Evolution of Structural Properties and Formation of N-N Split Interstitials in GaAs1-xNx Alloys

Using first-principles total energy calculations, we have studied the structural properties of the large lattice mismatched GaAs1-x Nx alloys. We first discuss the validity of Vegard's law, which assumes a linear variation of the alloy concentration with both the lattice constant and the volume. In the dilute-N limit, the calculated lattice constant coincides with Vegard's law for the lattice constant, but not for the volume variation. This deviation implies that using Vegard's law for the volume variation overestimates the N concentration. In the dilute-As limit, the calculated lattice constant is larger than that suggested by the Vegard's law for both the volume and the lattice constant, implying that using Vegard's law overestimates the As concentration. The calculated bulk moduli for GaAs1-x Nx, however, show almost linear behavior in the two region, increase monotonically with N concentration. We have also studied the effect of the split interstitial defect (N-N)spl, on the electronic and structural properties of the alloy. We find that the split interstitial is an amphoteric defect with (+ 0) transition at 0.2 eV and (0 -) transition at 0.3 eV above the valence band maximum. The concentration of the split nitrogen interstitial [N-N] is relatively small compared to the substitutional nitrogen [N], but it would be large if the system could be grown at the As-rich limit and the sample were doped n -type.