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

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.

Beyond enabling access to fundamental physics of conducting domain walls, the proposed research has excellent potential towards developing disruptive technologies and create impact across several spectrums. These avenues and the range of activities to realise impact across them are discussed below.

Economy : One of the direct consequences of this project will be cross-pollination of ideas between the PI and Asylum Research, the company supplying the bespoke HV module, towards realisation of new scanning modes being offered by the company to its >200 users across the world. Building on existing fruitful collaboration between PI and Asylum Research (including a commercialised patent for a scanning mode invented by PI), this proposal relies strongly on the existing working collaboration between them. Based on the success of this project, both modes being developed here, HV-KPFM and Quantitative HV-KPFM based Hall microscopy, could be patented jointly between the investigators and Asylum and further avenues for commercialization would be ascertained in that scenario. In the emerging field of reconfigurable nanoelectronics and neuromorphic circuitry which has stoked the interest of leading manufacturers such as Intel and Seagate, the quantitative Hall microscopy approach developed here could facilitate a robust and precise technique to access key carrier information for design and fabrication of such devices. Direct engagement with these industrial partners in the context of this project would be sought through the already existing collaborations with the Centre for Nanostructured Media at QUB.

People : Training the next generation of scientists in the evolving and competitive field of nanoscale functional materials would be an excellent outcome in terms of creating impact. 2 PDRAs funded by the critical mass grant and a tripartite US-Ireland DELNI/SFI/NSF grant respectively as well as 2
PhD students working on directly relevant projects will be trained as a part of this project. Regionally, the expertise in nanoscale electronic characterization of functional materials is likely to attract interest from high-tech manufacturing employers such as Seagate and Intel. These companies have shown strong interest in candidates with expertise in electronic, structural and functional characterisation of materials (especially on the nanoscale), and novel approaches towards devices.

Knowledge : Beyond domain wall physics and domain-wall based nanoelectronics, the quantitative approaches developed through this proposal would allow probing of phenomena across several branches of physical sciences. Current approaches offered by AFM suppliers to undertake HV-KPFM are sub-optimal and development of the proposed technique will lead to improved precision of the technique in the HV regime such that more reliable measurements can be made. The technique could also have ramifications for potential mapping of lateral devices where operating potentials in excess of normal range (-10V to +10V) are needed. Quantitative KPFM based Hall effect measurements, by way of facilitating access to carrier physics, may also open up new avenues in the field of silicon-based electronics industry where direct, fast and precise quantitative methods to extract carrier mobilities and densities are always in demand. Engagement with broader scientific community (beyond domain wall research) via the medium of conferences and inter-departmental seminars is envisioned to highlight our work and develop potential cross-disciplinary collaborations.

Society : The techniques developed through this project could allow profound insight into the underpinning physics of domain wall conductivity, thereby facilitating the invention of a new family of devices that would have significant impact on lifestyles, environment and health.