Classification of stacking faults and dislocations observed in nonpolar a-plane GaN epilayers using transmission electron microscopy

In this study, we presented comparative discrimination methods to identify various line and planar defects observed in nonpolar a-GaN epilayers on r-sapphire substrates. Unlike the case of conventional c-GaN, which is dominated by perfect threading dislocations, systematic identification of undistinguishable defects using transmission electron microscopy (TEM) is necessary to suppress the propagation of defects in nonpolar GaN epilayers. Cross-sectional TEM images near the [0 0 0 1] zone axis revealed that perfect mixed and pure screw type dislocations are visible, while pure edgeand basal stacking faults (BSFs) are not discernible. In tilted cross-sectional TEM images along the 12̄10 zone axis, the dominant defects were BSFs and partial dislocations for the g=1̄010 and 0 0 0 2 two-beam images, respectively. From plan-view TEM images taken along the 112̄0 axis, it was found that the dominant partial and perfect dislocations were Frank-Shockley and mixed dislocations, respectively. Prismatic stacking faults were observed as inclined line contrast near the [0 0 0 1] zone axis and were visible as band contrast in the two-beam images along the 12̄10 and 112̄0 zone axes.