Solid state: Molecular beam epitaxy: Streams of molecules in a vacuum chamber build devices ‘from the atom up’

Discusses the application of molecular beam epitaxy (MBE), a process for forming thin, single-crystal semiconductor structures in an ultrahigh vacuum. Microwave field-effect transistors, semiconductor lasers, millimeter-wave detectors, and a variety of other high-performance devices have been made from the multilayer crystals grown by the process. With MBE, the thickness of crystalline layers can be precisely controlled down to a few angstroms. Layer composition can be varied, and new semiconductor structures can be produced that are impossible to make by other means. The molecular beams are created when the required elements are heated in effusion ovens-small containers, each with an opening facing the substrate wafer. As many elements and effusion ovens are used as there are elements to be incorporated into the crystal structure. For gallium-arsenide devices, for example, ovens for gallium, for arsenic, and for each of the doping elements are needed.