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

合作：铁电薄膜的可靠性：点缺陷现象和局域电子结构效应的系统研究

• 批准号: 0335364
• 负责人: Nigel Browning
• 金额: $ 22.5万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-01-01 至 2006-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0335364&HistoricalAwards=false
• 关键词: Collaborative; Reliability; Ferroelectric; Thin; Films

This is the University of California at Davis (UCD) 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 UCD 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 UCD 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. A new outreach program will be established at UCD that is modeled after the successful program initiated by the PI at U IL at Chicago. In that program, research positions were provided for 32 Chicago-area high school students, from groups typically under-represented in engineering and the natural sciences. The program at UCD will make use of the strong links between the Davis campus and high-schools in the Sacramento area.

这是加州大学戴维斯分校 (UCD) 合作研究项目的一部分，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非易失性存储器件可靠性的疲劳和压印工艺之间的联系。该研究计划的一个关键目标是了解疲劳期间场致电子电荷注入/载流子捕获和带电氧空位重新分布的相对贡献以及最先进的 Pb(Zr,Ti)O3 (PZT) 的印记电影。 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状态密度的变化。 原子结构测定将使用 Z 对比度成像技术进行。 同时采集电子能量损失光谱将使光谱中的电子结构信息与 PZT 薄膜样品中的各个原子柱相关联。 我们在斯坦福大学的合作者将在 STEM/EELS 研究之前对 PZT 电容器进行电气测试。 对铁电/电极界面处的局域电子结构以及与点缺陷相关的载流子陷阱态的能量进行从头计算（也在斯坦福大学进行）将有助于对 EELS 特征的定量解释。铁电材料表现出自发极化，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，全球主要微电子公司目前正在开发（并且越来越多地推向市场）这种存储器。 在小尺寸薄膜结构中切换铁电极化状态的物理学也是具有基础科学意义的重要课题。 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偏振切换的现象提供了动力。 这些现象包括铁电疲劳、通过施加的电压脉冲重复切换后可切换极化的损失，以及印记、一种极性的重复电压脉冲导致的矫顽电压的偏移。 多年来，已经报道了许多关于铁电疲劳和印记的实验观察和理论模型。 然而，负责这些可靠性限制过程的详细机制仍然不确定。 该研究计划将研究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基本机制。该研究将由斯坦福大学和都柏林大学的三名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切换测量、使用电子显微镜的原子分辨率成像和光谱学以及电子特性模拟方面拥有互补的专业知识固体。研究的 UCD 部分将侧重于直接检查由 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。 都柏林大学将仿照芝加哥大学伊利诺伊大学 PI 发起的成功项目，建立一个新的外展项目。 在该计划中，为 32 名芝加哥地区的高中生提供了研究职位，他们来自工程和自然科学领域代表性不足的群体。 都柏林大学的该项目将利用戴维斯校区和萨克拉门托地区高中之间的紧密联系。