Free-carrier dynamics in low-temperature-grown GaAs at high excitation densities investigated by time-domain terahertz spectroscopy

We investigate the dynamics of the high-frequency conductivity of optically excited low-temperature-grown GaAs (LT-GaAs)—a material widely used for photoconductive switching in ultrafast optoelectronics—by time-resolved terahertz (THz) transmission spectroscopy. This contactless technique is insensitive to the nonlinear interband response of the material which complicates the analysis of all-optical pump/probe measurements. For material grown and annealed at or near temperatures, which are optimal for photoconductive switching, we find a slowing down of the decay of the conductivity (respectively, the carrier density) with rising excitation density. We associate the increase of the decay time with a saturation of the available trap states whose density is estimated to be on the order of $2\times {10}^{18}{\mathrm{cm}}^{\mathrm{-}3}.$