AbstractA new technique, designed to quantitatively determine the local Hall potential of surfaces using Kelvin Probe Force Microscopy (KPFM), has been developed and implemented in examining the carrier characteristics of conducting ferroelectric domain walls in the improper ferroelectric system ErMnO3. Preliminary measurements reveal that p-type carriers aggregate at tail-to-tail domain walls. QuickField modelling allowed for estimates of the effective cross-sectional area of the conducting channels, whilst direct measurement of the surface potential provided insight into the local electric fields that are responsible for driving an electrical current. Thus, estimates of carrier density and carrier mobility were formulated. Measurements would suggest effective carrier densities of 1013 cm-3, far beneath the requirements to electrostatically screen electric stray fields that arise due to the development of a polarization (≈ 1020 cm-3). Extracted carrier mobilities of hundreds of cm2Vs-1 (up to and above 670 cm2Vs-1) are somewhat surprising and provide further evidence that charged domain walls possess drastically different physical properties from the bulk material. Dimensional confinement effects might have a part to play in this phenomenon, as they do in LaO-STO interfaces (1,2). However, this possibility is only alluded to in this thesis and will not be explored thoroughly.
Subsequent measurements on an improved atomic force microscope platform, the Asylum Research MFP-3D, equipped with a variable field module for application of in-situ magnetic fields, provided scans with significantly enhanced stability and reduced drift that were free from significant image astigmatism. This has allowed for further development of the technique, to the degree where local Hall potential may now be fully spatially resolved and mapped. An increase in the effective signal-to-noise ratio reveals a Hall potential signal at head-to-head domain walls that is inverted with respect to the signal measured at tail-to-tail domain walls, indicating an aggregation of electrical carriers that are oppositely charged. Added interest arises when turning attention to the six-fold domain wall vertex structures that constitute a meeting point between oppositely charged domain walls. Preliminary electrical measurements, focused on directly measuring the electrical properties across the vertex, sought to verify the existence of diode-like behaviour and the possibility of naturally forming, one-dimensional PN-diodes. This work is still ongoing.
Finally, a new improper ferroelectric system, the hexagonal tungsten bronze CsNbW2O9, has been characterised through piezoresponse force microscopy (PFM), whereupon six-fold domain wall vertex structures, similar to those in ErMnO3, are observed. Interestingly, a second phase whose domain structure varies significantly from the clover-leaf domain patterns observed in the majority of grains, is also characterised. Unfortunately, direct electrical measurements of what appear to be charged domain walls is yet to be undertaken, due to limitations inherent in the sample structure. These are measurements that are fully intended and will thus stretch beyond the scope of this thesis. Complementary vector PFM and transmission electron micsroscopy studies reveal that the long axis of needle domains that form within these grains aligns with the polar axis. This means that domain walls appear to meander in the a-b plane but lie parallel to the c-axis. As a result, a-c and b-c domains take on needle-like patterns, which might be correlated to c-axis oriented hexagonal and trigonal channels that form during the ferroelectric phase transition. Polarization reversal experiments were also attempted and have been partially successful. Although it appears that relaxation takes place after the application of a direct switching bias, the system does not necessary return to its original state upon relaxation. This would suggest a domain structure in CsNbW2O9 that is malleable and perhaps adjustable, a property that adds fuel to the fire in terms of their implementation within reconfigurable nanoelectronics.
|Date of Award||Jul 2020|
|Supervisor||Marty Gregg (Supervisor) & Amit Kumar (Supervisor)|