First-principles study of Raman intensities in semiconductor systems

We present a general method for the theoretical determination of the off-re sonance Raman activity of semiconductor mixed systems such as substitutiona l alloys or superlattices, with the inclusion of strain effects. As basic t ool, the density-functional perturbation theory has been used. The calculat ion of the Raman cross-section requires the knowledge of the lattice dynami cs of the complete system. The description of semiconductor alloys or long- period superlattices requires large supercells, this makes the numerical ef fort of a direct ab initio calculation an unfeasible task. Simple procedure s using the virtual-crystal (VCA) and the mass approximation fail in descri bing the effect of both the polarizability variations and the change in the local environment due to the local strain. We used a perturbative scheme w hich includes the potential fluctuations responsible for chemical disorder and the variations of the atomic Raman tensors. The method is applied to so me III-V and Si/Ge semiconductor systems. (C) 2001 Elsevier Science B.V. Al l rights reserved.