The integration of electrical and optical components on a single chip, favourable silicon, is a major goal in research. Thereby, a bottleneck is the integration of active and passive optical elements. Graphene, with its electrically tuneable absorption and ultrafast photoresponse, is a promising candidate to move a step closer towards high-speed on-chip integration.
We fabricated a dual-gate tuneable pn-junction graphene phototdetector to investigate the relevant conversion mechanisms. The photodector is integrated on a silicon slot waveguide, which has twofold function. First, the two silicon strips of the slot waveguide are utilized as dual gate electrodes to create an electrically controllable pn-junction in the graphene. Second, the slot waveguide design allows confinement of light in subwavelength dimension. The confined light is directly absorbed in the slot between the n- and p-doped regions. At zero bias the conversion is dominated by the photo-thermoelectric effect, where we achieved a responsivity of 35 mA/W. While by applying a low bias of 300 mV, the responsivity increased to 76 mA/W due to an additional photoconductive contribution.
The photoresponse of photodetectors based on the photo-thermoelectric effect arises from hot electrons, rather than lattice heating. Therefore, we could demonstrate that our graphene integrated photodetector based on a tuneable pn-junction reaches a setup-limited 3dB-bandwidth of 65 GHz, which is the highest value reported for a graphene-based phototedetector.