Nucleation of threading dislocations in atomistic simulations of strained layer epitaxy of III-nitrides

Abstract

The early stage of heteroepitaxial growth of wurtzite III-nitrides on lattice-mismatched substrates produces epitaxial strain that is relieved by nucleating a network of misfit dislocations. However, these epilayers contain also a large density of threading dislocations that are nearly perpendicular to the interface, despite the fact that they do not relieve the epitaxial strain. To shed light onto the mechanism of nucleation of threading dislocations, we employ atomistic simulations to explore the heteroepitaxy between a GaN nanoisland and a rigid substrate whose lattice parameter is adjusted to produce the desired misfit. We observe that the ends of misfit dislocations experience large image forces that pull them toward the pyramidal facets of the nanoisland. This results in bending of the terminating ends of misfit dislocations which constitute segments of threading dislocations. The spacing of these threading dislocations would increase (and thus their density decrease) if the epitaxial growth could be manipulated such that the film was deposited in a layer-by-layer fashion.