Heterogeneously integrated III-V photodiodes on ultra-thin silicon nitride waveguides

The demand for compact and low-loss photonic integrated circuits continues to grow with the rapid expansion of artificial intelligence, data communication, sensing, and emerging quantum technologies. Many photonic systems rely on ultra-low-loss reference cavities, Mach-Zehnder interferometers (MZIs), and resonators based on ultra-thin silicon nitride (Si ₃ N ₄ ) waveguides for laser frequency stabilization and generating ultra-low phase noise signals. However, the realization of fully integrated stabilization circuits requires high-performance photodiodes (PDs) for efficient optical-to-electrical conversion and feedback, which remain challenging to integrate with ultra-thin Si ₃ N ₄ platforms. Here, we report the first heterogeneously integrated InGaAs/InP PDs on thin 80 nm Si ₃ N ₄ waveguides. Our fabrication process involves local removal of the top oxide cladding layer, InGaAs/InP die-to-wafer adhesive bonding, and cladding redeposition to enable efficient evanescent optical coupling while preserving optical confinement. The integrated PDs achieve a 3-dB bandwidth up to 2.1 GHz, with a responsivity of 0.61 A/W at 1550 nm. For ubiquitous integration of monitor PDs, we also demonstrate heterogeneous PDs on vertical grating couplers with 0.33 A/W that can be readily integrated on a 6-μm thick top cladding. Our results highlight the suitability of this heterogeneous ultra-low-loss Si ₃ N ₄ platform for fully integrated high-precision laser locking and frequency stabilization systems in applications such as quantum information science, atomic clocks, and metrology.