Electrical and optical properties of chalcogenide glasses

A number of chalcogenide glasses, representing the complete glass forming regions of the· three binary systems Selenium-Arsenic·, Selenium- Tellurium, and Tellurium-Arsenic were prepared in bulk form. For each of the glasses in each of the three systems, measurement of d.c. conductivity over a large temperature range, and optical absorption also over a large temperature range were completed. The Hall effect was looked for in the glasses from the Tellurium-Arsenic system. Other supporting experiments, namely, space charge limited current measurements in representative glasses from two systems, and density measurement of all glasses were carried out. X-Ray diffraction tests were· used to identify each material as amorphous. A description is given of the material and specimen preparation techniques, as are details of experimentation. A consideration of some current theoretical ideas is shown to point .to a density of states model which should apply to a .large range of amorphous semiconductors. Based upon this model certain computer calculations are made: The temperature dependence, of the density of free carriers in the conduction and valence bands, and the position of the Fermi energy as a function of temperature are determined. The calculations assume, acceptor, and donor, character for the localized states (proposed in the model) associated with the conduction and valence bands respectively. A second group of calculations is made to determine the variation of optical absorption coefficient with incident photon energy, assuming band to band transitions to be the dominant absorption mechanism. A comparison between the calculated and empirically determined functions for the above two cases plus those of the space charge limited current measurement is made so that a number of conclusions can be drawn. These include amongst others, the 'universal' nature of the density of states model, the existence and role of localized states, the nature of the conductivity, i.e. intrinsic or extrinsic, and the validity of the calculations. An anomolous low temperature, conductivity - temperature – electric field, characteristic is found in annealed specimens of certain glasses. This effect is discussed and is thought to be associated with 'hopping conduction'. Other questions concerning, variation of properties with composition, the difference bulk and thin film produced samples, and quality of electrical contacts are raised and discussed