Phase contrast scanning transmission electron microscopy imaging of light and heavy atoms at the limit of contrast and resolution

Emrah Yücelen,Ivan Lazić,Eric G T Bosch

doi:10.1038/s41598-018-20377-2

Abstract

Using state of the art scanning transmission electron microscopy (STEM) it is nowadays possible to directly image single atomic columns at sub-Å resolution. In standard (high angle) annular dark field STEM ((HA)ADF-STEM), however, light elements are usually invisible when imaged together with heavier elements in one image. Here we demonstrate the capability of the recently introduced Integrated Differential Phase Contrast STEM (iDPC-STEM) technique to image both light and heavy atoms in a thin sample at sub-Å resolution. We use the technique to resolve both the Gallium and Nitrogen dumbbells in a GaN crystal in [{bf{10}}bar{{bf{1}}}{bf{1}}] orientation, which each have a separation of only 63 pm. Reaching this ultimate resolution even for light elements is possible due to the fact that iDPC-STEM is a direct phase imaging technique that allows fine-tuning the microscope while imaging. Apart from this qualitative imaging result, we also demonstrate a quantitative match of ratios of the measured intensities with theoretical predictions based on simulations.

Highlights

Without any doubt transmission electron microscopy ((S)TEM) is one of the most powerful structural characterization tools available to numerous science and engineering disciplines
The result is an imaging technique that images roughly the square of the atomic number Z. This is why (HA)ADF-scanning transmission electron microscopy (STEM) is commonly referred to as Z-contrast imaging but it is at the same time the reason why it is insensitive to light elements such as O, N, C, B and Li when imaged together with heavier atoms like Si, Ga, Sr, Au etc
In this work we demonstrate imaging at the limit of contrast and resolution on Wurtzite GaN samples prepared in two different orientations ([1011] and [1120]) using ADF-STEM and iDPC-STEM simultaneously

Summary

Introduction

Without any doubt (scanning) transmission electron microscopy ((S)TEM) is one of the most powerful structural characterization tools available to numerous science and engineering disciplines Thanks to their negative charge, strong interaction occurs between the probing electrons and the electric field produced by the atoms, thereby making (S)TEM a tool that is uniquely suited for visualization of structures composed of different elements, from the very lightest to the very heaviest. The resolution limits imposed by the limitations of electromagnetic round lenses, have been overcome by successful construction and integration of aberration correction units into electron microscopes[2] This increase in resolution resulted in increased measurement precision and development of new possibilities for analysis at the atomic scale. Conventional (S)TEM imaging techniques provide atomic scale images of the phase of the transmission function, which can be interpreted as the projected potential for a thin sample, through different contrast transfer mechanisms. They involve sometimes iterative reconstruction schemes, which are not always converging[17], are restricted to the weak phase object approximation[14,15], and deal with ill-conditioned problems such as deconvolution[14,15,16]

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Conclusion