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Abstract
Bismuth ferrite (BiFeO3, BFO) has attracted recent attention due to its multi-functional properties, including multiferroism, resistive switching and photovoltaic effects. In particular, epitaxial BFO has been shown to demonstrate giant polarization, polarization-mediated resistive switching and unique magnetic properties. Until now, the most popular methods to obtain epitaxial BFO films with robust properties have been pulsed laser deposition (PLD) and radio frequency (RF) sputtering. Films made using these methods have been reported to have a high spontaneous polarization of up to 130 µC/cm2 and a switchable diode effect.
Chemical deposition techniques, such as chemical solution deposition (CSD), have attracted recent interest for the preparation of BFO films and owing to them offering a cost-effective and more convenient manufacturing method compared with PLD and RF sputtering, an aspect of particular importance in an industrial context. However, the large scale adoption of CSD-derived BFO thin films for a variety of applications has been stymied by a number of significant limitations and challenges including: (1) the imprecision of the starting chemical composition and the subsequent volatilisation of Bi during the annealing step leading to the formation of secondary phases and/or highly conductive films with very poor leakage resistance; (2) variable sintering and densification behaviour leading to films having porosity and poor microstructures; and, (3) limited epitaxy between the film and substrate. Collectively, these dramatically impair the structural and electromechanical properties of the BFO films rendering them unsuitable for practical application. Thus, there is the important need to optimize the CSD preparation process for obtaining pure-phase epitaxial BFO.
In this thesis, a non-aqueous CSD route was developed and studied with the aim to optimise it for the preparation of epitaxial (001) BFO thin films with robust (square) polarization hysteresis loops, high dielectric constant, strong piezoelectric response and distinct diode behavior.
Molecular changes in the organic precursors on heating (determined by NMR and FTIR) and the effects of gelation temperature–time and thickness on film morphology were studied to develop an optimal deposition–gelation process for the synthesis of homogenous, defect-free gel films suitable for subsequent crystallization. The key to obtaining a homogenous gel was control of the delicate balance between gelation and salt (metal nitrate) precipitation through solvent evaporation. The optimized synthesis route consists of spin-coating 0.25 M precursor on 70°C preheated substrate at 3000 rpm for 30 seconds then gelating at 90°C then drying at 270°C.
The crystallization of optimized gel films was studied as a function of Bi/Fe concentration and stoichiometry in the precursor solution, film thickness and single versus multiple depositions, crystallization temperature and atmosphere. Oxygen atmosphere was found to be essential for suppression of Bi volatilization and promotion of film epitaxial orientation. Pure-phase, epitaxial BFO thin film on (001)-strontium titanate (STO) substrate was obtained by rapidly heating the thin film to 650°C in an oxygen atmosphere and holding at the temperature for 30 minutes. A multi-layer deposition process for fabrication of films of various thicknesses was optimised by study of the deposition-heating sequence.
The ferroelectric properties of pure-phase, epitaxial BFO thin films on lanthanum strontium manganite buffered (001)-STO substrates were studied as a function of thickness (40, 70, and 150 nm). The 70 and 150 nm films exhibited exhibited square hysteresis loops at room temperature with high remanent polarization (2Pr up to 100 μC/cm2), low coercive field (2Ec down to 193 kV/cm), and high relative dielectric constant (up to 613). High-cycle fatigue tests showed that these films are resistant to polarization fatigue (up to 108 cycles). All thicknesses showed resistive switching behaviour and a polarization-mediated diode effect both of which became more pronounced with decreasing thickness. The CSD technique developed in this work yielded high-quality BFO thin films and offers a viable low-cost alternative to current BFO deposition techniques.