High-energy excitonic photoluminescence due to carrier injection from the barriers into the first excited subband in a GaAs/AlxGa1-xAs quantum well

Radiative recombination properties of excitons have been investigated for a GaAs quantum well confined by barriers with an energy gap slightly larger than the energy of the first excited (n = 2) subband exciton. The low-temperature photoluminescence (PL) spectra show a surprisingly intense emission line higher in energy and intensity than the ground-state exciton band. This high-energy PL line remains prominent for excitation power densities between 0.01 and 10 W/cm². It can be clearly discriminated from the barrier emission using PL excitation (PLE) spectra and calculations of the subband energies. The picosecond PL dynamics reveal that the high-energy PL line exhibits a decay time of 0.77 ns at 20 K, which is much longer than the expected carrier relaxation time. These results indicate that the high-energy PL line can be ascribed to the exciton population of the first excited subband, which is stabilized for certain barrier injection conditions due to the exciton-parity forbidden relaxation (from n - 2 to n = 1 excitons) pathway.