Subpicosecond carrier response of unannealed low-temperature-grown GaAs vs temperature

The subpicosecond carrier lifetime of an unannealed molecular beam epitaxial layer of GaAs grown at ∼210°C has been demonstrated between 10 and 290K through optoelectronic switching and all-optical pump-probe measurements. The low room-temperature resistivity of the as-grown layers, believed to arise from hopping conductivity through defect sites, has been observed to increase as the sample temperature was lowered, allowing ultrafast switching measurements to be performed using the low-temperature-as-grown GaAs as a photoconductive element. After illumination by 100 femtosecond optical pulses, photogenerated carriers in the sample have rapidly relaxed, returning the material to its high-resistivity state in less than 1 ps. This indicates that the precipitates present in post-annealed samples are not required for the fast relaxation of photoexcited carriers in low-temperature-grown GaAs. Ultrafast switching measurements on a post-annealed version of the GaAs film also resulted in the generation of similar, 0.6 ps full width at half maximum pulses.