By using a Green’s function technique which was based upon the linear muffin-tin orbital method in the atomic-spheres approximation, self-consistent calculations were made of the electronic structure of native defects and other impurities. Vacancies, antisite defects, interstitials, and some of the most common dopants were investigated in various charge states. In order to study lattice relaxation effects, the super-cell approach and a full-potential linear muffin-tin orbital method were applied to the Al vacancy and the N antisite in AlN. The effect of an hydrostatic pressure upon the energy positions of some defect states was also studied. Qualitative agreement was found upon comparing the present results, for native defects in GaN, with published pseudopotential results. Some discrepancies in the level positions were attributed to the fact that lattice relaxation was not usually taken into account here. This could lead to appreciable errors in cases where the defect involved a large atom (such as Ga) which was placed at an interstitial or anion site. Thus, in the case of GaN, the published pseudopotential result for the level position without relaxation was 0.9eV; instead of the 1.3eV which was predicted when relaxation was included. The present unrelaxed results gave 0.65eV. The super-cell calculations showed that, in the case of AlN, there was an outward lattice relaxation, around VAl, which was very similar to that reported for VGa. In more complex cases such as NAl, which appeared to be similar to EL2 in GaAs, the present calculations yielded results which were very similar to those that were obtained by means of first-principles pseudopotential calculations. It was concluded that, in both GaN and AlN, the pressure coefficients of the defect states depended upon the position of the state in the gap, but did not seem to depend upon the type of state (donor or acceptor).

I.Gorczyca, A.Svane, N.E.Christensen: Solid State Communications, 1997, 101[10], 747-52