Effect of N interstitial complexes on the electronic properties of GaAs1−xNx alloys from first principles

Although several approaches have been used in the past to investigate the impact of nitrogen (N) on the electronic structure of GaAs1−xNx alloys, there is no agreement between theory and experiments about the importance of the different N interstitial defects in these alloys, and their nature is still unknown. Here we analyse the impact of five different N defects on the electronic structure of GaAs1−xNx alloys, using density-functional methods: we calculate electronic states, formation energies, and charge transition levels. The studied defects include NAs, AsGa, AsGa-NAs substitutional defects and (N-N)As, (N-As)As split-interstitial complex defects. Our calculated defect formation energies agree with those reported by Zhang et al. [Phys. Rev. Lett. 86, 1789 (2001)], who predicted these defects. Among the interstitial defects, we found that (N-As)As emerges as the lowest energy configuration in comparison with (N-N)As, in agreement with recent experiments [Jen et al., Appl. Phys. Lett. 107, 221904 (2015)]. We also calculated the levels induced in the electronic structure due to each of these defects: defect states may occur as deep levels in the gap, shallow levels close to the band edges, and as levels resonant with bulk states. We find that the largest changes in the band structure are produced by an isolated N atom in GaAs, which is resonant with the conduction band, exhibiting a strong hybridization between N and GaAs states. Deeper levels in the band gap are obtained with (N-N)As split-interstitial defects. Our results confirm the formation of highly localized states around the N sites, which is convenient for photovoltaics and photoluminescence applications