Coexistence of ferroelectric and ferrielectric phases in ultrathin antiferroelectric PbZrO3 thin films

TL;DR

This study reveals that ultrathin PbZrO3 films exhibit coexistence of ferroelectric and ferrielectric phases, with a transition from antiferroelectric to ferroelectric behavior around 10-15 nm thickness, impacting device design.

Contribution

It provides the first detailed microstructural analysis showing coexistence of phases in ultrathin PbZrO3 films, highlighting the critical thickness for phase transition.

Findings

Transition from antiferroelectric to ferroelectric behavior at 10-15 nm thickness

Presence of ferrielectric phase with unique dipole configuration

Ferroelectric unit cell is half the size of the antiferroelectric phase

Abstract

Whereas ferroelectricity may vanish in ultra-thin ferroelectric films, it is expected to emerge in ultra-thin anti-ferroelectric films, sparking people's interest in using antiferroelectric materials as an alternative to ferroelectric ones for high-density data storage applications. Lead Zirconate (PbZrO3) is considered the prototype material for antiferroelectricity, and indeed previous studies indicated that nanoscale PbZrO3 films exhibit ferroelectricity. The understanding of such phenomena from the microstructure aspect is crucial but still lacking. In this study, we fabricated a PbZrO3 film with thicknesses varying from 5 nm to 80 nm. Using Piezoresponse Force Microscopy, we discovered the film displayed a transition from antiferroelectric behaviour in the thicker areas to ferroelectric behaviour in the thinner ones, with a critical thickness between 10 and 15 nm. In this critical thickness range, a 12 nm PZO thin film was chosen for further study using aberration-corrected scanning transmission electron microscopy. The investigation showed that the film comprises both ferroelectric and ferrielectric phases. The ferroelectric phase is characterized by polarisation along the pseudocubic [011] projection direction. The positions of Pb, Zr, and O were determined using the integrated differential phase contrast method. This allowed us to ascertain that the ferroelectric PbZrO3 unit cell is half the size of that in the antiferroelectric phase on the ab plane. The observed unit cell is different from the electric field-induced ferroelectric rhombohedral phases. Additionally, we identified a ferrielectric phase with a unique up-up-zero-zero dipole configuration. The finding is crucial for understanding the performance of ultrathin antiferroelectric thin films and the subsequent design and development of antiferroelectric devices.