Thermodynamic origin of the slow free exciton photoluminescence rise in GaAs

We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.