Structure and stability of threading edge and screw dislocations in bulk GaN

Abstract

Computational studies of a-type threading edge dislocations parallel to the [0001] direction in bulk GaN reveal three possible structures of their cores that are characterized as the 4-atom ring, the 8-atom ring and the 5/7-atom ring configurations. Similar calculations made on the c-type threading screw dislocation suggest the existence of a full core, although the precise structure is still a matter of debate. Most studies associate the position of the dislocation with only high-symmetry points in the lattice and do not discuss the core stability in non-equiatomic configurations. Here, we investigate how the core structures of a-type edge and c-type screw threading dislocations depend on the position in the (0001) plane at which the dislocation is inserted perpendicular to this plane. Furthermore, by successively removing the high-energy atoms from the cores of these dislocations, we track the stability of each core in non-equiatomic configurations and identify the core structures of minimum energy. The edge dislocation is shown to prefer the 5/7-atom ring structure, while the core of the screw dislocation transforms into a new Ga-rich configuration with the associated energy drop of nearly 0.4eV per one Angström of the dislocation length. For all types of dislocation cores found in this paper, we employ isotropic linear elasticity to estimate their core radii and core energies.