Hybrid Cavity–Plasmonic Germanium Photodetector Architecture for High-Responsivity Near-Infrared Detection

High-speed germanium photodetectors are essential for silicon photonics, but thin germanium absorbers typically exhibit limited absorption at 1550 nm. This work studies a resonant-cavity-enhanced germanium-on-silicon photodetector that combines dielectric cavity confinement, plasmonic near-field localization, and a transparent electrode to improve telecom-band detection. Alternating silicon dioxide and titanium dioxide multilayers form a bottom distributed Bragg reflector and a top anti-reflection coating, creating an optical cavity that boosts field intensity in the absorber. Plasmonic enhancement is achieved by embedding gold nanoparticles inside the germanium layer, with nanoparticle diameter and embedding depth optimized for strong optical localization. A graphene layer is integrated as a low-loss transparent electrode to support carrier extraction while preserving cavity response. Three-dimensional finite-difference time-domain simulations show that optimized structures achieve 0.78 - 0.8 absorption at 1550 nm for both symmetric and simple cavity designs. Photovoltaic operation is evaluated under 1550 nm illumination at 0.5 mW incident optical power. The symmetric cavity delivers 0.78 mA short-circuit current, 0.68 V open-circuit voltage, and 0.40 mW maximum electrical power at 0.58 V, corresponding to 1.56 A/W responsivity at zero bias. The simple cavity yields 0.52 mA, 0.65 V, and 0.225 mW at 0.52 V, corresponding to 1.04 A/W. These results demonstrate a scalable path to high-efficiency self-powered germanium photodetectors for silicon photonics receivers and infrared sensing.