Formation energies, bond lengths, and bulk moduli of ordered semiconductor alloys from tight-binding calculations

A tight-binding model is used to calculate the formation energies, bond lengths, and bulk moduli of a number of ordered III-V and II-VI semiconductor alloys. The parameters in the model are adjusted so that the bond lengths, cohesive energies, bulk moduli, and shear elastic constants for the constituent compounds are described correctly. The model is then applied to alloys without further adjustment. Based on the calculated excess energies, we conclude that none of the ordered alloys found experimentally is in its stable bulk equilibrium state at the growth temperatures. Although the alloy excess energies can be negative, if the reference constituent compounds are constrained to match the substrate lattice used in epitaxial growth, their magnitudes are not large enough to account for the observed ordering. A possible explanation of the observed states in terms of a barrier that prevents the metastable ordered alloy from decomposing into separate phases is presented. However, this explanation only applies to alloys with lattice-mismatched constituents. Detailed results on the bond lengths and bulk moduli are also discussed.