Ultrafast photoresponse and fabrication of freestanding LT-GaAs photoconductive devices

We report on fabrication and ultrafast photoresponse of novel, freestanding low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) devices. 1-mum-thick, LT-GaAs single-crystal films were grown by molecular beam epitaxy at the temperature range of 200 degreesC to 250 degreesC. Next, the films were patterned to the desired device sizes, lifted-off from their host substrates, and placed on predetermined places on either SiO2/Si or MgO wafers. Our freestanding LT-GaAs devices consisted of either approximately 20-mum by 20-mum PC switches, or 150-mum by 150-mum metal-semiconductor-metal (MSM) interdigitated structures with Ti/Au fingers patterned directly on top of the LT-GaAs film. For testing purposes, our devices were integrated with Ti/Au coplanar striplines, fabricated directly on SiO2/Si and MgO substrates. The test structures were illuminated with 100-fs-wide optical pulses and their time-resolved photoresponse was measured with an electro-optic sampling system, characterized by 200-fs time resolution and sub-millivolt sensitivity. Using 810-nm optical excitation, we recorded as narrow as 360-fs-wide electrical signals (1.25 THz, 3-dB bandwidth) for PC switches, resulting in 155 fs carrier lifetime in our freestanding LT-GaAs. For both types of devices, the photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Our freestanding photo-switches are robust and very reproducible. They are best suited for applications in hybrid optoelectronic and ultrafast electronic systems, since they can be placed at virtually any point on a test circuit.