Atomic structure and electronic properties of MgO grain boundaries in tunnelling magnetoresistive devices

Hideo Ohno,Hideo Sato,Jonathan J Bean,Shunsuke Fukami,Shoji Ikeda,Keith P Mckenna,Mitsuhiro Saito,Yuichi Ikuhara

doi:10.1038/srep45594

Abstract

Polycrystalline metal oxides find diverse applications in areas such as nanoelectronics, photovoltaics and catalysis. Although grain boundary defects are ubiquitous their structure and electronic properties are very poorly understood since it is extremely challenging to probe the structure of buried interfaces directly. In this paper we combine novel plan-view high-resolution transmission electron microscopy and first principles calculations to provide atomic level understanding of the structure and properties of grain boundaries in the barrier layer of a magnetic tunnel junction. We show that the highly [001] textured MgO films contain numerous tilt grain boundaries. First principles calculations reveal how these grain boundaries are associated with locally reduced band gaps (by up to 3 eV). Using a simple model we show how shunting a proportion of the tunnelling current through grain boundaries imposes limits on the maximum magnetoresistance that can be achieved in devices.

Highlights

In this study, we employ annular bright-field (ABF) and annular dark-field (ADF) imaging with an advanced scanning transmission electron microscopy (STEM) microscope (JEOL JEM-ARM200F) installed with a spherical aberration corrector (CEOS GmbH) to resolve the atomic structure of frequently occurring grain boundaries (GBs) within an ultrathin MgO film buried inside a multilayer stack
We prepare a sample for plan view imaging with STEM that consists of only the MgO layer from this device
Chains of structural units (SUs) of this type are analogous to segments of Σ5(210)[001] symmetric tilt GBs

Summary

Introduction

We employ annular bright-field (ABF) and annular dark-field (ADF) imaging with an advanced STEM microscope (JEOL JEM-ARM200F) installed with a spherical aberration corrector (CEOS GmbH) to resolve the atomic structure of frequently occurring GBs within an ultrathin MgO film buried inside a multilayer stack. Atomic level imaging of the structure of GBs in films has not been obtained since the [001] texture of the films means that atomic columns at GBs are unlikely to be aligned with the imaging direction. To image these GBs with atomic resolution we prepare samples for plan-view imaging by stripping all layers from the multilayer stack leaving only the MgO layer (e.g. see Fig. 1a). These results highlight how by combining advanced imaging and first principles theoretical calculations one can gain invaluable insight into the effect of GBs on the electronic properties of thin oxide films and guide improvements in device performance

Methods

Results

Discussion

Conclusion