Superior polarization retention through engineered domain wall pinning

Dawei Zhang,Valanoor Nagarajan,Xuan Cheng,Fan Ji,Matthew Weyland,Jan Seidel,Pankaj Sharma,Vivasha Govinden,Daniel Sando

doi:10.1038/s41467-019-14250-7

Abstract

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices.

Highlights

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field
Ferroelectric materials, characterized by a spontaneous polarization that can be switched by an external electric field, are currently widely investigated for developing lowvoltage non-volatile nanoelectronics[1,2,3]
Distinct directional polarization states (+P and −P) in ferroelectrics can represent the computational 0 and 1 states used in binary systems, utilized, e.g., in non-volatile ferroelectric random access memory (FeRAM)

Summary

Introduction

Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Increasing the film thickness to ~30 nm triggers a strain relaxation process, whereby a mixed-phase state comprising a tetragonal-like (T’) phase and a rhombohedral-like (R’) phase emerges[32,33] This mixed-phase BFO system is a fertile ground for intriguing physical properties including large piezoelectric responses[34,35] and fieldinduced strains[36], electrochromic effects[37], non-zero magnetic moments[38], electrical conductivity[39,40,41], interesting mechanical properties[42,43], and is a suitable system for reducing the polarization retention loss[30]. This effectively pins the domain walls and impedes domain backswitching as evidenced by a large increase in activation fields for domain wall motion, and very long polarization retention is achieved

Methods

Results

Conclusion