Some Aspects of Photoconductivity in Cadmium Selenide Crystals

Abstract

Vacuum annealing of insulating and insensitive cadmium selenide crystals grown from the vapor phase increases their photosensitivity to a value at least as high as that of crystals sensitized by impurity incorporation. Such annealing‐sensitized crystals have one hundred to one thousand times greater sensitivity than impurity‐sensitized crystals at low light intensities and/or high operating temperatures. A new infrared quenching band with maximum at 1.0 ev is found at elevated temperatures, correlated with the residual sensitivity of the annealing‐sensitized crystals. The same sensitizing centers are involved in the photoconductivity of annealing‐sensitized and impurity‐sensitized cadmium selenide crystals, regardless of whether the excitation is absorbed only at the surface or throughout the volume. In annealing‐sensitized crystals, the effective volume for photocurrent flow for volume excitation is one hundred times larger than that for surface excitation; this result indicates a fairly homogeneous distribution of sensitizing centers through the volume of the crystal. A tentative description of conductivity and photosensitivity characteristics is given in terms of the various types of vacancies likely to be participating.