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 (V Ga ) exist alongside dislocations and are stable up to 900 °C. It is suggested that dislocations are shallow positron traps. Upon annealing at ≤500 °C the decrease of dislocation density increases the effective positron diffusion length ( L + eff ) and the probability of trapping at V Ga . While L + eff continues to change, the trapping of positrons at V Ga is saturated upon annealing above 500 °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 °C dissociation of GaN at a rate of ~5 nm s -1 is observed. Oxygen clusters, stable up to 900 °C, appear to exist near the interface between the GaN film and the sapphire substrate.