Optical properties of non-crystalline materials. I. Optical absorption, photoemission and density of electron states

The distribution in energy of the electron states in a non-crystalline material has gross features that resemble those of the material in its crystalline form. Optical techniques such as u.v. absorption and photoemission can be used to probe the spectrum of electron levels and several examples are presented here. The k-conservation selection rule breaks down in amorphous materials and Van-Hove singularities are absent. Tight binding calculations based on a knowledge of local structure, and hence on the way the atomic orbitals interact, can be used to calculate the density of valence states which can then be compared with experimental data. Certain special topological features, not normally present in crystalline structures, e.g. the presence of five-membered rings of atoms, bond angle distortions etc. lead to distinctive features in the electron energy spectra, and these are discussed with particular reference to amorphous Ge and As. (author)