Effects of alloy disorder and confinement on phonon modes and Raman scattering in SixGe1-x nanocrystals : a microscopic modeling

Abstract

Confinement and alloy disorder effects on the lattice dynamics and Raman scattering in Si1-xGex nanocrystals (NCs) are investigated numerically employing two different empirical inter-atomic potentials. Relaxed NCs of different composition (x) were built using the Molecular Dynamics method and applying rigid boundary conditions mimicking the effect of surrounding matrix. The resulting variation of bond lengths with x was checked against Vegard's law and the NC phonon modes were calculated using the same inter-atomic potential. The localization of the principal Raman-active (Si-Si, Si-Ge and Ge-Ge) modes is investigated by analysing representative eigenvectors and their inverse participation ratio. The dependence of the position and intensity of these modes upon x and NC size is presented and compared to previous calculated results and available experimental data. In particular, it is argued that the composition dependence of the intensity of the Si-Ge and Ge-Ge modes does not follow the fraction of the corresponding nearest-neighbour bonds as it was suggested by some authors. Possible effects of alloy segregation are considered by comparing the calculated properties of random and clustered SixGe1-x NCs. It is found that the Si- Si mode and Ge-Ge modes are enhanced and blue-shifted (by several cm-1for the Si-Si mode), while the intensity of the Si-Ge Raman mode is strongly suppressed by clustering.