一阶回转曲线图谱法及其在HfO_(2)基铁电薄膜极化翻转行为研究中的应用

First-order reversal curve diagram and its application in investigation of polarization switching behavior of HfO_(2)-based ferroelectric thin films

铁电材料的极化翻转特性是铁电存储器实现"0,1"信息读写的物理基础,因此极化翻转的稳定性直接决定器件的服役可靠性.在交变电场循环载荷下,HfO_(2)基新型铁电薄膜存在唤醒(wake-up)、疲劳(fatigue)和极化翻转电流峰分裂(split-up)等极化翻转不稳定现象,严重制约了其在铁电存储器件中的实际应用.探明极化翻转行为复杂演变的微观机制,从而提出优化稳定性的可行措施是目前工作的重难点,但是基于传统测试手段的研究难以解决上述问题.一阶回转曲线图谱法被誉为迟滞系统研究中的"指纹鉴定",已在磁性材料特征参数演变规律的解析中得到成功应用.本文首先介绍一阶回转曲线图谱法的基本原理和实现方法,接着以Si掺杂HfO_(2)铁电薄膜为实验对象,利用该方法获得了薄膜内电畴极化翻转特征临界场的分布密度随外场加载历史的演变,为理解铁电材料的极化翻转行为提供了重要的微观物理机理信息.

From physical point of view,the"0,1"read/write operation of ferroelectric memory is based on the polarization switching of ferroelectric memory.Therefore,the reliability of device relies directly on the stability of polarization switching behavior.The polarization behaviors of HfO_(2)-based ferroelectric thin films subjected to bipolar cyclic electric field often exhibit wake-up,fatigue and split-up of transient switching current.These unstable switching properties seriously restrict the practical application of this new-type ferroelectric material in memory devices.It therefore becomes the critical task to explore the mechanism behind the complex evolution of polarization switching and find out possible approaches to optimizing the stability.However,it will be extremely difficult to accomplish the task by the traditional characterization methods.First-order reversal curve(FORC)diagram is regarded as"fingerprint identification"in the study of hysteresis systems,and has been used successfully to analyze the characteristic parameters of magnetic materials.The FORC diagram can intuitively determine the type,size and domain status of magnetic particles from distribution of both coercive field and interaction field.Moreover,it is also found that the FORC diagram is sensitive to measuring temperature.In this work,first,the Preisach model and implementation method of the FORC diagram are introduced.Then using Keithley 4200-SCS equipped with a remote pulse measurement unit,60 FORCs are recorded for Sidoped HfO_(2)ferroelectric thin films experiencing different external field loading histories.By the mathematical treatment,switching density distributions determined by FORC measurements are obtained to explore the evolution of coercive field and bias field.The FORC diagram of pristine film contains three distribution regions with different bias fields,which merge into one distribution with an almost zero bias field after 10^(4) wake-up cycles.Two oppositely biased regions can be observed after 2×10^(9) sub-cycling treatments.Surprisingly,the bias fields nearly vanish again after 10^(4) wake-up cycles.The main change of bias field instead of coercive field indicates that the migration of oxygen vacancies is likely to be the dominant mechanism behind the complex polarization switching behavior for HfO_(2)-based ferroelectric thin films.