The defect microstructures of thin wurtzite films which had been grown onto 6H SiC (basal plane) or sapphire (c-plane) substrates, via plasma-assisted molecular beam epitaxy, were studied by means of transmission electron microscopy. The predominant defects were found to be double positioning boundaries that originated at the interfaces between the substrate and the buffer layer or between the buffer and the GaN film. Since the stacking mismatch between coalescing domains was caused by substrate surface steps, these defects were also called stacking mismatch boundaries. Structural analyses implied that the presence of such boundaries was inevitable when wurtzite films were grown onto non-isomorphic substrates.

D.J.Smith, S.C.Y.Tsen, B.N.Sverdlov, G.Martin, H.Morkoç: Solid-State Electronics, 1997, 41[2], 349-52