By using high-resolution electron microscopy, anisotropic elasticity calculations and image simulations, typical contrasts were predicted for pure-a edge threading dislocations in layers which had been grown, via molecular-beam epitaxy, on sapphire. Their atomic structure was found to exhibit 5/7, or 8-atom, cycles. These configurations were observed, with similar frequencies, for isolated dislocations and low-angle boundaries. Coincidence grain boundaries were studied, and they were all found to be made up of a pure-edge dislocations having the above atomic structures. A topological analysis of high-angle boundaries was performed in order to determine the defect content at the interfaces. The defects which were introduced upon deviating from coincidence were associated with steps, and their Burgers vectors corresponded to the smallest vectors of the displacement-shift complete set. At the interfaces, defect-free steps which belonged to the sides or diagonal of the coincident-site lattice unit-cell occurred. Reconstruction of some boundaries was possible only by taking account of the occurrence of structural units which exhibited 4-atom ring cycles for the dislocation cores. In non-symmetrical interfaces, a new structural unit which was made up of 5/4/7 atom-rings was found to constitute the core of one grain-boundary dislocation.

Mosaic Growth of GaN on (0001) Sapphire - a High-Resolution Electron Microscopy and Crystallographic Study of Threading Dislocations from Low-Angle to High-Angle Grain Boundaries V.Potin, P.Ruterana, G.Nouet, R.C.Pond, H.Morkoç: Physical Review B, 2000, 61[8], 5587-99