Probing Charge Accumulation at SrMnO3/SrTiO3 Heterointerfaces via Advanced Electron Microscopy and Spectroscopy.

Hongguang Wang,Min Yi,Xijie Jiang,Peter A. van Aken,Jochen Mannhart,Vesna Srot,Rotraut Merkle,Robert W. Stark,Yi Wang,Hans Boschker

doi:10.1021/acsnano.0c01545

Abstract

The last three decades have seen a growing trend toward studying the interfacial phenomena in complex oxide heterostructures. Of particular concern is the charge distribution at interfaces, which is a crucial factor in controlling the interface transport behavior. However, the study of the charge distribution is very challenging due to its small length scale and the intricate structure and chemistry at interfaces. Furthermore, the underlying origin of the interfacial charge distribution has been rarely studied in-depth and is still poorly understood. Here, by a combination of aberration-corrected scanning transmission electron microscopy (STEM) and spectroscopy techniques, we identify the charge accumulation in the SrMnO3 (SMO) side of SrMnO3/SrTiO3 heterointerfaces and find that the charge density attains the maximum of 0.13 ± 0.07 e–/unit cell (uc) at the first SMO monolayer. Based on quantitative atomic-scale STEM analyses and first-principle calculations, we explore the origin of interfacial charge accumulation in terms of epitaxial strain-favored oxygen vacancies, cationic interdiffusion, interfacial charge transfer, and space-charge effects. This study, therefore, provides a comprehensive description of the charge distribution and related mechanisms at the SMO/STO heterointerfaces, which is beneficial for the functionality manipulation via charge engineering at interfaces.

Highlights

The last three decades have seen a growing trend toward studying the interfacial phenomena in complex oxide heterostructures
With the advancement of methodologies and instrumentation, analytical scanning transmission electron microscopy (STEM) has become a highly suitable method to study the distribution of charges at the interface, because it can reveal the charge density and ordering at the atomic scale by electron
A high-angle annular dark-field (HAADF)-STEM image at lower magnification shows several nearly equidistant cracks in the SMO thin film

Summary

Introduction

The last three decades have seen a growing trend toward studying the interfacial phenomena in complex oxide heterostructures. Many efforts have been devoted to figuring out the interfacial charge state and its impact on the properties of materials This task is still technically challenging, (i) because the characteristic scale for oxide interfaces is at the nanometer level, which makes direct observation of interface effects demanding, and (ii) because structural rearrangements and chemical environments at oxide interfaces are very complex. In a recent study, using core-loss EELS analysis, Cheng et al identified a type of charge ordering at an interfacial MnO layer and the induced polarization of thin films, while the origin of charge ordering is only discussed from the perspective of interfacial reconstruction.[29] Likewise, Garcia-Barriocanal and co-workers examined the charge density at a manganite/titanate (LMO/STO) interface and reported a charge-leakage process controlled by the relative thickness ratio.[30] the detailed mechanism for the charge distribution was not explained. It is essential to gain more insights into the detailed mechanism of interfacial charge distribution, which is vital for both fundamental understanding and application research

Methods

Results

Conclusion