In situ Characterization of Ferroelectric Oxide Thin Films during Aqueous Chemical Solution Deposition
Abstract
Ferroelectric materials are used in a wide range of applications, including capacitors and non-linear optical devices. Utilization of ferroelectric thin films is especially desirable due to the possibility to tailor the ferroelectric properties by controlling the degree of crystallographic texture in the films. Chemical solution deposition (CSD) of ferroelectric oxide thin films is a simple and environmentally friendly processing route. However, the decomposition and crystallization processes are complex and not well understood, making it challenging and time consuming to tailor the film properties and optimize processing conditions.
In this work, novel in situ characterization techniques were used to study the decomposition and crystallization of ferroelectric oxides thin films based on BaTiO3 and K0.5Na0.5NbO3 (KNN) during aqueous CSD. In situ synchrotron X-ray diffraction (XRD) was used to determine mechanisms for crystallization and texture development, while a diffuse reflection (DRIFTS) cell for in situ infrared (IR) spectroscopy was adapted for studying the decomposition reactions during thermal annealing.
For the BaTiO3-based films, the pyrolysis and removal of organic compounds in the precursor film occurred in the temperature range 400-500 °C, leaving only amorphous BaCO3-like and Ti-rich phases in the films. Upon further annealing, a metastable oxycarbonate phase with the proposed stoichiometry BaOx(CO3)1-x was formed, which has a crystal structure close to the calcite-type BaCO3 polymorph. Further heating lead to homogeneous nucleation of BaTiO3 above 530 °C by reaction of the oxycarbonate and Ti-rich phases. The nucleation temperature of BaTiO3 was influenced by the heating rate and type of substrate, while the microstructure depended solely on the heating rate during the nucleation stage. Films with modified chemical compositions were also investigated, and zirconium substitution was observed to increase the nucleation temperature and reduce the decomposition and texture formation kinetics, while the effect of calcium substitution was neglectable.
Mechanisms for development of crystallographic texture in the BaTiO3-based films were proposed. In a single layer deposited by CSD, texture was found to depend on the intermediate phases present in the film as a result of decomposition and pyrolysis of the precursors. Prolonged annealing below the nucleation threshold (500 °C) gave a crystalline film with high degree of texture, enhanced by a low heating rate during BaTiO3 nucleation. Texture could also be induced in films prepared by multiple depositions by CSD due to Oswald ripening of favourably oriented grains during the repeated annealing, independent of the substrate. In general, the film texture adhered to the substrate orientation. In case of high annealing temperatures (>1000 °C) and high heating rates (> 1 °C/s), the formation of epitaxy occurred in the BaTiO3 thin films. The observed lattice parameters showed that the films had no-uniform strain, which arise both from lattice and thermal mismatch, especially for multilayer films. A possible formation mechanism for epitaxy and cube-on-cube growth was suggested, and electrical characterization confirmed that multilayer cube-on-cube grown films were ferroelectric.
In the KNN films, the choice of niobium precursor affected the nucleation temperature, lattice parameter and secondary phase formation. KNN films based on a malic acid complexed niobium precursor nucleated at higher temperature and without formation of the secondary K4Nb6O17 phase, which was seen for the oxalate complexed niobium KNN films. However, all single layer KNN films were observed to be phase pure after annealing at 700-800 °C. No texture was seen in the single layer films, but texture formation by repeated annealing was suggested by the same mechanisms as for the BaTiO3-based films.
Has parts
Paper 1: K. Bakken*, A. B. Blichfeld, D. Chernyshov, T. Grande, J. Glaum, M.-A. Einarsrud, "Mechanisms for texturing in BaTiO3 thin films from aqueous chemical solution deposition", Submitted to Journal of Sol-Gel Science and Technology (2020). This paper is awaiting publication and is not included.Paper 2: A. B. Blichfeld, K. Bakken*, D. Chernyshov, J. Glaum, T. Grande and M.-A. Einarsrud, "Experimental setup for high-temperature in situ studies of crystallization of thin films with atmosphere control". Drafted manuscript.This paper is awaiting publication and is not included.