Construction of Strained SrTiO₃/BaTiO₃ Superlattices and Their Dielectric Properties

Abstract

Artificial dielectric superlattices of SrTiO₃/BaTiO₃(STO/BTO) and CaTiO₃/BaTiO₃ (CTO/BTO) have been formed by a pulsed laser ablation technique with an in situ monitoring of RHEED (reflection high energy electron diffraction) oscillation. The crystal structures can be controlled with atomic order accuracy and a large stress of 400–500 MPa is introduced at the interface between the BTO and STO layers. The superlattices show higher dielectric constant than that of (Sr_0.5Ba_0.5)TiO₃ films against the change of temperature or applied frequency. A large dielectric constant of 900 was observed for the superlattices with a stacking periodicity of 2 unit cells / 2 unit cells. The superlattices show drastically different electrical behavior from that of the solid solution (Sr,Ba)TiO₃ films, both with changing temperature and applied frequency. Broad maxima of the dielectric constants occur around 40–50 °C and the values remain large even for the temperature above 200 °C. On the contrary, in the case of CTO/BTO superlattices, lattice constants and dielectric constant do not change so much compared with STO/BTO cases. Lattice mismatch in the STO/BTO and the CTO/BTO superlattices are 2.5% and 5.5%, respectively. In the case of CTO/BTO, misfit dislocations such as stacking faults may occur at the interface between CTO and BTO layers owing to large lattice mismatch.Therefore, lattice strain is introduced effectively below the lattice mismatch of about 3%.