ELECTRICAL TRANSPORT IN CHALCOGENIDE (SE-TE-AS-GE) GLASSES

Electrical transport in bulk Se-Te-As-Ge glasses in the series ChxY1−x, where Ch = Se19.7 at. %, Teg80·3 at. %, YsAs74·4 at. %, Ge25·6 at. %, and a: = 0·45, 0·61 and 0·71, has boon studied. Measurements carried out wcro d.c. conductivity and thermoelectric power as a function of temperature and composition. The thermopower was found to be positive. The temperature coefficient of the optical gap in a hot-pressed Ch0·61Y0·39 glass was also measured and was found to be 3·1 × 10−4 eV K−1. The softening temperatures of the glasses in the above series is reported. It was found that the excessive heating of the Ch0·61Y0·39 glass in air enhanced its conductivity: This was associated with a thin conducting ‘ skin 1 on its surface. Varying degrees of curvature were found in all plots of log conductivity and thermoelectric power against 1/T. The data are examined in the light of the current theories, and are best fitted to the parallel conduction model in the extended and the localized edge valence states proposed by Nagels, Callaerts and Denayer (1974). Although the theoretical fits to the experimental data were satisfactory, the 300 K effective densities of states, Nv and NB, at the mobility edge and the localized band edge were found to be about 2·2 × 1019 and 1·8 × 1021 cm−3, respectively, with a localized band tail depth of 0 1 eV. It is argued that such a large NB is physically unrealistic. Considering variable-range hopping in the localized states as suggested by Grant and Davis (1974), reduces NB to about 6·8 × 1020 cm−3 at 300 K by assuming a hopping distance of 20 Å and a localization parameter of 0·1 Å−1. © 1979 Taylor & Francis Group, LLC.