High-speed high-responsivity low temperature grown GaAs detector

GaAs grown by molecular beam epitaxy (MBE) at low, near 200°C, temperature (LT-GaAs) has become the material of choice for ultra high speed, such as THz, detection due to its very short, carrier lifetime of around 1 psec [1-3]. By contrast, regular temperature GaAs (RT-GaAs) is grown at around 600°C and has carrier lifetime of ~1nsec. This short lifetime requires that optically generated carriers be collected quickly, however, LT-GaAs has very low carrier mobilities thus resulting in low photocurrent and hence responsivity. We have previously designed a novel structure that circumvents this limitation [4] and have reported speed of response measured optoelectronically that was limited by instrumentation. Here, we present electro-optic sampling (EOS) time response data that shows while maintaining the high speed of response in LT-GaAs, the device also achieves high responsivity, near that of RT-GaAs. This is done by utilizing (a) an AlGaAs heterojunction with RT-GaAs for better collection efficiency of carriers, (b) a thin (85 nm) LT-GaAs layer below this RT-GaAs channel that maintains high speed by capturing slow carriers, and (c) a vertical electric field transverse to cathode-anode direction that guides electrons to the high speed channel and eliminates the low speed holes. These features result in a photodetector device with dark current in tens of picoamps -hence large signal to noise ratio- and high speed of response with full width half max (FWHM) 6.3 psec, while maintaining high responsivity of 0.15 A/W comparable to RT-GaAs and much higher than previous LT-GaAs devices. Device simulation confirms the physical basis for the remarkable performance of this detector.