Epitaxial

Epitaxial refers to the growth of a crystalline layer on a crystalline substrate in such a way that the deposited layer adopts an ordered alignment with the substrate crystal lattice. The term comes from epi- meaning upon and taxis meaning arrangement. Epitaxial growth aims for a coherent or nearly coherent crystal relationship between the layer and the underlying material, typically over a substantial region. If the deposited material is the same as the substrate, the process is called homoepitaxy; if it is different, it is heteroepitaxy. Lattice matching between layer and substrate influences defect density; lattice mismatch can introduce strain and misfit dislocations, which may limit layer thickness via a critical thickness.

Growth techniques used to achieve epitaxy include molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and various vapor-phase methods such as hydride vapor phase epitaxy (HVPE) or chemical vapor deposition (CVD). These methods differ in range of temperatures, precursors, and control over composition, but all seek to control crystallographic orientation, surface morphology, and composition at the atomic scale. Substrates are typically single crystals chosen for their lattice parameters and surface quality, such as silicon, gallium arsenide, or sapphire, often with buffer or transition layers to improve alignment and reduce defects.

Epitaxy is central to modern electronics and optoelectronics. Applications include integrated circuits, laser diodes, light-emitting diodes, high-electron-mobility transistors, quantum wells and superlattices, and certain solar cells. The technique provides precise control over thickness, doping, composition, and strain, enabling engineered electronic and optical properties.

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