On contrast patterns produced by self-interstitial atoms in field ion microscope images of a b.c.c. metal

Abstract

A general method was presented for calculating the contrast pattern to be expected from a single self-interstitial atom (SIA) in a field ion microscope (FIM) image. It was assumed that the SIA was embedded inside a FIM specimen well below the surface. The SIA was uncovered and eventually removed by the field evaporation of successive atomic planes. Atoms on the surface were displaced outwards from the surface as a result of the dilatational field of the SIA. This outward displacement of atoms resulted in 3 possible contrast effects. These 3 contrast effects were: 1. (a) a bright spot; 2. (b) an extra bright spot; 3. (c) a vacant lattice site. A single SIA atom can produce all these contrast effects and the totality of these effects was termed a contrast pattern. The atom displacements were obtained from the α-iron inter-atomic potential of Johnson, without allowing for surface relaxation effects. Detailed atomic displacement maps were presented for the {111} planes with a 〈110〉 split and a 〈111〉 split crowdion SIA configurations located many interplanar spacings underneath these surfaces. It was shown that the resulting atomic displacement maps contained sufficient information to allow the configuration of the SIA to be determined. An atom displaced normal to a surface plane by a SIA was represented by an electric dipole whose strength was a function of the magnitude of the displacement. A local field enhancement factor was then calculated (the possibility of charge redistribution was allowed for in the enhancement factor) and converted to an extra ion current via the Chen-Seidman data on the ion current vs. electric field characteristic curves of individual planes. An application of the above principles was presented for a SIA contrast pattern detected in a (111) plane of tungsten irradiated with 20 keV W + ions at 18°K (Scanlan et al.). This contrast pattern contained the three contrast effects suggested by the model.