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: 2020
• 资助国家: 加拿大
• 起止时间: 2020-01-01 至 2021-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=716553
• 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.

该项目合并了我小组的两个活跃研究领域，因为我们正在将尖端增强拉曼光谱 (TERS) 部署到 HfxZr1-xO2 的铁电隧道结 (FTJ)，该隧道结只有三到四个晶胞厚（约 2 nm）。 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大大超过 SRAM 的写入速度相结合，同时又像闪存一样具有非易失性，所有这一切的能耗比闪存低约四个数量级。 2011 年在 HfxZr1-xO2 中发现铁电性是出人意料的，特别是因为几十年来 HfO2 和 ZrO2 都被单独用作电介质。尽管人们对这种第一个完全与 CMOS 兼容的铁电体产生了极大的兴趣，但所有解释这种铁电体物理起源的尝试仍处于起步阶段，并且缺乏通过直接、非侵入性技术的纳米级信息。尖端增强拉曼光谱是目前唯一能够产生纳米级化学、结构和功能信息的非侵入性成像技术。该技术基于通过紧邻贵金属尖端的局部表面等离子体共振介导的光学近场的增强和限制，贵金属尖端作为原子力显微镜的一部分进行扫描，并在样品表面上方的受控距离处进行反馈。我们选择了剪切力音叉配置，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，无论其是否导电，并且我们已经实现了 3 nm 的高光谱成像空间分辨率（即每个像素的完整拉曼和金发光光谱）。在 HfxZr1-xO2 中，体单斜相和薄膜四方相都不是铁电性的，并且都不易受到外延双轴应变的影响而诱发铁电性，例如在传统的钙钛矿铁电体中观察到。典型晶粒直径在 10 至 20 nm 之间，当前的工作模型是晶界的界面能量贡献诱发铁电性；然而该模型还有待实验验证。 HfxZr1-xO2 中铁电性的性质对于材料和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，并用 DFT 微扰计算来支持我们的实验，以便预测 TERS 几何结构中所有相的拉曼光谱，其中局部 k 向量很复杂，并且由于单键尺度上的强局部场梯度而违反了传统的拉曼选择规则。该研究计划涉及两项正在进行的战略合作伙伴资助和其他几个合作研究项目。