Die Kantenstreifung im elektronenmikroskopischen Bild würfelförmiger MgO-Kristalle bei Durchstrahlung in Richtung der Raumdiagonale

Abstract

In electron microscope images of MgO crystals orientated with their {111} lattice planes normal to the direction of the incident electron beam a sine-forme periodicity of fringe structure parallel to the crystal edges is mostly observed. This simple fringe structure cannot generally be expected since six strong diffractions (¯202), (0¯22) etc. are simultaneously excited. — In such micrographs spacing and contrast of the fringes vary strongly while from the projection images of the crystals there are no remarkable differences in crystal orientations. For this case, the region near the [111]-direction of incidence, the fringe structure was theoretically investigated by Eigenvalue method and Bethe approximation. In exact [111] direction, only two wave fields exist which produce sinusoidal fringes. Small deviations from the [111] direction keep the two wave fields prevailing, the fringe structure remaining nearly sinusoidal, but induce strong variation of spacing and contrast. The theoretical results are in qualitative agreement with the observed fringes of micrographs, but are not yet suitable for quantitative comparison because of the considerable aperture of illumination (10⁻³), used in the observation.