Characterization of photoconducting CdTe using acoustoelectric methods

Abstract

In semiconductors with bipolar photoconduction measurements of the electric conductivity and of the Hall effect are not sufficient for the determination of the carrier densities and mobilities. In this situation, the acoustoelectric effect may be used as an independent method to obtain further information for the analysis. In the present article the propagation of sound waves in a bipolar photoconductor containing traps is investigated. In this general case relations are derived for the dependence of the acoustoelectric current on the applied electric field and for the acoustoelectric Hall effect. The theoretical predictions are compared with experimental results obtained for photoconducting CdTe at different intensities of the band gap light. The experiments were performed at ultrasound frequencies between 10 and 30 MHz, on p-type CdTe crystals with a dark resistivity of 109 Ω cm at room temperature. These crystals were grown from the melt with excess Te using the Bridgman method and were compensated by doping with CdCl2. The ohmic Hall effect and the acoustoelectric current show inversion of sign with increasing photoexcitation due to bipolar character of the photoconduction. Both inversion points, however, do not coincide. This is explained by means of a theoretical model including different trapping parameters for electrons and holes. The observed electric field dependence of the acoustoelectric current can also be explained by additional consideration of carrier trapping.