  Several approaches have been used to investigate the impact of Nitrogen (N)
on the electronic structure of GaAs$_{1-x}$N$_{x}$ alloys, however, 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 analyze the impact of five different N defects on the electronic structure
of GaAs$_{1-x}$N$_{x}$ alloys, using density-functional methods: we calculate
electronic states, formation energies and charge transition levels. The studied
defects include N$_{As}$, As$_{Ga}$, As$_{Ga}$-N$_{As}$ substitutional defects,
and (N-N)$_{As}$, (N-As)$_{As}$ split-interstitial complex defects. Our
calculated defect formation energies agree with those reported by S.B. 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)$_{As}$, agreeing with recent experiments
[T. 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 between 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 bandgap 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.
