铁电薄膜畴超快背翻转动力学飞秒激光探测研究

The ultrafast back-switching kinetics of the domain in the ferroelectric thin film determines the retention properties of the ferroelectric memory, which is closely related to the reliability and lifetime of the device. However, it is very difficult to detect the information of the ultrafast back-switching kinetics by traditional domain detection methods, such as the piezoresponse force microscopy, because the signal capture speed in these methods is too slow. Therefore, it is very necessary to explore an ultrafast detection method with ultrahigh time resolution for the ultrafast kinetics of the ferroelectric domain back-swicthing. Femtosecond laser with ultrashort pulse can not only detect information in ultrahigh time resolution, but also stimulate the terahertz wave source to generate shaped THz electric field for domain writing. In this research project, an improved femtosecond laser detection method will be developed in order to observe the ultrafast back-switching kinetics of the domain in the ferroelectric thin film. Furthermore, we will calculate and simulate the ultrafast back-switching kinetics theoretically from the macro and micro perspective. Then, the effects of the domain writing conditions and the characteristics of ferroelectric thin film on the domain ultrafast back-switching will be also investigated. Based on the above-mentioned theoretical and experimental studies, the time scale, the physical process and main influence factors of domain ultrafast back-switching will be confirmed. These studies will uncover the main mechanisms of domain ultrafast back-switching, offering an extensively guidance for us to deeply understand and overcome the retention failure of the ferroelectric memory.

铁电薄膜畴超快背翻转动力学决定着铁电存储器的保持性能，直接关系到器件的可靠性及寿命。传统的畴变观测方法由于信号捕获速度慢等缺点无法捕捉到这个关键过程的动态信息。因此，无论是从科学研究还是实际应用的角度来讲，都迫切需要发展超高时间分辨率的探测方法来研究电畴超快背翻转动力学过程。飞秒激光提供的超短脉冲可以在实验上实现超高时间分辨率，同时还可以激发太赫兹光源产生太赫兹电场用于写畴。基于此，本申请项目拟发展铁电薄膜畴超快背翻转的飞秒激光探测方法，从而探测畴超快背翻转动力学过程，并从微观和宏观的角度对其背翻转过程进行理论计算与模拟，确定铁电薄膜畴超快背翻转的时间尺度，构建畴背翻转动力学过程的清晰物理图像。通过实验探测和理论模拟，探讨铁电薄膜特性和畴写入条件对畴超快背翻转动力学过程的影响，揭示决定畴寿命的主要因素及作用机理，厘清畴背翻转动力学机制，为深层次理解和解决铁电存储器保持性失效提供指导和依据。

本项目以铁电薄膜为研究对象，紧紧围绕铁电畴及其超快背翻转动力学过程进行了一系列研究。首先，建立了超高时间分辨率的铁电薄膜畴超快背翻转动力学探测系统。利用太赫兹脉冲电场的低光子能量、高峰值电场强度和亚皮秒脉宽等优点，并基于光学二次谐波与铁电材料畴结构强关联的特性，利用太赫兹脉冲电场写畴，并通过采集相对时间延迟的飞秒激光探测束经铁电薄膜表面反射产生的光学二次谐波，实现了铁电薄膜畴超快背翻转动力学过程的实时观测。其次，研究了铁电薄膜特性和电畴写入条件对畴超快背翻转动力学过程的影响。采集了不同电畴结构的外延铁电薄膜样品在飞秒激光作用下反射产生的光学二次谐波信号，研究了铁电薄膜的晶格失配应变和应变梯度等对光学二次谐波的影响，建立了铁电薄膜畴结构与光学二次谐波的对应关系。系统研究了太赫兹激励脉冲的电场大小、延迟时间和样品旋转方位角、铁电薄膜畴结构等参数对畴超快背翻转动力学过程的影响，通过总结提炼主要影响参数和影响规律，探讨了上述参数对畴超快背翻转动力学过程所起的作用。最后，分别基于软声子模型、相场方法和分子动力学方法建立了铁电薄膜畴超快背翻转动力学过程的理论模型，从宏观、介观、微观三个空间尺度上开展了模拟工作，探讨了畴演化机制及太赫兹电场参数、材料自身参数等的影响。这些工作可以为理解和解决铁电薄膜及其存储器的保持性失效提供一定的指导和依据。