  Self-consistent electronic structure calculations together with a structural
model are used to study the effect of short-range atomic order on the optical
properties of otherwise random Al(0.5)Ga(0.5)As, Ga(0.5)In(0.5)P, and
Al(0.5)In(0.5)As alloys. We find that clustering can reduce the direct band gap
of these alloys by as much as 100 meV. Furthermore, sufficiently strong
clustering is predicted to transform Al(0.5)Ga(0.5)As into a direct gap
material.
