Extrinsically controlled electronic behavior at improper ferroelectric domain walls

Abstract

Ferroelectric domain walls are natural interfaces, separating volumes with uniform polarization orientation. The walls have a width of a few Ångstrøm and they can develop completely different electronic properties than the surrounding domains, making them interesting as functional units for next-generation nanotechnolgy. While the intrinsic properties and underlying physics of ferroelectric domain walls are relatively well understood, much less is known about the influence of external parameters (e.g., oxygen partial pressure or contact effects). In this thesis, I explore how external parameters can be used to control the local electronic properties of ferroelectric domain walls and study their general potential for the development of domain wall-based sensor technology. Using the hexagonal manganites as a model system, the electronic domain wall-response to changes in temperature and oxygen partial pressure is monitored. By annealing in reducing and oxidizing conditions, the conductance of neutral domain walls can be reversibly changed from insulating to conducting. This sensitivity to environmental conditions demonstrates the general possibility to use domain walls for translating, e.g., changes in oxygen partial pressure or temperature into electrical signals. To learn about contact phenomena that are crucial for the read-out of such electronic signals, the influence of metal-semiconductor interfaces on the domain walls is explored. Interfacial barrier effects are found to have a significant influence on the bulk conductance, extending micrometers into the system. The domain wall conductance is, however, affected on much shorter length scales. This observation highlights the importance of interface-related effects when integrating domain walls into electronic devices and provides an additional degree of freedom that can be leveraged for their design. Another important aspect addressed in this thesis is the formation of scanning moiré fringes in conductive atomic force microscopy. The fringes co-determine the conductance measured at the nanoscale and arise due to a superposition of extrinsic and intrinsic phenomena. The results are crucial for the understanding of local transport measurements and emergent conduction phenomena at the nanoscale in general. The results gained as part of this thesis provide new insight into the impact of external parameters on the electronic domain wall properties and show novel possibilities for the integration of ferroelectric domain walls into future electronic devices. This is of particular interest for the development of atmospheric sensors, adding a new direction to the field of domain wall nanotechnology.