| Abstract: | Interfaces and heterojunctions which are incorporated into a crystal in well-defined geometrical and spatial arrangements can lead to a structuring or engineering of (semiconducting) solids down to atomic dimensions. The electrical and optical properties are then defined locally, and phenomena related to extremely small dimensions (“quantum size effects”) become more important than the actual chemical properties of the materials used. The technique of molecular beam epitaxy allows an atomic layer-by-layer deposition in a two-dimensional growth process, and crystalline materials in alternating layers of arbitrary composition and only a few atomic layers thickness are formed. The synthesis of microscopically structured solids by molecular beam epitaxy affords access to a new class of materials with accurately tailored electrical, optical, magnetic, dielectric, mechanical etc. properties. The semiconductor and metal superlattices described in this article, which are made of alternating thin layers of two different materials, symbolize just the beginning of a new area of materials engineering on a molecular (or atomic) scale. This periodic modulation of the chemical composition normal to the surface imposes an artificial periodicity on the semiconductor or metal crystal, a periodicity of one or two orders of magnitude larger than its natural lattice spacing. The synthesis of other materials combinations, including semiconductor/metal, semiconductor/insulator, metal/insulator, polymers, and magnetic materials, with entirely different properties and for completely different applications will certainly follow. Finally, a large variety of desired combinations of elements can be selected, and even metastable compounds with novel exciting properties can be synthesized by molecular beam epitaxy. |
