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

铁电畴壁（分隔均匀电极化区域的边界）传导电流的非凡能力为它们在纳米电子学中的应用开辟了巨大的可能性。可重构的铁电畴壁为基础的纳米电子学，其中导电和同时移动的畴壁的独特的电子性能可以被开发用于功能器件，代表了一个真正的新颖的和破坏性的方法，现有的电子规范。2017年，工程和物理科学研究理事会（EPSRC）资助了四个机构（贝尔法斯特，沃里克，圣安德鲁斯和剑桥）的团队的四年计划，以研究铁电和多铁性畴壁的新功能特性。这项工作的一个主要推动力是探索畴壁中的基本物理传输。作为这项工作的一部分，载体类型，密度和迁移率被映射为畴壁在一些不同的材料系统使用一种新形式的扫描探针显微镜，其中霍尔电压测量，与纳米级的空间分辨率，使用开尔文探针力显微镜（KPFM）。KPFM的工作原理是通过平衡样品上不同水平的表面电位与相等的尖端电位，由原子力显微镜（AFM）本身提供。在所有标准AFM中，可以提供给尖端的内部偏压范围为+/-10 V，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，在垂直磁场的存在下，电流被沿着壁驱动，并且所得到的霍尔电势被沿着畴壁和样品的顶表面之间的交叉线测量。对于畴壁电导率大的系统，可以使用适度的源极-漏极电势差来驱动足够的电流以允许可测量的霍尔信号。令人沮丧的是，对于具有较低电导率的畴壁，驱动足够的电流用于可测量的霍尔信号所需的源极-漏极电势差需要显著更大：高达50- 100 V的量级，并且超出内部AFM电子设备可以提供平衡偏压的范围，从而检测表面上的真实电势。因此，虽然我们已经能够进行分类测量的霍尔效应的畴壁具有良好的导电性，我们一直无法执行等效的测量系统，如Cu-Cl方铅矿，铌酸锂，锗酸铅和未掺杂的锰氧化物，其中缺乏等效的测量和物理洞察导电机制。如果可以通过为AFM配置的硬件/电子设备无缝地施加和检测更高的电压（> +/-10V），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最近开始提供能够施加-150V和+150V之间的电压的HV模块，该模块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。这项建议的目的是升级我们的原子力显微镜与HV模块，并随后适应它执行高电压KPFM的霍尔电压映射在导电铁电畴壁，使基本的洞察到物理传输在导电壁在一个显着更广泛的铁电体比目前可能的。开发的测量技术将消除一个重大障碍，直接提取相关的载体信息和机制的导电畴壁在大多数的散装和薄膜铁电体的利益为畴壁的纳米电子学。这里开发的技术也可以促进直接和相对易于使用的手段，在现有的电子工业中的纳米空间分辨映射的载体配置文件。

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.