A fast 2D MoS₂ photodetector with ultralow contact resistance

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), hold great promise for next-generation nanoelectronic and nanophotonic devices. While high photoresponsivity and broad spectral coverage (UV-IR) have been reported, the slow response time of MoS₂ photodetectors caused by their unfavorable RC characteristics is still a major limit in current devices. Once the RC limit issue is resolved, the intrinsic saturation drift velocity of electrons in TMDs (∼10⁶ cm s^-1) may enable GHz opto-electronic operations. Recent breakthroughs in device fabrication technology have enabled significant progress in exploring the possibilities of high-speed TMD photodetectors. In this work, using semi-metallic bismuth contacts to suppress metal-induced gap states (MIGS), an MoS₂ photodetector with ultra-low contact resistance (<400 Ω μm) was fabricated. The device exhibited a broad bandwidth and high photoresponsivity (>1 A W^-1). In particular, using an acousto-optic modulator (AOM)-modulated 532 nm laser, a -3 dB cutoff frequency of ∼70 kHz was obtained, which was corroborated by directly observed rise/fall times (on a scale of 10 μs). An extrinsic effect, where defective states of BN induce a negative shift in the photocurrent baseline was further identified and attributed to charge-induced screening, elucidating where a device can exhibit different dynamic and static response behaviors simultaneously. Our results may shed light for future GHz optoelectronic applications employing TMDs as a platform.