| Part One. Various aspects of the electronic theory of zincblende pseudobinary alloys are developed. The effects of chemical and bond-length disorder are modeled using the empirical tight binding method (ETBM). The choice of parameters is given special emphasis. A relativistic ETBM-CPA formalism is developed for systems with pure substitutional disorder. Its implementation is simplified through the development of a novel computational scheme (based on analytic continuation) which is also applicable in other types of Green's function calculations.; An application to Hg(,1-x)Cd(,x)Te explains the split-valence-band behavior seen in photoemission experiments and the observed bowings of the fundamental and E(,1) gaps in terms of strong cation s scattering far from the band edges. Alloy scattering is shown to have little effect on observed electron mobilities. An extension to semimagnetic Hg(,1-x)Mn(,x)Te is made possible in part by Harrison's parametrization scheme. Calculated s-p bands are similar to those in Hg(,1-x)Cd(,x)Te, but with larger disorder effects. Spin-polarized occupied Mn d states, treated perturbatively, are found to (1) lie (TURN) 3 eV below the gap, (2) have little effect on band edge properties, and (3) hybridize with Te p levels (with important implications for magnetic properties).; The additional effects of bond-length disorder are taken into account through a novel application of the molecular CPA. Realistic calculations for In(,1-x)Ga(,x)As indicate that such effects (1) are comparable in magnitude to those of chemical disorder, and (2) can either enhance or diminish the total scattering in a particular energy region.; Part Two. This part deals with the theory of interband optical absorption. A new method is presented for testing models of the electronic structure and chemical bonding in amorphous semiconductors based on a comparative analysis of moments of (epsilon)(,2) in amorphous materials and related crystals. An application to published data is shown to provide insight into (1) bonding differences between various classes of amorphous semiconductors and (2) the validity of the independent band model.; The thesis concludes with a simple two-band model CPA study of (epsilon)(,2) in alloy semiconductors illustrating the effects of (1) band distortions, (2) (')k non-conservation, (3) kinematic correlations and (4) random transition matrix elements. |
