| Although SrTiO3 is normally not ferroelectric at any temperature, predictions that predate this thesis based on thermodynamic analysis concluded that a biaxial tensile strain of order 1% would shift the paraelectric-to-ferroelectric transition temperature (Tc) of SrTiO3 to the vicinity of room temperature. In practice, uniformly straining SrTiO3 or related perovskite ferroelectrics to such strain levels is challenging and hitherto unheard of Nonetheless, using epitaxy and the misfit strain imposed by an underlying substrate, I have strained SrTiO 3 thin films to percent levels---far beyond where they would crack in bulk. Epitaxial ferroelectric films are often grown to thicknesses greatly exceeding their critical values, resulting in undesirable relaxation toward a zero-strain state by the introduction of dislocations. Dislocation densities of ∼1011 cm-2 are common in epitaxial ferroelectric films grown on lattice-mismatched substrates, and the resulting inhomogeneous strain smears out the ferroelectric phase transition. My approach to achieving the desired high strain levels in SrTiO3 films to assess strain predictions made use of new substrates (DyScO3 and GdScO3 ) that enabled the growth of uniformly strained SrTiO3 films below, or at least far closer to, the critical thickness for relaxation. The resulting strained SrTiO3 films have better structural perfection (narrower rocking curve widths) than the best bulk SrTiO3 single crystals. These films have the narrowest rocking curves ever reported for any heteroepeitaxial oxide thin film (6.5 arcsec).;Modeling of ferroelectrics under these strain levels predicts dramatic shifts in the transition temperature and enhancement of the polarization. Indeed, in our strained SrTiO3, a ferroelectric state was induced with a Tc near room temperature. These films also exhibit a peak dielectric constant near room temperature of ∼20,000, comparable to that seen at very low temperatures (∼4K) in bulk SrTiO3. Unexpectedly, the strained SrTiO3 films exhibit a frequency dependence of their dielectric constant consistent with relaxor ferroelectricity. Due to the anisotropy in the in-plane strain, the polarization develops at different temperatures along two orthogonal in-plane directions indicating an anisotropy in the dielectric properties due to the orhorhombicity of the substrate. These results, illustrate that in thin films strain is a viable alternative to the traditional method of chemical substitutions for shifting Tc by large amounts. |
