铁电薄膜自身热效应下的极化疲劳微缺陷机理研究

Under continuous external switching electric field cycling, the temperature rising resulting from the domain switching heating effect and the Joule heating effect, the interaction between the self-heating effect and the defect activities and the internal stress arising from thermal mismatch can strongly motivate the formations and accumulations of two types of micro defects such as oxygen vacancy and dislocation in ferroelectric thin films. The local depolarization field arising from the accumulations of defects can strongly pin the motions of ferroelectric domain walls and result in polarization fatigue. Because the operational reliability of the ferroelectric thin film memory can be badly damaged by the polarization fatigue, the reveal of the temperature dependent micro-defect mechanisms of the polarization fatigue is of great scientific importance. Firstly, beginning with the self-heating effect and considering the resistive switching effect, thermo-elastico-electrical coupling and size effect, we can analytically simulate the processes of the formation, the migration and the accumulation of oxygen vacancies and the nucleation of dislocations in ferroelectric thin films under electric field cycling, and determine the interaction between the self-heating effect and defect activities. Secondly, using the dislocation models of strain nucleus and generalized defects, we can simulate the interaction between domain switching and defects, and determine the quantitative relation between the accumulations of defects and the decrease of remanent polarization strength. Lastly, using the experimental verification, we can verify and modify the above analytical models and then provide quantitative scientific bases for the controlling of the micro-defect activities, prediction and optimization of the structural life of the ferroelectric thin films.

在持续外加交变电场作用下，铁电薄膜自身畴转热效应和焦耳热效应共同导致的温升，自热效应与微缺陷行为之间的交互作用，以及热失配内应力会强烈促进铁电薄膜结构中氧空位与位错等微缺陷的形成及其在界面的聚集。聚集后的缺陷通过局部退极化场强烈钉扎着铁电畴壁的运动，导致出现极化疲劳现象。因为极化疲劳会对铁电薄膜存储器工作的可靠性造成严重损害，所以揭示它的温度依赖微缺陷机制具有重要的科学意义。首先，本项目从铁电薄膜结构在循环荷载作用下的自身热效应入手，结合阻变效应、热力电耦合及尺寸效应，对结构中氧空位形成、迁移、聚集及位错形核过程进行解析模拟，确定微缺陷行为与自热效应之间的交互作用机理；其次，运用应变核、广义缺陷两类位错模型，模拟畴转与缺陷之间交互干涉作用，确定缺陷聚集与极化强度下降之间的定量关联；最后，通过实验验证手段完善解析模型，为控制铁电薄膜结构中的微缺陷行为，预估和优化结构寿命提供定量科学依据。

因为极化疲劳会对铁电薄膜存储器工作的可靠性造成严重损害，所以揭示它的温度依赖微缺陷机制具有重要的科学意义。基于此，本项目开展了下列研究：（1）提出了一个能描述耦合场中材料局部变形域的解析模型，计算了局部变形域的应变累积和内应力，分析了其对微结构演化的作用机理；（2）提出了能够用于描述畴反转过程的向错四极子模型，建立了向错模型与微缺陷演化和宏观性能的关联；（3）分析了压电耦合、几何尺寸、电畴及界面失配效应对铁电薄膜界面微缺陷成核及增殖过程的作用机理，讨论了上述效应对界面失效行为的影响；（4）研究了热电耦合效应对体、界面断裂作用机理，以及缺陷对智能结构性能的影响规律；（5）研究了几种具备压电/铁电性能晶体的声波传播特性，为发展缺陷探测声波技术奠定了基础。研究发现：畴通过局部应力场能够显著地干涉位错和微裂纹的演化行为，反过来位错的出现亦能影响电畴反转；向错四极子模型能很好地模拟交变电场作用下的电畴快速反转，畴反转过程及形貌对于外加载荷和材料微结构有强烈依赖性；界面的晶格失配程度和畴壁能会显著地导致界面位错增殖，位错密度会随着薄膜厚度下降而降低，所形成的位错主要集中分布在远离畴壁的界面上；热电耦合促进缺陷成核，而缺陷，尤其是裂纹，可显著地降低等效电导率和热导率；适当的材料匹配和几何尺寸能有效地增强界面强度和智能结构的性能可靠性；具备压电/铁电特性的CTGS晶体可应用于缺陷探测声波换能器。以上科学研究结果能够定量地指导铁电薄膜智能结构的设计和应用，提升结构的性能耐久性。