Sub-nm2 ferroelectric domains via charged 180 degree walls in ZrO2

TL;DR

This study reveals the formation of ultra-narrow ferroelectric domains in ZrO2 thin films, stabilized by charged 180-degree domain walls, with potential applications in low-voltage nanoelectronics.

Contribution

It provides direct observation and analysis of charged 180-degree domain walls in ZrO2, demonstrating their role in creating sub-nm2 ferroelectric domains and their electrical properties.

Findings

Domains as small as 1-2.75 nm thick and sub-nm2 in area.

Charged domain walls are stabilized by interstitial oxygen atoms.

HH walls are highly mobile with ultralow propagation barriers.

Abstract

Flat phonon bands in fluorite ferroelectrics (HfO2 or ZrO2) shrink polar domains laterally to an irreducible half-unit-cell width (0.27 nm) within which the vertical arrangement of dipoles is expected to remain uniform. We report on the direct observation of nonuniform and nearly discrete vertical arrangements of dipoles in ZrO2 thin films consisting of closely spaced head-to-head (HH) and tail-to-tail (TT) charged 180 degree walls, each exhibiting a distinct bulk-like structure. These charged domain walls (CDWs) further compress the irreducibly narrow, laterally stacked domains vertically to a thickness of 1-2.75 nm, yielding in-plane domains with sub-nm2 footprints-among the smallest ever reported for any ferroelectric material. The HH and TT walls form due to their flat longitudinal optical (LO) polar bands and are electrostatically stabilized by bound-charge compensation via interstitial oxygen atoms, which act as natural structural defects at the HH walls. Moreover, these walls are predicted to be conducting and to exhibit ultralow propagation barriers, with HH walls (1.6 meV) being far more mobile than TT walls (22.3 meV), indicating strong potential for low-voltage, domain-wall-based nanoelectronics.