THz spectroscopy and deviation from Drude model of LT-GaAs thin films with sub-picosecond charge-carrier lifetime

High-performance ultrafast terahertz devices such as modulators, photoconductive emitters, and detectors rely on an ultrafast change in conductivity. Semiconductors are suitable for this purpose since their charge-carrier density can be varied by external factors, such as incident photons or an applied electrical potential. High-performance ultrafast terahertz devices require semiconductors with high charge-carrier mobility but low charge-carrier lifetimes. The trade-off between charge-carrier mobility and the short charge-carrier lifetimes of standard semiconductors limits the signal-to-noise ratio of current terahertz devices. Hence, it is essential to develop high-performance materials to overcome these limitations and enable ultrafast terahertz technology. Low-temperature GaAs (LT-GaAs) has shown excellent performance in overcoming this limitation by providing a high charge-carrier mobility and, at the same time, a low charge-carrier lifetime. In this study, we have conducted terahertz time-domain spectroscopy (THz-TDS) and terahertz optical-pump terahertz-probe spectroscopy (OPTPS) on 2 µm-thick LT-GaAs thin films bonded with benzocyclobutene (BCB) to sapphire substrates. With these techniques, we demonstrate excellent sub-picosecond charge-carrier lifetimes across different infrared pump beam fluences. The measured samples showed a charge-carrier lifetime in LT-GaAs of 0.46 ps. We discuss observed saturation effects and how they can influence the shape of the photoconductivity spectrum. We explain the origin of this effect in the context of the experimental technique and charge-carrier dynamics in the material.