Epitaxial thin films were grown, onto the prismatic planes of sapphire substrates, by means of metalorganic chemical vapor deposition. It was found that the films contained 2 crystallographic variants, both of which were epitaxially oriented with respect to the substrate. The variants were related to each other by a 180 rotation about the c-axis (which lay in the interface plane). A rectangular misfit dislocation network at the interface was imaged by means of cross-sectional high-resolution transmission electron microscopy along 2 orthogonal directions, and by means of plan-view weak-beam dark-field transmission electron microscopy. The Burgers vectors of these 2 orthogonal sets of misfit dislocations were shown to be 1/3[1¯1•0] and 1/3[00•1]. The spacing of the dislocations was almost exactly equal to the ideal values; thus suggesting the existence of a very high degree of epitaxy. This was also confirmed by the almost equal values of the full-width at half-maximum of X-ray peaks. The perfection of the epitaxy was paradoxically attributed to the lack of multiplicity of slip systems, in the interface plane, for accommodating misfit.

H.Xie, Y.C.Lu, R.Raj: Journal of Applied Physics, 1996, 79[7], 3675-80