Transient photoconductivity in GaAs as measured by time-resolved terahertz spectroscopy

The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density (< 2 x 10(16) cm(-3)) with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz (far-infrared) pulses of several hundred Femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model (6540 cm(2) V-1 s(-1) at room temperature with N = 1.6 x 10(16) cm(-3), and 13600 cm(2) V-1 s(-1) at 70 K with N = 1.5 x 10(16) cm(-3)) are in good agreement with literature values. There an, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the valley due to LO-phonon-assisted intravalley absorption. At long delay times (5-900 ps), the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is 8.5 x 10(5) cm/s.