Stable structures of (VGaVN) n defects in bulk GaN

Vacancy-type defects in GaN can migrate and aggregate during post-growth annealing. Here, we perform large-scale first-principles calculations to clarify the stability of (VGaVN)n aggregates (n = 1–6) in bulk GaN. For several structure models at each n, the defect formation energy of (VGaVN)n and the energy gain for the aggregation of (VGaVN)n−1 and VGaVN are calculated to investigate the stability during the stepwise growth. The calculated results show that the aggregation is always favorable, and the energy gain for this process is significantly large at n = 3 and 6. This result is consistent with the recently reported positron annihilation experiments. The structural stability and the electronic structure of the various models are discussed with respect to the number of dangling bonds and lattice relaxations, depending on the morphology of (VGaVN)n.