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We report the first demonstration of a novel germanium (Ge) metal-semiconductor-metal (MSM) photodetector featuring asymmetrical Schottky-barrier height for low dark current and high-speed photodetection applications. Through co-implantation and segregation of valence-mending adsorbate such as sulfur at the NiGe/Ge interface, the germanide Fermi level can be pinned close to the conduction band edge. This results in an effective modulation of hole Schottky-barrier height, leading to a significant dark current suppression by >3 orders of magnitude over a conventional MSM photodetector. When operated at a bias voltage VA of 1.0 V, a detector with an area of 804 mum2 shows a spectrum response of ~ 0.36 A/W or a corresponding quantum efficiency of ~ 34%. In addition, a frequency response measurement reveals the achievement of a -3-dB bandwidth of ~15 GHz at an illumination photon wavelength of 1550 nm.