Optical near-field study of ferroelectric tunnel junctions

这个项目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HfxZr1-xO2的铁电隧道结(FTJ)，这些结只有大约三到四个单位单元厚度(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的成本效益和扩展潜力与写入速度显著超过SRAM，同时具有闪存的非易失性，所有这一切的能耗都比闪存低约四个数量级。2011年在HfxZr1-xO2中发现铁电性是意想不到的，特别是因为HfO2和ZrO2几十年来都分别用作介质。尽管人们对这种完全与cmos完全兼容的铁电材料产生了极大的兴趣，但所有解释这种铁电材料的物理起源的尝试仍处于初级阶段，缺乏通过直接、非侵入性技术获得的纳米级信息。尖端增强拉曼光谱是目前唯一一种在纳米尺度上获得化学、结构和功能信息的非侵入性成像技术。该技术基于通过贵金属尖端附近的局域表面等离子体共振来增强和限制光学近场，贵金属尖端作为原子力显微镜的一部分在样品表面上方的受控距离处进行反馈扫描。我们选择了剪切力音叉配置，以与几乎任何拉曼活性表面上电化学刻蚀的金和银尖兼容，无论它是否导电，我们已经为高光谱成像实现了3 nm的空间分辨率(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的，都不像在传统的钙钛矿铁电体中观察到的那样容易受到外延双轴应变的影响而产生铁电性。由于典型的颗粒直径在10-20 nm之间，目前的工作模型是晶界对界面能的贡献导致铁电性，但该模型尚未得到实验验证。HfxZr1-xO2中铁电性的性质对所有关于材料和沉积过程的优化程序都有巨大的影响。因此，我们打算采用尖端增强拉曼光谱，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矢量是复杂的，并且由于单键尺度上的强烈局部场梯度而违反了传统的拉曼选择规则。这项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。

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.