Stacking mismatch boundaries in GaN: Implications for substrate selection

The defect microstructure of thin wurtzite GaN films grown by plasma-assisted molecular beam epitaxy on 6H SiC (basal plane) and sapphire (c-plane) has been characterized using transmission electron microscopy. The predominant defects observed were double positioning boundaries that originated at the interfaces between the substrate and the buffer layer or the buffer and the GaN film. Since the stacking mismatch between coalescing domains is caused by substrate surface steps, these defects could also be termed stacking mismatch boundaries (SMBs). Structural analysis implies that these SMBs are inevitable whenever wurtzite films are grown on non-isomorphic substrates. It is therefore suggested that isomorphic materials such as bulk GaN or ZnO should be investigated as alternative substrates.