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

这项建议描述了一个连贯、协作的研究项目，该项目旨在研究铁电薄膜的点缺陷化学和电子结构与限制其可靠性的疲劳和压痕工艺之间的联系。该研究计划的一个关键目标是了解场诱导的电子电荷注入/俘获和带电氧空位再分布在最先进的PZT薄膜的疲劳和压痕过程中的相对贡献。将进行光学照明和DLTS测量下的疲劳和压痕测试，以表征薄膜中的光学和电活性载流子陷阱。原子分辨STEM/EELS研究将在未降解和疲劳/印迹样品上进行，以寻找退化引起的PZT电极界面的成键排列和局部电子结构的变化。在不同偏压条件下，通过铁电电容器的氧同位素深度分布将被用来表征氧空位运动。为了正确解释实验结果，我们将对铁电/金属界面的局域电子结构、与点缺陷有关的载流子陷阱态的能量以及缺陷的形成和迁移能进行从头计算。铁电材料表现出自发极化，这在微电子和通信中有各种不同的应用。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推动因素，目前全球主要微电子公司正在开发(并日益推向市场)非易失性半导体存储器。在小尺寸薄膜结构中转换铁电偏振态的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都为更好地理解干扰这些材料中可靠的极化开关的现象提供了动力。这些现象包括铁电疲劳，在施加的电压脉冲重复切换后失去可切换的极化，以及印记，即由单极性的重复电压脉冲引起的矫直电压的移动。多年来，关于铁电疲劳和压痕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。这项研究计划将调查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压痕的潜在机制。这项研究将由斯坦福大学和伊利诺伊大学芝加哥分校(UIC)的三名联合首席研究员指导。该计划建立在我们在测量铁电薄膜的带电缺陷迁移和极化开关特性、使用电子显微镜的原子分辨率成像和光谱分析以及固体电子性质的模拟方面的免费专业知识的基础上。它将加强针对芝加哥地区高中生的现有教育推广活动，并将包括斯坦福大学UIC本科生的暑期研究项目。这些暑期项目将与研究项目目标很好地结合在一起，并将加强我们两个机构之间的合作。

This proposal describes a coherent, 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 (FeRAM) devices. A key objective of the research program is to understand the relative contributions of field-induced electronic charge injection/trapping and charged oxygen vacancy redistribution during fatigue and imprint of state-of-the-art Pb(Zr,Ti)O3 (PZT) films. Fatigue and imprint testing under optical illumination and DLTS measurements will be pursued in order to characterize both optically- and electrically-active carrier traps in the films. Atomic resolution STEM/EELS studies will be performed on both undegraded and fatigued/imprinted specimens in order to look for degradation-induced changes in bonding arrangements and local electronic structure at the electrode interfaces with PZT. Oxygen isotope depth profiling through ferroelectric capacitors subjected to various electrical biasing conditions will be used to characterize oxygen vacancy motion. Ab initio calculations of the local electronic structure at ferroelectric/metal interfaces, the energies of carrier trap states associated with point defects, and defect formation and migration energies will be performed to properly interpret the experimental results.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 the University of Illinois at Chicago (UIC). The program builds on our complimentary expertise in measurements of charged defect migration and polarization switching characteristics of ferroelectric thin films, atomic resolution imaging and spectroscopy using the electron microscope, and simulations of the electronic properties of solids. It will strengthen existing educational outreach activities to Chicago-area high school students, and will include summer research projects for UIC undergraduates at Stanford. These summer projects will be well-integrated with the research program objectives and will strengthen the collaboration between our two institutions.