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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.
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 800
C (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 
-Ga
O
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.
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.