Kinetics of structural relaxations in the glassy semiconductor a–Se

Abstract

It has recently been suggested that the rate of structural relaxations in amorphous chalcogenide glasses is inversely proportional to the viscosity and the latter obeys the Vogel-Tammann-Fulcher relationship. In this paper, Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) experiments examining the glass transformation in a–Se are reported which have been carried out over as wide range of heating rates as practically possible on the available instrumentation covering more than three decades of heating rate variation from r = 0.05 °C/min to r = 100 °C/min. The observed glass transformation temperatures in DSC and TMA ranged from 30 °C to 65 °C. The a–Se films were vapor deposited by thermal evaporation onto A1 substrates (at ∼70 °C) under identical conditions and well aged over an equal period of time so that all the films had an identical initial structure. Analysis of the glass transition temperature, Tg, vs heating rate, r, data obtained from the DSC endotherms shows that the structural relaxation rate is non-Arrhenius and can be described reasonably well by a Vogel expression of the form exp[–A/(T – T0)] which may have a weak structure dependent factor of the form exp[–c(H – HE)] where (H – HE) is the deviation of the structural enthalpy from its equilibrium value. It was found that a phenomenological single parameter Vogel description for the retardation time over the whole temperature range accessed leads to A and T0 values comparable with those obtained from the Vogel analysis of the viscosity-temperature data of Cukiermann and Uhlmann (1972), as well as the recent thermomicrohardness data of the present authors (1989). Furthermore, it is shown that the wide differences reported for the crystallization activation energy in a–Se can be readily interpreted by a viscosity limited crystallite growth rate. The Vogel interpretation is also found to be consistent with the temperature dependence of the mean dielectric relaxation time in the Debye loss observed by Abkowitz, Pochan, and Pochan (1980). It is suggested that in the elemental chalcogenide a–Se, the relaxation rate of enthalpy, microhardness, and dielectric polarization, as well as the crystallization rate, all scale inversely with the viscosity. TMA measurements, on the other hand, although still interpretable in terms of a Vogel behavior, did not indicate a softening behavior which paralleled the viscosity.