Free-to-Bound and Bound-to-Bound Transitions in CdS

Abstract

Time-resolved photoluminescence spectra of two green broad emission bands in CdS have been observed as a function of temperature. New structure was discovered in both bands, and has been attributed to transverse optical phonons. From the wavelength shift of the emitted light with time and temperature, it is concluded that the shorter wavelength band (5130 \AA{}) is due to the recombination of free electrons with bound holes, while the longer wavelength band (5175 \AA{}) is due to the recombination of bound electrons with bound holes. Values for the donor and acceptor binding energies are obtained, as well as the bound-to-bound reaction constant, and an estimate of the donor concentration. The model is supported by independent information from the time decay of the bound-to-bound and the free-to-bound emission intensity, and its correlation with the low-temperature decay of photoconductivity as a function of time after excitation.