Characterization of dark current in Ge-on-Si photodiodes

Ge-on-Si photodiodes were fabricated from germanium films grown using low-pressure chemical vapor deposition. The mechanisms responsible for the dark current in these devices are studied using geometric analysis, temperature-dependent current-voltage characterization, deep level transient spectroscopy, and device modeling. It is found that an important source of leakage current is associated with the surface depletion region, which is impacted by the nature of the fixed charge at the Ge/dielectric interface. This source of leakage especially affects devices with smaller area, on the order of 10 × 10 μm2. Through a post-metallization anneal (PMA), the dark current of these devices can be reduced by ∼1000X. A similar reduction can be obtained by intentionally doping the top of the germanium film p-type. After the PMA, it is found that the dark current density of large devices is ∼1 mA/cm2, due mainly to generation of minority carriers in the depletion region of the device. The effect of reducing the threading dislocation density is also discussed.