Reversible Variations of Sensitivity in Certain Cadmium Sulfide and Selenide Photoconductors

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Abstract
Measurements of photoconductivity in certain CdS and CdSe crystals show the existence of relatively shallow acceptor levels with ionization energy in the 0.1 to 0.3 ev range. The photosensitivity normally associated with holes captured by centers with an ionization energy of 1.0 ev in CdS, and 0.6 ev in CdSe, is markedly decreased at room temperature when these shallow acceptor levels effectively replace the normal deeper levels. Such a photosensitivity decrease can be caused by illumination at room temperature for a period of time, or by heating to above 100°C, but less than about 150°C, followed by cooling to room temperature. The photosensitivity at room temperature can be increased again by heating the crystals to over about 150°C, and quenching. The process is completely reversible. In CdSe there appears to be a single level with ionization energy of 0.3 ev, whereas in CdS there appears to be a distribution of levels with ionization energies varying from about 0.15 ev to the normal 1.0 ev. The presence of the shallow levels makes possible a much faster decay of photocurrent at liquid nitrogen temperature than is usually found in sensitive crystals.

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