Crystallinity and Electronic Properties of Evaporated CdS Films

The effect of several processing methods on the crystallinity and electronic properties of evaporated CdS films has been investigated. Diffusion of Cu or Ag at temperatures above 450°C has yielded films composed of crystals of controllable size ranging from 10−5 to 1 cm in diameter. The electron mobility depends strongly on the reorientation of the crystallites but is only slightly affected by their size. The best films obtained have shown mobilities for photogenerated carriers of 300 cm2V−1 sec−1, characteristic of the bulk crystal. Typical values of the total trap density are in the range of 1019 to 1021 cm−3, compared to 1014 to 1016 in single crystals. The resistivity of the high mobility films can be controlled by the addition of Cl or Ga at the proper stage of the processing. In the photoconductive films, the mobility may vary with the illumination level by an order of magnitude. In films processed at temperatures above 400°C, a model of conducting crystallites separated by thin insulating barriers is insufficient account for the observed results.