Theory of Infrared and Raman Spectra of Amorphous Si and Ge

A theory of infrared and Raman spectra, based on a two-parameter elastic energy and simple local-bond optical coupling mechanisms, is developed for amorphous silicon (a-Si) and amorphous germanium (a-Ge). When the theory is applied to a random-network structural model the results are in satisfactory agreement with experiment, especially with regard to the qualitative differences between the infrared and Raman spectra. Results for simple microcrystalline models are also presented.