Dependence of the Hole Ionization Energy of Imperfections in Cadmium Sulfide on the Impurity Concentration

The variation of the hole ionization energy of imperfections in cadmium sulfide as a function of the impurity concentration has been measured using photoconductivity in a series of CdS: Ga: Cu powders. The Cu concentration varies from 4×{}10¹⁷ to 2×{}10²⁰ cm^-3, and each sample was prepared with a Cu-concentration to Ga-concentration ratio of 1.05. The hole ionization energy of the sensitizing centers, as determined from the thermal quenching of photoconductivity, decreases from about 1.0 ev for low Cu concentrations to about 0.3 ev for 2×{}10²⁰ Cu cm^-3. The results are analogous to other recent findings of small hole ionization energies in CdS and CdSe crystals.