Density of Localized States in the Gap of Non-Crystalline Semiconductors

Abstract

It is a natural consequence of the inherent lattice disorder of a non-crystalline semiconductor, that a distribution of electronic site energies will be generated, as well as a certain number of lattice defects. Anderson [1] and Mott [2] showed that the site energy distribution results in the appearance of bandtails at the edges of the electronic energy bands, and in the localization of the electronic wavefunctions representing those tail states. For semiconductors, where the phenomena of interest are related to the energy gap between the top of the valence band and the bottom of the conduction band, that gap will consequently be filled-in by a varying density of such localized states. A further contribution to the localized-state density in the gap will result from lattice defects. In amorphous structures these are in fact the coordination defects such as the silicon dangling bond or the over-or under coordinated chalcogenide negative-U centers. While it is easy to show in general terms that such defects will give rise to electronic energy levels in the gap, deducing the actual energy position and density of those levels from experimental data has been less straightforward.