Native defects and common dopants in the cubic phases were examined by means of ab initio calculations, using a super-cell approach and the full-potential linear muffin-tin orbital method. It was predicted that vacancies would be abundant defects. The high-pressure behavior of the defect states was also studied, and it was found that the pressure coefficients of the states depended mainly upon their position in the energy gap. In general, the defect levels lay somewhat deeper in the energy gap in AlN and BN than in GaN. In particular, some states that were resonant with the conduction or valence band in GaN, moved into the band-gap in AlN and BN. The super-cell calculations indicated marked outward lattice relaxations around cation antisites. In the case of the more complex N antisite, which seemed to be similar (in GaN and AlN) to EL2 in GaAs, a large inward relaxation was found. The distance from the N impurity atom to the nearest-neighbor was reduced by some 30% and became comparable to the N2 dimer bond-length. This effect did not occur in BN, and there was instead an symmetrical outward relaxation of the surrounding atoms by about 9%. A marked inward relaxation (about 18% of the bond length) was also found around a C impurity on the cation site in GaN or AlN. In the case of BN, the relaxation around a C impurity was small or negligible. Substantial outward relaxations were also found for Mg and Zn impurities in AlN. It was noticed that the outward relaxations were generally larger in AlN than in GaN; possibly because the Ga-3d electrons prevented the Ga-N bond-length from becoming too short. In BN, the relaxations were generally smaller (except for a B vacancy), and this was attributed to the similar atomic covalent radii of B, N and C. In all of these nitrides, the pressure coefficients of the defect states depended upon the position of the state in the gap, and not upon the type of state (donor or acceptor).

Theory of Point Defects in GaN, AlN and BN: Relaxation and Pressure Effects. I.Gorczyca, A.Svane, N.E.Christensen: Physical Review B, 1999, 60[11], 8147-57