SIMULTANEOUS MEASUREMENTS OF ELECTRON AND HOLE SWEEP-OUT FROM QUANTUM-WELLS AND MODELING OF PHOTOINDUCED FIELD SCREENING DYNAMICS

We measure both electron and hole escape times from a GaAs-AlGaAs quantum well in an electric field at room temperature. This gives important information for the design of high speed quantum-well modulator and optical switching devices. The measurements are made by picosecond optical pump-probe techniques on samples containing a single quantum well in a waveguide. The use of a single well avoids multiple well transport and resonant tunneling effects. Carriers excited in the quantum well by the pump beam result in a transient bleaching signal from excitonic saturation and, as they leave the well, a transient electroabsorption signal because the movement of charge partially screens the electric field. We model both processes, including important electrical equilibration processes (such as diffusive conduction) of the sample as a whole. This modeling and the use of two samples with asymmetric barrier heights allows the measurement of the electron and hole emission as a function of applied electric field. Both electrons and holes are emitted in a few picoseconds at high fields (e.g., 100 kV/cm) in low barrier (e.g., 20% Al) structures, and in hundreds of picoseconds at low fields (e.g., 20 kV/cm) in higher barrier (e.g., 40% Al) structures. Preliminary analysis suggests that the emission mechanism is thermionic rather than by tunneling, but the results are not well explained by conventional thermionic emission models.