Strain-Induced Elevation of the Spontaneous Polarization in BaTiO 3 Thin Films
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STRAIN-INDUCED ELEVATION OF THE SPONTANEOUS POLARIZATION IN BaTiO3 THIN FILMS
W. Tian*, J. H. Haeni**, D. G. Schlom**, and X. Q. Pan* *Department of Materials Science & Engineering, The University of Michigan, Ann Arbor, MI **Department of Materials Science & Engineering, Penn State University, University Park, PA ABSTRACT A manmade ferroelectric-paraelectric heterostructure, a BaTiO3 / SrTiO3 superlattice, was studied to explore the effect of strain on ferroelectricity. An atomically abrupt BaTiO3 / SrTiO3 superlattice was grown on a (001) SrTiO3 substrate by reactive molecular beam epitaxy. Both BaTiO3 and SrTiO3 layers were grown with their individual thicknesses less than the critical thickness for the formation of interfacial misfit dislocations, leaving the entire superlattice fully coherent with the substrate. This resulted in a uniformly and highly strained BaTiO3 layer to study the effect of strain on ferroelectricity. Quantitative high-resolution transmission electron microscopy was employed to examine the atomic positions of cations and anions in the strained BaTiO3 layers. It was found that the relative static displacement of cations (Ti4+, Ba2+) to anions (O2-) is larger than that of bulk BaTiO3. Our observation thus illustrates the strain-induced elevation of spontaneous polarization in BaTiO3 thin films. INTRODUCTION As miniaturization of modern semiconductor devices continues, the size of oxide components will be reduced to the nanometer scale. At this scale, the behavior of functional oxides is strongly size- and strain-dependent and interface-controlled. Specifically, integration of ferroelectric thin films with lattice-mismatched and thermal expansion mismatched materials will introduce significant strains in the ferroelectric thin films, which in turn affects the physical properties and device performance. The strain effect thus refers to the relationship between strain acting on the material and its properties, such as the paraelectric-ferroelectric phase transition temperature (TC), spontaneous polarization (Ps), and dielectric permittivity. Many attempts have been made to measure the effect of strain on ferroelectricity, most of them involving poly- and single-crystalline thin films [1-5]. Generally, biaxial tensile strain was found to depress the paraelectric-ferroelectric phase transition temperature, spontaneous polarization, and remanent polarization (Pr), while biaxial compressive strain had opposite effects. Theoretical work has been dedicated to calculating the strain effect [6-10]. However, the agreement between experimental results and theoretical calculations has only recently become quantitative [11,12], because of difficulties in determining the state of strain due to the polycrystalline nature of the films, residual stress, and inhomogeneity induced by defects in the lattice and film/substrate interfaces. The artificial heterostructures (superlattices) incorporating ferro- and para-electric components with relatively large mutual lattice mismatch, on the other hand, offer many op
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