The viscosity and structural relaxation rate of evaporated amorphous selenium

Abstract

Certain properties of vacuum‐deposited thin films of a‐Se were measured and compared with those of melt‐quenched Se glass. After annealing in the temperature range 25–50 °C, the creep, heat capacities, and far‐infrared behaviors of the vapor‐deposited films were indistinguishable from those of melt‐quenched specimens. The shear‐viscosity η measurements on the films were extended into a temperature range well below that where such data on melt‐quenched samples are available. Over this temperature range, 25–35 °C, the T dependence of η of the equilibrated films was well fit by an Arrhenius relation with an activation energy of 130 kcal/mole. The stress and thermal‐relaxation rates for annealed films were also measured as functions of T. They are controlled by different sets of relaxation processes and are not directly proportional to the equilibrium η. However, they are in good agreement with each other, except that the thermal‐relaxation times are more widely dispersed over a span of about two orders of magnitude. The isothermal relaxation of the films to their equilibrium creep rates, though not a simple exponential decay with a single time constant, can be simply described by a relaxation time τ (t), which is proportional to the instantaneous viscosity η (t). As deposited, the films exhibited glass‐transition temperatures as much as 10 °C below those of annealed films. Also they showed a broad exothermic peak at about 50 °C. These results, on annealed and as‐deposited films, are consistent with the hypothesis that the various molecular constituents, presumably eight‐membered rings and polymeric chains of a‐Se, readily interconvert to an equilibrium distribution at temperatures as low as 30 °C. This distribution is temperature dependent and the same in vapor‐deposited and melt‐quenched a‐Se.