Focus Dependence of the Black-White Contrast of Small Point Defect Clusters on Transmission Electron Microscope Images of Crystalline Specimens

Abstract

The diffraction contrast is investigated of small lattice defects on transmission electron microscope images taken with the objective lens focused onto a plane at distance ζ(|ζ|≧ 1μm) apart from the lower specimen surface (out‐of focus contrast). A method is developed for calculating the out of focus contrast in terms of the scattering matrix M as introduced by Häussermann et al. and Katerbau. Starting from an analytical representation of M, adjusted to a two‐dimensional set of data numerically calculated for a small dislocation loop of vacancy type, expressions for the strength of the black—white contrast of small lattice defects are derived. These expressions, which dapend on ζ and are essentially based on the symmetry relations of M as established by Katerbau, are worked out in particular for the so‐called thin‐foil case where the black‐white contrast of small lattice defects vanished under infocus conditions (ζ = 0).