Optical spectroscopy of free excitons in a CuInS2 chalcopyrite semiconductor compound

The spectra of reflectance and luminescence of high-quality CuInS2 single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy ∼1.5356 eV and doubly degenerate peak BC at the energy ∼1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, AUPB at ∼1.5361 eV and ALPB at ∼1.5347 eV, with a half-width ∼1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be ∼18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed