Defects in GaN films studied by positron annihilation spectroscopy

The annealing of n-type thin GaN films grown by metal-organic chemical vapour deposition in vacuum has been studied by beam-based positron annihilation spectroscopy. The results are consistent with a model in which Ga vacancies (VGa) exist alongside dislocations and are stable up to 900 deg. C. It is suggested that dislocations are shallow positron traps. Upon annealing at ≤500 deg. C the decrease of dislocation density increases the effective positron diffusion length (L+eff) and the probability of trapping at VGa. While L+eff continues to change, the trapping of positrons at VGa is saturated upon annealing above 500 deg. C. The formation of N vacancies near the surface at high temperatures is considered to introduce a potential that retards positron back-diffusion. At 900 deg. C dissociation of GaN at a rate of ∼5 nm s-1 is observed. Oxygen clusters, stable up to 900 deg. C, appear to exist near the interface between the GaN film and the sapphire substrate. (author). Letter-to-the-editor