Diffusion-Driven Exfoliation of Magneto-Optical Garnet Nanosheets: Implications for Low Thermal Budget Integration in Si Photonics

Rare-earth iron garnets are instrumental in the development of integrated nonreciprocal passive devices such as isolators and circulators in silicon photonics. Unfortunately, monolithic integration of garnet on-chip requires annealing temperatures much higher than the thermal budget of a semiconductor foundry. Here, we report the mechanical exfoliation of large area (0.2 mm × 0.2 mm) nanosheets of a high-gyrotropy cerium-doped terbium iron garnet (CeTbIG) enabled by a strain-enhanced vacancy diffusion process that follows the Nabarro-Herring (lattice diffusion) model. Diffusivities calculated from the strain rate-stress data (1.13 × 10-18 m2s-1) identify iron and rare-earth cations as the rate-determining lattice diffusants. Cross-section scanning transmission electron microscopy reveals an exfoliation gap located ∼30 nm into the film, comparable to the cation diffusion length, which appears to verify the model. With a saturation magnetization of 18 emu cc-1 and a Faraday rotation of -2900°cm-1 at 1550 nm, the magnetic and optical properties of the nanosheets are comparable to their thin-film values. Diffusion-driven exfoliation will open foundry-acceptable pathways for heterogeneous integration of garnets on photonic waveguides and protect devices from the high-temperature processes used in crystallizing garnet films.