Photoluminescence characterization of Cu2GeS3 bulk crystals

The optical properties of Cu2GeS3 bulk crystals grown by chemical vapor transport were characterized using photoluminescence (PL) spectroscopy. PL spectra from the Cu2GeS3 bulk crystals were analyzed as functions of excitation power and temperature. The crystal structure of the PL measurement samples was determined as monoclinic Cu2GeS3 by Raman spectroscopy. The PL spectra contained two main bands of exciton (1.59 eV) and donor-acceptor pair (DAP, 1.45–1.55 eV) recombination luminescence. In addition, phonon-assisted transitions of bound excitons were observed on the DAP recombination luminescence. The exciton band was composed of free- and bound-exciton luminescence. The thermal activation energy of the bound-exciton luminescence was estimated to be about 9 meV. The activation energy of bound-exciton luminescence was approximately identical to the peak energy difference of the free- and bound-exciton luminescence.