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Mizobata, Hidetoshi*; Tomigahara, Kazuki*; Nozaki, Mikito*; Kobayashi, Takuma*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Letters, 121(6), p.062104_1 - 062104_6, 2022/08
The interface properties and energy band alignment of SiO/GaN metal-oxide-semiconductor (MOS) structures fabricated on N-polar GaN(000) substrates were investigated by electrical measurements and synchrotron-radiation X-ray photoelectron spectroscopy. They were then compared with those of SiO/GaN MOS structures on Ga-polar GaN(0001). Although the SiO/GaN(000) structure was found to be more thermally unstable than that on the GaN(0001) substrate, excellent electrical properties were obtained for the SiO/GaN(000) structure by optimizing conditions for post-deposition annealing. However, the conduction band offset for SiO/GaN(000) was smaller than that for SiO/GaN(0001), leading to increased gate leakage current. Therefore, caution is needed when using N-polar GaN(000) substrates for MOS device fabrication.
Nozaki, Mikito*; Terashima, Daiki*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Japanese Journal of Applied Physics, 59(SM), p.SMMA07_1 - SMMA07_7, 2020/07
Times Cited Count:0 Percentile:12.83(Physics, Applied)AlGaN/GaN metal-oxide-semiconductor (MOS) structures were fabricated by low-power inductively coupled plasma reactive ion etching and chemical vapor deposition of SiO dielectrics on the etched surfaces, and they were systematically investigated by physical and electrical characterizations in an effort to develop a low-damage recessed gate process. The comprehensive research demonstrates the significant advantages of the proposed low-damage recessed gate process for fabricating next-generation AlGaN/GaN MOS-HFET devices.
Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Shih, H.-A.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Japanese Journal of Applied Physics, 57(6S3), p.06KA07_1 - 06KA07_6, 2018/06
Times Cited Count:6 Percentile:30.31(Physics, Applied)Thermal oxidation of AlGaN surface and its impact on the electrical properties of AlGaN/GaN MOS capacitors were investigated by means of synchrotron radiation photoelectron spectroscopy (SR-PES), atomic force microscopy (AFM) and C-V measurements. SR-PES analysis revealed that the AlGaN surface is oxidized even at low temperature of 400C, in contrast to no oxide formation on GaN surface. However, since no noticeable change in the surface morphology was observed at temperatures up to 800C, it can be concluded that an ultrathin oxide overlayer is formed on the AlGaN surface. On the other hand, for the oxidation treatments above 850C, the formation of small oxide grains was observed over the entire area of the AlGaN surface, and the growth of oxide grains significantly degraded the surface morphology. Therefore, the AlGaN/GaN MOS capacitors were fabricated on the AlGaN surface oxidized at moderate temperatures up to 800C. While we have confirmed that relatively good interface properties are obtained for direct AlON deposition without oxidation treatment, it was found that the oxidation treatment at 400C leads to further improvement of interface properties and reduction of C-V hysteresis.
Watanabe, Kenta*; Terashima, Daiki*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Ishida, Masahiro*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; et al.
Japanese Journal of Applied Physics, 57(6S3), p.06KA03_1 - 06KA03_6, 2018/06
Times Cited Count:10 Percentile:46.45(Physics, Applied)The advantage of SiO/AlON stacked gate dielectrics over SiO, AlON and AlO single dielectric layers was demonstrated. Our systematic research revealed that the optimized stacked structure with 3.3-nm-thick AlON interlayer is beneficial in terms of superior interface quality, reduced gate leakage current and C-V hysteresis for next-generation high frequency and high power AlGaN/GaN MOS-HFETs.
Nozaki, Mikito*; Watanabe, Kenta*; Yamada, Takahiro*; Shih, H.-A.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Japanese Journal of Applied Physics, 57(6S3), p.06KA02_1 - 06KA02_7, 2018/06
Times Cited Count:16 Percentile:66.02(Physics, Applied)We fabricated AlON dielectric films by repeating thin AlN deposition and in situ O oxidation for AlGaN/GaN MOS-HFETs. Uniform nitrogen distribution is achievable by the proposed ALD-based process and that nitrogen concentration can be precisely controlled by changing AlN thickness (ALD cycle number) in each step. It was found that AlON films grown by ALD system offers significant advantages in terms of practical application while keeping superior Vth stability and electrical properties at the insulator/AlGaN interface in AlGaN/GaN MOS-HFETs.
Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Hisashi*; Takahashi, Tokio*; Shimizu, Mitsuaki*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 11(1), p.015701_1 - 015701_4, 2018/01
Times Cited Count:33 Percentile:85.18(Physics, Applied)A simple and feasible method for fabricating high-quality and highly reliable GaN-based metal-oxide-semiconductor (MOS) devices was developed on the basis of systematic physical and electrical characterizations. Chemical vapor deposition of SiO films directly onto GaN substrates forming Ga-oxide interlayers was used to fabricate SiO/GaO/GaN stacked structures. Although well-behaved hysteresis-free GaN-MOS capacitors with extremely low interface state density below 10cmeV were obtained by post-deposition annealing, Ga diffusion into overlying SiO layers severely degraded the insulating property and dielectric breakdown characteristics of the MOS devices. However, this problem was found to be solved by employing rapid thermal processing, leading to superior performance of the GaN-MOS devices in terms of interface quality, insulating property and gate dielectric reliability.
Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Applied Physics Letters, 111(4), p.042102_1 - 042102_5, 2017/07
Times Cited Count:15 Percentile:60.82(Physics, Applied)AlGaN/GaN HFET (hetero-junction field-effect transitor) has gained much attention as next-generation high frequency and high power devices. In this study, we systematically investigated the interface reaction between Al-based dielectrics (AlO and AlON) and AlGaN layer during deposition and post-deposition annealing (PDA), and revealed high thermal stability of AlON/AlGaN interface.
Yamada, Takahiro*; Ito, Joyo*; Asahara, Ryohei*; Watanabe, Kenta*; Nozaki, Mikito*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; et al.
Journal of Applied Physics, 121(3), p.035303_1 - 035303_9, 2017/01
Times Cited Count:59 Percentile:91.81(Physics, Applied)Initial oxidation of GaN(0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of X-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy and X-ray diffraction measurements. It was found that, whereas initial oxide formation tends to saturate at temperatures below 800C, selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth leading to rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also revealed that polycrystalline - and -phase GaO grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. On the basis of these experimental findings, we also developed a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport.
Asahara, Ryohei*; Nozaki, Mikito*; Yamada, Takahiro*; Ito, Joyo*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
Applied Physics Express, 9(10), p.101002_1 - 101002_4, 2016/10
Times Cited Count:38 Percentile:83.94(Physics, Applied)The superior physical and electrical properties of AlON gate dielectrics on AlGaN/GaN substrates in terms of thermal stability, reliability, and interface quality were demonstrated by direct AlON deposition and subsequent annealing. Nitrogen incorporation into alumina was proven to be beneficial both for suppressing intermixing at the insulator/AlGaN interface and reducing the number of electrical defects in AlO films. Consequently, we achieved high-quality AlON/AlGaN/GaN metal-oxide-semiconductor capacitors with improved stability against charge injection and a reduced interface state density as low as 1.210 cmeV. The impact of nitrogen incorporation into the insulator was discussed on the basis of experimental findings.
Nozaki, Mikito*; Ito, Joyo*; Asahara, Ryohei*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
Applied Physics Express, 9(10), p.105801_1 - 105801_4, 2016/10
Times Cited Count:4 Percentile:24.97(Physics, Applied)Interface reactions between Ti-based electrodes and n-type GaN epilayers were investigated by synchrotron radiation X-ray photoelectron spectroscopy. Metallic Ga and thin TiN alloys were formed at the interface by subsequently depositing Al capping layers on ultrathin Ti layers even at room temperature. By comparing results from stacked Ti/Al and single Ti electrodes, the essential role of Al capping layers serving as an oxygen-scavenging element to produce reactive Ti underlayers was demonstrated. Further growth of the metallic interlayer during annealing was observed. A strategy for achieving low-resistance ohmic contacts to n-GaN with low-thermal-budget processing is discussed.
Oshima, Katsumi; Okano, Fuminori; Honda, Atsushi; Shinozaki, Shinichi; Usui, Katsutomi; Noto, Katsuya; Kawai, Mikito; Ikeda, Yoshitaka
JAEA-Technology 2007-044, 27 Pages, 2007/06
In the positive ion based NBI (P-NBI) system, we have developed a protection system to protect the power supply facilities from over load during long pulse operation. The protection system monitors the voltage (V) and current (I) in the power supply facilities, and calculates the parameters of V2t and I2t in real-time, where T is the pulse duration. It turns off the power supply facilities when V2t and I2t are beyond the critical values. After two development stages, we have completed the protection system using a package typed PLC (Programmable Logic Controller) which has a high expandability of multi-unit operation. Moreover, we have constructed a user-friendly system by using a SCADA (Supervisory Control and Data Acquisition) system.
Akino, Noboru; Hanada, Masaya; Kawai, Mikito; Usui, Katsutomi; Shinozaki, Shinichi
no journal, ,
no abstracts in English
Ito, Joyo*; Asahara, Ryohei*; Nozaki, Mikito*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Teraoka, Yuden; Hosoi, Takuji*; Shimura, Takayoshi*
no journal, ,
no abstracts in English
Nozaki, Mikito*; Ito, Joyo*; Asahara, Ryohei*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Teraoka, Yuden; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
Interface reactions between Ti-based multilayer electrodes on n-type GaN epilayers were investigated by means of synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). High-resolution SR-XPS analysis revealed formation of metallic Ga and TiN mixed interlayers at Ti/GaN interface even at room temperature by depositing Al capping layer. By comparing XPS spectra from the stacked Al/Ti electrode and single Ti electrode, an essential role of Al overlayers serving as oxygen diffusion barrier and oxygen scavenging element to produce reactive pure Ti underlayers on GaN was demonstrated. Moreover, growth of metallic interlayers consisting of Ga and Ti atoms during vacuum annealing was clearly identified from in situ SR-XPS analysis.
Asahara, Ryohei*; Nozaki, Mikito*; Yamada, Takahiro*; Ito, Joyo*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
AlGaN/GaN high electron mobility transistor (HEMT) with metal-oxide-semiconductor (MOS) gate structure has gained much attention as next-generation high frequency and high power devices. In this study, an improvement in thermal stability and interface properties of AlGaN/GaN MOS structure by using AlON gate dielectrics has been demonstrated. Synchrotron radiation photoemission spectroscopy (SR-PES) revealed that Ga and Al atoms were diffused from AlGaN into overlying AlO layer during annealing at 800 degrees. In contrast, the spectra for AlON sample remain almost unchanged even after the annealing, indicating high thermal stability of AlON/AlGaN structure. Furthermore, the negligible hysteresis in capacitance-voltage characteristics and interface state density as low as 1.210 cmeV were obtained for AlON/AlGaN/GaN MOS capacitors.
Yamada, Takahiro*; Ito, Joyo*; Asahara, Ryohei*; Nozaki, Mikito*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
Initial oxide growth on GaN epitaxial films by thermal oxidation in dry O ambient was investigated by means of synchrotron radiation X-ray photoemission spectroscopy and atomic force microscopy (AFM). High-resolution SR-PES analysis showed that the intensity ratio of O 1s/N 1s core-level peaks for the sample oxidized at 700 degrees is slightly higher than that for the sample after HCl wet-cleaning, indicating that GaN surface is oxidized. However, the O 1s/N 1s intensity ratio was nearly identical with increasing temperature up to 800 degrees and AFM observation revealed the preferential formation of small grains at the dark pits. On the other hand, the O 1s/N 1s intensity ratio drastically increased with further increasing temperature higher than 800 degrees, indicating significantly enhanced oxide formation on GaN surface.
Yamada, Takahiro*; Yoshigoe, Akitaka; Ito, Joyo*; Asahara, Ryohei*; Watanabe, Kenta*; Nozaki, Mikito*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; et al.
no journal, ,
Thermal oxidation process of self-standing GaN (ss-GaN) substrates with low-defect density were investigated by atomic force microscopy and synchrotron radiation photoelectron spectroscopy. ss-GaN sample showed a flat surface morphology without pits which are ascribed to dislocation defects in the measured area (1 mx1 m). Therefore, the preferential formation of Ga-oxides at the pits was not observed by thermal oxidation, and the flat surface morphology was kept in the sample oxidized at 800C (the root-mean-square (RMS) roughness of 0.14 nm). However, the ss-GaN sample oxidized at 900C showed rough surface morphology due to the formation of small Ga-oxide grains (RMS roughness of 0.62 nm). The grains drastically grew at 1000C, resulted in the surface morphology consisting of -GaO crystal.
Nozaki, Mikito*; Yoshigoe, Akitaka; Ito, Joyo*; Asahara, Ryohei*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
no journal, ,
Structural changes at Ti/GaN or Al/Ti/GaN interfaces were studied by using synchrotron radiation photoelectron spectroscopy. It was found that the Al capping layer plays an important role for oxygen diffusion barriers. The Al layer also stimulates interfacial reactions via introducing oxygen into Ti layers. We found that this reaction dramatically changes Ti/GaN interface structures.
Yamada, Takahiro*; Yoshigoe, Akitaka; Ito, Joyo*; Asahara, Ryohei*; Nozaki, Mikito*; Nakazawa, Satoshi*; Ishida, Masahiro*; Ueda, Tetsuzo*; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
To achieve high performance of GaN MOS devices, high quality interfaces between insulator and GaN are essentially needed. The precise control of native surface oxide and/or interfacial oxide layer is important. In this study, oxide formation process in thermally oxidized GaN surface is analyzed by using synchrotron radiation photoelectron spectroscopy.
Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
no journal, ,
Interface properties of SiO/GaN structure with post-oxidation treatment were investigated by synchrotron radiation photoelectron spectroscopy and C-V measurements. High-resolution Ga 2p core-level spectra for SiO/GaN structure oxidized at 800 degrees showed asymmetric feature with a shoulder at higher binding energies compared to that for wet-cleaned GaN surface. From peak deconvolution of the Ga2p spectrum, formation of GaOx layer at the SiO/GaN interface was revealed. C-V measurements of the SiO/GaOx/GaN MOS capacitor exhibited apparently low frequency dispersion and a small hysteresis of below 50 mV. Moreover, the C-V curves well agreed with an ideal C-V curve. These results indicate that good interface properties were achieved by the interfacial GaOx formation.