Transient Photoconductivity in Trigonal Selenium Single Crystals

Abstract

Measurements of recombination lifetimes in single crystals of trigonal selenium, which is a p-type extrinsic semiconductor, have been made by studying the decay of photoconductivity induced by a 1.5 μsec light pulse. The addition of sufficiently intense dc illumination allows traps to be filled and a simple exponential decay obtained, which is interpreted to be a direct measure of the recombination lifetime. Lifetimes were found to range between 10 μsec and 34 μsec. From a knowledge of these lifetimes and absolute dc photoconductive gain measurements, photoconductivity mobilities were found to be 40±8 cm2·V−1·sec−1 parallel to the c axis and 17±4 cm2·V−1·sec−1 perpendicular to the c axis. The temperature dependence of the majority-carrier drift mobility can be obtained by measuring the relative pulsed photoconductive gain as a function of temperature. These experiments are in good accord with studies of acoustoelectric current saturation. No evidence is found for a barrier-limited mobility. Other experiments indicate that electrons are mobile in trigonal selenium, allow an estimate of their drift mobility, and give evidence of multiple trapping of minority carriers in deep traps.