Free Energies and Equilibrium States of Mono- and Bi-Layer Superstructures of III-V Ternary Alloy Semiconductors

Equations for the free energies of mono- and bi-layer superstructures of III-V ternary alloy systems were derived as functions of the order parameter ϕ (random atom arrangement when ϕ = 0 and a complete ordered arrangement when ϕ = 1). It was assumed in the analysis that mixing enthalpy was a strain energy caused by mixing. By minimizing the free energy, we obtained the value of ϕ at the equilibrium state. The values are larger than zero for monolayer superstructures on (100) and (110) surfaces below certain temperatures. For monolayer superstructures on a (111) surface, the stable states are at ϕ = 0, i.e., random arrangements at all temperatures. Bilayer superstructures are less stable than monolayer superstructures of the corresponding surface orientation.