2.7 Defects at Surfaces

References

Luth, p.92,93;
Woodruff and Delchar,section 2.5;
Somorjai, Chapter 2;
Low Energy Electrons and Surface Chemistry , Ertl and Kuppers, Chapter 9.
Section 1.8 of Dr Roger Nix's Surface Chemistry course at Queen Mary & Westfield College

Up to now we have considered only ideal surfaces. On a real surface defects will always be present. For a clean surface (i.e. a surface without any additional adsorbed atoms or molecules) a number of defects may be present (see, for example, Fig. 5.7 of Prutton and Fig. 3.8 of Luth). Single atom defects include:

Vacancies - i.e. missing atoms;
Adatoms - i.e. additional atoms;
Anti site defects. These occur in compounds. For examples, in GaAs an antisite defect occurs when an As atom occupies a Ga site or vice versa.
Interstitials. These involve atoms that adopt positions between lattice sites.

A very common extended, one dimensional defect is the atomic step. Atomic steps separate atomic terraces. Although single atomic steps (with height=interplanar separation) are most common, depending on the surface and its prior processing, steps can also bunch together in multiples of the interplanar separation. Dislocations form another very common surface defect but we will not deal with dislocations in this module.

To see just how common single atomic defects and steps are, have a look at the STM image of the Si(100)(2x1) surface shown in Fig. 2.16(a). This is an image of a 150 nm x 150 nm region of a Si(100) sample where 6 atomic steps are visible.

Fig. 2.16(b) shows the atomic step and atomic scale defects in closer detail, where vacancies in the dimer rows on both the upper and lower atomic terrace are highlighted with circles. (Ignore the "1" and "2" labels on the image).

Both steps and atomic scale defects such as vacancies strongly affect the local properties of the surface and thus play a very important role in the adsorption of atoms and molecules and the growth of atomic and molecular layers. We will discuss these issues (and the type of defects that occur in adsorbed layers) in some detail in Section 4 of the module.

What is the height of a single atomic step on Si(100)? On Si(111)?

In Fig. 2.16(b) it is evident that the dimer rows on the upper terrace run in a direction perpendicular to the dimer rows on the lower terrace. Explain, with the aid of sketches of the Si(100) surface structure, why this is so.

End of Section 2
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