SUBPICOSECOND CARRIER RESPONSE OF UNANNEALED LOW-TEMPERATURE-GROWN GAAS VS TEMPERATURE

The subpicosecond carrier lifetime of an unannealed molecular beam epi taxial layer of GaAs grown at similar to 210 degrees C has been demons trated between 10 and 290K through optoelectronic switching and all-op tical pump-probe measurements. The low room-temperature resistivity of the as-grown layers, believed to arise from hopping conductivity thro ugh defect sites, has been observed to increase as the sample temperat ure was lowered, allowing ultrafast switching measurements to be perfo rmed using the low-temperature-as-grown GaAs as a photoconductive elem ent. After illumination by 100 femtosecond optical pulses, photogenera ted carriers in the sample have rapidly relaxed, returning the materia l to its high-resistivity state in less than 1 ps. This indicates that the precipitates present in post-annealed samples are not required fo r the fast relaxation of photoexcited carriers in low-temperature-grow n 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.