Optical near-field study of ferroelectric tunnel junctions

铁电隧道结的光学近场研究

• 批准号: RGPIN-2019-07023
• 负责人: Ruediger, Andreas
• 金额: $ 2.48万
• 依托单位: Institut national de la recherche scientifique
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=685325
• 关键词: Optical; near; field; study; ferroelectric

This program merges two active research areas of my group as we are deploying tip-enhanced Raman spectroscopy (TERS) to ferroelectric tunnel junctions (FTJs) of HfxZr1-xO2, which are only three to four unit cells thick (approx. 2 nm). In 2017, we were the first to demonstrate the operation of these FTJs for non-volatile memory applications, thus paving the way for a resistance-based readout memory combining the cost efficiency and scaling potential of DRAM with a write speed that considerably exceeds SRAM while at the same time being non-volatile like Flash, all this at an energy consumption about four orders of magnitude inferior to Flash. The discovery of ferroelectricity in HfxZr1-xO2 in 2011 was unexpected, in particular as both HfO2 and ZrO2 had been individually used as dielectrics over several decades. Despite the tremendous interest in this first fully cmos-compatible ferroelectric, all attempts to explain the physical origin of this ferroelectricity are still in their infancy and lack nanoscale information through a direct, non-invasive technique. Tip-enhanced Raman spectroscopy is currently the only non-invasive imaging technique to yield chemical, structural and functional information at a nanometer scale. The technique is based on the enhancement and confinement of an optical near field mediated through a localized surface plasmon resonance in direct proximity of a noble metal tip that scans as part of an atomic force microscope in feedback at a controlled distance above the sample surface. We have chosen a shear-force tuning fork configuration to be compatible with electrochemically-etched gold and silver tips on virtually any Raman active surface, regardless of whether it is conducting or not and we have achieved a spatial resolution of 3 nm for hyperspectral imaging (i.e. a complete Raman and gold luminescence spectrum at each pixel). In HfxZr1-xO2, neither the bulk monoclinic phase nor the thin film tetragonal phase are ferroelectric and neither is susceptible to epitaxial biaxial strain to induce ferroelectricity as e.g. observed in conventional perovskite ferroelectrics. With a typical grain diameter between 10 and 20 nm, the current working model is that an interfacial energy contribution from grain boundaries induces ferroelectricity; the model is however yet to be experimentally verified. The nature of ferroelectricity in HfxZr1-xO2 has tremendous implications for all optimization procedures regarding the material as well as the deposition process. We therefore intend to deploy tip-enhanced Raman spectroscopy and to back up our experiments with DFT perturbation calculations in order to predict the Raman spectra for all phases in a TERS geometry where the local k-vector is complex and where conventional Raman selection rules are violated due to strong local field gradients at the scale of single bonds. This research program relates to two ongoing strategic partnership grants and several other collaborative research projects.

该计划合并了我的团队的两个活跃的研究领域，因为我们正在将尖端增强拉曼光谱（TERS）部署到HfxZr 1-xO 2的铁电隧道结（FTJ）中，这些隧道结只有三到四个单元格厚（约100 nm）。2nm）。2017年，我们率先展示了这些FTJ在非易失性存储器应用中的操作，从而为基于电阻的读出存储器铺平了道路，该存储器将DRAM的成本效率和扩展潜力与写入速度相结合，大大超过SRAM，同时又像闪存一样具有非易失性，所有这一切的能耗都比闪存低四个数量级。2011年在HfxZr 1-xO 2中发现铁电性是出乎意料的，特别是因为HfO 2和ZrO 2几十年来都被单独用作超导体。尽管对这种第一个完全兼容cmos的铁电体有着巨大的兴趣，但所有试图解释这种铁电性的物理起源的尝试都还处于起步阶段，缺乏通过直接的非侵入性技术获得的纳米级信息。针尖增强拉曼光谱是目前唯一的非侵入性成像技术，以产生在纳米尺度上的化学，结构和功能信息。该技术是基于增强和限制的光学近场介导的局部表面等离子体共振直接接近的贵金属尖端，扫描作为原子力显微镜的一部分，在样品表面上方的控制距离的反馈。我们已经选择了一个剪切力音叉配置与几乎任何拉曼活性表面上的电化学蚀刻金和银尖端兼容，无论它是否导电，并且我们已经实现了3 nm的空间分辨率用于高光谱成像（即，在每个像素处的完整拉曼和金发光光谱）。在HfxZr 1-xO 2中，体相单斜相和薄膜四斜相都不是铁电的，并且都不容易受到外延双轴应变的影响而诱发铁电性，如在常规钙钛矿铁电体中所观察到的。在典型的晶粒直径在10和20 nm之间的情况下，当前的工作模型是来自晶界的界面能贡献诱导铁电性;然而，该模型还有待实验验证。HfxZr 1-xO 2中的铁电性的性质对于关于材料以及沉积过程的所有优化程序具有巨大的影响。因此，我们打算部署尖端增强拉曼光谱和备份我们的实验与DFT微扰计算，以预测的拉曼光谱的所有阶段的TERS几何形状的地方k-向量是复杂的，传统的拉曼选择规则被违反，由于强的局部场梯度在单键的规模。该研究计划涉及两个正在进行的战略伙伴关系赠款和其他几个合作研究项目。