Synthesis and Properties of Barium Titanate Solid Solution Thin Films on Copper Substrates

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Title: Synthesis and Properties of Barium Titanate Solid Solution Thin Films on Copper Substrates
Author: Ihlefeld, Jon F.
Advisors: Jon-Paul Maria, Committee Chair
Angus Kingon, Committee Member
Robert Nemanich, Committee Member
Zlatko Sitar, Committee Member
William Borland, Committee Member
Abstract: Barium titanate thin films were deposited via chemical solution deposition using a hybrid-chelate chemistry directly on copper foil substrates. A process was developed to crystallize and densify the ferroelectric films at 900C by using a reductive atmosphere containing nitrogen, hydrogen, water vapor, and oxygen impurities such that film constituents were oxidized to form barium titanate and the foil substrate remained metallic. The crystallized films are polycrystalline with equiaxed morphology and average grain diameters in excess of 100 nm. The dielectric properties exhibit permittivities in excess of 1800 at room temperature and zero bias with tunabilites of greater than 90% and high field loss tangents of less than 1%. A series of samples was prepared with varying grain and crystallite sizes by dividing and processing a single film over a range of temperature from 700 to 900C. This ensures that the chemical composition and film thickness is invariant for each sample. It is shown that the grain size increases with higher process temperatures and results in a concomitant increase in permittivity and tunability. These enhancements, combined with the constant paraelectric⁄ferroelectric phase transition temperature, indicated that a combination of film crystallinity and grain size is responsible for diminished performance. The phase transition temperature and temperature coefficient of capacitance modified by partially substituting zirconium, hafnium, and tin for titanium. The resulting films were single phase and the phase transition shifts were consistent with bulk materials. A reduction in permittivity was observed for increasing substituent level and was attributed to a reduction in grain size for both barium titanate zirconate and barium titanate hafnate. Processing conditions were chosen to stabilize Sn2+ during the firing process in an attempt to flux the system and increase grain size. The barium titanate stannate films had less reduction in grain size per substituent level than either zirconium or hafnium, however a similar reduction in permittivity was observed. The diminished dielectric response was explained by a defect reaction involving divalent tin and oxygen vacancies that quenched the extrinsic domain response to the dielectric constant. Defect equilibria were investigated with respect to processing atmosphere, stoichiometry, and dopant concentration. The solubility of excess barium and titanium was found to be greater in the films than is expected in the bulk, however it is unclear that equilibrium is achieved in the process. It was demonstrated that dopants could successfully eliminate the necessity of a reoxidation anneal to compensate for oxygen point defects resulting from the low pO2 atmospheres. The dopant levels necessary and insulation resistance of pure BaTiO3 were greater than expected from thermodynamic calculations. It was suggested that this is the result of a reduction in the enthalpy of reduction, stemming from an increase in grain boundary volume. Barium borate fluxes were used to improve densification and crystallinity. Barium borate additions between 0 and 3% uniformly increased grain size and density, while levels greater than 3% resulted in anomalous grain growth. Films with exaggerated grains show tetragonal peak splitting in the X-ray diffraction patterns, consistent with bulk barium titanate. In materials without exaggerated grain growth, dielectric measurements revealed permittivities in excess of 3000 at room temperature (for average grain sizes of approximately 160 nm). This value is equivalent to the finest-prepared bulk ceramics and substantially greater than any polycrystalline film ever reported. This has been attributed to in improvement in film crystallinity. These two accomplishments — tetragonal crystal symmetry and permittivities in excess of 3000 — represent dramatic breakthroughs in ferroelectric thin film technology.
Date: 2006-09-26
Degree: PhD
Discipline: Materials Science and Engineering

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