Transient acoustoelectric interaction in CdS and ZnS crystals

The paper describes an essentially new experimental approach to the study of the interaction between drifting charge carriers and piezoelectric lattice modes. An excitation pulse of fast electrons or light generates electron-hole pairs close to the top electrode of a highly resistive platelet specimen. A thin space charge layer of one type of carrier is drawn across the crystal and the drift velocity ν d is measured directly. Acoustoelectric interaction can thus be investigated during a single transit. The field dependence of ν d for electrons has been measured in CdS and both cubic and hexagonal ZnS. A sharp discontinuity in the slope of the graphs occurs at a a critical drift velocity which is in close agreement with the appropriate sound velocity. In CdS the experiments lead to a clear distinction between longitudinal and shear wave interaction. Similar results have been obtained for interaction with generated holes. The effect of shallow traps has been investigated and it is found that interaction disappears abruptly if the drift mobility falls below about 0·37 of the lattice mobility. An interesting general result is the surprisingly fast build-up of the interaction within a fraction of the transit time ([less, similar] 10 nsec). This is discussed in some detail in connection with a number of possible mechanisms and it is shown that the effect of non-linear terms in the amplification theory (White 1962) cannot account for this result. It is likely that under the above experimental conditions the interaction mechanism is basically different.