Large tensile-strained BaTiO3 films grown on a lattice-mismatched La-doped BaSnO3 bottom electrode

Abstract

Perovskite BaTiO₃ has been widely studied and utilized in various applications owing to its high permittivity, ferroelectricity, and stability. However, its low ferroelectric-paraelectric phase transition temperature (T_C, 120 °C) limits its application. The T_C can be increased by applying an epitaxial strain provided by a lattice-mismatched substrate. However, applying large tensile strain on BaTiO₃ is difficult, especially when a bottom electrode is present. In this study, we successfully fabricated large tensile-strained BaTiO₃ films using La-doped BaSnO₃ bottom electrodes. A tensile strain of 2% was achieved, which is three times larger than that previously reported for BaTiO₃ films grown on bottom electrodes. By adjusting the thickness of the BaTiO₃ layer between 20 and 300 nm, tensile strain can be varied within the range of 0.6–2%. Remarkably, the T_C of the obtained films exceeds 400 °C. In addition, although it was considered that tensile-strained BaTiO₃ films have ferroelectric polarization in the in-plane direction, the 0.6% tensile-strained film showed ferroelectric polarization in the out-of-plane direction. This finding reveals that the ferroelectric polarization direction was slightly tilted away from the predominant in-plane direction of the film.