Transition metal defects in group-III nitrides: Anab initiocalculation of hyperfine interactions and optical transitions

We present ab initio calculations for $3d$ transition metal point defects and for defect pairs in group-III nitrides with a special emphasis on charge transfer energies, hyperfine interactions, and internal optical transition energies. Our LMTO-ASA Green's function total energy calculations show VacN-TM pairs to be tightly bound in semiconducting and in n-type GaN and AlN with pair formation energies in excess of $1$ eV. We show that in the framework of density functional theory reliable total energies are obtained for excited states, if the levels involved are represented by a single determinantal state within a 3dN basis. Our optical transition energies agree fairly well with the experiments.