Unexpected band-gap collapse in quaternary alloys at the group-III-nitride/GaAs interface: GaAlAsN

We present first-principles relativistic pseudopotential calculations of the structural and electronic properties of the quaternary alloy GaAlAsN. We have found a surprising band-gap collapse, in contrast to the empirical and first-principles virtual-crystal approximation (VCA) calculations that predict a wide band gap. Unusually strong structural relaxation caused by large differences in the strength of the chemical bonds between the III-V ions in the alloy invalidates the VCA and leads to the band-gap collapse. These results have important implications for other complex semiconductor alloys.