Dislocation contrast in high-angle hollow-cone dark-field TEM

Abstract

Dislocations and grain boundaries have been imaged by high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM). The characteristics of image contrast in HADF-TEM are similar to those in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, or Z-contrast STEM), but different contrast mechanisms could be involved in each case because of differences in the respective scattering geometries. In HADF-TEM, the image contrast depends not only sensitively on the local mass-thickness of the specimen (i.e., Z-contrast), but also much more sensitively on the diffracting conditions that are initially set up for the corresponding on-axis bright-field TEM imaging, as well as on the position of the dislocation in the foil. The dislocations appearing in the HADF-TEM images have a one-to-one correspondence with those present in the corresponding BF-TEM image. The technique thus shares many properties with conventional diffraction contrast imaging. The dislocation contrast in HADF-TEM is generated by a two-step mechanism — the creation of diffuse scattering due to lattice distortion around the dislocation cores and the subsequent channelling propagation of the diffusely scattered electrons parallel to the optic axis, as governed by dynamical diffraction effects. The creation of local diffuse scattering is determined by the first step, and the final image contrast is essentially determined by the second step. It is the last step that makes HADF-TEM imaging analogous in many respects to conventional diffraction contrast imaging. The bright contrast of grain boundaries in HADF-TEM and HAADF-STEM images is at least in part due to high-angle diffuse scattering created by the local atomic rearrangement or relaxation. This is Huang scattering contrast, which usually does not carry atomic-number-sensitive information. It has been demonstrated that HADF- TEM is an easy method for determining the positions of dislocations in the foil thickness direction.