A Study On Thermodynamic Properties Of Epitaxial Perovskite Ferroelectric Thin Film

With the development of modern micro-electronics and semiconductor technologies, high-speed and high-volume materials have been widely investigated. Perovskite-type ferroelectric materials have been recognized as the most promising candidates for application because of high dielectric constants, great tenability and low dielectric loss. As a compound, the properties of Ba_xSr_1-xTiO₃ (BST) materials are determined by pure BaTiO₃ and SrTiO₃. Ferroelectricity appears when appropriate ferroelectric BaTiO₃ is added to quantum paraelectric SrTiO₃, which means dopant induces phase transition from paraelectric to ferroelectric. As a result, Ba_xSr_1-xTiO₃ may exhibit two structural phases with the variation of BaTiO₃ concentration. The ferroelectric phase transition is still not observed even near OK in pure SrTiO₃ single crystals because of the large ground state quantum fluctuation of soft mode. However, the phase transition temperature of BaTiO₃ is 400K. Therefore, by varying Sr²⁺ and Ba²⁺ content, Ba_xSr_1-xTiO₃ films may be tailed and fabricated to meet different requirements. The Curie temperature(Tc=280K) of Ba_0.6Sr_0.4TiO₃ films is close to room temperature, which expects a large dielectric constant. However, the thermodynamic properties of epitaxial Ba_xSr_1-xTiO₃ films are greatly different from their bulk and single crystal samples which may be attributed to compositional and microstructural inhomogeneities, higher concentration of defect, internal stresses arising from lattice misfit and the difference of thermal expansion coefficient between substrates and films.Internal strains, including normal strains and shear strains, are intrinsic mechanism to have effects on epitaxial ferroelectric films. The normal strains comprise compressive strains and tensile strains. The compressive strain appeares when substrate lattice parameter is smaller than that of thin films. On the contrary, the tensile strain occurs. The shear strains may be neglected when ferroelectric materials are epitaxially grown on thicker substrates. Epitaxial BaTiO₃ thin films may have five stable low-temperature...