Ultrafast Hot Carrier Cooling Enables van der Waals Photodetectors at Telecom Wavelengths

With the increasing demand for optical data transmission and processing by modern communication systems, high-speed optoelectronics operating at telecom wavelengths are greatly desired. Two-dimensional (2D) materials with high carrier mobility and picosecond intrinsic response times exhibit large potential in this respect. However, most reported 2D material photodetectors for the telecom C-band either suffer from a slow response speed or rely on integrated silicon waveguides, which is a consequence of the inefficient utilization of hot carriers in these devices. Here, we report a high-performance and waveguide-free WS2/graphene photodetector operating at 1560 nm enabled by hot carrier injection and long-lived charge separation. The efficiently injected hot electrons from graphene to WS2 exhibit ultrafast cooling dynamics with 3 ps intrinsic response time. Simultaneous hole trapping in WS2 ensures a long circulation of injected electrons, enabling a high responsivity of 0.26 A/W. In continuation, we present a vertical WS2/graphene/WSe2 device with a built-in electric field and the same detection mechanism, for which a photocurrent on-off ratio of 3500 and an extrinsic response time of 1.71 ns are obtained. Our study highlights the benefits of combing 2D semiconductors and semimetals for high-speed photodetection.