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Contribution of space charges to the polarization of ferroelectric superlattices and its effect on dielectric properties

机译：空间电荷对铁电超晶格极化的贡献及其对介电性能的影响

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摘要

A theoretical model is developed for ferroelectric bilayers and multilayer heterostructures that employs a nonlinear Landau-Devonshire formalism coupled with a detailed analysis of the depolarizing fields arising from the polarization mismatch across interlayer interfaces and the electrical fields of localized space charges at such interfaces. We first present how space charges alter the free-energy curves of ferroelectrics and then proceed with a numerical analysis for heteroepitaxial (001) PbTiO_3-SrTiO_3 (PTO-STO) bilayers and (001) superlattice structures on (001) STO substrates. The switchable (ferroelectric) and nonswitchable (built-in) polarizations and the dielectric properties of PTO-STO bilayers and superlattices are calculated as a function of the planar space-charge density and the volume fraction of the PTO layer. Similar to the temperature dependence of a monolithic ferroelectric, there exists a critical volume fraction PTO below which the bilayer or the superlattice is in the paraelectric state. This critical volume fraction is strongly dependent on the density of trapped charges at the interlayer interfaces. For charge-free (001) PTO-STO heteroepitaxial bilayer and super-lattices, the critical fraction is 0.40 for both constructs but increases to 0.6 and 0.72, for the bilayer and the superlattice, respectively, for a planar space-charge density of 0.05 C/m~2. Furthermore, our results show that close to the vicinity of ferroelectric-paraelectric phase transition, there is a recovery in ferroelectric polarization. The dielectric-response calculations verify that there is sharp ferroelectric phase transformation for charge-free bilayers and superlattices whereas it is progressively smeared out with an increase in the charge density. Furthermore, our analysis shows that the dielectric constant of these multilayers at a given volume fraction of PTO decreases significantly in the presence of space charges.
机译：针对铁电双层和多层异质结构，开发了一种理论模型，该模型采用了非线性Landau-Devonshire形式主义，并详细分析了由跨层界面上的极化失配引起的去极化场以及此类界面处的局部空间电荷的电场。我们首先介绍空间电荷如何改变铁电体的自由能曲线，然后对（001）STO衬底上的异质外延（001）PbTiO_3-SrTiO_3（PTO-STO）双层和（001）超晶格结构进行数值分析。根据平面空间电荷密度和PTO层的体积分数，计算出PTO-STO双层和超晶格的可切换（铁电）和不可切换（内置）极化以及介电特性。类似于单片铁电体的温度依赖性，存在临界体积分数PTO，低于该临界体积分数，双层或超晶格处于顺电状态。该临界体积分数在很大程度上取决于层间界面处俘获电荷的密度。对于无电荷（001）PTO-STO异质外延双层和超晶格，对于两种构造，临界分数均为0.40，而对于双层和超晶格，当平面空间电荷密度为0.05时，临界分数分别增加到0.6和0.72 C / m〜2。此外，我们的结果表明，在铁电-顺电相变附近，铁电极化得以恢复。介电响应计算证明，无电荷的双层和超晶格具有明显的铁电相变，而随着电荷密度的增加，它逐渐被抹去。此外，我们的分析表明，在存在空间电荷的情况下，在给定的PTO体积分数下，这些多层的介电常数会大大降低。

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  • 来源
    《Physical review》 |2010年第9期|p.094115.1-094115.7|共7页
  • 作者

    M. B. Okatan; rnI. B. Misirlioglu; rnS. P. Alpay;

  • 作者单位

    Materials Science and Engineering Program, Department of Chemical, Materials, and Biomolecular Engineering,University of Connecticut, Storrs, Connecticut 06269, USA;

    rnFaculty of Engineering and Natural Sciences, Sabanci University, Tuzla-Orhanh 34956, Istanbul, Turkey;

    rnMaterials Science and Engineering Program, Department of Chemical, Materials, and Biomolecular Engineering,University of Connecticut, Storrs, Connecticut 06269, USA Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA;

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