GaN and related alloys are one of the promising materials for applications in optoelectronic devices. These materials are typically grown on sapphire (α-Al₂O₃) substrates. Surface nitridation of α-Al₂O₃ is one of effective methods to obtain high-quality GaN films. However, there still has a lattice mismatch of more than 3 % between GaN and AlON_x formed by the surface nitridation of α-Al₂O₃. Recently, the growth of high-quality α-Ga₂O₃ by mist chemical vapor deposition (CVD) was reported on a sapphire substrate [1]. When the α-Ga₂O₃ film is utilized as a novel buffer layer for the growth of GaN films, the lattice constant of α-Ga₂O₃ surface is expected to match with that of GaN after the surface nitridation of α-Ga₂O₃. This may lead the realization of higher-quality GaN films. In this study, GaN films were grown by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) on the α-Ga₂O₃ buffer layers deposited on α-Al₂O₃ substrates by mist CVD. Effect of α-Ga₂O₃ surface nitridation was also investigated.

α-Ga₂O₃ buffer layers were grown on (0001) α-Al₂O₃ substrates by mist CVD. Typical full-width at half maximum (FWHM) value of (0006) α-Ga₂O₃ rocking curve measured by X-ray diffraction (XRD) was approximately 100 arcsec. After the growth of α-Ga₂O₃, the samples were introduced to the MBE chamber. Since α-Ga₂O₃ is a metastable phase in Ga₂O₃ structures, the surface of the α-Ga₂O₃ buffer layer was nitridated by irradiating a nitrogen radical beam under the temperature where no transformation of α-Ga₂O₃ to stable β-Ga₂O₃ phase occurred. Then, GaN films were grown on the nitridated α-Ga₂O₃ buffer layers by RF-MBE. As in-situ observation, reflection high-energy electron diffraction (RHEED) was utilized. The samples were also measured ex-situ by XRD analysis.

When the α-Ga₂O₃ buffer layer was annealed in a vacuum, the RHEED diffraction pattern intensity of α-Ga₂O₃ started to weaken at 620°C. From the result of XRD θ-2θ scan measurement, the new diffraction peaks different from the peaks of α-Ga₂O₃ were observed in the sample annealed at over 660°C. This suggests that the transformation of α-Ga₂O₃ to other structures occurred at over 620°C in a vacuum. When the surface nitridation of α-Ga₂O₃ buffer layer was carried out, the distance between streaks of the α-Ga₂O₃ RHEED pattern became close with that between streaks of the GaN RHEED pattern after the surface nitridation of 2 hours. The GaN films grown on the nitridated α-Ga₂O₃ buffer layers were found to have the epitaxial relationship of [0001]_GaN//[0001]_α-Ga2O3//[0001]_α-Al2O3 from the profile of XRD θ-2θ scan. The in-plane epitaxial relationship between GaN, α-Ga₂O₃ and α-Al₂O₃ was also found to be <10-10>_GaN//<11-20>_α-Ga2O3//<11-20>_α-Al2O3 from the results of XRD φ-scan measurements. These epitaxial relationships were the same as the cases without the use of α-Ga₂O₃ buffer layer. Details on the crystal quality of the GaN film will also be reported.

The authors would like thank Prof. Fujita and Dr. Kaneko of Kyoto Univeristy for technical supports of mist CVD growth. This work was partly supported by JSPS KAKENHI Grant Numbers #25706020, #25420341 and #25390071, TEPCO Memorial Foundation and ALCA project of JST.

[1] D. Shinohara and S. Fujita, Jpn. J. Appl. Phys. 47, 7311 (2008).

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1. Polarized Raman Spectra in β-Ga₂O₃ Crystals