Channeling of Aberration-corrected STEM Probes at the "Sub-atomic" Scale
Microscopy and Microanalysis
The phenomenon of fast electron channeling in atomic crystals has long been appreciated as an important factor in TEM characterization, being particularly critical for consideration in scanning transmission electron microscopy (STEM). As such, electron channeling effects have been examined to understand the thickness-dependence of annular-dark-field (ADF) STEM image contrast , the emergence of atomic-scale core-loss electron-energy-loss spectroscopy (EELS) spectrum image contrast , the
... ontrast , the orientation-dependence  and spatial localization  of X-ray energy dispersive spectroscopy (XEDS) signals, and the detectability of dopant atoms  ; STEM imaging has, in turn, recently been used to experimentally measure electron channeling behavior  . The scattering of on-column intensity is the primary contributor to ADF and spectrum image signals in STEM, thus electron channeling studies of crystals have focused on the on-column intensity oscillations of converged probes centered on (or very slightly off of) atomic columns. However, to better understand the evolution of channeling behavior of fine aberration-corrected STEM probes when scanned over a crystal, we have employed multislice [7, 8] simulations of probe propagation wherein a two-dimensional intensity map is saved at every z-slice for each incident probe position. The TEMSIM multislice package  was used to simulate aberration-corrected STEM probes (accelerating voltages 100-300 kV, probe convergence semi-angles 20-25 mrad) in various crystals, averaging over many frozen phonon configurations at room temperature. Simulation inputs were generated both by using the default parameterization tables of TEMSIM and by parameterizing projected atomic potentials calculated via the Quantum Espresso package  . Simulated data for <100>-oriented SrTiO 3 (Figure 1 ) and <2110>-oriented wurtzite AlN (Figure 2 ) both exemplify the "sub-atomic" lateral complexity of channeling behavior, especially of off-column probes. Implications of such channeling behavior for high resolution ADF-STEM imaging, EELS and EDS spectrum imaging, and EELS fine structure features will also be discussed  .