Imaging 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).