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

Using high-resolution electron microscopy, anisotropic elasticity calculations, and image simulations, typical contrast was identified for the pure edge threading dislocations in GaN layers grown by molecular-beam epitaxy on sapphire. Their atomic structure was found to exhibit 5/7, or 8 atom cycles. The two configurations were observed at a similar frequency for isolated dislocations and low-angle boundaries. Coincidence grain boundaries have been studied and they are all made of pure-edge dislocations with the above identified atomic structures. A topological analysis of high-angle boundaries has been carried out in order to determine the defect content at the interfaces. The defects introduced on deviation from coincidence are associated with steps and their Burgers vectors correspond to the smallest vectors of the displacement shift complete set. At the interfaces, defect free steps, which belong to the sides or diagonal of the coincident site lattice unit cell, have been found to occur. The reconstruction of some boundaries was only possible by taking into account the occurrence of structural units which exhibit 4-atom ring cycles for the dislocation cores. In nonsymmetric interfaces, a new structural unit made of 5/4/7 atom rings has been found to constitute the core of one grain boundary dislocation.