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

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 10⁴cm ^-1. The residual donor concentration was 10¹⁴-10 ¹⁵cm^-3, with a very low density (2.5 10 ¹³cm^-3) of electron traps located at E_c- 0.6 eV, 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 1 eV 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 10¹³cm^-3, with 2DEG mobility of 80 cm²/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.