Effects of disorder on the optical properties of the (Zn,Mg)(S,Se) quaternary alloy

The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close substantially the gap with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap vs. composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures typical for MBE growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy.