the discovery of elegant new fundamental physics. These advances are illustrated for the gallium arsenide (GaAs) system in the following subsection.
A third example of crystallography leading to major discoveries in both science and technology is the development of gallium arsenide-based heterostructures. These single-crystal GaAs films are a natural extension of the deep experience of the scientific community with single-crystal Si. This work was chosen to illustrate the essential coupling between high-quality crystal growth and the discovery of completely new and unexpected physical phenomena. It also illustrates how the ability to produce single-crystal films of ever higher quality is the determining factor in making progress in this field of physics.
A search by physicists for a more perfect semiconductor-insulator interface than Si-SiO2 led to the development of the GaAs-aluminum arsenide (AlAs) system, in which both the conducting GaAs and the insulating AlAs are incorporated in the same single crystal. Interestingly, these crystallize in the same tetrahedrally bonded diamond structure as that of Si, having the same average valence electron count. Using advances made in techniques to epitaxially grow single-crystal films, near-perfect crystal interfaces can be produced. Figure 1.4 is an atomic-scale micrograph showing 12 atomic layers of semiconducting GaAs (darker layer) sandwiched between layers of semi-insulating AlAs (lighter layers). Notice that in this figure the atomic layers of GaAs cleanly link up to the adjacent atomic layers of AlAs,