Electrical conductivity of ZnS crystals: ZnS crystals as quasi-one-dimensional conductors

Due to their special morphology many vapor-grown ZnS crystals behave as quasi-one-dimensional conductors. These faulted single crystals display a highly anisotropic dc conductivity, the conductivity at 90° to the c axis being in some cases up to ${10}^7$ times larger than its counterpart along the c axis. In the latter direction there exist intrinsic builtin electrical fields of different polarities and field strengths. These internal fields are due to charged dislocation layers appearing between different structures and polytypes which have different basal-plane densities. The influence of these phenomena on the ac conductivity has been investigated for frequencies between 5 kHz and 150 MHz which was found to be both direction and frequency dependent. The peculiar properties of these crystals made it possible to distinguish between effects of the electrodes, surface conductivity, dislocations, and bulk conductivity proper. Even at microwave frequencies up to 3 GHz there is a small anisotropy in the electrical conductivity. A theoretical approach developed for the excitation dynamics in random one-dimensional systems which was previously applied to a random barrier model in the case of the anomalous low-frequency behavior of the ionic conductivity in a quasi-one-dimensional superionic conductor, has been found to be appropriate for a quantitative evaluation of the described experiments.