Waking up metallic-like conduction in nominally neutral domain walls in Pb(Zr,Ti)O₃ films

Domain wall electronics is an attractive concept for circuits based on two-dimensional domain-wall channels formed within a normally insulating ferroelectric material. Domain wall (DW) responsiveness to the electrical and mechanical stimuli, and their plasticity show a promise for neuromorphic networks. Charged domain walls are particularly attractive because of high conductivity, however they are often unstable, or require special poling conditions, which limits their use. Neutral DWs form more stable and controllable configurations, however they typically show low conductivity making single DW elements problematic. A promising solution comes from thin films where the interplay between the ferroelectric lattice distortion and the mismatch of substrate and film lattice parameters transform nominally neutral DWs to partially charged ones. We studied this phenomenon using tetragonal Pb(Zr,Ti)O₃ (PZT) with simple and well studied domain patterns. We show robust non-thermally-activated metallic-like DW conduction at 180° ferroelectric DWs and 90° ferroelastic DWs using scanning probe microscopy under controlled environment and ultra-high vacuum, within a wide temperature range.
The results suggest that that domain wall conduction in tetragonal PZT films is an omnipresent phenomenon occurring within a range of strain conditions, for PZT deposited on different substrates. However, DW conduction in PZT is rarely observed on as-grown specimens and typically require activation via a combination of special temperature profile and gas environment. In this context, the previously proposed
analysis and phase-field simulations showing an interplay between a- and c-domains in strained PZT have been completed with defect chemistry considerations. In particular we focus on the role of oxygen vacancies and other charged defects in activation of metallic-like DW conduction. Our results suggest that
metallic like conduction in nominally uncharged DWs can be a widespread phenomenon, and it is possible to
to activate and stabilize it in a way relevant to information processing applications.