  With its electrically tunable light absorption and ultrafast photoresponse,
graphene is a promising candidate for high-speed chip-integrated photonics. The
generation mechanisms of photosignals in graphene photodetectors have been
studied extensively in the past years. However, the knowledge about efficient
light conversion at graphene pn-junctions has not yet been translated into
high-performance devices. Here, we present a graphene photodetector integrated
on a silicon slot-waveguide, acting as a dual-gate to create a pn-junction in
the optical absorption region of the device. While at zero bias the
photo-thermoelectric effect is the dominant conversion process, an additional
photoconductive contribution is identified in a biased configuration. Extrinsic
responsivities of 35 mA/W, or 3.5 V/W, at zero bias and 76 mA/W at 300 mV bias
voltage are achieved. The device exhibits a 3 dB-bandwidth of 65 GHz, which is
the highest value reported for a graphene-based photodetector.
