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

利用脉冲激光烧蚀技术制备了SrTiO_3/BaTiO_3 （STO/BTO）和CaTiO_3/BaTiO_3 （CTO/BTO）人工介电超晶格，并对反射高能电子衍射（RHEED）振荡进行了原位监测。晶体结构可以用原子有序精度控制，并在BTO层和STO层之间的界面处引入400-500 MPa的大应力。随着温度和施加频率的变化，超晶格的介电常数高于（Sr_<0.5>Ba_<0.5>） TiO_3薄膜。超晶格的介电常数为900，其堆叠周期为2个单元格/2个单元格。随着温度和施加频率的变化，超晶格的电学行为与固溶体（Sr, Ba） TiO_3薄膜的电学行为有很大的不同。介电常数的最大值大约在40-50？糖化血红蛋白，即使温度超过200？糖化血红蛋白。相反，在CTO/BTO超晶格中，晶格常数和介电常数与STO/BTO相比变化不大。STO/BTO和CTO/BTO超晶格的晶格失配率分别为2.5%和5.5%。在CTO/BTO的情况下，由于大的晶格错配，CTO和BTO层之间的界面可能会发生错配位错，如层错。因此，在晶格失配约3%以下，有效地引入了晶格应变。

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 %.