Theoretical and observed electron microscope images of impurity atoms in thin crystals formed by L-shell ionization electrons

Abstract

Theoretical electron microscope images and electron diffraction patterns of aluminum and silicon crystals formed by electrons that have undergone losses at the inner shells of an aluminum or a silicon atom are discussed. The inelastic scattering factors for AlL 1 and SiL 3 ionizations are derived by the program of modified hydrogenic L-shell cross section. By embedding the inelastic event in a multi-slice computation for elastic scattering the intensity distribution of electron waves at the bottom surfaces of the crystals are derived. Then the contrast of the electron microscope images and intensity distribution of electron diffraction patterns are obtained by taking into account the spherical aberration of the lens and the coherence of the electron waves. It is concluded that bright spots appear at individual atomic columns containing atoms that have undergone inner-shell ionizations. 200 kV electron microscope images of Al-1.2wt%Si crystals containing precipitated Si crystals are recorded by using 119 ± 2.5 eV loss electrons, corresponding to the losses at inner shells of AlL i and SiL 2,3. From these images have been subtracted the images of the same area formed by using 88 ± 2.5 eV electrons (background of EELS). The obtained image contrast is discussed in the light of calculated results. We conclude that bright spots appearing in the images of Si single crystals and in Al matrices thinner than 15 nm are the images of individual atomic columns containing Si atoms.