Quantitative Hall Voltage mapping at conducting Ferroelectric domain walls: A novel approach to extracting conduction mechanisms on the nanoscale

铁电畴壁，即分隔均匀电极化区域的边界，具有非凡的导电能力，这为其在纳米电子学中的应用开辟了巨大的可能性。可重构的基于铁电畴壁的纳米电子学，其中导电和同时移动畴壁的独特电子特性可以用于功能器件，代表了对现有电子学规范的真正新颖和颠覆性的方法。2017年，工程与物理科学研究委员会（EPSRC）为四个机构（贝尔法斯特、华威、圣安德鲁斯和剑桥）的团队资助了一项为期四年的计划，以研究铁电和多铁畴壁的新功能特性。这项工作的主要推动力是探索在畴壁中看到的输运的基本物理。作为这项工作的一部分，载流子类型、密度和迁移率正在使用一种新的扫描探针显微镜在许多不同材料系统的畴壁上进行映射，其中使用开尔文探针力显微镜（KPFM）以纳米尺度的空间分辨率测量霍尔电压。KPFM的工作原理是通过原子力显微镜（AFM）本身提供的相等的尖端电位来平衡样品上不同水平的表面电位。在所有标准afm中，可提供给尖端的内部偏置范围为+/- 10V，限制了可以映射到相同范围的表面电位。对于我们的纳米级畴壁测量，电流沿着畴壁驱动，在垂直磁场的存在下，产生的霍尔电位沿着畴壁和样品顶表面之间的交点测量。对于畴壁电导率大的系统，足够的电流允许可测量的霍尔信号，可以使用适度的源漏电位差来驱动。令人沮丧的是，对于电导率较低的畴壁，为可测量的霍尔信号驱动足够电流所需的源漏电位差需要显着更大：高达50-100V的数量级，并且超出内部AFM电子设备可以提供平衡偏置的范围，从而检测表面上的真实电位。因此，虽然我们已经能够对具有良好导电性的畴壁进行霍尔效应的分类测量，但我们无法在Cu-Cl硼砂，LiNbO3，锗酸铅和未掺杂锰矿等系统中进行等效测量，这些系统缺乏等效测量和对导电性机制的物理见解。如果可以应用更高的电压（> +/- 10V），并通过为AFM配置的硬件/电子设备无缝检测，则可以克服霍尔电压显微镜方法的这种限制。AFM的制造商Asylum Research最近开始提供一种高压模块，能够应用-150V和+150V之间的电压，可以通过我们在霍尔电压显微镜方面的相关专业知识进行调整，以在更高的电压范围内执行完全定量的霍尔电位映射。本提案旨在用HV模块升级我们的AFM，并随后使其适应在导电铁电畴壁上执行基于KPFM的高压霍尔电压映射，以便在比目前可能的范围更大的铁电体范围内对导电壁的输运物理进行基本的了解。开发的测量技术将消除直接提取相关载流子信息和导电机制的重大障碍，这些信息和机制适用于大多数基于畴壁的纳米电子学感兴趣的大块和薄膜铁电体。在这里开发的技术也可以为现有电子工业中载流子轮廓的纳米尺度空间分辨映射提供直接和相对容易使用的方法。

The remarkable ability of ferroelectric domain walls, boundaries that separate regions of uniform electrical polarisation, to conduct electrical current has opened up dramatic possibilities for their use in nanoelectronics. Reconfigurable ferroelectric domain wall-based nanoelectronics, where unique electronic properties of conductive and simultaneously mobile domain walls can be exploited towards functional devices, represents a truly novel and disruptive approach to existing norms of electronics. In 2017, the Engineering and Physical Sciences Research Council (EPSRC) has funded a four year programme for teams across four institutions (Belfast, Warwick, St Andrews and Cambridge), to investigate novel functional properties in ferroelectric and multiferroic domain walls. A major thrust of this effort is the exploration of the fundamental physics of transport seen in domain walls. As part of this work, carrier types, densities and mobilities are being mapped for domain walls in a number of different materials systems using a new form of scanning probe microscopy, in which the Hall voltage is measured, with nanoscale spatial resolution, using Kelvin Probe Force Microscopy (KPFM). KPFM works by balancing different levels of surface potential on the sample with equal tip potentials, supplied by the atomic force microscope (AFM) itself. In all standard AFMs, the range of internal bias that can be supplied to the tip is +/- 10V, limiting the surface potential that can be mapped to the same range. For our nanoscale domain wall measurements, current is driven along the walls, in the presence of a perpendicular magnetic field, and the resultant Hall Potential is measured along the lines of intersection between the domain walls and the top surfaces of the samples. For systems in which the domain wall conductivity is large, sufficient current to allow a measurable Hall signal, can de driven using modest source-drain potential differences. Frustratingly, for domain walls with lower conductivities, the source-drain potential difference needed to drive sufficient current for measurable Hall signals needs to be significantly larger: up to the order of 50-100V and beyond the range at which internal AFM electronics can supply a balancing bias and hence detect the true potential on the surface . Thus, while we have been able to make categorical measurements of the Hall Effect for domain walls with good conductivity, we have been unable to perform equivalent measurements in systems such as Cu-Cl boracite, LiNbO3, lead germanate and undoped manganites, where equivalent measurements and physical insight into conductivity mechanisms are lacking. This limitation of the Hall voltage microscopy approach can be overcome if a higher voltage (> +/- 10V) can be applied and detected seamlessly by the hardware/electronics configured for the AFM. The manufacturers of the AFM, Asylum Research, have recently started offering a HV module capable of applying voltages between -150V and +150V which could be adapted by our relevant expertise in Hall voltage microscopy to perform fully quantitative Hall potential mapping in the higher voltage regime. This proposal aims to upgrade our AFM with a HV module and subsequently adapt it to perform high-voltage KPFM based Hall voltage mapping at conducting ferroelectric domain walls to allow fundamental insight into the physics of transport at conducting walls across a significantly wider range of ferroelectrics than currently possible. The developed measurement techniques will remove a significant hurdle in directly extracting relevant carrier information and mechanisms of electrical conduction at conducting domain walls in the majority of bulk and thin-film ferroelectrics of interest for domain wall based nanoelectronics. The techniques developed here could also facilitate direct and relatively easy-to-use means for nanoscale spatially resolved mapping of carrier profiles in the existing electronics industry.