REFLECTIVITY MEASUREMENTS OF FEMTOSECOND CARRIER AND FIELD-DYNAMICS IN SEMICONDUCTORS

Carrier and field dynamics in a photoconductive switch are investigated by voltage modulated reflectivity. Measurements are performed with single, femtosecond pulsed and cw probe beams, and with pulsed pump and probe beams. We investigate the spatial dependence of the reflectivity changes and show that for mode-locked probe light the signal at the positive electrode completely dominates the response. The comparison of above and below bandgap excitation and reflectivity changes resolves the contributions of carrier-induced field changes and those due to the modulation of the applied field. Photoconductive switches on p- and n-doped substrates show that field enhancement at the Schottky barriers has only secondary influence. We explain the results in terms of a two-dimensional field distribution which produces the largest field gradient and hence acceleration of carriers at the electrodes and that the higher mobility of electrons accounts for the dominant signal appearing at the positive electrode.