Epitaxy refers to the deposition of an overlayer on a crystalline substrate, where the overlayer is in registry with the substrate. The overlayer is called an epitaxial film or epitaxial layer.

Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed crystal, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal.

It gives high purity and low defects.

• It can be classified as:

1. Homoepitaxy – The film and the substrate are the same material. Often used in Si on Si growth. Epitaxially grown layers are purer than the substrate and can be doped independently of it.
2. Heteroepitaxy – Film and substrate are different materials. Example, AlAs on GaAs growth. Allows for optoelectronic structures and band gap engineered devices.

Requirements of general epitaxial deposition are surface preparation and surface mobility. A clean surface is required as the defects of the surface gets duplicated in epitaxial layer i.e. some defects on surface will still remain even after applying film.

Epitaxial layer can be chemically pure than the substrate.

There are basically three types of Epitaxy:

1. Molecular Beam Epitaxy
2. Vapor phase Epitaxy
3. Liquid phase Epitaxy