Energy of excitons in CuInS2 single crystals

High-quality single crystals of CuInS2, grown by the traveling heater method in an indium solvent, were characterized using photoluminescence (PL) and reflectance (RF) at temperatures from 4.2to300K. A number of well-resolved sharp excitonic peaks have been observed in the near-band-edge region of the PL and RF spectra at 4.2K. The lines at 1.536 and 1.554eV in the RF spectra were associated with A and (B,C) free-excitonic transitions, respectively. In the PL spectra the A exciton revealed a well-resolved splitting into two peaks at 1.5348 and 1.5361eV assigned to the lower and upper branches of exciton polariton, respectively. Other sharp lines were assigned to excitons bound at shallow impurities. The experimental temperature variation of the band gap was analyzed using the Bose–Einstein model. Two deeper bands in the PL spectra were identified as free-to-bound optical transitions followed by phonon replicas.