Techniques for studying surfaces in TEM

There are two basic modes of TEM operation, namely the bright-field mode, wherein the (000) transmitted beam contributes to the image, and the dark-field imaging mode, in which the (000) beam is excluded. The size of objective aperture in bright-field mode directly determines the information to be emphasized in the final image. When the size is chosen so as to exclude the diffracted beams, one has the configuration that is normally used for low-resolution defect studies, the so-called diffraction contrast. In this case, a crystalline specimen is oriented to excite a particular diffracted beam, or systematic row of reflections. This imaging mode is sensitive to the differences in specimen thickness, distortion of crystal lattices due to defects, strain and bends. High-resolution imaging is usually performed in bright-field mode by including a few Bragg-diffracted beams within the objective aperture. The lattice images are the result of interference between Bragg reflected beams, the so-called phase contrast. In this section, we outline a few techniques that have been extensively developed for studying surfaces in high-resolution TEM (HRTEM) (Cowley, 1986; Smith, 1987).

Imaging using surface-layer reflections

The dark-field imaging technique is favorable for imaging surfaces when the surface layers have different periodicity from that of the bulk of the crystal (Figure 5.1 (a)). Then ‘superlattice’ satellite reflections appear in the diffraction pattern and dark-field imaging with these spots can give high-contrast images of surface structure.

There are two basic mechanisms for generating the superlattice reflections. A sharp termination of the crystal bulk at the surface may form a stacking sequence that is not a complete unit cell.