A low-energy positron beam was used to study epitaxial Si-doped GaN layers, in which the grain size ranged from 0.2 to 5µm. Negatively charged Ga vacancies were found in n-type samples. Their concentration was independent of the grain size; thus suggesting that Ga vacancies existed in the grain interior. Positrons were also observed to become trapped at other negatively-charged centers. The positron trapping rate at these defects was related to the grain-boundary density. Observed shallow positron traps, which did not contain open volume, were attributed to negatively charged edge-type dislocations which defined the grain boundaries.
Ga Vacancies and Grain Boundaries in GaN. J.Oila, K.Saarinen, A.E.Wickenden, D.D.Koleske, R.L.Henry, M.E.Twigg: Applied Physics Letters, 2003, 82, 1021-3