Investigation of ferroelectric domain structure and nonlinear optical properties in barium titanate epitaxial thin films

The ferroelectric material barium titanate exhibits exceptional electro-optic and dielectric properties, making it desirable for electro-optic applications, especially in thin film form. The effective nonlinear optical and dielectric properties of BaTiO3 films are determined by the ferroelectric domain structure. The ferroelectric domain structures of epitaxial BaTiO3 films on MgO (100) substrates were investigated using x-ray diffraction (XRD), scanning probe microscopy (SPM), and second harmonic generation (SHG) techniques.; XRD measurements indicated the epitaxial BaTiO3 films were multi-domain. Evidence supporting co-existence of both the tetragonal and orthorhombic ferroelectric phases was observed, in agreement with recent theoretical calculations by Li and Chen for weakly strained BaTiO3 films. This strain state is consistent with complete lattice mismatch relaxation during film growth. For tetragonal films, typical domain volumes of 1/3 a-oriented and 2/3 c-oriented were determined from integrated peak intensities. XRD peak broadening analysis indicated enhanced microstrain accommodation in the orthorhombic phase.; The spatial relationships and size distributions of domains in BaTiO 3 films were quantified for the first time, using an SPM-based technique. Typical a-domain widths were ∼100 nm. Domain fractions calculated from the c- to a-domain width ratio agreed with those from XRD. The hierarchical domain structure consisted of alternating c- and a-domains grouped into 1-2 mum2 "tiles" of two types, which in turn alternated to reduce macroscopic strains. The domain size distributions had fractal dimensions of 2.2-2.5, in excellent agreement with theoretical predictions based on poling dynamics experiments.; SHG polarization rotation measurements examined net domain fractions of parallel and antiparallel domains. In-plane net polarizations in the as-grown films were small (2-5%). The SHG tensor coefficient ratio d33/d 31 of the thin films was larger than bulk BaTiO3. SHG hysteresis curves obtained using coplanar electrodes revealed the presence of an internal electric field. Saturation of the hysteresis loops at 1.25 MV/m indicated complete 180° poling, verified by comparing the net polarization areas to the total domain volumes (from XRD). The remnant deff was ∼2/3 the maximum, indicating considerable stability of the poled domain structure. Large nonlinear optical coefficients were observed. The maximum deff of 18.4 pm/V is the largest ever reported for a BaTiO3 thin film.