Tests on the validity of the atomic column approximation for STEM probe propagation

In this contribution we examine the channelling properties of electron probes with a diameter of a few Angstroms in order to determine limits to atomic resolution analysis as they are caused by the interaction between the electron probe and the specimen. We show that if the probe size is not considerably smaller than the interatomic spacings the light atomic columns act as electron traps. We examine the effects of small misplacements of the probe from the atomic column positions and of small specimen tilts on the shape of the channelling peak. Finally, we show that if the columns are not perfectly straight but have a correlated displacement as a function of z (as often occurs close to an interface), the channelling characteristics of the probe can be seriously affected. Depending on the form of the atomic column, and the atomic number of the element occupying the column, a strong dispersion of the probe may occur even at small thicknesses. This has serious consequences for the analytical signal, but also almost certainly for the intensity distribution in high annular dark-field images.