Date of Completion

8-2-2018

Embargo Period

8-3-2018

Keywords

Ferroelectric, Dielectric, Nanocomposite

Major Advisor

Dr. S. Pamir Alpay

Associate Advisor

Dr. Serge M. Nakhmanson

Associate Advisor

Dr. Mark Aindow

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Nanocomposites consisting of a ferroelectric oxide embedded within a non-ferroelectric matrix are a unique class of materials with potential to improve performance in next-generation electronics and non-volatile memory. Dielectric losses and breakdown due to processing and intrinsic properties of a monolithic ferroelectric point to low figures of merit. Therefore, it is necessary to develop composite materials with an active ferroelectric that is physically confined within a low-loss dielectric matrix while maintaining significant dielectric and pyroelectric properties inherent to ferroelectrics. The purpose of this work is to explore the behavior of ferroelectric nanocomposites as a function of dimensionality, and composition. The theoretical studies concentrate on a thermodynamic model while experimental work considers two different nanocomposite configurations. Thermodynamic calculations based on a Landau-Devonshire theory of phase transformation for PbZr0.2Ti0.8O3films with various interposed dielectrics reveal a dielectric constant-dependent critical thickness where subcritical thicknesses show enhanced dielectric and pyroelectric coefficients due to Curie temperature suppression. These calculations guide experiments where PbZr0.4Ti 0.6O3films with a thin HfO2dielectric layer were grown using chemical solution deposition and atomic layer deposition, respectively. Experiments reveal that the polarization, coercive field and dielectric constant vary within the sample set following a model that describes capacitors-in-series, in contradiction to the theoretical work which assumes large electrostatic interactions at the interface. Furthermore, these films show over an order of magnitude difference in resistivity, with retention time greater than 1000 seconds making them attractive for memristor applications. Finally, thin films were fabricated by the electrospray evaporation of BaTiO3particles followed by the atomic layer deposition of HfO2which permeated and adhered to the porous particle film. Dielectric properties were found to be tunable between pure BaTiO3and the composite capacitance. This work spans from theory to fabrication to understand how ferroelectric and dielectric materials can synergistically be combined to create a tunable, functional composite material.

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