Transient electron-phonon interaction and ultrasonic generation in CdS.

This thesis describes experimental work on the interaction of the drifting charge carriers with the piezoelectric lattice modes in highly resistive CdS crystals. In these experiments the specimens were fitted with evaporated metal electrodes on opposite faces, and a fast electron or light pulse generated electron-hole pairs in a narrow region below the top electrode. The crystals were mounted on a silica buffer rod with a quartz transducer on its opposite end. A synchronized field pulse drew a thin space charge layer out of this region and the transit time and drift velocity were obtained directly. The new feature of this work is the simultaneous detection of the generated piezoelectric wave in a frequency range from 3 to 75 Mc/s. The results show a close correlation between the ultrasonic measurements and those features of the drift velocity experiments associated with the transient acoustoelectric interaction. The amplitude of the piezoelectric wave has been studied as a function of the applied field, temperature, excitation pulse length, electron beam current and beam energy (5-35 keV) and also as a function of steady, highly absorbed light incident on the specimen top surface. From the measurements it has been possible to estimate the value of the piezoelectric field. This is sufficient to explain the non-ohmic behaviour of the drift velocity versus applied field curves. On the basis of the above results a microscopic model is proposed. A single incident electron is considered which simultaneously generates an elementary piezoelectric wavelet and a distribution of free electrons within a few microns of the surface. The local interaction of the electrons with the wavelet leads to ultrasonic amplification in accordance with the White theory. The significance of the surface barrier in connection with the amplification is discussed. A quantitative estimate is made of the local density of the bunched carriers which, on the basis of the White theory, leads to a build-up time of the interaction of less than 10 nsec. The results of this analysis resolve one of the main difficulties in the understanding of the transient acoustoelectric interaction.