  Graphene, owing to its zero bandgap electronic structure, is promising as an
absorption material for ultra-wideband photodetection applications. However,
graphene-absorption based detectors inherently suffer from poor responsivity
due to weak absorption and fast photocarrier recombination, limiting their
viability for low intensity light detection. Here we use a
graphene/WS$_2$/MoS$_2$ vertical heterojunction to demonstrate a highly
sensitive photodetector, where the graphene layer serves dual purpose, namely
as the light absorption layer, and also as the carrier conduction channel, thus
maintaining the broadband nature of the photodetector. A fraction of the
photoelectrons in graphene encounter ultra-fast inter-layer transfer to a
floating monolayer MoS$_2$ quantum well providing strong quantum confined
photogating effect. The photodetector shows a responsivity of $4.4\times 10^6$
A/W at 30 fW incident power, outperforming photodetectors reported till date
where graphene is used as light absorption material by several orders. In
addition, the proposed photodetector exhibits an extremely low noise equivalent
power ($N\!E\!P$) of $<4$ fW/$\sqrt{Hz}$ and a fast response ($\sim$
milliseconds) with zero reminiscent photocurrent. The findings are attractive
towards the demonstration of graphene-based highly sensitive, fast, broadband
photodetection technology.
