Effect of vicinal off-cut substrates on the basal stacking fault density in nonpolar a-GaN epilayers

Nonpolar a-GaN epilayers were grown on r-plane sapphire substrates with vicinal off-cut angles by metalorganic chemical vapor deposition. The effect of the off-cut angle on the formation of basal stacking faults (BSFs) observed in the a-GaN epilayers was investigated using transmission electron microscopy (TEM). The vicinal off-cut angles of the r-plane sapphire, which were used, were ±0.4° toward the [1 1 0 1]_Sapp direction. Based on high-resolution TEM images, the values for the tilt angle of the epilayers were -0.3 and 3.0 for the -0.4° and 0.4° off-cuts, respectively, considering the spontaneous -2.0° tilting observed in the film on the on-axis substrate. The vicinal off-cut induced a slight difference in step height on the substrate surface. Consequently, this resulted in periodical surface steps of one monolayer and formed BSFs due to the variations in the stacking sequence of the (1 1 2 0)_GaN planes. It was observed that the relatively high tilt angle of the epilayer observed in the 0.4° off-cut substrate was responsible for the reduction in the BSF density of the a-GaN epilayers.