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Journal Articles

SiO$$_{2}$$/AlON stacked gate dielectrics for AlGaN/GaN MOS heterojunction field-effect transistors

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:4 Percentile:32.09(Physics, Applied)

The advantage of SiO$$_{2}$$/AlON stacked gate dielectrics over SiO$$_{2}$$, AlON and Al$$_{2}$$O$$_{3}$$ 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.

Journal Articles

Implementation of atomic layer deposition-based AlON gate dielectrics in AlGaN/GaN MOS structure and its physical and electrical properties

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:4 Percentile:32.09(Physics, Applied)

We fabricated AlON dielectric films by repeating thin AlN deposition and in situ O$$_{3}$$ 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.

Journal Articles

Control of Ga-oxide interlayer growth and Ga diffusion in SiO$$_{2}$$/GaN stacks for high-quality GaN-based metal-oxide-semiconductor devices with improved gate dielectric reliability

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:9 Percentile:12.47(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$$_{2}$$ films directly onto GaN substrates forming Ga-oxide interlayers was used to fabricate SiO$$_{2}$$/GaO$$_{x}$$/GaN stacked structures. Although well-behaved hysteresis-free GaN-MOS capacitors with extremely low interface state density below 10$$^{10}$$cm$$^{-2}$$eV$$^{-1}$$ were obtained by post-deposition annealing, Ga diffusion into overlying SiO$$_{2}$$ 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.

Journal Articles

Design and control of interface reaction between Al-based dielectrics and AlGaN Layer in AlGaN/GaN metal-oxide-semiconductor structures

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:10 Percentile:22.28(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 (Al$$_{2}$$O$$_{3}$$ and AlON) and AlGaN layer during deposition and post-deposition annealing (PDA), and revealed high thermal stability of AlON/AlGaN interface.

Journal Articles

Comprehensive study on initial thermal oxidation of GaN(0001) surface and subsequent oxide growth in dry oxygen ambient

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:28 Percentile:5.61(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 800$$^{circ}$$C, 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 $$alpha$$- and $$beta$$-phase Ga$$_{2}$$O$$_{3}$$ 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.

Oral presentation

In situ photoemission study of interface reaction in Al/Ti/n-GaN structures

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

Oral presentation

Synchrotron radiation X-ray photoelectron spectroscopy study of interface reactions in Al/Ti/GaN ohmic contacts

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.

Oral presentation

Investigation of Initial Oxide Growth on GaN Epitaxial Films

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$$_{2}$$ 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.

Oral presentation

Investigation of thermal oxidation process in low-defect density GaN substrate

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 $$mu$$mx1 $$mu$$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 800$$^{circ}$$C (the root-mean-square (RMS) roughness of 0.14 nm). However, the ss-GaN sample oxidized at 900$$^{circ}$$C showed rough surface morphology due to the formation of small Ga-oxide grains (RMS roughness of 0.62 nm). The grains drastically grew at 1000$$^{circ}$$C, resulted in the surface morphology consisting of $$beta$$-Ga$$_{2}$$O$$_{3}$$ crystal.

Oral presentation

Synchrotron radiation X-ray photoelectron spectroscopy study of interface reactions in Al/Ti/GaN Ohmic contacts

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.

Oral presentation

Investigation of oxide formation process in thermally oxidized GaN surface

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.

Oral presentation

Improved interface properties of SiO$$_{2}$$/GaN MOS capacitor by interfacial GaOx formation with post-oxidation treatment

Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; Watanabe, Heiji*

no journal, , 

Interface properties of SiO$$_{2}$$/GaN structure with post-oxidation treatment were investigated by synchrotron radiation photoelectron spectroscopy and C-V measurements. High-resolution Ga 2p$$_{3/2}$$ core-level spectra for SiO$$_{2}$$/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$$_{3/2}$$ spectrum, formation of GaOx layer at the SiO$$_{2}$$/GaN interface was revealed. C-V measurements of the SiO$$_{2}$$/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.

Oral presentation

Formation of thin GaOx interlayer by thermal oxidation of SiO$$_{2}$$/GaN and its effect on electrical properties

Yamada, 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$$_{2}$$/GaN structure was investigated by spectroscopic ellipsometry and synchrotron radiation photoelectron spectroscopy. In the SiO$$_{2}$$/GaN structure before thermal oxidation, GaOx interlayer (about 4 nm) was already formed. This is considered to be attributable to O$$_{2}$$ plasma exposure to GaN surface during SiO$$_{2}$$ 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$$_{2}$$ capping layer. C-V characteristics of SiO$$_{2}$$/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.

Oral presentation

Interface engineering of Al based gate insulators in AlGaN/GaN MOS-HFETs

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, Al$$_{2}$$O$$_{3}$$ 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 (Al$$_{2}$$O$$_{3}$$ and AlON) and AlGaN layer during deposition and post-deposition annealing (PDA), and revealed high thermal stability of AlON/AlGaN interface.

Oral presentation

Synchrotron radiation photoemission study of thermal oxidation of AlGaN surface

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 O2 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.

Oral presentation

Design and control of interface reaction between Al-based dielectrics and AlGaN layer for hysteresis-free AlGaN/GaN MOS-HFETs

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, Al$$_{2}$$O$$_{3}$$ 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 Al$$_{2}$$O$$_{3}$$ layer by applying positive gate bias is often observed for Al$$_{2}$$O$$_{3}$$/AlGaN/GaN MOS structures. We have recently reported that N incorporation into Al$$_{2}$$O$$_{3}$$ significantly reduces electron traps in Al$$_{2}$$O$$_{3}$$ layer for Si and SiC MOS devices. In this study, we systematically investigated the interface reaction between Al-based dielectrics (Al$$_{2}$$O$$_{3}$$ and AlON) and AlGaN layer during deposition and post-deposition annealing (PDA), and revealed high thermal stability of AlON/AlGaN interface.

Oral presentation

Influence of deposition power and temperature on SiO$$_{2}$$/AlGaN interface property deposited by PECVD

Terashima, 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$$_{2}$$ deposition by plasma-enhanced CVD on SiO$$_{2}$$/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$$_{2}$$ gate insulator deposited at low input power shows relatively better SiO$$_{2}$$/AlGaN interface quality, despite degraded interface quality for the sample with SiO$$_{2}$$ deposited at high input power. These results suggest that AlGaN surface oxidation during oxide deposition should be controlled to obtain good interface property.

Oral presentation

AlON gate dielectrics formed by repeating ALD-based thin AlN deposition and in situ oxidation for 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$$_{3}$$ 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.

Oral presentation

Physical and electrical characterization of AlGaN/GaN MOS gate stacks with AlGaN surface oxidation treatment

Yamada, Takahiro*; Watanabe, Kenta*; Nozaki, Mikito*; Shih, H.*; Nakazawa, Satoshi*; Anda, Yoshiharu*; Ueda, Tetsuzo*; Yoshigoe, Akitaka; Hosoi, Takuji*; Shimura, Takayoshi*; et al.

no journal, , 

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 degrees, in contrast to no oxide formation on GaN surface. However, since no noticeable change in the surface morphology was observed at temperatures up to 800 degrees, it can be concluded that an ultrathin oxide overlayer is formed on the AlGaN surface. On the other hand, for the oxidation treatments above 850 degrees, 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 800 degrees. 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 400 degrees leads to further improvement of interface properties and reduction of C-V hysteresis.

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