Image calculations for REM are usually difficult because it is necessary to incorporate surface defects, such as steps and dislocations. This, in principle, can be done with the PeTSM theory; but, in practice, the huge amount of computation required and the sensitivity of REM image contrast to focus, beam convergence and diffracting conditions make the calculations rather involved and difficult compared with experimental observations. Attempts have been made to simulate surface step images (Peng and Cowley, 1986; Ma and Marks, 1990), but the results are still not very satisfactory. For this reason, in this chapter, the contrast mechanisms of REM imaging are described based on simplified models, in order to illustrate the physical concepts.

There are four basic contrast mechanisms in REM. Phase contrast (or Fresnel contrast) is produced by the path-length difference (or phase shift) of electron waves due to the change in surface morphology. This contrast dominates the images of atom-high surface steps. High-resolution information can be obtained from the phase contrast mechanism. Diffraction contrast (or Bragg contrast) is produced by the variation of local Bragg reflection angles due to lattice distortion from dislocations, local strain and crystal boundaries. Compositional contrast is produced by a variation in scattering power of different elements. This type of contrast appears on different surface domains or regions, depending on the local composition, and usually has relatively lower resolution. Finally, geometrical (or morphology) contrast is produced by variation in surface geometry, such as large surface steps and facets.