In Situ Characterisation of Ephemeral p-n Junctions inside Ferroelectric Domain Walls

Ferroelectric materials are renowned for their distinctive microstructure, characterised by a patchwork of domains where local electrical dipoles align, separated by domain walls that act as interfaces between these domains. When subjected to applied fields, certain domain orientations become stabilised and tend to grow, while unstable ones diminish. Consequently, the movement of domain walls becomes an inherent necessity. The creation of new domains necessitates the formation of domain walls, while the disappearance of domains leads to the disappearance of walls. Therefore, domain walls exhibit both mobility and transience in response to varying field conditions. What's particularly noteworthy is that while most ferroelectrics possess inherent electrical insulation properties, domain walls can exhibit conductivity [1]. In such cases, domain walls serve as quasi-2D electrical pathways within an otherwise insulating ferroelectric matrix. This intriguing characteristic paves the way for envisioning dynamically reconfigurable domain walls capable of creating, erasing, and rewriting entire nanoscale circuits in different configurations.<br/>Our team has devoted several years to the study of conducting domain walls in thin film heterostructures of lithium niobate (LNO) [2-4]. It is now recognised that when a modest positive bias is applied, the domain walls exhibit a remarkable behaviour, tilting away from the polar axis and forming what are known as charged head-to-head (n-type) domain walls, which exhibit strong conductivity. In this study, we perform <i>in situ</i> cross-sectional transmission electron microscopy (TEM) imaging of bias-induced domain wall dynamics, allowing us to “live” image the domain wall tilting under applied electric fields.<br/>This investigation has yielded a remarkable finding: certain sections of conducting domain walls transition from n-type to p-type as the direction of the applied bias changes - a result that has never been observed before and is incredibly exciting. It seems that we are creating a new kind of dynamically formed domain wall p-n junction. Although p-n junctions are commonly found in improper ferroelectric systems like boracites and rare-earth manganites, what we have observed in focused ion beam (FIB) slices of LNO represents a unique phenomenon. The in-wall p-n junctions (which could be described as ‘zero-dimensional’ entities due to the one-dimensional nature of the domain walls) only exist within a finite bias range, and hence are truly ephemeral in nature.<br/>Through monitoring the current-voltage response, we observe striking diode-like characteristics. Moreover, thanks to the dynamic nature of these <i>in situ</i> TEM studies, we have been able to evaluate the switching mechanisms involved in the formation of these conductive pathways and transient p-n junctions. This work is expected to provide valuable insights into in-wall p-n junctions and contribute significantly to the advancement of domain wall nanoelectronics.<br/><br/>[1] J. Seidel <i>et al.</i> <i>Nat. Mater.</i> 8, 229 (2009)<br/>[2] J. P. V. McConville et al. Adv. Funct. Mater. 30, 202000109 (2020)<br/>[3] C.J. McCluskey <i>et al.</i> <i>Adv. Mater.</i> 34 2204298 (2022)<br/>[4] A Suna et al. <i>Advanced Physics Research</i>, 2200095 (2023)