COHERENT OVERLAPPING LACBED PATTERNS IN 6H SIC

In a previous publication, it was shown that interference fringes due to the coherent addition of amplitudes were observed in the overlap regions of adjacent discs in convergent beam electron diffraction (CBED) patterns recorded with a defocused probe in a conventional transmission electron microscope equipped with a cold field-emission source (Hitachi HF-2000). This research has been extended to describe coherent large-angle CBED (LACBED) patterns from 6H SiC, where the angular field of an overlap region is expanded by use of a small (1 mum) selected-area aperture that transmits only a restricted set of beams. The number of fringes visible within an overlap is increased to over 100, where the fringe shifts and rotations are directly related to angular variations in the relative phase of the two beams. In a direct analogy with processing of images produced by electron holography, it is shown that coherent LACBED patterns encode the amplitude product and the phase difference of the overlapped beams. Examples are discussed where the glide plane in 6H SiC produces a pi phase shift of fringes across the Gjonnes-Moodie dark bar, equivalent to a direct representation of the glide operator. The minimum demagnified source size in the object plane is 3.5 angstrom, suggesting that fringes with 5 angstrom period should be visible in the diffraction plane.