Preparation of Bismuth Ferrite (BiFeO3) Ferroelectric Thin Film by Sol-Gel Processing
Peggy Zhang, Owen Standard and Nagarajan Valanoor
Bismuth ferrite (BiFeO3) belongs to the class of multiferroic materials because it exhibits simultaneously ferroelectric, antiferromagnetic, piezoelectric properties. Furthermore, owing to its high ferroelectric Curie temperature (TC=820-850°C) and antiferromagnetic Neel temperature (TN=350-380°C), the multiferroic properties of bismuth ferrite are relatively stable thus making the material a promising candidate for applications in magnetic and ferroelectric devices. Commonly, such devices are in the form of thin films on substrates with the films being deposited by techniques such as such as chemical vapour deposition, pulsed-laser deposition, sputtering etc. However, such techniques are not amenable for commercial production owing to need for expensive, specialised equipment and the inability to coat large quantities of material. Sol-gel deposition by spin coating is a widely-used chemical deposition method for ceramic films and offers the following advantages over the other methods: low-temperature deposition, high compositional and microstructural uniformity over relatively large areas, high preparation efficiency and low cost. Collectively, these advantages make it an attractive technique for commercial production.
Only limited work on sol-gel deposition of BiFeO3 films has been reported in the literature and most of this has indicated that the complex chemistry and chemical preparation route make it difficult to obtain pure, dense and defect-free thin films (Fig.1). In particular, the presence of porosity in films invariably lead to current leakage ("short-circuiting") between upper and bottom electrodes during electrical measurement and thus renders the films useless for practical electromechanical applications. The overall objective of the present work was to develop a viable sol-gel deposition route for the fabrication of high-quality, leakage-free BiFeO3 films. A non-aqueous sol precursor system based on 2-methoxyethanol and acetic anhydride as the solvent phases with Bi and Fe nitrate as the chelating metal species was used. Extensive studies of the chemistry (using techniques such as Fourier-transform infrared spectroscopy and nuclear magnetic resonance) underlying the initial gelation behaviour of the system were completed. Formation of a continuous Bi-Fe-gel coating was dependent on the competing kinetics of solvent gelation and solvent evaporation. Gelation involving the metal nitrates and the solvent is needed to form a coherent, continuous gel layer but, for the system used in this work, this is driven by the evaporation of the solvent itself. If the solvent evaporation is too slow then this promotes crystallisation of non-stoichiometric phases within the film. If the solvent evaporation is too fast then there is insufficient time for gelation and, instead, a powdery coating is obtained. By optimisation of the heating rate, gelation temperature, and metal nitrate to organic solvent ratio, it was possible to produce a continuous gel coating of relatively high Bi and Fe concentration which, on subsequent heat-treatment, yielded a single-phase, near fully-dense, and continuous thin film of BiFeO3. An example of a film ~45 nm thick, is shown in Fig. 1. The crystallography, nanostructure, and electromechanical properties of the coatings will be examined in the next stage of the project, with the intention to produce coatings for potential commercial applications.
Fig. 1. Sol-gel derived bismuth ferrite thin film on a selected substrate showing: (a) film surface and (b) film cross section.