In-plane Ferroelectricity in Barium Titanate Films
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Ferroelectric materials are irreplaceable materials in electronics used everywhere in the modern society. The ferroelectric perovskites are the most widely used class of ferroelectric materials, and the use is dominated by PbZr1−xTixO3 (PZT). The use of lead in is an environmental concern, and this has led to an intensive search for lead-free alternatives, including research for enhancement of the properties of well-known lead-free alternatives such as BaTiO3 (BTO). Strain-engineering is one possible route to enhanced material properties, and in this work the in-plane ferroelectric and dielectric properties of BTO films under tensile strain were explored, with emphasis on the effect of crystallographic orientation. BTO thin films were prepared on single-crystal SrTiO3 (STO) substrates using chemical solution deposition (CSD). This is an attractive fabrication route to thin films due to low costs and high scalability. An aqueous CSD route to BTO films was developed, demonstrating that organic solvents are not required to synthesize high quality BTO films by CSD. Relaxed epitaxial BTO films were prepared on (100), (110), and (111) oriented STO substrates. In-plane tensile strain caused by a mismatch in the thermal expansion between the BTO film and the STO substrate was confirmed by X-ray diffraction for all the three crystallographic orientations. Interdigitated electrodes (IDEs) were deposited on the surface of all films and used to investigate the in-plane ferroelectric and dielectric properties. Using IDEs to characterize the film required a model to distinguish the capacitive contribution of the film from the contribution of the substrate. In this thesis, a new model was developed centered on preserving correct boundary conditions at all relevant interfaces. The model was demonstrated by successfully calculating the in-plane dielectric constant and polarization of the BTO films. The impact of crystallographic orientation on the in-plane electrical properties of the (100), (110), and (111) oriented BTO films was investigated using the new model for IDEs. The properties were found to depend strongly on the orientation of the film and on the in-plane direction probed.Strong in-plane anisotropy was found for the (110) oriented BTO film, corresponding to the low in-plane rotational symmetry of the (110) STO substrate, while low in-plane anisotropy was recorded for the (111) orientation. The dominant ferroelectric domain pattern was determined for each substrate orientation by piezoresponce force microscopy (PFM), and the domain patterns were consistent with the anisotropy in the remnant polarization observed by the IDEs. Further investigation of a (100) oriented film was performed by in situ piezoresponse force microscopy combined with the measurement of capacitance-voltage curves. It was found that domain switching was always associated with a 90° rotation of the domain walls. It was also found that the film preferentially switched in bands spanning between the electrodes, where multiple superdomains switch together.