Local vibrational modes of Ge-richc-Si1−xGexalloys

The local vibrational modes of Ge-rich c-${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ge}}_{\mathit{x}}$ alloys (x≥0.75) are calculated using a parameter-free, mass-defect Green’s-function approach. A microscopic expression of the Raman-scattering efficiency is then evaluated for inelastic light scattering from these modes. Particular attention is paid to the modes which are responsible for the fine structure in the phonon spectra which is observed in addition to the strong Ge-Ge, Si-Si, and Si-Ge modes. The corrections to the extended Ge-Ge modes due to alloy disorder are calculated with use of the coherent-potential approximation and also included in the Raman susceptibility.