Surface characteristics

Within the crystal the bonding orbitals of all the atoms are satisfied and each type of atom is in the same environment as its counterparts. In contrast to this, the surface atoms have dangling bonds that are not satisfied, and this deficiency of chemical bonding can be characterized quantitatively by a surface energy per unit area.

The [111] surface of a silicon or germanium crystal consists of a hexagonal array of Si atoms in the layer below the surface. The energy is lowered by the surface reconstruction in which surface atoms move together and bond to each other in pairs to accommodate and satisfy the broken dangling bonds. Another phenomenon that sometimes takes place is surface structure relaxation in which the outer layer of atoms moves slightly toward or a short distance away from the layer below. Contraction takes place with most metal surfaces. The Silicon[111] surface layer contracts by about 25% and the three interlayer spacings further below compensate for this by expanding between 1% and 5%.