Diffusivity of native defects in GaN

The diffusion of relevant native point defects in wurtzite GaN crystals is investigated using first-principles density-functional pseudopotential calculations. Our reexamination of the ground state of the defects, using a higher level of convergence than was previously used, yields results in good agreement with earlier published results [J. Neugebauer and C. G. Van de Walle, Phys. Rev. B $50,$ 8067 (1994)]. Gallium interstitials are stable at the octahedral interstitial site and can occur in 1+, 2+ (metastable), or 3+ charge states. They migrate via an interstitialcy mechanism with an unexpectedly low barrier of 0.9 eV, consistent with the annealing of the $L5$ signal in electron-paramagnetic-resonance experiments [K. H. Chow et al., Phys. Rev. Lett. $85,$ 2761 (2000)]. For the nitrogen interstitial the ground-state configuration is a split interstitial, occurring in charge states ranging from 1- to 3+. Migration also proceeds via an interstitialcy mechanism, with barriers of 2.4 eV or lower, depending on the charge state. The nitrogen vacancy has two stable charge states 1+ and 3+. The migration barrier for VN⁺ is high (4.3 eV), while the migration barrier for VN³⁺ is significantly lower, at 2.6 eV, consistent with recent positron-annihilation experiments [S. Hautakangas et al., Phys. Rev. Lett. $90,$ 137402 (2003)]. The gallium vacancy, finally, can occur in charge states 0, 1-, 2-, and 3-, and migrates with a barrier of 1.9 eV. For all these defects the lowest-energy migration path results in motion both parallel and perpendicular to the c axis; no anisotropy in the diffusion will therefore be observed. Applications to point-defect-assisted impurity diffusion will also be discussed.