EPITAXY

the deposition of a crystalline substance, usually in a thin layer, upon the surface of a single crystal (q.v.) of another substance. The process does not involve a chemical reaction between the two materials, but the crystalline structure and orientation of the substance determines how the epitaxial layer is deposited. Several kinds of solid-state electronic devices, such as high-speed transistors and integrated circuits, are fabricated by production of an epitaxial layer of one semiconductor on a wafer of another that has a different type of electrical conductivity. One technique used in preparing epitaxial layers is called chemical vapour deposition. Here the clean surface of a semiconducting single crystal, commonly silicon or germanium, is exposed to the action of a vaporized mixture of a compound of the semiconductor and a compound of another element, called a dopant (q.v.), often gallium or arsenic. The vapours condense and decompose on the surface, forming an orderly crystalline layer in which a small proportion of the sites in the atomic lattice of the major elemental component are occupied by atoms of the dopant.