Epitaxy

epitaxy, the process of growing a crystal of a particular orientation on top of another crystal, where the orientation is determined by the underlying crystal. The creation of various layers in semiconductor wafers, such as those used in integrated circuits, is a typical application for the process. In addition, epitaxy is often used to fabricate optoelectronic devices.

The word epitaxy derives from the Greek prefix epi meaning “upon” or “over” and taxis meaning “arrangement” or “order.” The atoms in an epitaxial layer have a particular registry (or location) relative to the underlying crystal. The process results in the formation of crystalline thin films that may be of the same or different chemical composition and structure as the substrate and may be composed of only one or, through repeated depositions, many distinct layers. In homoepitaxy the growth layers are made up of the same material as the substrate, while in heteroepitaxy the growth layers are of a material different from the substrate. The commercial importance of epitaxy comes mostly from its use in the growth of semiconductor materials for forming layers and quantum wells in electronic and photonic devices—for example, in computer, video display, and telecommunications applications. The process of epitaxy is general, however, and so can occur for other classes of materials, such as metals and oxides, which have been used since the 1980s to create materials that display giant magnetoresistance (a property that has been used to produce higher-density digital storage devices).

In vapour phase epitaxy the deposition atoms come from a vapour, so that growth occurs at the interface between gaseous and solid phases of matter. Examples include growth from thermally vaporized material such as silicon or from gases such as silane (SiH₄), which reacts with a hot surface to leave behind the silicon atoms and to release the hydrogen back into the gaseous phase. In liquid phase epitaxy layers grow from a liquid source (such as silicon doped with small amounts of another element) at a liquid-solid interface. In solid phase epitaxy a thin amorphous (noncrystalline) film layer is first deposited on a crystalline substrate, which is then heated to convert the film into a crystalline layer. The epitaxial growth then proceeds by a layer-by-layer process in the solid phase through atomic motion during the recrystallization at the crystal-amorphous interface.

There are a number of approaches to vapour phase epitaxy, which is the most common process for epitaxial layer growth. Molecular beam epitaxy provides a pure stream of atomic vapour by thermally heating the constituent source materials. For example, silicon can be placed in a crucible or cell for silicon epitaxy, or gallium and arsenic can be placed in separate cells for gallium arsenide epitaxy. In chemical vapour deposition the atoms for epitaxial growth are supplied from a precursor gas source (e.g., silane). Metal-organic chemical vapour deposition is similar, except that it uses metal-organic species such as trimethyl gallium (which are usually liquid at room temperature) as a source for one of the elements. For example, trimethyl gallium and arsine are often used for epitaxial gallium arsenide growth. Chemical beam epitaxy uses a gas as one of its sources in a system similar to molecular beam epitaxy. Atomic layer epitaxy is based on introducing one gas that will absorb only a single atomic layer on the surface and following it with another gas that reacts with the preceding layer.