The electrical and optical properties of cadmium selenide

Abstract

Measurements of thermally stimulated currents, photo conductivity and photoluminescence have been used to study the imperfection centres in photosensitive CdSe crystals. These crystals were grown from the elements using a flow technique. Seven closely spaced, discrete sets of traps, with concentrations between 10(^12) and 10(^14) cm(^-3), lie between 0.15 and 0.36 ev below the conduction band. They empty in the temperature range 90 to 210 K. Two other groups of traps are found after illumination at various temperatures with intense white light.(1) Illumination between 250 and 275 K creates four new sets of traps with associated T.S.C. peaks at 215, 230, 250 and 270 K. Also in this temperature range holes are thermally freed from sensitising centres which lie 0.6 ev above the valence band. The consequent increased recombination substantially modifies the T.S.C. peak shapes and no sensible values of the trapping parameters can be obtained from T.S.C. curve analysis. Illumination at higher temperatures destroys these four centres.(2) Three, further T.S.C. peaks appear at 295, 335 and 365 K as a result of illumination above 250 K. They increase in height with increasing temperature of illumination. Such behaviour can be attributed to either the photochemical creation of traps or the existence of traps surrounded by repulsive potential barriers. The traps have depths of 0.43, 0.52 and 0.63 ev. They have capture cross sections of about l0(^-20) cm(_2) and densities of up to 10(^19) cm(_2).Illumination at increasing temperatures in the range 90 to 400 K results in a progressive reduction in the free electron lifetime. This is due to the photochemical creation of class 1 fast recombination centres. Simultaneously the intensity of a 1.15 p luminescence emission band increases and that of a 0,95 p band decreases. There is no direct evidence to suggest that these effects are related.