Band-gap narrowing in ordered and disordered semiconductor alloys

Either spontaneous or artificial ordering of semiconductor alloys into CuAu-like, chalcopyrite, or CuPt-like structures is predicted to be accompanied by a reduction in the direct band gaps relative to the average over the binaries. In this letter calculated results are presented for seven III-V and II-VI alloys. We identify the mechanism for this band-gap narrowing as band folding followed by repulsion between the folded states. The latter is coupled by the non-zinc-blende component of the superlattice potential. The same physical mechanism (but to a different extent) is responsible for gap bowing in disordered alloys.