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

Abstract

SummaryWe 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.

Space rocks and optimising scanning electron channelling contrast

“…The present work links to both the excellent single crystal analysis of Winkelmann et al [7], who explore the role of channelling-in and channelling-out within single crystal semiconductors; as well as the identification of the relationship between channelling-in and channelling-out contrast due to the effect of crystal rotations of Kaboli and Gauvin [13]. Importantly for the present study, Winkelmann et al [7] show that the electrons received with a virtual detector placed towards the top of the EBSD phosphor screen highlight terracing on a single crystal growth surface, whereas virtual detectors placed towards the bottom of the EBSD phosphor screen highlight local strain and orientation variations due to threading growth dislocations.…”

“…The present work links to both the excellent single crystal analysis of Winkelmann et al [7], who explore the role of channelling-in and channelling-out within single crystal semiconductors; as well as the identification of the relationship between channelling-in and channelling-out contrast due to the effect of crystal rotations of Kaboli and Gauvin [13]. Importantly for the present study, Winkelmann et al [7] show that the electrons received with a virtual detector placed towards the top of the EBSD phosphor screen highlight terracing on a single crystal growth surface, whereas virtual detectors placed towards the bottom of the EBSD phosphor screen highlight local strain and orientation variations due to threading growth dislocations. The benefit of virtual detector analysis is also highlighted in the work of Nolze et al [14] who explore a range of detection modalities using (largely) electron backscatter pattern (EBSP) based approaches, including a specific note that a significant amount of contrast within virtual FSD detectors is common between the raw EBSP analysis and analysis of only the background signal (and thereby also supporting an assertion that a significant amount of contrast within FSD images is from channelling in phenomena).…”

“…(5) Electron scattering & effective density -where the scattering efficiency is proportional to the electron density of the material within the interaction volume, giving rise to Z-contrast and variable electron energies in the escaping electrons [5]. (6) The position, size, and form of the diode(s) with respect to the sample -which can bias the signal due to variable emission, detection efficiency, noise, and angle subtended by the diode(s) [7]. (7) Voltage of the incoming beam, as this will affect the depth of penetration, and spatial resolution (due to electron optics); as well as channelling-in and channelling-out behaviour (due to diffraction effects related to the wavelength of the electron beam).…”

“…(6) The position, size, and form of the diode(s) with respect to the sample -which can bias the signal due to variable emission, detection efficiency, noise, and angle subtended by the diode(s) [7]. (7) Voltage of the incoming beam, as this will affect the depth of penetration, and spatial resolution (due to electron optics); as well as channelling-in and channelling-out behaviour (due to diffraction effects related to the wavelength of the electron beam).…”

Improving EBSD precision by orientation refinement with full pattern matching

“…An improvement in orientation precision when using a pattern matching approach for standard‐resolution patterns has also been discussed with respect to the orientation data delivered by different EBSD systems (Nicolaÿ et al ., 2018). Increased topographical and structural detail have been shown to be produced from the analysis of the EBSP background intensity and the channelling‐in effects on this parameter (Winkelmann et al ., 2017).…”