Characteristics of a-GaN films and a-AlGaN/GaN heterojunctions prepared on r-sapphire by two-stage growth process

• Published in: Journal of applied physics 2011-11, Vol.110 (9), p.093709-093709-6
• Main Authors: Polyakov, A. Y., Jang, Lee-Woon, Smirnov, N. B., Govorkov, A. V., Kozhukhova, E. A., Yugova, T. G., Reznik, V. Y., Pearton, S. J., Baik, Kwang Hyeon, Hwang, Sung-Min, Jung, Sukkoo, Lee, In-Hwan
• Format: Article
• Language: English
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.3658026

The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was ∼5×10 4 cm −1 . The residual donor concentration was 10 14 -10 15 cm −3 , with a very low density (2.5×10 13 cm −3 ) of electron traps located at E c −0.6eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands associated with SFs. In contrast to previously studied nonpolar GaN films, the a-GaN layers showed a high concentration of gallium-vacancy-related acceptors near E v +1eV and a strong yellow luminescence band, both indicating that growth conditions were effectively N-rich. a-AlGaN/GaN heterojunctions with thin heavily Si doped AlGaN barriers made on a-GaN substrates showed two-dimensional electron gas (2DEG) concentrations of 1.2×10 13 cm −3 , with 2DEG mobility of 80 cm 2 /Vs. Capacitance-voltage profiling of Schottky diodes on these HJs suggest that the 2DEG is fully depleted by the built-in voltage of the Schottky diode.