Variations of microstructures and electrical properties of Bi4Ti3O12/SrTiO3/(La0.5, Sr0.5)CoO3/MgO epitaxial thin films by annealing

Epitaxial thin films of a heterostructure with Bi4Ti3O12(BIT)/SrTiO3(ST) were successfully grown with a bottom electrode consisting of La0.5Sr0.5CoO3(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.