Pulsed Ultrasonic Studies of the Acoustoelectric Effect, Ultrasonic Attenuation, and Trapping in CdS

Abstract

Ultrasonic attenuation and electron trapping in cadmium sulfide have been studied by means of the acoustoelectric effect. Pulses of 30-MHz ultrasound, short compared to the sample length, generate acoustoelectric current signals with a portion corresponding to the sound pulse entirely inside the sample. This portion should decay exponentially with a time constant proportional to the total ultrasonic attenuation, independent of bond losses, transducer efficiencies, etc. By varying the conductivity, the electronic and lattice parts of the attenuation can be separated. Available CdS crystals have rather inhomogeneous spatial impurity distributions which appear as spatial variations in the electron density. These variations distort the exponential decay of the acoustoelectric current, forcing us to formulate a model for the nonuniformity in order to explain the observed waveforms. This model gave good quantitative agreement with the data on one sample. The ultrasonic attenuation due to electrons was found to be accurately described by the theory of Hutson and White, and lattice attenuations of about 2 dB/cm were measured. Information on trapping parameters can be obtained from the magnitude of the acoustoelectric current. One sample showed strong evidence of trapping which could be explained by the presence of a trap 0.4 eV below the conduction band with a density of about 4×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$.