Nanoscale Tera-Hertz Metal-Semiconductor-Metal Photodetectors

Metal-semiconductor-metal photodetectors (MSM PD's) with finger spacing and width as small as 25 nm were fabricated on bulk and low-temperature (LT) grown GaAs and crystalline Si using ultrahigh resolution electron-beam lithography. High-speed electrooptic characterization with a 100 fs pulsed laser showed that the fastest MSM PD's had finger spacing and width, full width at half maximum response time, and 3 dB bandwidth, respectively, of 300 nm, 0.87 ps, and 0.51 THz for LT-GaAs; 100 nm, 1.5 ps, and 0.3 THz for bulk GaAs; and 100 nm, 10.7 ps, and 41 GHz for crystalline Si. To our knowledge, these detectors are the fastest nanoscale MSM PD's on each of these materials reported to date. Monte Carlo simulation was used to understand the impulse response of the MSM PD's and to explore the ultimate speed limitation of transit-time-limited MSM PD's on GaAs and Si. Factors that are important to detector capacitance were identified using a conformal mapping method. Based on the experimental data, Monte Carlo simulation, and calculation of detector capacitance, scaling rules for achieving high-speed MSM PD's are presented.