Dependency of a localized phonon mode intensity on compositional cluster size in SiGe alloys

Using molecular dynamics, we found that the localized phonon-mode spectrum in SiGe alloys, which was recently discovered by an inelastic x-ray scattering experiment, changes according to the size distribution of compositional clusters in alloys. By varying the spatial distributions of Si and Ge, alloy models with differing compositions of Si and Ge clusters were able to be produced. For a range of alloys comprising 20%-80% Ge, a mixture of small and intermediate sized clusters will give the highest intensities of the local mode. The Si-Ge optical mode intensity increases with the local mode intensity, but the Si-Ge bond alone is not sufficient to produce the local mode. Si rich alloys with small Ge clusters produce the highest local mode intensities, suggesting that this mode is caused by small Ge clusters surrounded by Si pairs.