Electro-optical properties of barium titanate films epitaxially grown on silicon

BaTiO3 (BTO) single crystals exhibit one of the largest Pockels coefficients (r(42) > 1000 pm/V) among oxides. This makes BTO an excellent active material for electro-optical (EO) devices such as switches, modulators or tuning elements. However, in order to harness these properties in silicon photonics circuits, the challenge is to integrate BTO as high quality thin films onto Si substrates. The effective Pockels coefficients can be enhanced in epitaxial films due to their tight relationship with the crystallographic symmetry and microstructure. We report on the EO properties of epitaxial BTO thin films on Si. The growth of BTO layers on Si(001) is performed by molecular beam epitaxy (MBE). A thin single-crystalline strontium titanate seed layer is grown on Si, followed by a 130 nm thick BTO layer. Electrodes to provide an electrical field parallel to the surface are patterned on the films using photolithography. Throughout this process, the BTO keeps an epitaxial relationship to the Si-substrate. Considering the tensor nature of the Pockels effect, the optical behavior of the BTO layers upon applying an electrical field is simulated, taking into account the films' crystalline multi-domain structure. An experimental way to access these EO properties is discussed, which utilizes polarization changes of a transmitted laser beam upon applying an electrical field to the film. Simulations of the measurement signals demonstrate the capability of resolving the expected EO response of the samples, which serves as a promising base for future experiments.