Deep electronic gap levels induced by isovalent P and As impurities in GaN

The electronic and atomic structure of isovalent substitutional P and As impurities in GaN is studied theoretically using a self-consistent plane-wave pseudopotential method. In contrast with the conventional isovalent III-V systems, GaṈ:P and GaṈ:As are shown to exhibit deep gap levels. The calculated donor energies are ε(+/0)=εᵥ+0.22 and εᵥ+0.41 eV, respectively, and the double donor energies are ε(++/+)=εᵥ+0.09 and εᵥ+0.24 eV, respectively. The p-like gap wave function is found to be strongly localized on the impurity site. Outward atomic relaxations of ~13% and ~15% are calculated for the nearest-neighbor Ga atoms surrounding neutral GaṈ:P⁰ and GaṈ:As⁰, respectively. The relaxation increases by ~1% for the positively charged impurities. The impurity-bound exciton binding energy is calculated at Eb=0.22 and Eb=0.41 eV for GaṈ:P and GaṈ:As. The former is in good agreement with the experimental data (Eb=0.232 eV) whereas the latter is offered as a prediction. No clear Jahn-Teller symmetry lowering (Td→C₃ᵥ) distortion, suggested by the one-electron configuration, is found for GaṈ:P⁺ and GaṈ:As⁺.