Synthesis and electrical properties of thin film transition metal oxides

Abstract

BaTiO3 thin films were deposited onto polycrystalline Pt using a dip-coating technique, with calcination temperatures of 750 to 900 °C. To avoid film imperfections such as cracking or regions of zero BaTiO3 coverage (pinholes), key conditions, including aging periods, water content, and stirring speeds, were refined to produce pinhole free, uniform films with some porosity. Whilst those coated a single time short circuited during electrical characterisation, this could be avoided if films were produced by multiple coating cycles. The relative permittivity of a 600 nm BaTiO3 film was measured at 290 by fitting solid state impedance data in the frequency range 100 Hz to 1 MHz. Electrochemical impedance with an aqueous electrolyte allowed evaluation of the porosity, which remained fairly constant between one and five coating cycles. Using this method, it was possible to estimate the effective permittivity of the BaTiO3 itself as 374 and hence to evaluate the increase in the relative permittivity that could be achieved by minimising porosity.Strontium and zirconium doping of thin film BaTiO3 was performed on the refined sol-gel synthesis of BaTiO3 by replacing Ba(OAc)2 and Ti(OiPr)4 with Sr(OAc)2 and Zr(OiPr)4, respectively. Producing phase pure (Ba1-xSrx)TiO3, Ba(Ti1-yZry)O3 and (Ba1-xSrx)(Ti1-yZry)O3 where x ≤ 0.30 and y ≤ 0.15. The inclusion of each dopant affected the sols aging process, where strontium decreased the aging time, most likely due to the inclusion of extra absorbed water. Zirconium, on the other hand, increased the aging duration, this was because the main component in the gelation process, Ti(OiPr)4, was being replaced. The change in relative permittivity was measured as the concentration of dopant species was increased. The addition of either species individually, or together decreased the relative permittivity, this was because isovalent dopants of barium titanateiiiwith a smaller cation size (Sr), or larger cations (Zr) retards the vibration of the ions along <111>, and the <100>, respectively. This affects the Curie temperatures phase transition and the crystals structures ability to polarise.1A stable, alkoxide-based electrolyte for the electrochemical deposition of TiO2 thin films was developed. Films with controllable thicknesses between 60 nm and 2.4 μm were electrochemically deposited onto polycrystalline Pt thin film substrates. Films of around 80 nm thickness were smooth, crack-free and well adhered. Annealing at temperatures between 300 and 1000 °C resulted in anatase or rutile-structured TiO2, with crystallite sizes increasing with temperature from 5 to 50 nm.Barium titanate was electrodeposited under the same conditions as the TiO2 by incorporating BaCl2 into the sol, and by substituting methoxy ethanol, potassium nitrate and nitric acid with methanol, potassium chloride and hydrochloric acid with the resulting film annealed at 750 °C. This allowed for the co-deposition of titanium and barium by gelation and precipitation mechanism, respectively. The crystallite size was estimated as 8.8 +/- 3.6 nm, and the BaTiO3 had a lattice parameter a=b=c= 4.019 Å which matched the literature value of 4.014 Å,2 with an overall film thickness was 340 nm.

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ORCID for Andrew L. Hector: orcid.org/0000-0002-9964-2163

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