180 Gbit/s Si3N4-waveguide coupled germanium photodetector with improved quantum efficiency.

A high quantum efficiency (QE) and high-speed silicon nitride (Si3N4) waveguide coupled germanium-on-silicon photodetector (Ge-on-Si PD) is presented. The proposed device is fabricated in a commercial 90 nm silicon photonics process platform. By decreasing the spacing between the tapered Si3N4 waveguide and the bottom Si to 200 nm and the Si3N4 thickness to 300 nm, the QE is significantly improved. Although the theoretical responsivity can reach up to 0.92 A/W at 1550 nm, the measured value is calculated to be approximately 0.61 A/W. The maximum experimental responsivity is about 0.9 A/W at 1485 nm. The 3 dB optoelectrical bandwidth of up to 54 GHz is demonstrated at a -3.3V bias. Additionally, the 80, 90, 100, and 105 Gbit/s non-return-to-zero on-off-keying and the 150, 160, 170, and 180 Gbit/s four-level pulse amplitude modulation clear openings of the electrical eye diagrams are attained. Overall, the Si3N4-waveguide coupled Ge-on-Si PD in this work possesses higher QE and operates at the highest data rates reported so far.