Investigation of the electro-optic and dielectric properties of epitaxial ferroelectric thin films

The electro-optic properties of epitaxial BaTiO₃ and KTa _xNb_1−xO₃ thin films were investigated to determine their utility as the active medium in electro-optic (EO) devices. The relationship between the EO properties, both static and dynamic, and the ferroelectric (FE) domain structure of the films was determined. A theoretical model was developed to describe the magnitude and dynamic response of the EO effect and dielectric properties of multidomain FE films. The effective linear EO coefficient of each film, r_eff, was modeled as a linear superposition of the EO response of each FE domain variant, and the dynamic EO response was described in terms of a statistical distribution of domain relaxation times.; EO coefficients were measured using a transverse technique. For unpoled films of BaTiO₃ and KNbO₃, r_eff was between 1 and 10 pm/V, more than an order of magnitude lower than in bulk materials. The lower coefficients were attributed to reverse, or 180°, FE domains. Calculations indicated 1 to 10% domain alignment in the unpoled films. Consequently, the EO response of the unpoled films is limited by the presence of reverse FE domains. The largest r_eff values in the BaTiO₃ and KNbO₃ films, 260 and 480 pm/V respectively, were observed under DC bias fields applied along the ⟨110⟩ axes of the cubic substrates. The larger r_eff values and hysteretic bias response were attributed to poling of the FE domain structure. An in-plane EO anisotropy was observed in the BaTiO₃ and KNbO₃ films and was attributed to their multi-variant domain structure.; The transient response of EO, polarization, and dielectric measurements was described by a power law, At−m, extending from ≤6 ns to 1 s in both the BaTiO₃ and KNbO ₃ film systems. This non-exponential relaxation was attributed to the reorientation of FE domains having a power law distribution of FE domain sizes spanning 9 orders of magnitude. The value of the exponent m was directly related to the power of the size distribution, γ, by the expression, m = γ − 2. The average exponent, m, determined from transient EO and polarization measurements ranged from 0.16 to 0.33. Values were within the predicted limits of the model, 0 ≤ m ≤ 0.5, and similar to that expected for a distribution of domain sizes given by percolation theory, m = 0.19.