INPLANE TRANSPORT OF PHOTOEXCITED CARRIERS IN GAAS QUANTUM-WELLS

We have measured the picosecond-time-resolved in-plane motion of carriers in a multiple quantum well by a spatially scanned pump-probe technique at bath temperatures between 8.5 and 60 K. The effective ambipolar diffusivity measured from the spatial expansion of a nonequilibrium carrier distribution is found to increase with pump power, i.e., carrier density. We believe that this effect is in part due to the formation of a degenerate semiconductor plasma and to screening of scattering centers. An increase in the diffusivity is observed with the change in the bath temperature from 8.5 to 60 K that is consistent with a simple impurity-limited scattering model. The diffusivity at 0-200 ps is found to increase with the incident photon energy, while the diffusivity measured at 500 ps remains relatively constant with increasing photon energy. The decrease in the expansion rate with time is ascribed to the decreasing kinetic energies of the carriers.