Nanoscale heterostructure InGaAs MSM photodetectors

Abstract

In this work, we investigate the DC and high-frequency performance of heterostructure InGaAs/InAlAs metalsemiconductor-metal (MSM) photodetectors fabricated using e-beam nanolithography. The device finger electrodes are approximately 250 nm in width with gaps of 100 nm, and are arranged interdigitally in a circular geometry with ~4.5 µ m outer diameter. The InGaAs absorption layer is 200 nm thick. High-bandgap material layers are utilized to form carrier transport barriers and to enhance Schottky barrier heights at the contacts. Also, a buried Bragg reflector (resonant cavity enhanced structure) serves to increase the photodetector responsivity given the very thin absorption layer thickness. Current-voltage measurements of seven devices were taken at room temperature under dark and illuminated conditions. Additionally, the frequency performance was evaluated using a 1550-nm diode laser with an integrated 50-GHz electroabsorption modulator and a microwave network analyzer. A fairly large range of dark currents from 100 pA-1 µ A is observed. External responsivities for these small devices also vary widely between 0.05 -0.24 A/W. Under high-frequency intensity modulation, the devices exhibit an RC-limited and/or diffusion-limited performance, with usable bandwidths to 15 GHz. Physical origins of the device and parasitic capacitances are discussed as well as optical diffraction effects, both of which are thought to contribute to the limited high-frequency performance.