Abstract
This thesis attempts to investigate and characterise the nature of the conducting domain walls within the improper ferroelectric system ErMnO3. ErMnO3 belongs to a family of rare-earth manganites (h-RMnO3).They form energetically costly, charged domain walls through a process of trimerization. Enhanced conduction has been demonstrated along all types of charged domain walls: tail-to-tail (T-T), head-to-head (H-H) and neutral. The complex and meandering domain microstructure found within the hexagonal rare-earth manganites make them fascinating for their potential application within nanoelectronic devices. The six-fold vertex junction is of special interest as a candidate for a one-dimensional domain wall P-N junction.
The vertex occurs at the point in which a conducting, p-type carrier domain wall meets a conducting, n-type carrier domain wall, and offers a new type of functional, novel nanoelectronic circuit element. This thesis seeks to provide evidence of the existence of such a device through experimentation, and develop enough supporting processes such that this proof-of-concept can be applied to other proper ferroelectric systems.
PFM was used to map and align the axis of polarisation for a bulk single crystal of Er{1-x}(Ca/Zn)xMnO3, grown and co-doped with the group II element calcium and the group XII transition metal zinc. The relative conductance of the T-T and H-H domain walls were characterised through cAFM with analysis in agreement with previous literature of un-doped ErMnO3: showing no enhanced conductivity of H-H domain walls at low applied voltages. This in contrast to the literature of enhancing conduction through use of these dopants. The background domain conductivity was found to be enhanced in comparison to its known and expected insulating behaviour.
A process was developed to fabricate and adequately recover the surface of a pseudo-2D thin film lamellae structurally etched from bulk single crystal co-doped Er{1-x}(Ca/Zn)xMnO3 by focused ion beam. This enabled the mapping of domain structure through PFM and measurement of the conductivity of charged domain walls through cAFM. Low ion beam energies were used at low incident angles in conjunction with acid etching to completely remove the amorphous layer typically formed during lamella preparation. Enhanced electronic transport was observed at charged domain walls on thin lamellae subjected to an applied field with conductive atomic force microscopy measurement. This demonstrated the translation of enhanced domain wall conductivity in bulk to lamellae and provided the pathway to isolating and characterising individual domain walls in ErMnO3.
An un-doped ErMnO3 lamella was fabricated utilising this method and placed into a geometry which enabled the isolation and electrical characterisation of individual domain walls. KPFM potential mapping was conducted through a range of currents as each individual domain wall was isolated and current was constrained to flow through a single-charged conducting domain wall. Calibrated potential maps enabled access to multi-probe measurements along the length of the conducting domain wall in a method similar to that of four-probe resistance measurements. Current-electric field functions were constructed which showed Ohmic behaviour for p-type walls, with an intrinsic room temperature conductivity value of ~0.4 Sm-1. The n-type walls showed non-Ohmic behaviour and a significantly lower conductivity. The current-field response in these walls is known to be complex and we suspect that the fields used were not great enough to access the strongly conducting, potentially metallic, state previously observed in literature; an upper bound for the room temperature conductivity of the domains themselves could, however, be confidently established (~6 * 10-6 Sm-1 at 0.1 MVm-1).
Finally, an experiment was derived to isolate and electrically characterise an individual vertex junction within un-doped ErMnO3. Two charged, conducting domain walls of opposing sense were electrically wired on either side through electron beam-assisted chemical vapour deposition of platinum electrodes. Smaller individual square platinum electrodes were locally deposited along the length of the domain walls under study. KPFM potential mapping of the surface through a range of applied currents -600nA to +600nA constrained through one junction pathway enabled the extraction of the absolute potential from the platinum voltage sensing electrodes in a four-probe resistance method. Surface potential was extracted from a line segment crossing the vertex in a multi-probe method such that current (I)-electric field (E) spectra were plotted. Both methods demonstrated similar non-linear, asymmetric spectra characteristic of that of a diode, providing strong evidence for the existence of a P-N junction at the centre of a six-fold domain wall vertex in ErMnO$_3$.
This would add it to the list of materials that are rightfully gaining interest for their potential in thin film transistors and application in nanoelectronic devices.
| Date of Award | Jul 2024 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Sponsors | Engineering & Physical Sciences Research Council |
| Supervisor | Marty Gregg (Supervisor) |
Keywords
- ferroelectric
- ferroelectrics
- domain wall
- ferroelectric domain wall
- Kelvin probe force microscopy
- conductive atomic force microscopy
- Focused Ion Beam
- Scanning Electron Microscopy