Composition Dependence of Photoconductivity in Amorphous Thin Films of Se.80-xTe.20Gex

The present paper reports the dependence on composition of photoconductivity in vacuum evaporated thin films of amorphous Se.80-xTe.20Gex (x = .05, .10, .15 and .20). Temperature dependence of conductivity in dark as well as in presence of light show that conduction is through a thermally activated process in both cases. The activation energy is found to decrease with increase in light intensity, indicating shift of the Fermi level with intensity. A correlation between activation energy and the pre-exponential factor is observed in all the compositions, which could be fitted to the Meyer-Neldel rule. Measurements on the dependence of photoconductivity on intensity show that photoconductivity increases with intensity as a power law, where the power is found to be between 0.5 and 1.0. The photosensitivity sigma(ph)/sigma(d) increases with Ge concentration. This is explained in terms of the decrease in the density of defect states with increase of Ge content in a-Se.80-xTe.20Gex. This is consistent with the conclusions reported in the literature by dielectric loss measurements.