Abstract

The spectral distribution and the temperature dependence of quenching of the photoconductivity in cadmium sulfide crystals have been investigated. Taft and Hebb reported two quenching bands in the infrared region and suggested that infrared radiation excites trapped holes from their ground states to levels of higher energy, from which the excited holes ionize thermally and recombine with trapped electrons. In the present experiments, the following has been observed. Of the two quenching bands which are resolved at room temperature in “pure” crystals, one, the 1.4-micron band, is no longer resolved at −75°C and lower, although some quenching is still observed in this wavelength region at −185°C. In copper-doped crystals, on the other hand, only one quenching band has been observed, at 1.4-microns, which completely disappears at temperature below −80°C. These results imply that the quenching process at the 1.4 micron peak involves excitation of the trapped holes and thermal ionization of the excited holes. The quenching peak at 0.9 micron, on the other hand, remains at −185°C in “pure” crystals, although the quenching efficiency reduces to some extent. Addition of infrared radiation to green light on CdS crystals causes at first a transient increase in photoconductivity in the crystals followed by a decay. Measurements of the temperature dependence of this infrared stimulation of photocurrent have been carried out. The results, together with the results of “glow curve” measurements, indicate that the transient increase in photoconductivity on application of infrared radiation is due to the temporary increase of conduction electrons created by infrared light which ejects trapped electrons from traps into the conduction band.

© 1956 Optical Society of America

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