Three-dimensional strain-field information in convergent-beam electron diffraction patterns

Abstract

Convergent-beam electron diffraction (CBED) patterns contain diffracted beams from higher-order Laue zones (HOLZ), in addition to the more often observed diffracted beams from the zeroth-order reciprocal lattice zone (ZOLZ) that contains the origin of the reciprocal lattice. Since HOLZ diffraction vectors have non-zero components along the incident electron beam direction, they can detect components of static real-lattice displacement fields that lie along the incident electron beam direction, an event not possible for diffracted beams normal to the incident electron beam direction (i.e. ZOLZ diffracted beams). This effect is used in the present work to determine Burgers vectors of straight dislocations and loops in silicon from observations of splitting of HOLZ lines within the forward scattered beam Bragg disk, and from splitting of Kikuchi lines associated with HOLZ Bragg reflections. The method was also applied with limited success to dislocations in aluminum; here splitting was more difficult to observe because of the rather strong diffuse background in the CBED patterns. Calculations of HOLZ line splitting due to the presence of a dislocation in the irradiated crystal volume were in good qualitative agreement with the experimental observations. Effects on CBED patterns to be expected from some partial dislocations are discussed. This CBED method can be very useful for the determination of non-ZOLZ fault vector components in an atomic resolution structure-imaging microscope. These instruments usually have specimen tilt ranges limited to about 10°; thus conventional Burgers-vector analysis is not generally possible. Finally, the present CBED results show clearly that the projection approximation generally used to interpret structure fringe images is not strictly valid. the changes in fringe images or weak-beam images from HOLZ excitations remain to be evaluated.