Enhanced Quantum Efficiency in Vertical Mixed-Thickness n-ReS2/p-Si Heterojunction Photodiodes

We fabricated few-layer, multilayer, and mixed- thickness rhenium disulfide (ReS2) based on a vertical van der Waals n-p junction for photosensing applications. ReS2 flake deposition onto a p(+2)Si substrate led to the formation of a n-p heterojunction with rectifying characteristics and good photosensing ability under reverse bias. A thin ReS2 layer with a Si heterojunction showed weak photosensing performance with a fast response, whereas a thick multilayer ReS2/Si showed an improvement in photocurrent, but an overall degradation of the response time. To overcome the trade-off between responsivity and speed, a mixed-thickness ReS2/Si was fabricated. This heterojunction was found to exhibit the best photoresponse, with a short response time and high quantum efficiency. A high photoresponsivity (at 3 V) of , similar to 33.47 A/W at a high-speed operation of 80 mu s was recorded, making this one of the fastest reported transition metal dichalcogenides with silicon photodiodes with high responsivity. The heterointerface of Si with thickness-independent direct-bandgap ReS2 of mixed thicknesses enabled more gain related to photogenerated carrier trapping, resulting in the observed high photoresponsivity and fast (mu s) response. This work demonstrates that a mixture of different thicknesses of ReS2-based n-p junctions results in improved photoresponsivity and speed in optoelectronics and sensor applications.