Calculation of the Raman spectra in compositional and topologically disordered semiconductors

A calculation of the Raman spectrum of a highly impurified tetrahedral semiconductor is presented. This sort of alloy gives rise to compositional and topological disorder. The theory used for the calculation of the vibrational modes is base d on the coherent potential approximation (CPA). Very simple self-consistent equations are obtained if a Born-type Hamiltonian is used. The corresponding calculation of the polarised Raman spectrum is made using a simple model for the local polarisability of the bonds. The observed softening of the Raman mode is discussed in terms of the combined effects of the impurities, of the disorder introduced by vacancies and of force constant changes.