Electrical transport in chalcogenide (Se-Te-As-Ge) glasses

Abstract

Electrical transport in bulk Se-Te-As-Ge glasses in the series Ch _x Y_1−x, where Ch ≡ Se₁₉.₇ at. %, Te₈₀.₃ at. %, Y ≡ As₇₁.₄ at. %, Ge_25.6 at. %, and x = 0.45, 0.61 and 0.71, has been studied. Measurements carried out were d.c. conductivity and thermoelectric power as a function of temperature and composition. The thermo-power was found to be positive. The temperature coefficient of the optical gap in a hot-pressed Ch_o.₆₁Y_o.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’ on its surface.

Varying degrees of curvature were found in all plots of log conductivity and thermoelectric power against I|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 Xagels, Callaerts and Denayer (1974). Although the theoretical fits to the experimental data were satisfactory, the 300 K effective densities of states, Nv and N_B, 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 bandiail depth of ∼ 0.1 eV. It is argued that such a large N_B is physically unrealistic. Considering variable-range hopping in the localized states as suggested by Grant and Davis (1974), reduces N_B 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.