INFLUENCE OF SURFACE ELECTRIC-FIELD ON EXCITON PHOTOLUMINESCENCE OF CDS

The structure of luminescence spectra of free excitons in CdS single c rystals at T = 77 K is investigated. The dip formation is found in the top part of free A and B exciton resonant peaks of photoluminescence (PL) spectra after heating of the CdS crystals followed by fast coolin g down to 77 K. Similar effects are observed when an external electric field is applied. It is shown that the structure of exciton PL spectr a is due to self-absorption of resonance radiation in the near-surface layer with low exciton concentration. Our calculations demonstrate th at the formation of an exciton concentration gradient may be caused by the influence of a surface electric field on the binding of electron- hole pairs to excitons. It is assumed that the increase of surface ele ctric fields after heat treatment is caused by the thermostimulated ad sorption of oxygen, which results in the formation of electron trap le vels.