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 $(<\mathrm{}2\mathrm{}\times {10}^{16}\hspace{0.5em}{\mathrm{cm}}^{-3})$ 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 $(6540\mathrm{}\hspace{0.5em}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature with $N=1.6\mathrm{}\times {10}^{16}{\mathrm{cm}}^{-3},$ and $13600\hspace{0.5em}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 70 K with $N=1.5\mathrm{}\times {10}^{16}{\mathrm{cm}}^{-3})$ 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 $\Gamma$ 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 $8.5\times {10}^5\mathrm{c}\mathrm{m}/\mathrm{s}.$

• Received 6 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.15764