Abstract
The lack of high-purity GaN is illustrated by the n-type character of the as-grown material, indicative of the presence of shallow donors, or the strong yellow luminescence of all GaN crystals, indicative of the presence of deep levels. The theoretical techniques that have been used so far to study defect centers in GaN are based on the density-functional theory, the Hartree–Fock methodology, and molecular-dynamics simulations. In these calculations, host crystal is approximated by periodic supercells or hydrogen-terminated molecular clusters. In some ways, the band structure of GaN resembles those of II–VI semiconductors, e.g., the valence band is split into lower and upper parts. Further, the difference in electronegativities implies a large ionic character, i.e., a large contribution of the long-ranged Madelung energy. Although the antisites have large predicted formation energies in the otherwise perfect crystal, they may play a substantial role in imperfect regions where they could alleviate compressive and tensile stresses due to grain boundaries, film-substrate lattice mismatches, etc.