Optical Properties of Disordered Condensed Matter

This chapter reviews the understanding of the fundamental optical properties in some disordered semiconductors. It presents two approaches that are used for calculating the absorption coefficient in amorphous semiconductors. In the first approach, one assumes that the transition matrix element is independent of the photon energy. In the second approach, contrary to the first one, using the constant dipole approximation, the transition matrix element is found to be photon-energy dependent. Applying the first approach, one obtains the well-known Tauc's relation for the absorption coefficient of amorphous semiconductors. It is shown that, through the first approach, both the fractal and effective medium theories are useful in explaining the optical properties of disordered forms of condensed matter. The chapter also discusses the effect of the compositional variation on the optical gap of amorphous chalcogenide alloys. The bandgap varies with the composition and often exhibits extrema at certain stoichiometric compositions.