Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications.

A Winkelmann,G Naresh‐Kumar,S Vespucci,M Vos,C Trager‐Cowan,A.J Wilkinson,G Nolze,A Vilalta‐Clemente

doi:10.1111/jmi.12571

Abstract

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modelling of the energy- and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

Highlights

The image in the scanning electron microscope (SEM) can change completely if the position of the collector is changed.O
In order to shortly recall the basic understanding of the combined role of the diffraction of incident and outgoing pathways in EBSD measurements (Reimer, 1998; Wells, 1999), we show in Figure 2 measurements that were taken under controlled conditions for incident and exit directions using a special electron spectrometer with a fixed geometry between the incident beam and the detected outgoing directions
The modulated total number of backscattered electrons is subsequently diffracted on the outgoing pathway (→ EBSD)

Summary

Introduction

The image in the SEM can change completely if the position of the collector is changed.O. With the widespread availability of advanced two-dimensional (2D) imaging detectors in the SEM, an improved experimental characterization of the pertinent effects involved and their correct theoretical description by computational simulations acquires renewed importance (Wells et al, 2012; Jaksch, 2012; Walker et al, 2016; Forbes & Allen, 2016) This is the more so important in the case of crystalline materials, for which the signal generation process can obviously be influenced by diffraction effects due to the local crystal lattice, providing orientation contrast in the SEM (Prior et al, 1996; Day & Quested, 1999; Wright & Nowell, 2006; Day, 2009; Schwarzer & Sukkau, 2013; Wright et al, 2015)

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Conclusion