Photoelectronic Properties of Defects in CdSe Single Crystals

Abstract

The properties of electronically active imperfections in CdSe single crystals have been investigated through measurements of dark conductivity, Hall effect, photoconductivity, and optical absorption. The same apparent photoconductivity sensitizing centers with an energy level 0.6 eV above the valence band were found in all photosensitive crystals, regardless of the method of heat treatment. Various procedures for producing high-resistivity photosensitive crystals from high-conductivity nonstoichiometric crystals introduce characteristic recombination centers that compete with the sensitizing centers in determining the magnitude and temperature dependence of the photoconductivity. The diffusion of silver acceptors results in a much lower photosensitivity than diffusion of copper acceptors. Annealing in selenium introduces sensitizinglike recombination centers with levels closer to the valence band than 0.6 eV, which dominate the temperature dependence of the photosensitivity, so that the highest sensitivity is found only below liquid-nitrogen temperature. Subsequent vacuum annealing at an elevated temperature or room-temperature annealing for several months removes the influence of these centers. The only center definitely associated with Cu impurity has a level 1.05 eV above the valence band. No evidence was obtained associating Cu or Ag impurity with the sensitizing center at 0.6 eV. A defect involving native acceptors, such as Cd vacancies, provides the most consistent explanation for the sensitizing center.