Collaborative: Reliability of Ferroelectric Thin Films: A Systematic Study of Point Defect Phenomena and Local Electronic Structure Effects

这是伊利诺伊大学芝加哥分校（UIC）的一个合作研究项目的一部分，该项目涉及铁电薄膜的点缺陷化学和电子结构与限制其在非易失性存储器件中的可靠性的疲劳和压印过程之间的联系。该研究计划的一个关键目标是了解场致电子电荷注入/载流子捕获和带电氧空位再分布在疲劳和最先进的Pb（Zr，Ti）O3（PZT）薄膜的压印过程中的相对贡献。 我们将使用原子分辨率STEM和EELS来研究铁电疲劳和压印电测试导致的原子排列和局部电子结构的变化。这种变化的例子可能包括电极界面和晶界附近局部高氧非化学计量的发展，以及这些界面处键合排列和局部态密度的变化。 将使用Z-对比成像技术进行原子结构测定。 电子能量损失谱的同时采集将允许在光谱中的电子结构信息与PZT薄膜试样中的单个原子柱相关。 在STEM/EELS研究之前，PZT电容器的电气测试将由我们在斯坦福大学的合作者进行。 EELS功能的定量解释将有利于从头计算（也在斯坦福大学进行）的局部电子结构在铁电/电极界面和载流子陷阱态的能量与点defects.Ferroelectric材料表现出自发极化，可用于在微电子和通信的各种不同的应用。 例如，薄膜铁电材料是新一代非易失性半导体存储器的关键使能器，其目前正由全球主要微电子公司开发（并且越来越多地推向市场）。 在小尺寸薄膜结构中，铁电极化态的切换物理也是一个重要的基础科学研究课题。 铁电薄膜的科学和技术为更好地理解这些材料中干扰可靠极化切换的现象提供了动力。 这样的现象包括铁电疲劳，在通过施加电压脉冲重复切换之后可切换极化的损失，以及印记，由一个极性的重复电压脉冲导致的矫顽电压的偏移。 铁电疲劳和压痕的实验观察和理论模型的主机已经报道了多年。 然而，负责这些可靠性限制过程的详细机制仍然不确定。 本研究计划将探讨铁电疲劳的基本机制，并在我们的合作者提供的最先进的铁电薄膜在半导体行业的印记。这项研究将由三个共同的主要研究人员在斯坦福大学和UIC与互补的专业知识，在带电缺陷迁移和铁电薄膜的极化开关，原子分辨率成像和光谱使用电子显微镜的测量，和模拟固体的电子性质。UIC部分的研究将集中在直接检查的PZT薄膜的疲劳和压印电气测试引起的局部键合和电子结构的变化。这项研究计划还将加强我们现有的教育推广活动，以芝加哥地区的高中学生，这一努力，在过去的4年中，提供了研究职位，为32名学生从群体通常代表不足的工程和自然科学。

This is the University of Illinois at Chicago (UIC) portion of a collaborative research project on the connections between the point defect chemistry and electronic structure of ferroelectric thin films and the fatigue and imprint processes that limit their reliability in non-volatile memory devices. A key objective of the research program is to understand the relative contributions of field-induced electronic charge injection/carrier trapping and charged oxygen vacancy redistribution during fatigue and imprint of state-of-the-art Pb(Zr,Ti)O3 (PZT) films. We will use atomic resolution STEM and EELS to study the changes in atomic arrangements and local electronic structure that result from ferroelectric fatigue and imprint electrical testing. Examples of such changes might include development of locally-high oxygen non-stoichiometry near electrode interfaces and grain boundaries, and changes in bonding arrangements and the local density of states at these interfaces. Atomic structure determinations will be made using the Z-contrast imaging technique. Simultaneous acquisition of electron energy loss spectra will allow electronic structure information in the spectrum to be correlated with individual atomic columns in PZT thin film specimens. Electrical testing of the PZT capacitors prior to STEM/EELS studies will be performed by our collaborators at Stanford. Quantitative interpretation of EELS features will be facilitated by ab initio calculations (also performed at Stanford) of the local electronic structure at ferroelectric/electrode interfaces and the energies of carrier trap states associated with point defects.Ferroelectric materials exhibit a spontaneous polarization which can be used in a variety of different applications in microelectronics and communications. For example, thin film ferroelectric materials are the key enabler for a new generation of non-volatile semiconductor memories which are currently being developed (and, increasingly, brought to market) by major microelectronics firms worldwide. The physics of switching the ferroelectric polarization state in small-dimension, thin film structures is also an important topic of fundamental scientific interest. Both the science and the technology of ferroelectric thin films provide motivation for better-understanding phenomena that interfere with reliable polarization switching in these materials. Such phenomena include ferroelectric fatigue, the loss of switchable polarization after repeated switching by applied voltage pulses, and imprint, a shift in coercive voltage resulting from repeated voltage pulses of one polarity. A host of experimental observations and theoretical models for ferroelectric fatigue and imprint have been reported over the years. However, the detailed mechanisms responsible for these reliability-limiting processes remain uncertain. This research program will investigate the underlying mechanisms of ferroelectric fatigue and imprint in state-of-the art ferroelectric films provided by our collaborators in the semiconductor industry. The research will be directed by three co-principal investigators based at Stanford University and UIC with complimentary expertise in measurements of charged defect migration and polarization switching of ferroelectric thin films, atomic resolution imaging and spectroscopy using the electron microscope, and simulations of the electronic properties of solids. The UIC portion of the research will focus on direct examination of local bonding and electronic structure changes induced by fatigue and imprint electrical testing of PZT thin films. This research program will also strengthen our existing educational outreach activities to Chicago-area high school students, an effort that has, over the last 4 years, provided research positions for 32 students from groups typically under-represented in engineering and the natural sciences.