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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：30.01(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 400
C, 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.
/AlON stacked gate dielectrics for AlGaN/GaN MOS heterojunction field-effect transistorsWatanabe, 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：45.99(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：18 Percentile：65.6(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.
/GaN stacks for high-quality GaN-based metal-oxide-semiconductor devices with improved gate dielectric reliabilityYamada, 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：39 Percentile：84.89(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 10
cm
eV
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：16 Percentile：60.44(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：66 Percentile：91.84(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：40 Percentile：83.77(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 1.2
10
cmeV. The impact of nitrogen incorporation into the insulator was discussed on the basis of experimental findings.
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.
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.
/GaN MOS capacitor by interfacial GaOx formation with post-oxidation treatmentYamada, 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.
/GaN and its effect on electrical propertiesYamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*
no journal, ,
Formation of thin GaOx interlayer by thermal oxidation of SiO/GaN structure was investigated by spectroscopic ellipsometry and synchrotron radiation photoelectron spectroscopy. In the SiO/GaN structure before thermal oxidation, GaOx interlayer (about 4 nm) was already formed. This is considered to be attributable to O plasma exposure to GaN surface during SiO deposition by plasma CVD. Thickness of the GaOx interlayer slightly increased about 1 nm by thermal oxidation up to 1000 degrees. This behavior indicates that the oxidation of GaN surface was markedly suppressed by the SiO capping layer. C-V characteristics of SiO/GaOx/GaN MOS capacitor with the thin GaOx layer, except that prepared at 1000 degrees, exhibited low frequency dispersion and hysteresis of below 10 mV. The measured C-V curves also well agreed with ideal C-V curves. These results demonstrate that the excellent GaN MOS interface properties were realized by the insertion of thin GaOx interlayer.
Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
AlGaN/GaN HFET (hetero-junction field-effect transitor) has gained much attention as next-generation high frequency and high power devices. Since AlGaN/GaN HFET with Schottky gate is restricted in device application due to large gate leakage current and normally-on operation, MOS gate stack with deposited gate insulator has been widely investigated to overcome these limitations. Among various insulating materials, AlO is one of the potential candidates because of its wide bandgap and high thermal stability. 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.
Watanabe, Kenta*; Yamada, Takahiro*; Nozaki, Mikito*; Nakazawa, Satoshi*; Shih, H.*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
Initial oxidation process of AlGaN surface during thermal oxidation was investigated by synchrotron radiation photoelectron spectroscopy. Thermal oxidation of AlGaN/GaN/Si(111) substrates was carried out for 30 min in the temperature range from 200 to 1000 degrees in O ambient. The obtained Ga 2p spectra exhibited peak shift toward the higher binding energy with increasing oxidation temperature, suggesting the growth of Ga oxides on AlGaN surface. The peak deconvolution of Ga 2p spectra into Ga-O and Ga-N components revealed that the formation of Ga oxides was formed on AlGaN surface at above 400 degrees while GaN surface was stable below 600 degrees. These results indicate that AlGaN surface is easy to be oxidized compared to GaN surface.
Watanabe, Kenta*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ishida, Masahiro*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
AlGaN/GaN HFET has gained much attention as next-generation high frequency and high power devices. Among various insulating materials, AlO is one of the potential candidates. However, large amount of electrical defects at MOS interfaces severely degrade both drive current and threshold voltage stability. Positive Vth shift due to electron trapping in AlO layer by applying positive gate bias is often observed for AlO/AlGaN/GaN MOS structures. We have recently reported that N incorporation into AlO significantly reduces electron traps in AlO layer for Si and SiC MOS 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.
/AlGaN interface property deposited by PECVDTerashima, Daiki*; Watanabe, Kenta*; Yamada, Takahiro*; Nozaki, Mikito*; Shih, H.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; et al.
no journal, ,
Impact of input power and substrate temperature of SiO deposition by plasma-enhanced CVD on SiO/AlGaN interface was investigated by means of electrical characterization of MOS capacitors. Synchrotron radiation photoemission spectroscopy revealed that Ga oxide component on AlGaN surface increases with increasing input power of PECVD. MOS capacitors with SiO gate insulator deposited at low input power shows relatively better SiO/AlGaN interface quality, despite degraded interface quality for the sample with SiO deposited at high input power. These results suggest that AlGaN surface oxidation during oxide deposition should be controlled to obtain good interface property.
/AlON stacked gate dielectrics for AlGaN/GaN MOS-HFETWatanabe, Kenta*; Terashima, Daiki*; Nozaki, Mikito*; Yamada, Takahiro*; Nakazawa, Satoshi*; Ishida, Masahiro*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; et al.
no journal, ,
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.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.
no journal, ,
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.
