Real materials, however, most exhibit mixtures of these types of bonds. In the case of many semiconductors which are tetrahedrally-bonded such as silicon, germanium, GaAs, and InP, the directionality of the bonds in the bulk will dramatically affect and may result in directed bonds at the surface. Figure 10 below shows some possible arrangements of atoms that are typically observed on surfaces. The figure caption explains much of what is being shown in the figure. Relaxation occurs when there is a reduction or expansion in the atomic spacing in the direction perpendicular to the surface, the c direction.
Reconstruction occurs when there is an in plane reconfiguration of the atoms.
A schematic view of the characteristic rearrangements of surface atoms of the simple cubic lattice is shown. The lattice constant of the bulk material is a. As one might expect, real surfaces can be quite a bit more complex than those in Figure 10. The reconstructed surface that was shown in Figure 2 is the result of the strong, directional nature of the covalent bonding in GaAs. Determining these atomic postions is difficult and involves a variety of experimental techniques, some of which we'll learn about in the next sections of the course.