Crystallographic orientation of epitaxial BaTiO3 films: The role of thermal-expansion mismatch with the substrate

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Abstract
Expitaxial ferroelectric BaTiO3 thin films have been grown on (001) MgO and MgO‐buffered (001) GaAs substrates by pulsed laser deposition to explore the effect of substrate lattice parameter. X‐ray‐diffraction studies showed that the BaTiO3 films on both MgO single‐crystal substrates and MgO‐buffered (001) GaAs substrates have a cube‐on‐cube epitaxy; however, for the BaTiO3 films grown on MgO the spacing of the planes parallel to the substrate was close to the c‐axis dimension of the unconstrained tetragonal phase, whereas the BaTiO3 films on MgO/GaAs exhibited a spacing closer to the a‐axis dimension of the unconstrained tetragonal phase. The cube‐on‐cube epitaxy was maintained through the heterostructures even when thin epitaxial intermediate buffer layers of SrTiO3 and La0.5Sr0.5CoO3 were used. The intermediate layers had no effect on the position of the BaTiO3 peak in θ‐2θ scans. Together, these observations indicate that, for the materials combinations studied, it is the thermal‐expansion mismatch between the film and the underlying substrate that determines the crystallographic orientation of the BaTiO3 film. Preliminary measurements indicate that the BaTiO3 films are ‘‘weakly’’ ferroelectric.