Steplike spectral distribution of photoelectrons at the percolation threshold in heavily p -doped GaAs

The origin of the steplike shoulder on the high-energy side of the low-temperature photoluminescence spectrum of heavily p-doped GaAs is studied experimentally. It is shown that it is controlled by both the Fermi-Dirac distribution of the holes and the energy distribution of the photoexcited electrons exhibiting a sharp steplike dependence. The latter results from abrupt changes in the energy relaxation rate at the percolation threshold separating localized from delocalized electron states. A comprehensive set of optical techniques based on spin orientation of electrons, namely, the Hanle effect, time- and polarization-resolved photoluminescence, as well as transient pump-probe Faraday rotation, are used for these studies. Two different electron ensembles with substantially different lifetimes of 20 and 280 ps are identified. Their spin relaxation times are longer than 2 ns, so that the spin lifetime is limited by the electron lifetime. The relative contribution of short- and long-lived photoexcited electrons to the emission spectrum changes abruptly at the step in the high-energy photoluminescence tail. For energies above the percolation threshold, the electron states are empty due to fast energy relaxation, while for lower energies the relaxation is suppressed and the majority of photoelectrons populates the states located there.