hashizume, T. and Hasegawa, H. : "Successful Surface Passivation of Air-Exposed AlGaAs by a Silicon Interface Control Layer-Based Technique", Japanese Journal of Applied Physics, 36 : 1756-1762 (1997)*
    1997年 [査読無し][通常論文]
  • Sato, T., Uno, S., Hashizume, T. and Hasegawa, H. : "Large Schottky Barrier Height on Indium Phosphide-Based Materials Realized by In-Situ Electrochemical Process", Japanese Journal of Applied Physics, 36 : 1811-1817 (1997)*
    1997年 [査読無し][通常論文]
  • Iizuka, K., Hashizume, T. and Hasegawa, H. : "Small-Signal Response of Interface States at Passive InGaAs Surfaces from Low Frequencies up to Microwave Frequencies", Solid State Electronics, 41 : 1463-1468 (1997)*
    1997年 [査読無し][通常論文]
  • Hashizume, T., Shiobara, S. and Hasegawa, H. : "Dominant Electron Trap with Metastable State in Molecular Beam Epitaxial GaAs Grown at Low Temperatures", Japanese Journal of Applied Physics, 36 : 1775-1780 (1997)*
    1997年 [査読無し][通常論文]
  • Kudoh, T., Okada, H., Hashizume, T. and Hasegawa, H. : "Controlled Formation of Metal-Semiconductor Interface to 2DEG Layer by in-situ Electrochemical Process and Its Applicationto In-Plane Gated Electron Waveguide Devices", Applied Surface Science, 11・・・
    1997年 [査読無し][通常論文]
     
    Kudoh, T., Okada, H., Hashizume, T. and Hasegawa, H. : "Controlled Formation of Metal-Semiconductor Interface to 2DEG Layer by in-situ Electrochemical Process and Its Applicationto In-Plane Gated Electron Waveguide Devices", Applied Surface Science, 117/118 : 342-346 (1997)*
  • 1997年 [査読無し][通常論文]
     
    Hasegawa, H., Sato, T. and Hashizume, T. : "Evolution Mechanism of Nearly Pinning-Free Platinum/n-type Indium Phosphide Interface with a High Schottky Barrier Height by in situ Electrochemical Process", Journal of Vacuum Science and Technology, 15-B : 1227-1235 (1997)*
  • T Hashizume, S Koyanagi, H Hasegawa JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 14 (4) 2872 -2881 1996年07月 [査読無し][通常論文]
     
    Electronic properties of the interfaces between Si and ultrathin (less than or similar to 10 Angstrom) oxides formed by various low-temperature processes were characterized in contactless fashion, using contactless capacitance-voltage and photoluminescence surface state spectroscopy techniques together with x-ray photoelectron spectroscopy measurement. Hydrogen (H) terminated Si(111) surfaces were used as the initial surface. Ultrathin oxides were formed at low temperatures by chemical oxidation processes (hot HNO3, H2SO4+H2O2), long-time air exposure, and low-temperature oxidation processes below 350 degrees C. The initial H-terminated surfaces showed presence of Fermi-level pinning at E(0)=E(V)+0.65 eV due to high density of amphoteric discrete state probably originating from Si dangling bonds. On the other hand, all the ultrathin oxide-Si interfaces exhibited very limited capacitance variations with voltage at low capacitance levels similar to GaAs metal-insulator-semiconductor systems, and indicated that the Fermi level is pinned near the hybrid orbital charge neutrality level E(HO) due to presence of interface states with narrow U-shaped continuous distributions. Low-temperature oxidation at 350 degrees C slightly weakens such pinning. The present work indicates difficulty of realizing unpinned ultrathin oxide-silicon interface by low-temperature processes. (C) 1996 American Vacuum Society.
  • S Uno, T Hashizume, S Kasai, NJ Wu, H Hasegawa JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 35 (2B) 1258 -1263 1996年02月 [査読無し][通常論文]
     
    Pt Schottky barrier diodes (SBDs) with a high Schottky barrier height of 0.86 eV and an ideality factor of near unity were successfully realized by a novel in situ electrochemical process. Applying this novel technique to InP metal semiconductor field effect transistors (MESFETs), good gate control of drain current with pinch-off, an effective channel mobility of 1,840 cm(2)V(-1)s(-1) and no drain current drift behavior were achieved. The InP MESFET operates even under a positive gate bias, showing feasibility of enhancement-mode operation as well as depletion-mode operation.
  • K. Jinushi, H. Okada, T. Hashizume and H. Hasegawa : "Novel GaAs-Based Single Electron Transistors with Schottky In-Plane Gates Operating up to 20K", Japanese Journal of Applied Physics 35 : 1132-1139 (1996)*
    1996年 [査読無し][通常論文]
  • H. Tomozawa, K. Jinushi, H. Okada, T. Hashizume and H. Hasegawa : "Design and Fabrication of GaAs/AlGaAs Single Electron Transistors Based on In-Plane Schottky Gate Control of 2DEG", Phisica B, 227 : 112-115 (1996)*
    1996年 [査読無し][通常論文]
  • S. Kasai, T. Hashizume and H. Hasegawa : "Electron Beam Induced Current Characterization of Novel GaAs Quantum Nanostructures Based on Potential Modulation of Two-Dimensional Electron Gas by Schottky In-Plane Gates", Japanese Journal of Applied Physics・・・
    1996年 [査読無し][通常論文]
     
    S. Kasai, T. Hashizume and H. Hasegawa : "Electron Beam Induced Current Characterization of Novel GaAs Quantum Nanostructures Based on Potential Modulation of Two-Dimensional Electron Gas by Schottky In-Plane Gates", Japanese Journal of Applied Physics, 35 : 6652-6658 (1996)*
  • S. Koyanagi, T. Hashizume and H. Hasegawa : "Contactless and Nondestructive Characterization of Silicon Surfaces by Capacitance-Voltage and Photoluminescence Methods", Japanese Journal of Applied Physics, 35 : 946-953 (1996)*
    1996年 [査読無し][通常論文]
  • S. Shiobara, T. Hashizume and H. Hasegawa : "Deep Level and Conduction Mechanism in Low-Temperature GaAs Grown by Molecular Beam Epitaxy", Japanese Journal of Applied Physics, 35 : 1159-1164 (1996)*
    1996年 [査読無し][通常論文]
  • T. Hashizume, H. Okada and H. Hasegawa : "Quantum Transport in A Schottky In-Plane-Gate Controlled GaAs/AlGaAs Quantum Well Wires", Phisica B, 227 : 42-45 (1996)
    1996年 [査読無し][通常論文]
  • H. Fujikura, S. Kodama, T. Hashizume and H. Hasegawa : "Surface Passivation of In0.53Ga0.47As Ridge Quantum Wires Using Silicon Interface Control Layers", Journal of Vacuum Science and Technology, B-14 : 2888-2894 (1996)*
    1996年 [査読無し][通常論文]
  • Tamotsu Hashizume, Hiroshi Okada, Keiichiro Jlnushi, Hideki Hasegawa Japanese Journal of Applied Physics 34 (5) L635 -L638 1995年 [査読無し][通常論文]
     
    A novel Schottky in-plane gate (IPG) quantum wire transistor has been successfully fabricated for the first time on a GaAs/AlGaAs quantum-well (QW) wafer, using a low-damage in situ electrochemical process. In comparison with previous IPG transistors of insulated-gate type, the present Schottky IPG device exhibited much better gate control of drain currents. In spite of large device dimensions of several hundred nanometers, sharp quantized con­ductance steps in units of 2e2/h were observed up to 40 K as a result of the inherent strong electron confinement capability of the present IPG structure. © 1995 The Japan Society of Applied Physics.
  • Hiroshi Okada, Tamotsu Hashizume, Hideki Hasegawa Japanese Journal of Applied Physics 34 (12) 6971 -6976 1995年 [査読無し][通常論文]
     
    The low-temperature transport properties of the novel Al0.3Ga0.7As/GaAs Schottky in-plane gate (IPG) quantum wire transistor realized by the in-situ electrochemical process were studied. The device showed good field-effect transistor (FET) characteristics with excellent gate control and complete pinch-off. Quantized conductance in the units of 2e2/h was clearly seen at 4 K. The first plateau persisted up to 40 K. Shubnikov-de Haas oscillationmeasurements showed an extremely weak dependence of the carrier density on the gate bias, indicating that theSchottky IPG controls the wire width without changing the sheet carrier density. Application of a magnetic fieldwidened the conductance plateaus. Under harmonic approximation for the electrostatic confinement potentials, a subband spacing of hωo = 4-5 meV was obtained. © 1995 The Japan Society of Applied Physics.
  • Hiroshi Okada, Kei-Ichiroh Jlnushl, Nan-Jian Wu, Tamotsu Hashizume, Hideki Hasegawa Japanese Journal of Applied Physics 34 (2S) 1315 -1319 1995年 [査読無し][通常論文]
     
    A novel wire transistor structure with Schottky in-plane gates (IPG) to the AlGaAs/GaAs quantum well (QW) has been successfully fabricated and characterized. An in situ selective electrochemical process is utilized to form direct Schottky contacts to the edge of the QW. Details of the novel process for transistor fabrication, as well as field-effect characteristics of the novel device, are presented and discussed. Field-effect transistor (FET) characteristics with excellent gate control and clear pinch-off are obtained at room temperature and they are compared with a theory newly developed for the IPG FET. © 1995 The Japan Society of Applied Physics.
  • Tamotsu Hashizume, Hiroshi Okada, Nan-Jan Wu, Hideki Hasegawa Japanese Journal of Applied Physics 34 (2S) 1149 -1152 1995年 [査読無し][通常論文]
     
    Depletion properties of direct Schottky/quantum well (QW) contacts formed by the in situ selective electrochemical process were systematically characterized by the C-V and electron-beam-induced current (EBIC) techniques. The EBIC images clearly showed that the barrier exists at the edge of the QW layer. It was found thatthe capacitance of the Schottky/QW contact depends linearly on In (l/(Vu — V)), where the Vbi is built-in voltage, and that the depletion width obtained from the EBIC signal is proportional to the applied voltage. These resultsdemonstrate that well-behaved depletion characteristics of the Schottky/QW systems can be realized. © 1995 The Japan Society of Applied Physics.
  • T HASHIZUME, H HASEGAWA, G TOCHITANI, M SHIMOZUMA JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 31 (12A) 3794 -3800 1992年12月 [査読無し][通常論文]
     
    Thermal stability of the SiO2/GaAs interface structures prepared by 50-Hz plasma-assisted chemical vapor deposition (PCVD) and the electrical activation of Si implanted into GaAs with SiO2 cap were investigated. X-ray photoelectron spectroscopy (XPS) analysis showed that the 50-Hz PCVD method at 200-degrees-C allowed the presence of an As-enriched layer, intentionally inserted by HF treatment before SiO2 deposition, at the SiO2/GaAs interface. After rapid thermal annealing (RTA) at 950-degrees-C for 20 s, in fact, the As-enriched layer still remained and it was effective in suppressing interfacial reactions which caused various shortcomings. In addition, carrier concentration profiles in excellent agreement with Lindhard-Scharff-Schiott (LSS) curves were reproducibly obtained by RTA in Si-implanted GaAs capped with both the As-enriched layer and the SiO2 film, demonstrating that this process is applicable to post-annealing of ion-implanted GaAs.

受賞

  • 2018年09月 応用物理学会 フェロー
     
    受賞者: 橋詰 保
  • 2016年09月 2016 Paper Award, The Japan Society of Applied Physics
     
    受賞者: 橋詰 保
  • 2016年09月 応用物理学会 優秀論文賞
     
    受賞者: 橋詰 保
  • 2014年05月 CS-MANTECH 2013 Paper Awards (He Bong Kim Award)
     
    受賞者: 橋詰 保
  • 2014年04月 応用物理学会 APEX/JJAP編集貢献賞
     
    受賞者: 橋詰 保
  • 2007年04月 応用物理学会 JJAP編集貢献賞

共同研究・競争的資金等の研究課題

  • 特異構造を含む異種接合の界面制御と電子デバイス展開
    科学研究費補助金:新学術領域研究(研究領域提案型)
    研究期間 : 2016年04月 -2020年03月 
    代表者 : 橋詰 保
  • 高い安定性を有するGaN-MOSトランジスタスイッチ
    JST:戦略的国際共同研究プログラム(SICORP)
    研究期間 : 2015年09月 -2019年03月 
    代表者 : 橋詰 保
  • GaN縦型パワーデバイスの基盤技術開発
    NEDO:SIP(戦略的イノベーション創造プログラム)
    研究期間 : 2014年 -2019年03月 
    代表者 : 橋詰 保
  • 異種接合GaN横型トランジスタのインバータ展開
    科学技術研究機構(JST):戦略的創造研究推進事業(CREST)
    研究期間 : 2009年10月 -2015年03月 
    代表者 : 橋詰 保
  • 電子準位制御と新ゲート構造による窒化ガリウム系トランジスタの高信頼化
    科学研究費補助金:基盤研究(A)
    研究期間 : 2009年04月 -2013年04月 
    代表者 : 橋詰 保
  • Nano-scale surface control of semiconductors for device applications
    研究期間 : 1994年

教育活動情報

主要な担当授業

  • 電子情報工学演習Ⅱ
    開講年度 : 2019年
    課程区分 : 学士課程
    開講学部 : 工学部
  • 情報エレクトロニクス特別講義
    開講年度 : 2019年
    課程区分 : 修士課程
    開講学部 : 情報科学院
    キーワード : 電子材料、超高速電子デバイス、エネルギー変換回路、光デバイス、先進無線通信技術、高度集積化技術
  • 情報エレクトロニクス特別講義第一
    開講年度 : 2019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 : 電子材料、超高速電子デバイス、エネルギー変換回路、光デバイス、先進無線通信技術、高度集積化技術
  • 情報エレクトロニクス特別講義第二
    開講年度 : 2019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 : 電子材料、超高速電子デバイス、エネルギー変換回路、光デバイス、先進無線通信技術、高度集積化技術
  • 情報エレクトロニクス特別講義
    開講年度 : 2019年
    課程区分 : 博士後期課程
    開講学部 : 情報科学院
    キーワード : 電子材料、超高速電子デバイス、エネルギー変換回路、光デバイス、先進無線通信技術、高度集積化技術
  • 応用物性工学
    開講年度 : 2019年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 発電、再生可能エネルギー、変圧器、パワーエレクトロニクス、パワー半導体デバイス、整流回路、インバータ、コンバータ
  • 電気エネルギー工学
    開講年度 : 2019年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 発電、再生可能エネルギー、変圧器、パワーエレクトロニクス、パワー半導体デバイス、整流回路、インバータ、コンバータ

大学運営

学内役職歴

  • 2014年4月1日 - 2016年3月31日 量子集積エレクトロニクス研究センター長
  • 2016年4月1日 - 2018年3月31日 量子集積エレクトロニクス研究センター長
  • 2018年4月1日 - 2020年3月31日 量子集積エレクトロニクス研究センター長

委員歴

  • 2012年04月 - 2014年03月   応用物理学会   北海道支部長
  • 2012年04月 - 2014年03月   応用物理学会   理事
  • 2009年04月 - 2011年03月   電子情報通信学会電子デバイス専門委員会   委員長
  • 2007年   電子情報通信学会   電子デバイス研究会 副委員長   電子情報通信学会


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