Large oscillator strength excitons in PbGa2S4 crystals

Exciton states with a large oscillator strength (transverse–longitudinal splitting of 50 meV) and binding energy of 290 meV have been observed in PbGa2S4 crystals. The ground exciton states are stable up to the room temperature. The ground (n = 1) exciton state and two excited (n = 2 and n = 3) states of the short-wavelength B-excitons series were observed at low temperature (T = 10 K). The ground (n = 1) and the excited (n = 2) states of the long-wavelength A-exciton series were also revealed. The exciton Bohr radius of the ground exciton state was determined to be 70 Å for the A-exciton and 10 Å for the B-exciton. The band splitting and the main parameters of excitons have been determined from the calculation of the reflectivity spectra contours. The group theory analysis of the band symmetry was performed and the scheme of bands responsible for exciton transitions in the center of the Brillouin zone was proposed.

► Exciton states with unusually large oscillator strength have been observed in PbGa2S4.
► The exciton Bohr radius is equal to 70 Å and 10 Å for two exciton series in PbGa2S4.
► The band splitting and the main parameters of excitons have been determined.
► Optoelectronic devices based on the exciton-light interaction can be developed on PbGa2S4.