Native defects and common dopants in the cubic phases were studied by means of ab initio calculations, using a super-cell approach and the full-potential linear muffin-tin orbital method. The atomic positions, electronic structure and defect formation energy were calculated. Vacancies were predicted to be abundant defects. The high-pressure behavior of the defect states was studied, and it was found that the pressure coefficients of the defect states depended mainly upon their position in the energy gap. The calculations predicted similar trends in the positions of the defect levels in all 3 materials, but the absolute values were different. This was due mainly to the differing values of the energy gaps. The defect levels lay somewhat deeper in the energy gap, in AlN and BN, than in GaN. The super-cell calculations indicated marked outward lattice relaxations around cation antisites. In the more complex case of the N antisite which seemed to be similar (in GaN and AlN) to the EL2 defect in GaAs, large inward relaxations were found. The distance between the N impurity and the nearest-neighbour was reduced by 30% and became comparable to the N2 dimer bond-length. This effect did not occur in BN. Instead, there was an outward symmetrical relaxation (about 9%) of the surrounding atoms. A marked inward relaxation (of about 18% of the bond length) was also found around a C impurity on cation sites in GaN and AlN. In BN, the relaxation around a C impurity was small or negligible. Substantial outward relaxation was also found for Mg and Zn impurities in AlN.

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