Investigation of the dependence of the radiative properties of cadmium sulfide single crystals on the equilibrium carrier density

An investigation was made of the photoluminescence spectra (T=77°K) of CdS crystals as a function of the equilibrium carrier density n=10¹³–10¹⁸cm^–3. The effective optical gain and the threshold current density were determined at T=85°K when the same crystals were excited by an electron beam. The highest gain, amounting to 70–100 cm^–1 (corresponding to j=1 A/cm²), and the lowest threshold current density were recorded for undoped crystals (n=10¹³–10¹⁴cm^–3), whose photoluminescence spectra included the line due to free A excitons and the luminescence resulting from the recombination of these excitons accompanied by the emission of one or two longitudinal optical phonons. The results indicated that CdS crystals with n>5×10¹⁶cm^–3 were unsuitable as the active material for electron-beam-excited lasers.