Epitaxially Integrated Electro-Optic KNbO₃ Thin Films on Silicon

The absence of a native electro-optic effect in silicon remains a fundamental limitation for integrated photonics, motivating the integration of ferroelectric oxides as active materials. Here, we report the first epitaxial integration of KNbO₃ thin films on silicon, establishing a new materials platform for silicon-integrated electro-optics. Silicon integration of KNbO₃ has remained unexplored due to potassium volatility challenges during synthesis. Employing suboxide molecular-beam sources assisted by in situ RHEED monitoring and guided by thermodynamic simulations, we establish an adsorption-controlled growth window for phase-pure KNbO₃ films. Temperature dependent x-ray diffraction, optical second-harmonic generation, piezo-response force microscopy, and transmission electron microscopy aided by phase-field simulations confirm high-quality epitaxial films with sharp interfaces and structural phase transitions mirroring KNbO₃ single crystals. Electro-optic measurements demonstrate a linear Pockels response with an effective electro-optic response of 146  ±  14 pm V^-1 at 1550 nm. Phase-field simulations further predict that modest compressive strain can drive the electro-optic response to ∼900 pm V^-1. Beyond KNbO₃, this integration pathway provides access to the broader (K,Na)(Ta,Nb)O₃ family, opening pathways to harness their strong electro-optic response, nonlinear optical response, piezoelectric response and ferroelectric properties for next-generation silicon photonics and electronics beyond current BaTiO₃-based approaches.