7.4.6 Molecular beam epitaxy (MBE)
Where a suitable ALD chemistry cannot be found, or where cleanliness and high crystallinity are required, molecular beam epitaxy may be necessary. This is more akin to evaporation than to CVD, with multiple molecular beams of the separate chemical constituents each focused onto the hot wafer surface. Deposition is performed under extreme vacuum conditions (10−11 mbar) to prevent any contaminants from being incorporated, and the substrate must present a perfect cleaved crystal face so that a matching lattice can be seeded in the deposited film. MBE has found its major application for transistor structures in III-V semiconductors such as GaAs, as it allows multiple layers of chemically similar layers (e.g. GaAs, AlxGa1−xAs, AlAs) to be grown in a single process with a perfect interface between them.