Modeling the extended defect evolution in lateral epitaxial overgrowth of GaN: Subgrain stability

The topic of extended defect structure stability in lateral epitaxial overgrowth (LEO) of GaN is addressed. We first summarize experimental results of growth morphology and dislocation structure in LEO GaN on sapphire substrates in the context of the crystallographic orientation of the stripe window openings. For 〈112̄0〉-oriented stripes, the LEO material often exhibits a triangular cross-section morphology with inclined ${101̄1}$ facets. The threading dislocations (TDs) propagate vertically in the window region and then become unstable and bend toward the inclined sidewalls, to eventually lie in the basal plane. In the case of 〈11̄00〉-oriented stripes, vertical ${112̄0}$ sidewalls may develop and, when realized, yield the lowest density of extended defects in the wing (overgrown) regions. Most of the TDs propagate vertically in the window regions and remain in loosely organized cell walls (subgrain boundaries). For this stripe orientation, a discernable crystallographic tilt of the LEO wing regions relative to the window region is also observed. This tilt is related to the appearance of edge dislocation arrays in the transition region between windows and wings. To understand the growth of dislocation-free wing regions, we present a model for dislocation subboundary cell structure stability during lateral overgrowth. The model provides a relation between structural parameters (such as cell size and misorientation) under which lateral defect-free growth is energetically favorable.