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

Abstract

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