Screening and Stark Effects Due to Impurities on Excitons in CdS

Abstract

Properties of intrinsic and bound excitons in CdS with Cl donor concentrations over a wide range are investigated by measuring emission and reflection spectra at liquid helium temperature. As a result two characteristic properties of excitons are observed. 1) The binding energy of excitons decreases with increasing donor concentration, but the dependence on the concentration is different between intrinsic and bound excitons. 2) The emission line due to bound exciton shows asymmetric broadening with a tail extending to lower energies at high donor concentrations. According to Hanamura’s theoretical work in the succeeding paper, the first behavior of the decrease of binding energy is ascribed to the screening effect of donor electrons on excitons. The second feature of anomalous line shape is concluded to be a consequence of the Stark effect due to charged impurities on bound exciton. On the basis of these interpretations very good agreement has been obtained between experiment and theory.