Role of open volume defects in Mg-doped GaN films studied by positron annihilation spectroscopy

We have performed a systematic study by positron annihilation spectroscopy of magnesium-doped, codoped, and annealed GaN films made by metal organic chemical vapor deposition. GaN:Mg films are free of detectable vacancy defects up to [Mg]~3×10¹⁸0.3em0excm^-3, but at doping levels above 10¹⁹0.3em0excm^-3 vacancies are observed. Two defects are identified: VN-MgGa pairs and vacancy clusters, where the amount of the missing atoms is estimated to be about 60. The defects have an inhomogeneous depth profile with a layer of higher defect concentration starting 1000.3em0exnm below the surface. Thermal annealing dissociates the VN-MgGa pairs and the vacancy clusters, which migrate and form a homogeneous distribution of smaller clusters through the film. The identified defects play an important role in the electrical compensation and activation of the Mg acceptors in GaN films, and show correlations with results from transmission electron microscopy and photoluminescence studies.