Suppression of Mixing of Metastable Zincblende Phase in GaN Crystal Grown on ScAlMgO4 Substrates by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB-GaN is generated as stacking faults during the coalescence of the initially grown wurtzite- (WZ-) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ-GaN. Therefore, high-temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB-GaN also decreases with an increase in the growth time. Based on these results, a 940 nm-thick GaN layer is grown at 750 degrees C on the SAM substrate with a terrace width of 820 nm. A pure WZ-GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 x 10(9) cm(-2) is obtained.