Transient Luminescence, Transport and Photoconductivity in Chalcogenide Glasses

Abstract

The chalcogenide glasses are perhaps the most thoroughly studied amorphous semiconductor systems. In the past two decades their measured properties have been repeatedly compared with model predictions and served as guides in the search for fundamental concepts and principles necessary for amorphous semiconductors. An example is the synthesis of the Mott-CFO model which is a low carrier density and high carrier mobility picture that involves the concepts of band tail states and mobility edge. On the other hand, Emin proposed that the charged carriers in chalcogenide glasses form small polarons and necessarily implies low carrier mobility because of small band width caused by atomic displacements associated with self-trapping. The small-polaron model can explain, in addition, the Hall effect sign anomaly and the difference in activation between the Peltier heat and conductivity. In spite of the fact that the Mott-CFO model and the small-polaron model are orthogonal to each other, the experimental data prior to 1978 have not been able to discriminate which is the correct model. In this work, transient optical and transport data which have rapidly accumulated since 1979 are considered. These recent data enable us to narrow down the possible states that can exist in chalcogenide glasses. As a result of this analysis, a minimal set of states has been proposed that can explain the totality of optical and transport data. It confirms that transport occurs by small polaron hopping. The proposed minimum set of states is consistent with Anderson’s bipolaronic ground state. The transient transport data, transient optical data, the dynamical dielectric relaxation data, and the volume and enthalpy recovery data of chalcogenide glasses are shown to conform to a universal pattern predicted by a recent unified model of relaxation at low frequencies/long times of condensed matter in general.