Elastic energies and order in epitaxial Si-Ge alloys

Abstract
Elastic energies, and configurational and phonon entropies of Si-Ge alloys epitaxially grown on Si substrates are calculated as a function of long-range order. A Keating-type potential is used to describe interactions within large (≤512 atom) periodic unit cells with configurations appropriate to random and partially ordered alloys. We find an order-disorder transition into the experimentally observed rhombohedral structure at ∼60 K. No reasonable change in potential parameters could increase this temperature to the experimental value ∼800 K, implying that the transition is not elastically driven.