Single-photon detection in Ge- & GeSn-on-Si avalanche photodiodes

Inhalt:
We report advancements in germanium (Ge) and germanium-tin (GeSn) avalanche photodiodes (APDs) monolithically integrated on silicon (Si) for single-photon detection in the short-wave infrared regions. By solving key challenges in epitaxial growth, electric-field engineering, and surface passivation, we demonstrate high-performance, complementary metal-oxide-semiconductor (CMOS)-compatible APDs suitable for quantum and classical photonics. Using a two-step molecular beam epitaxy process, we realize high-quality Ge virtual substrates enabling both lattice-matched Ge or pseudomorphic GeSn layers. A double mesa structure effectively suppresses peripheral electric fields, reducing dark current by up to three orders of magnitude, while an optimized α-Si/Al2O3/SiO₂ passivation achieves further reduction. Ge-on-Si APDs achieve single-photon sensitivity at 1310 nm with moderate dark count rates. Extending this approach to GeSn-on- Si enables detection at 1550 nm with responsivities exceeding 5 A/W and successful Geiger-mode operation. These results position Ge- and GeSn-on-Si APDs as promising platforms for scalable, low-cost single-photon detectors in Si photonics.