Hydrothermal Deposition of 〈001〉 Oriented Epitaxial Pb(Zr,Ti)O3 Films under Varying Hydrodynamic Conditions

Abstract

The influence of the hydrodynamic effects of fluid flow on the hydrothermal deposition of 〈001〉 oriented epitaxial Pb(Zr0.7Ti0.3)O3 (PZT) thin films onto single-crystal SrTiO3 (100) substrates was investigated. The films were grown at 150 °C from lead acetate, zirconyl chloride, and titanium dioxide precursors and potassium hydroxide mineralizer under batch quiescent conditions and at varying stirring rates in the range of 200−1700 rpm. To ensure that the hydrodynamic effects of the stirring rate were not unique to a specific set of thermodynamic variables, two sets of reaction conditions were chosen corresponding to a precursor concentration of 0.3 m (Zr + Ti) and mineralizer concentrations of 6 and 10 m KOH. Thermodynamic computations predicted that the perovskite phase was stable at >99% yield under both sets of conditions. Characterization of the films by field-emission scanning electron microscopy, Rutherford backscattering, and X-ray diffraction showed that fluid flow derived from stirring significantly altered the microstructure, compositional homogeneity, phase purity, and crystal quality of the films. It was found that films deposited at higher stirring rates were thinner but also exhibited a higher degree of stoichiometry, phase purity, and epitaxy with the substrate. The study of the PZT film and particles growing simultaneously in the hydrothermal reactor provided useful insight into the crystallization mechanism. The mechanistic implications of these observations for the nucleation and growth of epitaxial films under hydrothermal conditions at low temperatures are discussed and related to mass transport effects, such as particle motion and chemical diffusion. A new crystallization mechanism is proposed based on particle encounters with a crystallizing surface and overlap of electrical double layers.