Electrophotoconductivity in Polycrystalline Films of Cadmium Sulfide.

The studies of the modulation of the photoconductivity of polycrystalline films of CdS by acoustically induced electric fields are presented. The CdS films were deposited on lithium niobate (LiNbO(,3)) substrates by vacuum evaporation which was followed by a proper annealing to yield films of high photosensitivity. The change in photoconductivity of these films due to the penetration of electric fields associated with elastic waves propagating on their substrates was demonstrated and studied. The origin of this phenomenom was established by investigating the light intensity and temperature dependences of the conductivity change. The elastic waves traveling on the piezoelectric substrate of the film were generated by interdigital transducers. The relationship between the acoustic electric field and its induced change in conductivity of the film was found to be a nonlinear one. The fractional change in conductivity is strongly dependent on the light intensity and the film temperature showing a prominent maximum as a function of these quantities. The largest recorded fractional change in conductivity was about 25% at electric fields of the order of 10('3) volts per centimeter. A phenomenological model is developed to explain our observations of the electric field induced changes in photoconductivity of the CdS films. This model is based on the interaction between the space charge created by the electric field and the electron trapping states in the photoconductor. We have assumed the existence of a quasi-continuous distribution of impurity states in forbidden gap in our film and that these impurity states act either as traps or as recombination centers depending on the excitation intensity and the film temperature.