A photoluminescence study of excitonic grade CuInSe₂ single crystals irradiated with 6 MeV electrons

High-quality single crystals of CuInSe₂ with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 10¹⁵ to 3 × 10¹⁸cm⁻², and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 10¹⁶cm⁻². The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Se_i) and copper on indium site (Cu_In). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation