Excitons in high-quality CuInS2 single crystals

High purity single crystals of CuInS2, grown by the traveling heater method using an indium solvent, were characterised using photoluminescence and reflectance at temperatures from 4.2 to 300 K. A number of sharp peaks, associated with excitons, have been resolved in the near-band-edge region of the optical spectra. The peaks at 1.536 and 1.554 eV, clearly resolved in the reflectance spectra at temperatures up to 240 K, were associated with A and (B, C) free excitons, respectively. Two sharp lines at 1.5348 and 1.5361 eV (near the A exciton) were assigned to the lower and upper branches of exciton–polariton, respectively. The other sharp lines at 1.5309, 1.5255, 1.5228, 1.5203 and 1.5162 eV were related to excitons bound on either shallow donor or acceptor impurities. The temperature variation of the band gap was measured from 4.2 to 300 K.