Optical properties of non-crystalline materials. II. Fundamental absorption edge, luminescence, states in the gap and photoconductivity

Non-crystalline materials contain defects in the form of dangling bonds and unusual chemical bonding arrangements, in addition to impurities, vacancies, voids etc. An understanding of these defects is important for the interpretation of several optical properties, e.g. those associated with the fundamental absorption edge, luminescence and photoconductivity. A recent model for certain types of defects in chalcogenide glasses are described in some detail and shown to account for many experimental results. Even in the absence of specific defects, the nature of the electron states, particularly near band edges, differs from that in crystals: in the absence of long range order the states become localized, leading to the concepts of mobility edges and hopping conduction. Bond angle distortions and internal microfields also affect band edges and the fundamental absorption edge of several materials are described and discussed with reference to these. (author)