弯曲铁弹畴壁伸缩的钳制应力调控及存储特性研究

Ferroelectric domain wall memory with the advantages of adjustable design, high density and non-destructive read-out, becomes an important candidate for the non-volatile memory. The conductivity, stability without applied field, and the removability with applied field of the ferroelectric domain wall are the basic performances for their application in memory. In this project, the bent ferroelastic domain wall with high stability, conductivity and good extending-retraction characteristic modulated by the clamping stress is proposed to storage the data of “1” and “0” based on its extending-retraction under an external electric field to connect-disconnect the two electrodes in a memory cell. Theoretically, the model based on the phase field method combined with semiconductor theory will be developed to investigate extend-retract process and the conductivities of the bent ferroelastic domain wall. Experimentally, the bent ferroelastic domain wall will be realized in the lead zirconate titanate epitaxial ferroelectric films prepared by the pulsed laser deposition and its characteristic of extend-retract and the conductivities will be investigated by piezoresponse force microscopy (PFM) along with conductive atomic force microscopy (C-AFM). Then, combined with the theoretical and experimental studies, the relationship between clamping stress and the characteristic of extend-retract and the conductivities will be studied. The corresponding micro process and the mechanism of the effect by the clamping stress for extend-retract will be revealed essentially. This project will shed the light for developing the ferroelectric domain wall memory and improving the memory characteristics.

铁电畴壁存储器具有器件设计灵活、存储密度高、非破坏性读出等优势，是一种有前景的非易失性存储器。导电性良好、无外场时的稳定性好和在外场作用下可移动性好是铁电畴壁可应用于数据存储的基本条件。本申请项目提出通过调制钳制应力使弯曲铁弹畴壁兼备良好的导电性、稳定性和可伸缩移动性，并基于其在外电场下伸缩连通/断开存储单元来存储数据。理论上，基于相场理论和半导体理论，建立弯曲铁弹畴壁的相场分析模型，模拟计算其伸缩和导电性；实验上，利用脉冲激光沉积法制备锆钛酸铅外延铁电薄膜，通过控制铁电薄膜连续性、组分和单晶衬底的种类、取向和斜切角等调控钳制应力，获得弯曲铁弹畴壁，并利用导电原子力和压电力显微镜技术等表征其导电性和在外电场作用下的伸缩特征；最后，综合理论和实验研究钳制应力与弯曲铁弹畴壁伸缩特征及导电性的关联，剖析伸缩过程，揭示弯曲铁弹畴壁伸缩的钳制应力调控机理，为铁电畴壁存储器的开发及其性能优化提供指导。

基于存储单元中弯曲铁弹畴壁的伸缩可望实现高密度、高速、低功耗的数据存储。调制钳制应力使弯曲铁弹畴壁兼备良好的导电性、稳定性和可伸缩移动性是获得优异存储特性的基础。本项目从理论和实验方面研究了钳制应力对弯曲铁弹畴壁宽度、导电性及其在外电场下的伸缩运动特性的影响，并从电场分布、空间电荷、极化取向等方面探索了优化存储特性的方法。理论方面，相场模拟结果表明，在失配压应变作用下，随着钳制应力的增大，铁弹畴的宽度逐渐减小，逐步形成倒立的楔形畴，铁弹畴壁出现轻微的弯曲现象，且随着钳制应力增大畴壁弯曲的程度增大，导电性增强。在面外方向外电场作用下，楔形铁弹畴及弯曲铁弹畴壁发生可逆的伸缩运动，伸缩前后存储单元的开关电导比超过10^2，各伸缩状态能稳定保持，伸缩运动速度随着电场的增加而先减小后增加。实验方面，采用脉冲激光沉积法在SrTiO3单晶衬底上制备了外延SrRuO3(底电极层)和外延Pb(Zr,Ti)O3(PZT)铁电薄膜，通过改变单晶衬底的晶面取向和脉冲激光能量密度，调控了钳制应力。结果表明在一定的压应力下，PZT铁电薄膜中得到分布较稀疏的铁弹畴，铁弹畴呈倒立的楔形，畴壁弯曲，且表现出一定的导电性。该铁弹畴在c-AFM探针扫描电场作用下，在表面可观察到可逆的伸缩运动，且各伸缩状态静置超过6 h不发生变化。存储特性优化方面，面内和面外双向电场作用下，铁弹畴壁变为“头碰头”和“尾碰尾”的结构，导电性大幅提高，有利于开关比的提高，但其稳定性受限；相比于加载均匀电场，加载探针电场可减少伸缩时间，即提高存储速度；当薄膜中存在合适的空间电荷时，存储速度也可提高约5%左右，其他性能保持良好；弯曲铁弹畴壁两侧a/c畴的极化取向对存储特性几乎没有影响。为理解铁弹畴壁的导电性及稳定性，开展了第一性原理计算，结果表明含氧缺陷铁弹畴壁的畴壁能低、氧缺陷的引入使费米能级右移约2eV，导电性增强。这些结果可为铁电畴壁存储器的开发及其性能优化提供一定指导。