What is it for?

Molecular Beam Epitaxy (MBE) is generally used to grow semiconductors as thin film stacking. The low deposition rate allows for epitaxial growth of single crystals. The lattice constants of materials grown on each other should be really close as lattice mismatches could imply defects formation in the materials. The ultra-high vacuum of MBE results in very low levels of impurity and this technique offers the highest achievable purity among grown films.

Examples of structures usually grown by MBE: quantum wells, III-V materials, transistors, atomically flat surfaces/abrupt interfaces, nanowires.

How does it work?

High-purity materials are heated in effusion or e-beam chambers to slowly sublime. The atoms of the source material travel immediately towards the vacuum chamber and directly reach the sample without reacting with other atoms or molecules (hence ‘molecular beam’ as the mean free path of the atoms is much higher than the chamber dimensions). The nucleation process at the surface of the seed wafer is complex science. The gaseous elements impinging on the substrate may condense and forms desired compounds, may absorb, may rebound, etc. Growth can also be located at specific locations thanks to nucleation centers as for nanowire growth. Precise monitoring through RHEED allows for deposition of single atomic layers.