Model for Imaging in High-Resolution Secondary Electron Microscopy

We introduce a model for secondary electron imaging at high resolution, obtained as an atomic-sized probe is scanned across the specimen. We consider the kinematics of the underlying ionization events that are the origin of the signal providing the atomic resolution images. We argue that the short mean-free path for inelastic scattering of the ejected electrons inside the specimen largely renders the directional dependence of the ejected electrons, inherent in the ionization kinematics, unimportant and it suffices to consider appropriately scaled total ionization cross sections. Dielectric screening of the ionization, particularly pertinent for loosely bound electrons, is taken into account using a simple parameterization requiring only the input of a characteristic screening energy. The model employs a straight-forward description of the attenuation of the secondary electron signal from different depths in the specimen, requiring only an effective attenuation length as input. The model is illustrated by application to secondary electron imaging of the surface of the MCM-22 zeolite. Using a generally accepted surface structure, there is good agreement with experiment.