It was shown how the sign of a screw dislocation, or of the screw component of a mixed dislocation in a thin elastically isotropic foil viewed end-on, could be deduced from the dark-field weak-beam diffraction contrast arising from surface relaxation displacements. The contrast consisted of black-white lobes, with the line of no-contrast parallel to g; similar to that found previously for the bright-field imaging of screw dislocations in thick foils. Unlike the weak-beam images of inclined dislocations, the image profiles were very broad (about 10nm for the strongest) due to the long-range nature of the surface-relaxation strain-field. For dislocations with a spacing of about 10nm or less, the overlap of the strain-fields from nearby dislocations had to be taken into account. The nature of the contrast from mixed dislocations which were slightly tilted from the incident beam direction, when contrast from the edge component was expected, was considered together with the possibility of determining the sign of the screw component in this case.

Determination of the Sign of Screw Dislocations Viewed End-On by Weak-Beam Diffraction Contrast. P.B.Hirsch, Z.Zhou, D.J.H.Cockayne: Philosophical Magazine, 2007, 87[34], 5421-34