  The minimum energy paths for the migration of interstitial Mg in wurtzite GaN
are studied through density functional calculations. The study also comprises
Li, Na, and Be dopants to examine the dependence on size and charge of the
dopant species. In all cases considered, the impurities diffuse like ions
without any tendency of localizing charge. Li, Mg, and to some extent Na,
diffuse almost isotropically in GaN, with average diffusion barriers of 1.1,
2.1, and 2.5 eV, respectively. Instead Be shows a marked anisotropy with energy
barriers of 0.76 and 1.88 eV for diffusion paths perpendicular and parallel to
the c-axis. The diffusion barrier generally increases with ionic charge and
ionic radius, but their interplay is not trivial. The calculated migration
barrier for Mg is consistent with the values estimated in a recent beta-
emission channeling experiment.
