Key points are not available for this paper at this time.
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 (<210^16 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^2V^-1s^-1 at room temperature with N=1. 610^16cm^-3, and 13600 cm^2V^-1s^-1 at 70 K with N=1. 510^16cm^-3) are in good agreement with literature values. There are, 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. 510^5cm/s.
Beard et al. (Fri,) studied this question.