Optical spectroscopy of excitonic states in CuInSe2

Optical properties of structurally perfect CuInSe2 single crystals were studied in the temperature range of 4.2–300 K with the use of photoluminescence, optical absorption, optical reflection, and wavelength-modulated optical reflection (WMOR). The intense lines of free excitons A (∼1.0414 eV) and B (∼1.0449 eV) with a half-width of ∼0.7 meV at 4.2 K are found to be related to two extrema of valence band split by a crystal field. The excitons emission line C (∼1.2779 eV) in WMOR spectra are related to a lower valence band split-off by spin-orbit interaction. Within the context of the quasi-cubic Hopfield model, the parameters of valence band splitting ΔCF=5.2 meV and ΔSO=234.7 meV defined by the crystal and spin-orbit interaction, respectively, are calculated. In the region of the fundamental absorption edge, the lines of bound excitons are found with a half-width ∼0.3 meV that is indicative of a high quality of grown CuInSe2 crystals.