  We report an on-chip integrated metal-graphene-silicon plasmonic Schottky
photodetector with 85mA/W responsivity at 1.55 um and 7% internal quantum
efficiency. This is one order of magnitude higher than metal-silicon Schottky
photodetectors operated in the same conditions. At a reverse bias of 3V, we
achieve avalanche multiplication, with 0.37A/W responsivity and avalanche
photogain~2. This paves the way to graphene integrated silicon photonics.
