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最近的研究结果表明 ,由L MBE生长的铁电BaTiO3 /SrTiO3 超晶格的二次谐波产生系数(d3 3 可达 1 5 6 .5pm/V)比体材料BaTiO3 高出一个数量级。为了探讨该材料超常的光电性能与微结构的关系 ,我们对其界面结构进行了详细研究。样品的名义结构为 :BaTiO3 (n) /{SrTiO3 (n) /BaTiO3 (n) }m/SrTiO3 ( 0 0 1 ) ,其中n为晶胞数 ,m为周期数。由于样品相当薄 (单层厚度仅为 2～ 1 6nm) ,而且界面失配大 ,使应变和晶格常数的测定比较困难 ,为此我们采用了X射线掠入射反射率和漫散射等多种实验方法。X射线掠入射反射率和漫散射的理论模拟结果表明 ,样品的表面、界面粗糙度为σ =0 .1nm至 0 .2nm ,Ispec/Idiffuse≈ 1 0 3 ,Bragg衍射除了多级卫星峰外还出现了干涉条纹 ,表明超晶格样品具有原子级平滑的表面和界面。在分析 ( 0 0 2 )、( 30 3)、( 0 33)和 ( 1 0 3)对称和非对称衍射及相应的倒易空间图的基础上 ,确定了样品的水平和垂直晶格常数。第一次采用掠入射衍射(GID)技术 ,直接测定了BaTiO3 /SrTiO3 超晶格的界面应变的水平弛豫。结构分析结果显示 :随周期厚度的变化 ,超晶格水平应变量与二次谐波产生系数的变化具有对应关系。即随周期厚度减小 ,界面水平弛豫减小 ,晶格内应力增大 ,从而使得自发极化增大 ,并?
Recent research shows that the second harmonic generation coefficient (d3 3 up to 165.6 Vpm / V) of the ferroelectric BaTiO3 / SrTiO3 superlattice grown by L MBE is one order of magnitude higher than that of the bulk material BaTiO3. In order to investigate the relationship between the extraordinary optical properties of the material and the microstructure, we conducted a detailed study of the interface structure. The nominal structure of the sample is: BaTiO3 (n) / {SrTiO3 (n) / BaTiO3 (n)} m / SrTiO3 (001) where n is the cell number and m is the number of cycles. As the sample is quite thin (single layer thickness of only 2 ~ 16nm), and the interface mismatch, the determination of strain and lattice constants more difficult, for which we use the X-ray grazing incidence of reflectivity and diffuse scattering and other experimental method. X-ray grazing incidence reflectivity and diffuse scattering theoretical simulation results show that the sample surface, the interface roughness σ = 0 .1nm to 0.2nm, Ispec / Idiffuse ≈ 103, Bragg diffraction in addition to multi-level satellite peak Interference fringes appear, indicating that the superlattice samples have atomically smooth surfaces and interfaces. Based on the analysis of (0 0 2), (30 3), (0 33) and (1 0 3) symmetric and asymmetric diffraction and corresponding reciprocal space plots, the horizontal and vertical lattice constants of the samples were determined. For the first time, grazing incidence diffraction (GID) technique was used to directly measure the horizontal relaxation of interfacial strain of BaTiO3 / SrTiO3 superlattice. The results of structural analysis show that the horizontal strain of superlattice has a corresponding relationship with the variation of the second harmonic generation coefficient with the variation of the periodic thickness. That is, as the thickness decreases, the interface relaxation decreases and the stress in the crystal lattice increases, which increases the spontaneous polarization.