Heteroepitaxial growth of ferroelectric films on Si substrates and their applications in waveguides and electro-optics

We introduce a general route to integrate epitaxially ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ (SCNN) thin films on Si(001) substrates by pulsed laser deposition. The In:Mg_0.8Zn_0.2O (In:MZO)/TiN buffer layer is induced to overcome the large lattice mismatch between SCNN films and Si(001) substrates. The In:MZO layer acts as the buffer layer to bridge the Si and ferroelectric, the buffer layer with low refractive index for waveguide structure and the bottom electrode for electro-optic (EO) modulators. Waveguide and EO properties of the epitaxial integrated heterostructure were characterized by a prism coupler technique. The observed sharp and distinguishable TM and TE propagation mode lines revealed that favorable light confinements were achieved within the SCNN waveguide layer. The extraordinary and ordinary refractive indices were measured to be n_e = 2.1802 and n_o = 2.2164 at 633 nm, respectively, implying a birefringence of 0.0362 was obtained. This large birefringence indicates that a large optically anisotropic property existed in the epitaxial SCNN films. An angular shift of guided mode was observed under applied electric field along with c-axis direction. The linear EO coefficient tensor r₃₃ was measured to be about 860 pm/V. All of these results indicate that the integrated SCNN/In:MZO/TiN/Si epitaxial heterostructure has great potential for integrated optical EO and waveguide applications.