Misfit Strain Driven Phase Transformations in Epitaxial Barium Strontium Titanate Films
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Misfit Strain Driven Phase Transformations in Epitaxial Barium Strontium Titanate Films Z.-G. Ban and S. P. Alpay Department of Metallurgy and Materials Engineering and Institute of Materials Science University of Connecticut, Storrs, CT 06269 ABSTRACT We develop phase diagrams for single domain epitaxial barium strontium titanate films on cubic substrates as a function of the misfit strain based on a Landau-Devonshire phenomenological model similar to the one developed by Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)]. Unusual ferroelectric phases that are not possible in single crystals and bulk ceramics are demonstrated in epitaxially constrained BST films. The misfit strain is correlated with the film thickness quantitatively by taking into account the formation of misfit dislocations that relieve epitaxial stresses during deposition. Theoretical estimation of the dielectric constant of (001) Ba0.7Sr0.3TiO3 and Ba0.6Sr0.4TiO3 films grown on Si, MgO, LaAlO3, and SrTiO3 substrates as a function of film thickness is provided. It is shown that the selection of the substrate and the film thickness can be chosen as design parameters to manipulate the internal stress level in the film to achieve enhanced dielectric response. INTRODUCTION Thin films of barium strontium titanate (BaxSr1-xTiO3, BST) are attractive for applications in dynamic random access memories (DRAM) and tunable microwave devices due to their desirable ferroelectric and dielectric properties such as high dielectric permittivity and strong dependence of permittivity on electric field [1] [2]. However, compositional and microstructural inhomogeneities, defects, and internal stresses reduce the physical properties of ferroelectric thin films compared to bulk ceramics or single crystals. A variety of reasons account for the emergence of internal stresses in thin films. The most important one is the lattice mismatch between film and the substrate in the case of epitaxial films. Recent experiments have shown that epitaxy induced stresses have a profound effect on physical properties of ferroelectric films especially in the vicinity of a phase transformation [3, 4]. Landau-Devonshire (LD) phenomenological theory has been widely used to model the observed dependence of physical properties on misfit strain [2, 3, 5, 6]. These models, however, do not take into account the possibility of formation of unusual phases that cannot form in bulk ceramics and single crystals due to the internal stresses. Recently, the appearance of the unusual phases in epitaxial BaTiO3 (BT) and PbTiO3 (PT) films has been theoretically predicted by Pertsev et al. [7]. The modified LD thermodynamic model incorporated with ā2-Dā clamping effect in epitaxial thin films was used to construct the misfit strain ā temperature phase diagrams. In this article, we develop similar phase diagrams for single domain epitaxial (001) Ba0.6Sr0.4TiO3 (BST 60/40) and (001) Ba0.7Sr0.3TiO3 (BST 70/30) films on (001) cubic substrates as a function of the misfit strain based on the LD phenomenological mod
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