Luminescence from Exciton-Free Carrier Collisions in GaSe

Photoluminescence spectra of GaSe at moderately high densities of electron-hole pairs are reported for lattice temperatures between 2 to 200 K. We observed, at energies below the exciton recombination line, an additional narrow emission which has been attributed to exciton-free carrier scattering. Its narrowness at low temperatures is due to the selection rules for optical interband transitions in GaSe. Using numerical calculations based on second order perturbation theory, we fit quite well the experimental spectra. We find that the carriers which participate in the scattering process are electrons and holes at the center of the Brillouin zone (BZ), while the contribution of the electrons located at the M points of the BZ is negligible.