Construction of Ferroelectic Oxide Superlattices by a Laser Ablation Technique and Their Properties

用脉冲激光烧蚀技术原位制备了SrTiO_3/BaTiO_3（STO/BTO）和CaTiO_3/BaTiO_3（CTO/BTO）人工介电超晶格。晶体结构可以用原子级精度控制，并且在BTO和STO层之间的界面处引入400-500 MPa的大应力。随着温度或外加频率的变化，超晶格的介电常数比（Sr_<0.5>Ba_<0.5>）TiO_3薄膜的介电常数大。对于堆叠周期为2个晶胞/2个晶胞的超晶格，观察到900的大介电常数。超晶格薄膜的电学行为与固溶体（Sr，Ba）TiO_3薄膜的电学行为在温度和频率变化时有很大的不同。宽的介电常数的最大值发生在40-50？A1 C和值仍然很大，即使在200？相反，在CTO/BTO超晶格的情况下，晶格常数和介电常数与STO/BTO情况相比变化不大。STO/BTO和CTO/BTO超晶格的晶格失配分别为2.5%和5.5%。在CTO/BTO的情况下，由于大的晶格失配，在CTO和BTO层之间的界面处可能发生失配位错，例如堆垛层错。因此，在低于约3%的晶格失配的情况下有效地引入晶格应变。

Artificial dielectric superlattices of SrTiO_3/BaTiO_3 (STO/BTO) and CaTiO_3/BaTiO_3 (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 layrs. The superlattices show higher dielectric constant than that of (Sr_<0.5>Ba_<0.5>) TiO_3 films against the change of temperature or applied freqency. 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) TiO_3 films, both with changing temperature and applied frequency. Broad maxima of the dielectric constants occur around 40-50? A1C and the valucs remain large even for the temperature above 200? A1C.On the contrary, in the case fo CTO/BTO superlattices, lattice constans 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 layrs owing to large lattice mismatch. Therefore, lattice strain is introduced effectively below the lattice mismatch of about 3 %.