铁电隧道结可靠性及失效机理的研究

Ferroelectric tunnel junction (FTJ) is an emerging memory technology which uses the ferroelectric polarization to modulate the tunneling conductance. Because of many advantages such as high access speed, low power consumption and nondestructive readout, the FTJs may have potential applications in the next-generation memory and logic devices. However, currently the reliabilities of FTJs (including performance controllability, fatigue and retention) are still low, which cannot meet the requirements of practical device application. In order to improve the reliabilities of FTJs, it is urgently needed to understand the microscopic factors causing the reliability issues. Therefore, this project proposes to use piezoresponse force microscopy in combination with synchrotron X-ray diffraction, to in situ study the evolutions of domain structures and microscopic defects with the voltage amplitude, voltage cycle, and retention time. Based on these experiments, one can understand the resistive switching mechanisms in different voltage regions, and the failure mechanisms of fatigue and retention. This can provide guidance on how to optimize the fabrication procedures and working conditions of FTJs, so as to address the reliability issues. We believe that this project will lay the groundwork for developing highly reliable FTJs, and moreover, promote the application of FTJs in the next-generation nonvolatile random access memories.

铁电隧道结(FTJ)是一种利用铁电极化调控隧穿电导的新型存储器，它具有高访问速度、低能耗、非破坏性读取等优点，有望应用于下一代内存及逻辑器件中。然而，目前FTJ在可靠性方面(包括性能可控性、疲劳和保持特性)仍存在不足，未达到实际器件应用的要求。为了提升FTJ的可靠性，首先需要对造成可靠性问题的微观因素有深入的认识。因此，本项目拟利用压电力显微镜和同步辐射X射线衍射技术，原位研究FTJ的畴结构和微区缺陷随外加电压幅值、电压循环次数、保持时间的演变过程，弄清FTJ在各电压区间主导的阻变机制以及疲劳、保持的失效机理。这将为有针对性地优化FTJ的制备工艺和工作条件提供指导，从而解决FTJ可靠性不高的难题。本项目的研究为开发高可靠性FTJ奠定了基础，也可能推动FTJ在下一代非挥发性随机存取存储器中的应用。

铁电隧道结(FTJ)是一种利用铁电极化调控隧穿电导的新型存储器，它具有高访问速度、低能耗、非破坏性读取等优点，有望应用于下一代内存、逻辑和类脑计算等器件中。然而，目前FTJ的可靠性 (包括性能可控性、疲劳和保持特性)仍存在不足，并且造成其可靠性不足的微观机理也不清楚。本项目利用了同步辐射X射线衍射(XRD)技术原位研究了FTJ中铁电薄膜在外场作用下的结构演变，据此推断出氧空位的迁移情况并分析其对阻变效应和可靠性的影响。首先，制备出纯相、外延、平整度高的BaTiO3 (BTO)超薄膜，并且利用压电力显微镜证明其极化可以翻转。然后，通过顶、底电极优化，获得了高开关比、非易失的BTO基FTJ器件，并阐明了顶、底电极的屏蔽长度对隧穿势垒和开关比的影响规律。以Pt/BTO/LaNiO3 FTJ为样本，利用同步辐射XRD原位研究证实了BTO结构在外加电场作用下的往复变化：-5 V/1 ms电压脉冲诱导其面外晶格常数减小，而+3 V/1 ms电压脉冲诱导其面外晶格常数增大。据此推断出足够大负电压作用下氧空位被吸引至上界面并被氧离子填充，而足够大正电压作用下氧离子被抽出并产生氧空位。这种电场诱导氧空位迁移可能对阻变效应有一定贡献，也可能是造成FTJ可控性较差、疲劳性能不佳的原因之一。本项目为开发高开关比FTJ提供了一些思路，还揭示了氧空位迁移对FTJ可靠性的影响，为最终解决其可靠性问题奠定了一些基础。