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）合作研究项目的一部分，该项目研究铁电薄膜的点缺陷化学和电子结构与疲劳和压印过程之间的联系，这些过程限制了铁电薄膜在非易失性存储设备中的可靠性。研究计划的一个关键目标是了解在最先进的Pb(Zr,Ti)O3 （PZT）薄膜疲劳和压印过程中场致电子电荷注入/载流子捕获和带电氧空位重新分配的相对贡献。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，以及这些界面上键的排列和局部态密度的变化。原子结构测定将使用z对比成像技术。同时获取电子能量损失谱将允许光谱中的电子结构信息与PZT薄膜样品中的单个原子柱相关联。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。通过对铁电/电极界面的局部电子结构和与点缺陷相关的载流子陷阱态的能量的从头计算（也在斯坦福大学进行），将有助于对EELS特征的定量解释。铁电材料具有自发极化特性，可用于微电子和通信领域的各种不同应用。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推动者，这些存储器目前正在由世界各地的主要微电子公司开发（并越来越多地推向市场）。在小尺寸薄膜结构中切换铁电极化状态的物理学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开关的现象提供了动力。这些现象包括铁电疲劳，在施加电压脉冲反复开关后可切换极化的损失，以及压印，由一个极性的重复电压脉冲引起的矫顽力电压的移位。多年来，关于铁电疲劳和印记的大量实验观察和理论模型已经被报道。然而，负责这些可靠性限制过程的详细机制仍然不确定。本研究计划将研究由我们在半导体行业的合作者提供的最先进的铁电薄膜中铁电疲劳和印记的潜在机制。该研究将由斯坦福大学和UIC的三位联合首席研究员指导，他们在铁电薄膜的带电缺陷迁移和极化开关测量、电子显微镜的原子分辨率成像和光谱以及固体电子特性模拟方面具有互补的专业知识。UIC部分的研究将集中在PZT薄膜的疲劳和压印电测试引起的局部键合和电子结构变化的直接检查。这个研究项目还将加强我们现有的针对芝加哥地区高中生的教育推广活动，在过去的4年里，这项努力为32名来自工程和自然科学领域代表性不足的学生提供了研究职位。

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.