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 degrees toward the [(1) over bar 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 degrees and +0.4 degrees off-cuts, respectively, considering the spontaneous -2.0 degrees 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) over bar 0)(GaN) planes. It was observed that the relatively high tilt angle of the epilayer observed in the +0.4 degrees off-cut substrate was responsible for the reduction in the BSF density of the a-GaN epilayers. (C) 2011 Elsevier B.V. All rights reserved.