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This thesis investigates the crystallography, domain morphology and electromechanical behaviour of epitaxial PbZrxTi(1-x)O3 (PZT) bilayer heterostructures deposited on silicon. As a first step, PZT bilayers are deposited on SrTiO3 crystalline substrates by pulsed laser deposition, and their high temperature crystallography is investigated by X-ray diffraction (XRD). The profile of rhombohedral and tetragonal PZT structures with temperature shows excellent bulk-like behaviour with clear crystallographic phase transformations and coefficients of thermal expansion that are comparable with literature. Comparisons to theoretical relaxed films reveal how strain within the system is enough to modify the thermal properties of the films including increasing the Curie temperature for extended operation temperatures in practical applications. Next, thin films of tetragonal PbZr0.3Ti0.7O3 (PZT-T) of varying thickness are deposited above a 40 nm rhombohedral PbZr0.54Ti0.46O3 (PZT-R) film, all grown on silicon (100) substrates. XRD is performed on the samples and compared to the crystallographic dataset for bilayers on SrTiO3 substrates. XRD and transmission electron microscopy provide evidence that the PZT-T layer has a modified, pseudo-tetragonal structure due to substrate induced strain but maintains its in-plane a-axis orientation. The structure is observed to pass through a ferroelectric-paraelectric phase transformation with an increased Curie temperature. The domain morphology, characterised by piezoresponse force microscopy, reveals that the unequal lattice parameters of the pseudo-tetragonal structure are insufficiently different to reorient the polarisation. Instead, the film is arranged as ferroelastic a1/a2 tetragonal nanodomains within a larger array of mosaic superdomains. This remains unchanged with reduced film thickness, except for a smaller periodicity of the a1/a2 twins. Interestingly, the domain pattern gives rise to a series of topological defects including vortex/anti-vortex pairs at the surface and multi-phase coexisting core structures within the bulk of the PZT-T film. Finally, the electromechanical properties of the thin films on silicon are investigated, in both an out-of-plane and in-plane configuration. Ferroelectric polarisation switching experiments show a square hysteresis loop of saturation ~32 μC/cm2 and clear capacitance switching peaks. The experiments show that capacitance is doubled when measured in-plane, compared to out-of-plane, since ferroelectric properties are a function of electrode spacing, rather than film thickness, which can be three orders of magnitude larger. Single frequency piezo-hysteresis loops provide evidence of a ~250% improvement of the effective d33 response compared to a standard rhombohedral PZT film on SrTiO3 substrate. This is attributed to the coexisting multi-phases and mobile topological defects which demonstrate the ability to migrate the film surface or annihilate one another in the presence of a local bias. An improvement of this magnitude demonstrates the opportunity to implement bilayer technology while exploiting a functional silicon substrate for enhanced, industry-ready, smart material applications. |
