12 CHAPTER 1. STRUCTURAL PROPERTIES OF SEMICONDUCTORS
Figure 1.8: (b) This transmission electron microscope picture shows the precision with which
semiconductor compositions can be altered by epitaxial growth techniques. Individual semicon-
ductorlayersasthinas10A can be produced. ̊
1.2.5 Surfaces : Ideal Versus Real
The arrangement of atoms on the surface can be quite different from that in the bulk. The
bulk crystal structure is decided by the internal chemical energy of the atoms forming the crystal
with a certain number of nearest neighbors, second nearest neighbors, etc. Since the surface,
the number of neighbors is suddenly altered, the spatial geometries which were providing the
lowest energy configuration in the bulk may not provide the lowest energy configuration at the
surface. Thus, there is a readjustment or “reconstruction” of the surface bonds toward an energy
minimizing configuration.
An example of such a reconstruction is shown for the GaAs surface in figure 1.9. The figure
(a) shows an ideal (001) surface where the topmost atoms form a square lattice. The surface
atoms have two nearest neighbor bonds (Ga-As) with the layer below, four second neighbor
bonds (e.g., Ga-Ga or As-As) with the next lower layer, and four second neighbor bonds within
the same layer. We could denote the ideal surface by the symbol C(1×1), representing the fact
that the surface periodicity is one unit by one unit along the square lattice along [110] and[ ̄110].