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RF-MBE growth of GaN films on nitridated α-Ga2O3 buffer layer

Tomohiro Yamaguchi 1Takumi Hatakeyama 1Daiki Tajimi 1Yohei Sugiura 1Takeyoshi Onuma 2Tohru Honda 1

1. Kogakuin University, 2665-1 Nakanomachi, Hachioji-city, Tokyo 1920015, Japan
2. Tokyo National College of Technology (TNCT), 1220-2, Kunugida, Hachioji 193-0997, Japan


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 AlONx 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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Related papers

Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 3, by Tomohiro Yamaguchi
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-14 17:33
Revised:   2013-06-03 05:38