Tapered Cladding Design for Monolithic Waveguide-Photodetector Coupling in Si-Based Integrated Photonics.

Silicon photonics offers a powerful route to leverage existing microelectronics infrastructure to enhance performance and enable new applications in data processing and sensing. Among the available material platforms, silicon nitride (Si3N4) provides significant advantages due to its wide optical transmission window. A key challenge, however, remains the monolithic integration of passive nitride-based photonic components with active electronic devices directly on silicon wafers. In this work, we propose and demonstrate a tapered bottom-cladding design that enables efficient coupling of visible light from Si3N4/SiO2 core-cladding waveguides into planar p-n junction photodiodes fabricated on the silicon surface. Si3N4/SiO2 waveguides were fabricated using fully CMOS-compatible processes and materials. Controlled reactive ion etching (RIE) of SiO2 allowed the formation of vertically tapered claddings, and finite-difference time-domain (FDTD) simulations were carried out to analyze coupling efficiency across wavelengths from 509 nm to 740 nm. Simulations showed transmission efficiencies above 90% for taper angles below 30°, with near-total coupling at 10°. Experimental fabrication achieved angles as low as 8°. Responsivity simulations yielded values up to 311 mA W-1 for photodiodes without internal gain. These results demonstrate the feasibility of fabricating monolithic Si-based waveguide-photodetector systems using simple, CMOS-compatible methods, opening a scalable path for integrated photonic-electronic devices operating in the visible range.