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

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其在非易失性存储器件中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/载流子陷阱和带电氧空位再分布在最先进的Pb(Zr，Ti)O_3(PZT)薄膜疲劳和压痕过程中的相对贡献。我们将使用原子分辨STEM和EELS来研究铁电疲劳和压印电学测试引起的原子排列和局域电子结构的变化。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比的发展，以及这些界面上的成键排列和局部态密度的变化。原子结构的测定将使用Z对比度成像技术。同时获取电子能量损失谱将允许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。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。通过从头计算(也是在斯坦福大学进行的)铁电/电极界面的局域电子结构和与点缺陷相关的载流子陷阱态的能量，将有助于对电致发光特性的定量解释。铁电材料具有自发极化，可用于微电子和通信中的各种不同应用。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UCD的三名联合首席研究人员指导，他们拥有互补的专业知识，包括测量铁电薄膜的带电缺陷迁移和极化转换、使用电子显微镜进行原子分辨率成像和光谱分析，以及模拟固体的电子性质。这项研究的UCD部分将集中于直接检查PZT薄膜的疲劳和压痕电学测试引起的局部成键和电子结构变化。UCD将仿照伊利诺伊州立大学芝加哥分校PI发起的成功计划，建立一个新的外展计划。在该项目中，为芝加哥地区的32名高中生提供了研究职位，这些学生来自工程和自然科学领域的代表性通常较低的群体。加州大学戴维斯分校的项目将利用戴维斯校区与萨克拉门托地区高中之间的紧密联系。

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.