Cooperative kinetics of defects and domain structures in ferroelectrics

该项目的总体目标是确定在块状或表面存在可移动离子或电子电荷载流子的情况下，铁电体中铁电体中磁畴结构的形成和协同演化的主要特征。具体地说，我们将(A)建立由带电缺陷(氧空位、离子等)动力学驱动的磁畴结构演化的自洽描述；(B)设计铁电体中由电中性弹性缺陷驱动的合作现象的理论；(C)建立非线性表面屏蔽对自发极化反转的影响；在Landau-Ginzburg-Devonshire方法的框架下，利用三维有限元建模和分析计算，包括极化和电场之外的化学应变、压电、电致伸缩和弯曲电耦合。理论预测将通过与观察到的现象和专门设计的宽温度范围内的电物理、极性和介电磁滞性质的测量、X射线和具有本征或引入点缺陷的普通铁电材料(Pb2In)P2(S，Se)6和Sn2P2(S，Se)6的X射线和光谱表征相比较来验证。这两种方法将使我们能够建立体缺陷和表面缺陷以及相关屏蔽现象对铁电薄膜中协作电畴结构动力学和极化反转机制的影响。此外，我们还将分析铁电极化、空间和表面电荷分布的磁滞回线，以及相应的磁畴和电荷态的驰豫动力学。将对描述带电和电中性弹性缺陷的一般模型、自由载流子(电子和空穴以及离子输运)的体屏蔽和通过Bardeen表面态、Fermi-Dirac 2D电子气或Stephenson-Highland吸收离子和空位的非线性表面屏蔽进行解析和数值计算。结果表明，单区和多区铁电区、混合区和非区铁电区将被描绘为缺陷浓度、形成能和电离度、薄膜厚度、外加电压和温度的函数。我们将进一步建立具有2D半导体电极的铁电材料中电畴结构与电荷输运之间的相互作用。这种方法将使我们能够探索各种含体缺陷和表面缺陷的铁电薄膜的一般特征、开关动力学和电畴演化的细节，并为铁电薄膜在下一代场效应管、调制器、存储元件、电传感器和压阻元件中的应用提供机会。

The general objective of the project is to establish the main features of the formation and cooperative evolution of domain structures in ferroelectrics in the presence of mobile – ionic or electronic – charge carriers in bulk or at the surface. Specifically, we will (a) develop a self-consistent description of domain structure evolution driven by the kinetics of charged defects (oxygen vacancies, ions, etc); (b) devise a theory of cooperative phenomena driven by electro-neutral elastic defects in ferroelectrics; (c) establish the impact of nonlinear surface screening on the spontaneous polarization reversal; and (d) elaborate a self-consistent description of domain structure kinetics in ferroelectric structures with 2D-semiconducting electrodes.The proposed methodology will use 3D finite element modeling and analytical calculations in the framework of the Landau-Ginzburg-Devonshire approach including, beyond polarization and electric field, chemical strain, piezoelectric, electrostrictive and flexoelectric coupling. Theoretical predictions will be verified by comparison with observed phenomena and specially designed measurements of electro-physical, polar and dielectric hysteresis properties in a wide temperature range, X-ray and spectroscopic characterization of generic ferroelectrics (Pb,Zr)TiO3, BaTiO3, BiFeO3, (Cu,In)P2(S,Se)6 and Sn2P2(S,Se)6 with intrinsic or introduced point defects.The combined methodologies will allow us to establish the influence of bulk and surface defects and related screening phenomena on the cooperative domain structure kinetics and polarization reversal mechanisms in ferroelectric thin films. Moreover, we will analyze the hysteresis loops of ferroelectric polarization, space and surface charge distributions, as well as relaxation dynamics of corresponding domain and charge states. Analytical and numerical calculations will be performed for generic models describing charged and electro-neutral elastic defects, bulk screening by free carriers (electrons and holes, as well as ionic transport) and nonlinear surface screening by e.g. Bardeen surface states, Fermi-Dirac 2D-electron gas or Stephenson-Highland absorbed ions and vacancies. As a result the regions of single- and poly-domain ferroelectric, mixed and non-ferroelectric states will be delineated as a function of defect concentration, formation energy and ionization degree, film thickness, applied voltage and temperature. We further will establish the interaction between domain structure and the charge transport in ferroelectrics with 2D-semiconducting electrodes. The proposed approach will allow us exploring the general features, specifics of switching kinetics and domain evolution in various ferroelectric films with bulk and surface defects, and open opportunities to use ferroelectric films in next generation field effect transistors, modulators, memory elements, electrical transducers and piezo-resistive elements.