Effects of site correlations on the local structure of strain-relaxed semiconductor alloys

In semiconductor alloys, the length mismatch between the constituent materials leads to elastic strain and structural distortions in local atomic environments. A computational method for performing global strain relaxations based on supercells is applied to several random, ordered, and correlated ternary alloy systems of the type ${\mathit{A}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{B}}_{\mathit{x}}$C. Site correlations are generated using Monte Carlo simulations of the face-centered cubic second-neighbor Ising model. Various structural quantities for the semiconductor alloy are calculated, including first- and second-neighbor distances, strain energies, and next-nearest-neighbor coordination numbers. From these calculations it is found that a sensitive experimental probe of correlations in the alloy is the cation-cation distance as a function of the composition x, which can be measured using the extended x-ray absorption fine-structure spectroscopy technique. This distance is found to bow inward from the random line for alloys that order, and outward from the random line for alloys that phase separate.