Tuning the Tunability in Epitaxial Barium Strontium Titanate Film via Internal Stresses
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Tuning the Tunability in Epitaxial Barium Strontium Titanate Film via Internal Stresses 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 The tunability of epitaxial barium strontium titanate films is analyzed theoretically for the first time using a modified phenomenological Landau-Devonshire approach taking into account the formation of unusual ferroelectric phases that cannot form in bulk and single-crystal ceramics. It is shown that enhanced tunability can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. The internal stresses in epitaxial films as characterized by the misfit strain can be controlled by changing the substrate material and/or varying the film thickness. The latter is due to the possibility of stress relaxation by formation of misfit dislocations that relieve epitaxial stresses at film growth temperature. Based on the thermodynamic model, we provide quantitative estimations of tunability of (001) Ba0.5Sr0.5TiO3 films on (001) LaAlO3 (LAO) and SrTiO3 (STO) as a function of film thickness. The analysis indicates that films on STO substrates should be as thick as possible in order to achieve optimum tunability. To obtain maximum tunability on LAO substrates, the thickness of films should be as close as possible to a critical film thickness (~120 nm). INTRODUCTION Barium strontium titanate (BaxSr1-xTiO3, BST) is a solid-solution ferroelectric material that exhibits unique properties, such as a high dielectric permittivity, reasonably low dielectric loss, and high tunability. Accordingly, barium strontium titanate (BaxSr1-xTiO3, BST) in the thin film form is being investigated widely as suitable candidates for applications such as storage capacitor dielectrics for dynamic random access memories (DRAM) as well as variable elements in the tunable microwave devices [1,2]. The primary objective of research in ferroelectric thin film has been to reliably reproduce the properties of bulk ceramics or single crystals in thin film form for device applications. However, compared to their bulk or single crystal counterparts, generally inferior electrical and electromechanical properties are observed in ferroelectric thin films [3]. The reason for this is believed to be due to compositional and microstructural inhomogeneities, defects, and internal stresses in ferroelectric thin film. A variety of reasons account for the emergence of internal stresses in thin films, the lattice mismatch between film and the substrate being the most important one in the case of epitaxial films. For epitaxial BST films used in tunable microwave devices, tunability is one of the key design parameters and ideally, a large tunability accompanied by a small dielectric loss is desired. Recent experiments have shown that epitaxy induced stresses have a profound effect on tunability of epitaxial BST films [2, 4, 5]. The dependence of the structural, electrical
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