Excitons bound to nitrogen pairs in GaAs

We have observed radiative recombinations of excitons bound to different nitrogen-pair complexes in GaAs by applying hydrostatic pressure. The samples investigated have relatively high nitrogen concentrations and were grown using a chloride method. By carefully tuning the pressure, we can not only make the ${\mathrm{NN}}_{\mathit{i}}$ pairs (1≤i≤10) appear successively in the band gap of GaAs and then become the major exciton recombination channel, but also change the binding energy of these levels. A study of thermal quenching of the radiative recombination of excitons bound to a center with different binding energies due to different pressures shows that the quenching mechanism changes from the thermal excitation of the whole exciton when the binding energy is small, to the excitation of the hole when the binding energy of the exciton is greater than that of the hole. A pressure-tuned thermal-state-selection method is developed from the thermal quenching properties. It artificially selects the state to be thermally quenched and is similar to below band-gap excitation or selective excitation spectroscopy. However, it can be applied to a defect the concentration of which is too low for a resonant excitation, as is the case for the GaAs:N samples here. The exciton-phonon coupling strength in GaAs:N is observed to increase slightly with binding energy, and, in agreement with the pressure dependence of ${\mathrm{NN}}_{\mathit{i}}$ energy levels, indicates an increase in the localization of the nitrogen exciton trap potential. The exciton-phonon coupling factor is observed to be independent of temperature.