[Paper Review] Conduction at domain walls in insulating Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ thin films

Jill Guyonnet, Iaroslav Gaponenko+2 more|arXiv (Cornell University)|May 1, 2012

Ferroelectric and Piezoelectric MaterialsMaterials Science20 citations

TL;DR

This study demonstrates electric conduction along 180° ferroelectric domain walls in insulating Pb(Zr₀.₂Ti₀.₈)O₃ (PZT) thin films, using conductive-AFM and PFM, revealing highly nonlinear, asymmetric, temperature-dependent, and stable current flow exclusively at domain walls. The conduction is attributed to defect segregation at partially charged domain wall segments, suggesting a general mechanism beyond multiferroic materials like BiFeO₃.

ABSTRACT

Among the recent discoveries of domain wall functionalities, the observation of electrical conduction at ferroelectric domain walls in the multiferroic insulator BiFeO3 has opened exciting new possibilities. Here, we report evidence of electrical conduction also at 180° ferroelectric domain walls in the simpler tetragonal ferroelectric PZT thin films. The observed conduction shows nonlinear, asymmetric current-voltage characteristics, thermal activation at high temperatures and high stability. We relate this behavior to the microscopic structure of the domain walls, allowing local defects segregation, and the highly asymmetric nature of the electrodes in our local probe measurements.

Motivation & Objective

• To investigate whether 180° domain walls in purely ferroelectric PZT exhibit conduction, independent of multiferroic effects.
• To clarify the microscopic origin of domain wall conduction in a simpler system than BiFeO₃, where competing mechanisms like band gap narrowing or polarization discontinuity are less likely.
• To determine whether conduction is intrinsic to the domain wall structure or mediated by defects and charge screening.
• To assess the stability and reproducibility of domain wall conduction for potential device applications.

Proposed method

• Epitaxial PZT thin films (60–70 nm) were grown on SrRuO₃/SrTiO₃ substrates via pulsed laser deposition.
• Domain walls were created by applying a positive DC bias with a conductive AFM tip to reverse polarization.
• Piezoresponse force microscopy (PFM) was used to image domain structure and confirm polarization orientation.
• Conductive-AFM (c-AFM) with a bias up to 10 V was used to map local current flow at the domain walls.
• Measurements were performed in ultra-high vacuum (~3×10⁻¹⁰ mbar) to minimize surface contamination and environmental effects.
• Current-voltage characteristics were analyzed for nonlinearity, asymmetry, and temperature dependence to distinguish domain wall conduction from bulk or switching currents.

Experimental results

Research questions

• RQ1 Does conduction occur specifically at 180° domain walls in insulating PZT, a purely ferroelectric perovskite?
• RQ2 What is the microscopic origin of domain wall conduction in PZT, given the absence of multiferroic or complex polarization discontinuities?
• RQ3 How does the conduction behavior differ from bulk or switching currents in terms of voltage dependence and hysteresis?
• RQ4 What role do defects, particularly oxygen vacancies, play in enabling conduction at domain walls?
• RQ5 How stable is the domain wall conduction over time, and what does this imply for device applications?

Key findings

• Conduction was observed exclusively at 180° domain walls under negative DC bias, with an average current of 25 pA at -1.5 V, while the bulk remained insulating.
• The current signal spatially coincided with the PFM-identified domain wall width, confirming localization to the interface.
• The conduction exhibited strong nonlinear and asymmetric voltage characteristics, with no hysteresis at low bias, indicating stable, pinned domain wall states.
• The current remained stable for up to 4 days at room temperature, demonstrating high temporal stability of the conductive path.
• The observed conduction is attributed to defect segregation at partially charged domain wall segments, particularly oxygen vacancies, which provide trap states for electron tunneling and hopping.
• The results suggest that domain wall conduction is a general phenomenon in ferroelectrics, driven by defect-mediated screening at charged domain wall junctions, not band gap narrowing.