AC conductivity and capacitance measurements on evaporated cadmium telluride thin films

Abstract

The ac electrical properties of evaporated cadmium telluride thin films sandwiched between ohmic aluminum electrodes have been investigated for temperatures of approximately 200 - 400 K over the frequency range 10 2 - 2 multiplied by 10 4 Hz. The ac conductivity (sigma) ((omega) ) was found to follow a (sigma) ((omega) ) varies direct as (omega) N law, where (omega) is the angular frequency and the exponent s systematically decreases with increasing temperature from a maximum value close to unity at approximately 200 K. This low temperature value is consistent with values of s expected from the model of Elliott for correlated barrier hopping, and other features predicted by the model, such as the increase in (sigma) with temperature and the strong frequency dependence at low temperatures, were also observed. Measurements of (sigma) as a function of temperature for various fixed frequencies showed that in the higher temperature range the conductivity activation energy was typically 0.66 eV depending on the frequency, whereas at low temperatures its value was of the order of 0.01 eV. These values are indicative of a free-band conduction process at higher temperatures and confirm the existence of hopping conduction at lower temperatures. In the hopping region the density of localized states was estimated to be approximately 9.9 multiplied by 10 22 m -3 , and was of a similar order to the trap concentration determined previously from dc space-charge-limited conductivity measurements. Measurements of capacitance C as a function of frequency and temperature showed a decrease in C with increasing frequency tending towards a constant value for all temperatures. C also increased with increasing temperature, with the most significant increase occurring at low frequencies. This type of behavior is consistent with an existing equivalent circuit model which assumes the existence of ohmic contacts to the films and a thermally activated conduction process.