Internal Photoeffect and Exciton Diffusion in Cadmium and Zinc Sulfides

Abstract

The spectral response of photoconductivity of ZnS and CdS for both single-crystal and sintered-powder samples was measured for several distances between the area of illumination and the electrode region. The energy transport process for indirect illumination was studied by measuring the photocurrent pulses produced by high-intensity light flashes and by experiments with light transmission and electric fields. These studies demonstrate energy transport by electrically neutral entities other than photons, which we interpret as diffusion of excitons or mobile electron-hole pairs.This indirect or transfer photocurrent occurs predominantly at frequencies below the absorption edge of the crystal, with the detailed response being sensitive to the impurity content of the samples. The diffusion parameters in this region generally lie in the following range: lifetime $\ensuremath{\tau}={10}^{\ensuremath{-}4} \mathrm{to} {10}^{\ensuremath{-}6}$ sec, diffusion constant $D={10}^{3} \mathrm{to} {10}^{4}$ ${\mathrm{cm}}^{2}$/sec, and diffusion length $L=0.1 \mathrm{to} 1$ cm.The transfer photocurrent for steady illumination shows a nearly linear dependence on the incident light intensity. The slow decay of the photocurrent indicates that trapping processes are important in the photocurrent mechanism. Measurement of photocurrent pulses for flash illumination shows a greater than linear dependence on intensity, indicating a second-order or bimolecular process. A proposed mechanism for the transfer photoeffect involves initial absorption of the light with creation of an exciton, which diffuses to a trap in the lattice. A second exciton reaching the trap then dissociates, using energy transferred from the trap.