Position dependence of the visibility of a single gold atom in silicon crystals in HAADF-STEM image simulation

A modified multislice method was applied to simulate high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) images of a single Au atom embedded in Si(100) Crystals. The minimum size of the electron probe in the optimum defocus condition was small enough to resolve (400) lattice planes of Si (d = 0.136 nm) at 200 kV accelerating voltage. The Au atom had a high image contrast even if the thickness of Si crystals increased up to 54.2 nm. The Si-thickness dependence of the contrast of the Au atom located al the entrance surface was explained in terms of the reciprocal of the image intensity of Si-atomic column. When the Au atom was located at the exit surface, the contrast was strongly affected by the intensity-modification of the probe electrons due to the channelling along Si-atomic column. It was clarified that the intensity of the probe electrons mainly affects the image contrast when the Au atom was located at various depths in the Si crystals.