Thermalization of Photoexcited Localized Excitons in GaSe Samples with Stacking Disorder

We have studied the photoluminescence of GaSe at 80 K under energy-selective excitation conditions. For excitation energies on the low-energy side of the n=1 direct free-exciton absorption line we find that the emission line due to the n=1 direct free excitons follows rigidly the excitation energy, whereas for excitation energies on the high-energy side the respective emission spectrum shifts to lower energies and the linewidth increases. Similar behavior is found on exciting into the excited n=2 direct exciton states. At room temperature and for excitation energies larger than the direct gap the emission spectra become independent of the excitation energy. We show that the experimental findings can be understood in terms of an extended version of the multiple-trapping model, which accounts for the localization of excitons in the direction perpendicular to the layers of GaSe. This localization is a consequence of the stacking disorder present in our samples. The resulting physical picture is that at low temperatures and for low excitation energies thermalization effects can be neglected within the recombination lifetime, whereas at high temperatures or high excitation energies the thermalization takes place within the recombination lifetime.