Transient acoustoelectric interaction in single crystals of CdS and ZnS.
thesisposted on 19.11.2015, 09:17 by Peter George Le Comber
This thesis describes experimental work on the interaction of the drifting charge carriers with the piezoelectric lattice modes in CdS and ZnS crystals. The investigation has been carried out by drift mobility techniques which represent a very direct and essentially new approach to the study of the acoustoelectrie interaction. In the experiments highly resistive thin platelet crystals 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. A synchronised field pulse drew thin space charge layer of one type of carrier out of this region and the transit time and drift velocity were obtained directly; it was thus possible to study the acoustoelectric interaction during a single transit. The field dependence of the electron drift velocity was studied in CdS with the applied field parallel and perpendicular to the c-axis, and in both cubic and hexagonal ZnS specimens. In all cases a sharp discontinuity in the slope of the graph occurred at a critical drift velocity vd', which agreed with the piezoelectric sound velocity in the particular crystallographic direction. A clear distinction between the longitudinal and shear wave interactions could be made. In CdS the interaction for both electrons and holes was observed. The drift velocity measurements have been checked by transient current pulse experiments. The effect of shallow trapping on the acoustoelectric interaction has been investigated by drift velocity experiments at lower temperatures. It is found in all cases that the interaction disappeared when the drift mobility was reduced to about 0.38 of the lattice mobility. The interesting result of all these experiments was the surprisingly strong interaction between the drifting carriers and the piezoelectric modes. This was found to build up within a fraction of a transit time, certainly in less than 20 nsecs. The possible meohanisms to explain this strong interaction are discussed and it is concluded that the non-uniform excitation is at least partly responsible. The results of some preliminary ultrasonic measurements indicate that the stress wave associated with the excitation pulse may be amplified in a region close to the top electrode.