First-principles study of Raman intensities in semiconductor systems

We present a general method for the theoretical determination of the off-resonance Raman activity of semiconductor mixed systems such as substitutional alloys or superlattices, with the inclusion of strain effects. As basic tool, the density-functional perturbation theory has been used. The calculation of the Raman cross-section requires the knowledge of the lattice dynamics of the complete system. The description of semiconductor alloys or long-period superlattices requires large supercells, this makes the numerical effort of a direct ab initio calculation an unfeasible task. Simple procedures using the virtual-crystal (VCA) and the mass approximation fail in describing the effect of both the polarizability variations and the change in the local environment due to the local strain. We used a perturbative scheme which includes the potential fluctuations responsible for chemical disorder and the variations of the atomic Raman tensors. The method is applied to some III–V and Si/Ge semiconductor systems.