Vibrational properties of amorphous silicon-germanium alloys and superlattices

Abstract

Densities of states and localization characteristics have been calculated for vibrational modes in a-${\mathrm{Si}}_{\mathrm{x}}$ ${\mathrm{Ge}}_{1\mathrm{-}\mathrm{x}}$ alloys and a-Si/a-Ge superlattices, using amorphous tetrahedral networks generated by molecular-dynamics methods. For the alloys, the calculated densities of states agree very well with Raman scattering experiments at all concentrations 0≤x≤1. We explicitly show that an x-dependent feature at 390 ${\mathrm{cm}}^{\mathrm{-}1}$ arises primarily from Si-Ge bond-stretching motions. The localization of vibrational modes is enhanced in magnitude and in frequency range for the alloys as compared with the pure amorphous materials, but extended modes are still present at all x. To account for the Raman measurements on thin a-Si/a-Ge superlattices, a mixed Si-Ge interfacial region about 5 Å in thickness is indicated by our calculations.