Charged ferroelectric domain walls

: The next generation in 2D functional materials

Abstract

In this thesis I present my investigations into the properties of charge carriers in conducting ferroelectric domain walls in single crystals of YbMnO3, ErMnO3 and lithium niobate.

Through the use of atomic force microscopy (AFM), electron microscopy and electrical measurements I measured Hall mobility in these systems. This revealed that carriers in domain walls are significantly more mobile than charge carriers measured in bulk ferroelectrics. In the rare-earth hexagonal manganites, YbMnO3 and ErMnO3, I worked on the development of local potential mapping techniques in AFM such as Kelvin probe force microscopy (KPFM) to spatially map the Hall potential on the nanoscale. This allowed the behaviour of the wall to be established independently of the influence of the surrounding domain and the contact resistances at electrodes.

Mobilities of 60cm2V−1s−1 and 600cm2V−1s−1 were measured at conducting domain walls in YbMnO3 and ErMnO3 respectively.

In lithium niobate, I have worked on a range of systems with conducting domain walls and explored different approaches to measuring the Hall mobility. Initially I attempted to establish temperature dependence of domain wall conduction. This demonstrated that ionic migration and aggregation reduces conduction in domain walls. The KPFM approach was also developed further.

Finally I worked on ion-sliced samples of lithium niobate. The small thickness of these samples enabled electric field control of the material’s DC resistance state over several orders of magnitude. Exploration of the microstructure led to a new experiment. The closed topology of these domain walls was shown to behave equivalently to a Corbino disc geometry known to exhibit magnetoresistance and a Hall mobility of 2970 ± 230cm2V−1s−1 has been measured. This Hall mobility is greater than any carrier mobility measured in lithium niobate, suggesting that carriers at the domain wall experience a very different potential environment to those in bulk.

Date of Award	Jul 2021
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Marty Gregg (Supervisor) & Solveig Felton (Supervisor)