Water-soluble CaO sacrificial layer heteroepitaxially grown on yttria-stabilized zirconia substrate for large ferroelectric BaTiO 3 sheets

Ferroelectric perovskite oxide sheets with a high orientation and crystallinity are suitable for various applications. A promising method for their synthesis involves the combination of epitaxial growth with sacrificial layers that can be dissolved in solvents. However, the practical applications of these sacrificial layers are limited. They are typically grown on substrates such as SrTiO 3 single crystals, which have not yet been scaled up for large-area applications. In addition, during the lift-off process, a high density of cracks are often formed in the ferroelectric sheets. To address these issues, we used a CaO sacrificial layer in this study. We deposited a ferroelectric BaTiO 3 epitaxial layer on the CaO sacrificial layer, which was epitaxially grown on a [110]-oriented yttria-stabilized zirconia (YSZ) substrate, together with a protective Al 2 O 3 glass layer. After dissolving the CaO layer in pure water, we successfully obtained a crack-free freestanding BaTiO 3 sheet with a lateral size of 5 mm × 2 mm. The resulting BaTiO 3 sheet exhibited room-temperature ferroelectricity and piezoelectric properties with a large d 33 value of 270 pm V −1 owing to the release of strain from the substrate. In addition, the YSZ substrate could be reused. Our study expands the range of water-soluble sacrificial layers and substrates for obtaining large ferroelectric and piezoelectric oxide epitaxial sheets.