Computer-simulated electron microdiffraction patterns from MgO crystal surfaces

Abstract

Dynamical diffraction calculations have been carried out for an incident 100 kV electron wavepacket (STEM case), 12 Å in diameter, running parallel to a flat (100) face of a MgO crystal. The microdiffraction patterns and the electron density distributions have been obtained. A surface potential field, representing the modified image force, was added to the crystal potential to study its effect on the small electron probe. The calculation results show that electron microdiffraction patterns are very sensitive to the surface potential for crystal thickness larger than 120 Å. The streakings of the central disk in the microdiffraction patterns can be accounted for as the contributions from surface channeling electrons and electrons being deflected in the surface potential field. Both contributions have their characteristic features in the streakings of the central spot. The forbidden spots, (100) and ( 100 ), which are experimentally observed, can be obtained only after the replacement of the vacuum potential by the modified surface image potential. With the incident beam outside the surface, the presence of the surface image potential field greatly enhances the electron surface monolayer channeling.