Nanoscale ultrafast metal-semiconductor-metal photodetectors

Summary form only given. MSM PDs (metal-semiconductor-metal photodetectors) with finger spacing and width as small as 25 nm have been fabricated on bulk GaAs, LT (low-temperature-grown)-GaAs, and bulk Si, using a custom-built electron-beam lithography system. Measurements using an electrooptic sampling system with a 100-fs pulse laser showed that the shortest full width at half maximum impulse response and the highest 3-dB bandwidth of these nanoscale MSM PDs are, respectively, 0.87 ps and 480 GHz for MSM PDs on LT-GaAs, 1.5 ps and 300 GHz for bulk GaAs, and 10.7 ps and 40 GHz for bulk Si. Monte Carlo simulation was used to understand the impulse response of the MSM PDs and to explore the ultimate speed limitation of transit-time-limited MSM PDs on GaAs and Si. Comparison with experimental data showed that the long tail in responses of the MSM PDs on bulk GaAs and Si was primarily due to slow-moving holes instead of photoexcited carriers deep inside the semiconductor. >