Conventional high speed optical communication components predominantly rely on the utilization of compound semiconductor due to its excellent light-emission and absorption properties. However, III-V devices are generally deemed to be too costly for implementation in server-based optical interconnects. Consequently, an alternative technology in utilizing Group-IV based optical components has been actively pursued for low cost communication applications. In particular, germanium (Ge) has been identified as an excellent alternative material suitable for near-infrared photodetections due to its small band gap energy and its integration compatibility with current CMOS technology. By leveraging on the advantage of lower capacitance, Ge-on-SOI metal-semiconductor-metal (MSM) photodetector has been demonstrated with high speed performance. However, high dark current issue observed in these devices due to the low hole Schottky barrier generally impedes its practicability for typical receiver applications.

To overcome this issue, IME researchers have reported 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 absorbate 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 the 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 µm2 shows a spectra 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, making it an attractive option for exploitation in high speed telecommunication applications.

Fig. 1 (a) Photo-response characteristics of the asymmetrical Schottky barrier MSM photodetector with sulfur-segregation at the NiGe/Ge interface. Good spectra response of ~0.36 A/W was achieved. (b) Normalized frequency response measured at a photon wavelength of 1550 nm. A -3dB bandwidth of ~15 GHz was achieved at an applied bias of 1.0 V.