Hall Potential Mapping of Conducting Ferroelectric Domain Walls

Abstract

The phenomenon of distinct conductivity at certain ferroelectric domain walls has the potential to revolutionise nanoelectronics. Complete control over their local carrier properties would allow two-dimensional analogues of semiconducting devices to be dynamically written, effectively supplanting modern printed circuit design. However, despite a growing array of ferroelectric materials found to exhibit domain wall conductance, the most fundamental carrier measurements have not been performed with neither local carrier types, mobilities or densities explicitly determined for any domain wall system. This thesis details the development of a scanning probe microscopy technique that facilitates spatial mapping of the Hall effect within conducting domain walls. The measurements presented herein represent the first explicit characterisation of the carriers involved in conducting ferroelectric domain walls. 
Studying YbMnO3 single crystals, the Hall potential developed within tail-to-tail charged domain walls was imaged by its influence on an intermittent-contact atomic force microscopy topography profile, confirming the local conduction was mediated by p-type carriers. By calibration of the AFM signal, an estimate of ~ 1x10¹⁶ cm⁻³ is calculated for the mobile carrier density in the wall, around four orders of magnitude below that required for complete screening of the polar discontinuity. A carrier mobility of ∼ 60cm²V⁻¹s⁻¹ is calculated, about an order of magnitude below equivalent carrier mobilities in p-type silicon, but sufficiently high to preclude carrier-lattice coupling associated with small polarons. 
The technique was further developed by utilising Kelvin probe force microscopy to map the topographic and electrostatic signal in isolation. Surface potential mapping of an ErMnO₃ single crystal confirms quantitative accuracy is maintained under a range of magnetic and electric field states, with distinct wall contrast observed at both charged wall types. Lastly, the alternating polarity method was demonstrated as an simple means of signal demodulation to precisely measure the typically minute conductance of individual domain walls.

Date of Award	Sept 2017
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Marty Gregg (Supervisor) & Amit Kumar (Supervisor)