SURFACE SEGREGATION AND ORDERING IN III-V SEMICONDUCTOR ALLOYS

Using the first-principles total-energy pseudopotential method, we hav e studied the formation energy of the (001) Ga1-xInxP alloy surface as a function of composition and reconstruction. The results are present ed as T=0 surface stability diagrams that show the lowest energy recon struction and cation occupation pattern as functions of the chemical p otentials. Slightly different stability diagrams emanate depending on whether or not there is equilibrium between the surface and the bulk. The stability diagrams show a pronounced asymmetry between the Ga- and In-rich regions. The asymmetry is interpreted in terms of the size di fference between In and Ga and the effect of this size difference on t he bonding geometry. For surfaces in equilibrium with the bulk, we fin d a strong dependence of surface segregation on the surface reconstruc tion, and we predict a Ga enrichment of the surface in the moderate ca tion-rich limit and In enrichment in the anion-rich region. This resul t suggests a way to achieve abrupt interfaces in semiconductor heteros tructures. For surfaces not in equilibrium with the bulk, we identify regions in the stability diagram where surface-induced CuPt ordering ( both type A and type B) occurs.