Convergent-beam imaging—a transmission electron microscopy technique for investigating small localized distortions in crystals

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Abstract
A new transmission electron microscopy technique is described, convergent-beam imaging (CBIM), in which an image is formed using a convergent beam focused above or below a thin crystalline specimen. Superimposed on the image are higher-order Laue zone (HOLZ) lines, the displacements of which map the spatial variation of strains, lattice parameters and crystallographic rotations. The image has its normal resolution, the superimposed diffraction information has an angular resolution of about 10−4 rad, and this diffraction information comes from regions of the specimen defined approximately by the cross-over size of the electron probe size, broadened by beam spreading. The visibility of HOLZ lines in the CBIM image is significantly improved by cooling the specimen. The CBIM technique is illustrated by its application to a Si/GexSi1−x strained-layer structure, where spatial changes in the lattice parameter and crystal symmetry are revealed by the relative positions of the HOLZ lines in the Si and alloy layers. Moreover, distortions arising from surface relaxation effects near the Si/GexSi1−x interfaces can be directly visualized from the curvature of HOLZ lines as these lines cross an interface. The CBIM technique has general application to the investigation of crystalline systems where localized variations in strain, lattice parameter, crystal symmetry and crystallographic rotation are of interest.