Nanoscale tera-hertz metal-semiconductor-metal photodetectors

Metal-semiconductor-metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm were fabricated on bulk and low-temperature (LT) grown GaAs and crystalline Si using ultra-high-resolution electron-beam lithography. High-speed electrooptic characterization with a 100-fs pulsed laser showed that the fastest MSM PDs 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. Monte Carlo simulation was used to understand the impulse response of the MSM PDs and to explore the ultimate speed limitation of transmit-time-limited MSM PDs 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 PDs are presented