Variations of microstructures and electrical properties of Bi ₄Ti₃O₁₂/SrTiO₃/(La_0.5, Sr_0.5)CoO₃/MgO epitaxial thin films by annealing

Epitaxial thin films of a heterostructure with Bi₄Ti ₃O₁₂(BIT)/SrTiO₃(ST) were successfully grown with a bottom electrode consisting of La_0.5Sr_0.5CoO ₃(LSCO) on MgO(001) substrates using pulsed laser deposition. The grown BIT and ST (001) planes were parallel to the growth surface with the orientation relationship of BIT <110>//ST <010>. In the as-deposited film, the BIT (001) plane appeared to expand to relieve a lattice mismatch with the ST (001) plane. However, annealing for 20-40 min induced the BIT (001) plane to contract horizontally with its c-axis expanding, which was associated with a local perturbation in the layer stacking of the BIT structure. This structural distortion was reduced in the film annealed for 1 h, with restoration of the periodicity of the layer stacking. Correspondingly, the dielectric constant of the as-deposited film was increased from 292 to 411 by annealing for 1 h. In parallel, the film was paraelectric but became more ferroelectric, with the remanent polarization and the coercive field changing from 0.1 μC/cm² and 14 kV/cm to 1.7 μC/cm² and 69 kV/cm, respectively.