Some Properties of Green and Red-Green Luminescing CdS

A series of electro-optical experiments at room temperature have been made with two types of high-purity single crystals of CdS. Those of type $A$, believed to have near-perfect stoichiometry, luminesce green (5145 A) under 3650 A ultraviolet irradiation, have low dark conductivity (2.5×${10}^{-11\mathrm{}}$ mho ${\mathrm{cm}}^{-1\mathrm{}}$ at 300°K), exhibit no photopeak, and have a relatively short carrier lifetime (about 4 microseconds or less). Those of type $B$, believed to have a sulfur deficiency, luminesce first strongly red (≅7200 A), then strongly green (>5145 A) as the exciting radiation is increased in intensity. Simultaneously both emissions shift toward longer wavelengths. Also, as the red emission intensity begins to saturate, the intensity-rate change of conductivity abruptly changes. It is suggested that red luminescence is the result of a free hole capture by one of the electrons residing in a sulfur vacancy. This trapping sensitizes the crystal but at the expense of increasing carrier lifetime to millisecond magnitudes. Furthermore, some of these vacancies (∼0.4 ev below the conduction band) act as donors to increase dark conductivity (about ${10}^{-9\mathrm{}}$ mho ${\mathrm{cm}}^{-1\mathrm{}}$ at 300°K).