ICLs are fabricated from semiconductor heterostructures grown using molecular beam epitaxy (MBE). Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor deposition. In vapor-phase epitaxy, a gas containing the negative dopant is passed over the substrate wafer. Commercially, GaN crystals can be grown using molecular beam epitaxy or metalorganic vapour phase epitaxy. Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. Molecular beam epitaxy is a particularly sophisticated form of thermal evaporation. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductors. The synthesis of n-type semiconductors may involve the use of vapor-phase epitaxy. Early examples used costly molecular beam epitaxy processes. Devices have been grown by molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE). This MOSFET is made using a P-type substrate, with a P epitaxy. Examples are thin films of crystalline rubrene prepared by hot wall epitaxy. The material was created via molecular beam epitaxy.