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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:39 Percentile:90.77(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.

Journal Articles

Effect of nitrogen incorporation into Al-based gate insulators in AlON/AlGaN/GaN metal-oxide-semiconductor structures

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:31 Percentile:85.09(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 Al$$_{2}$$O$$_{3}$$ 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$$times$$10$$^{11}$$ cm$$^{-2}$$eV$$^{-1}$$. The impact of nitrogen incorporation into the insulator was discussed on the basis of experimental findings.

Journal Articles

Synchrotron radiation X-ray photoelectron spectroscopy of Ti/Al ohmic contacts to n-type GaN; Key role of Al capping layers in interface scavenging reactions

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:25.92(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.

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

Effect of nitrogen incorporation into Al-based gate insulator in AlGaN/GaN MOS-HEMT

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 Al$$_{2}$$O$$_{3}$$ 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 1.2$$times$$10$$^{11}$$ cm$$^{-2}$$eV$$^{-1}$$ were obtained for AlON/AlGaN/GaN MOS capacitors.

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.

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