Electronic structure of the substitutional versus interstitial manganese in GaN

Density-functional studies of the electron states in the dilute magnetic semiconductor GaN:Mn reveal major differences for the case of the Mn impurity at the substitutional site (Mn-Ga) versus the interstitial site (Mn-I). The splitting of the twofold and the threefold degenerate Mn(d) states in the gap are reversed between the two cases, which is explained in terms of the symmetry-controlled hybridization with the neighboring atoms. In contrast to Mn-Ga, which acts as a deep acceptor, Mn-I acts as a donor, suggesting the formation of Coulomb-stabilized complexes such as MnGaMnIMnGa, where the acceptor level of Mn-Ga is passivated by the Mn-I donor. Formation of such passivated clusters might be the reason for the observed low carrier-doping efficiency of Mn in GaN. Even though the Mn states are located well inside the gap, the wave functions are spread far away from the impurity center. This is caused by the hybridization with the nitrogen atoms, which acquire small magnetic moments aligned with the Mn moment. Implications of the differences in the electronic structure for the optical properties are discussed.