Epitaxy

Epitaxy is a process in which a crystalline layer, called an epilayer, is grown on a crystalline substrate such that the layer's lattice orientation follows that of the substrate. The technique is used to create highly ordered layered materials with controlled thickness and composition. Epitaxial growth requires a crystallographically matched interface and is distinct from deposition on amorphous surfaces, where the overlayer does not inherit substrate orientation.

There are two main categories: homoepitaxy, where the epilayer is the same material as the substrate, and heteroepitaxy, where they are different. Heteroepitaxy enables functional material combinations but introduces lattice mismatch that can induce strain and defects.

Growth modes depend on interfacial energies and lattice mismatch. In layer-by-layer growth (Frank-van der Merwe), complete monolayers form before the next layer. In island growth (Volmer-Weber), islands form directly on the substrate. In Stranski-Krastanov growth, initial layers form followed by islands after several monolayers due to strain. Strain from lattice mismatch can be accommodated up to a critical thickness, beyond which misfit dislocations relieve the strain.

Common epitaxial techniques include molecular beam epitaxy (MBE), in which atomic or molecular beams are directed at the substrate in ultrahigh vacuum, and metal-organic chemical vapor deposition (MOCVD or MOVPE), which uses metal-organic precursors. Chemical vapor deposition (CVD) can also be used for epitaxy, as can liquid phase epitaxy for certain materials. Epitaxy requires a crystalline substrate with a defined orientation, such as GaAs on GaAs, GaN on sapphire, or silicon-based layers, and is widely used to engineer quantum wells, superlattices, and high-quality semiconductor junctions.

Applications span optoelectronics, microelectronics and materials research, enabling devices such as laser diodes, LEDs and high-mobility transistors, as well as strained-layer structures, buffers, and quantum well architectures.