Direct observation of localized exciton states in CdS$_{1-x}$Se$_x$ solid solutions

A system of isolated localized exciton states corresponding to particular energies from the tail of the density of states is observed in CdS$_{1-x}$Se$_x$ crystals. From an analysis of the obtained photoluminescence excitation spectra, the boundary between the free and localized excitons and the length of the tail region of the density of localized states that make an essential contribution to radiative recombination are determined. It is shown that the localized and free excitons prove to be separated and represent two exciton subsystems. The energies of actual optical photons are accurately determined to be $23.6\pm 0.2$ and $35.6\pm 0.3\,$meV for CdS$_{0.70}$Se$_{0.30}$ and $20.7\pm 0.2$ and $31.4\pm 0.3\,$meV for CdS$_{0.50}$Se$_{0.50}$.