Thermodynamic modeling of ferroelectric epitaxial films and polarization graded ferroelectrics

Date of Completion

January 2003

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Thermodynamic modeling based on the Landau-Ginzburg-Devonshire theory has been performed for epitaxial ferroelectric thin films and polarization graded ferroelectrics. The aim of this thesis is to provide a fundamental and systemic understanding of the role of internal stresses in the physical properties of epitaxial ferroelectric thin films as well as an explanation for the unconventional behavior of polarization graded ferroelectrics, thereby establishing a framework for materials/device design to enhance their properties. ^ The thermodynamic analysis of single domain epitaxial barium strontium titanate (BST) thin films leads to the construction of misfit strain/electric field phase diagrams. The dielectric response, tunability and pyroelectric properties of the films are calculated as a function of the misfit strain. Analysis shows that high dielectric response, tunability, and pyroelectric coefficient can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. Theoretical approach not only predicts general trends, but is also in good quantitative agreement with the experimental data reported in the literature. The stress relaxation by the formation of misfit dislocations is taken into account. It is suggested 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 electrical properties. ^ A generalized Landau-Ginzburg-Devonshire model is constructed and used to develop a methodology for analyzing polarization graded ferroelectrics. A systematic variation in the polarization arises from composition, temperature, or stress gradients. These spatial non-uniformities are shown to give rise to local order parameters having corresponding spatial variation. Polarization graded ferroelectrics exhibit a displacement of the hysteresis curve along the polarization axis with “up” or “down” charge offsets due to the polarization gradient. A quantitative agreement between the theoretical calculations and the experimental data reported in the literature is found. Theoretical analysis of the dielectric and pyroelectric properties of polarization graded ferroelectrics shows that the polarization gradient can be used to adjust these properties and thereby provides a means to achieve enhanced properties. Extension of the thermodynamic approach to graded ferromagnets, ferroelastics, and other ferroic systems is also studied in this thesis. ^

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