Interlayer Charge Transfer and Photodetection Efficiency of Graphene–Transition-Metal-Dichalcogenide Heterostructures

Graphene and transition-metal-dichalcogenide- (TMD) based van der Waals heterostructures in field-effect-transistor (FET) architecture exhibits extremely high sensitivity to optical radiation due to transit and physical separation of the photogenerated carriers across the heterointerface. Both the sensitivity and speed of these detectors depend on the kinetics of charge transfer, but their interdependency at room temperature (T), where these detectors would be most useful, remains largely unexplored. Here we systematically measure the T dependence of the magnitude (gain) and timescale (bandwidth) of photoresponse in graphene-TMD heterostructures well up to the room T. The gain-bandwidth product is found to be strongly dependent on the power of optical illumination and increases with decreasing power (P), becoming as large as $1\phantom{\rule{0.2em}{0ex}}\mathrm{M}\mathrm{Hz}$ in the low-P limit. We find that thermally activated back transfer of charge from graphene to the TMD determines the response time of the detector at higher temperatures under continuous illumination. Our experiment reveals the impact of charge-transfer pathways on the performance in a broad class of graphene-TMD detectors.