Finite size effects of dielectric constant in BaTiO₃/SrTiO ₃ superlattice

BaTiO₃/SrTiO₃ superlattices have been deposited onto (La,Sr)CoO₃/MgO (100) substrate using pulsed laser deposition. As the thickness of BTO/STO superlattice was varied from 100 nm to 10 nm, the dielectric constant decreased, which is know to be the finite size effects. The finite size effect was investigated with structural characteristics such as lattice distortion and volume change. These structural analyses were accomplished by synchrotron X-ray scattering. The anisotropic lattice distortion of the superlattice was significantly influenced by the variation in the thickness. Moreover, the superlattice exhibited the volume change of BTO and STO layers while maintaining the volume of a supercell unchanged, suggesting that the hydrostatic strain (i.e., volume change) as well as the anisotropic lattice distortion exist in the consisting BTO and STO layers. Oxygen vacancy defect, possibly causing the hydrostatic strain, did not affect the dielectric constant of the superlattice significantly, excluding the oxygen nonstoichiometry in thin superlattices from the cause of the finite size effect. The finite size effect observed in BTO/STO superlattices is mainly attributed to the lattice strain (i.e., hydrostatic and anisotropic strains) developed in the superlattices.