Dynamics of tetrahedral networks: Amorphous Si and Ge

Abstract

Experimental data are reported for depolarized and fully polarized components of the Raman spectra of variably ordered states of a-Ge and a-Si films. These spectra are compared to theoretical Raman calculations for relaxed Polk-model-derived structures with different degrees of bond-angle disorder. In addition, the experimental and theoretical densities of states of a-Ge in highly ordered and disordered states are reported. The Raman spectra for the fully polarized component are found to yield good agreement at high frequencies, as well as the observed trend of decreased width with increasing bond-angle order. At low frequencies the theoretical spectra are more intense than that observed experimentally, indicating limitations of the three-parameter phenomenological Raman polarizability model. The depolarized Raman component may, in contrast, be fitted reasonably well with a fixed ratio of the D${̃}_1$ and D${̃}_2$ mechanisms at intermediate and higher energies. The two-parameter lattice-dynamical calculation of the density of states also yields good agreement with experiment, except at intermediate frequencies, where a somewhat more intense scattering is observed. A comparison of experimental and theoretical results allows estimates of the bond-angle disorder in a-Ge and a-Si to be obtained. The results are consistent with estimates based on radial-distribution-function and calorimetry studies.