Misfit Strain-Temperature Phase Diagram of Ferroelectric BaTiO3/SrTiO3 Superlattices
Thin films of ferroelectric barium titanate (BaTiO3) and strontium titanate (SrTiO3) [(BaTiO3)8(SrTiO3)4]x40 superlattice structures are epitaxially grown on novel rare-earth scandate substrates in order to induce a systematically varied strain in the superlattice. Strained thin-film superlattices exhibit phase transition temperatures that differ from the same materials in their bulk form. A thorough understanding of the misfit strain-temperature phase diagram is needed to engineer the material parameters and physical properties of these superlattices. Ultraviolet Raman spectroscopy, transmission electron microscopy, and x-ray diffraction are utilized to develop a misfit strain-temperature phase diagram for BaTiO3/ SrTiO3 superlattices. Results indicate that the superlattices experience strain- and temperature-dependent phase transitions from rhombohedral-like phases to tetragonal or orthorhombic-like phases and then again to paraelectric cubic phases. Once the misfit strain-temperature relationship is understood, phases of the superlattices can be engineered based on the chosen substrate, the film thickness, and number of layers in a superlattice.