Fig.1 Image of a Reconstructed Si (001)-2 × 1 Surface with Terraces. Note that the rows form in alternate directions per terrace. The top terrace (that in the lower left corner of the image) meets the second level with a predominately type B step edge, while the second and third levels are bordered by a predominately type A step edge. Since dimer diffusion along surface rows is highly favored over diffusion across the rows, the two types of step edge may be expected to display different growth rates, etc. Schematics of the different step edge types and the surface reconstruction are given in Figs. 1A and 1B.

Fig.2 Close-up image of a Reconstructed Si (001)-2 × 1 Surface with Defects. Note the individual dimer beads that make up the dimer rows. Many of the dimers are buckled as a result of the presence of defects. The defects in this image are predominately of two main varieties: what have come to be known as the A-type, a single dimer vacancy, and the C-type, a feature that images as if two neighboring dimers were deficient one dimer. The feature in the lower left corner of the image is a C-type defect, while the feature in the lower right is one variety of A-type and the furthest right feature in the third row is another. M. Uchikaway et al. propose that while the former is the site of a single missing dimer, the latter is actually a dimer depressed bellow the surface layer.

M. Uchikawa, M. Ishida, K. Miyake, K. Hata, R. Yoshizaki, H. Shigekawa. Surface. Sci. 357-358 (1996) 468-471.