ADF STEM imaging of screw dislocations viewed end-on

This paper presents annular dark-field scanning transmission electron microscope image simulations of a screw dislocation viewed end-on in a thin single crystal of Mo, taking into account surface relaxation (the Eshelby twist). The image contrast can be understood in terms of the effects of the displacements normal to the dislocation arising from the Eshelby twist on the channelling behaviour and interband scattering of the incident beam. With the beam focussed at the entrance surface, the image peak positions reflect the positions of the atoms at the entrance surface. For atomic columns at distances from the core less than the foil thickness, the image peak positions are predicted to lie between the perfect crystal and actual surface atom positions. The predicted intensity distribution of the image is qualitatively similar to that of a published experimental image of a screw dislocation in GaN [Phys. Rev. Lett. 91 (2003) p. 165501]. An assessment is made of the possibility of imaging core displacements by focussing near the foil centre, where surface relaxation effects should be minimised.