Nitrogen vacancies as major point defects in gallium nitride.

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.