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Epitaxy - MBE
Molecular Beam Epitaxy
If high-tech wizardry excites you, then Molecular Beam Epitaxy (MBE) is your ticket to a good time. The name is fairly self-explanatory. This technique for epitaxial growth involves shooting a beam of high-energy molecules at a target. The technique involves some of the most stringent mechanical and electrical demands of any research device. It is often one of the most expensive pieces of hardware in a lab as well.
Originally invented by Alfred Y. Cho in the late 1960's at the AT&T Bell Laboratories, Molecular Beam Epitaxy paved the way for today's advanced semiconductor devices. In fact, most lasers are fabricated by an MBE process.
A molecular beam epitaxy machine doesn't actually shoot molecular beams. Instead, the beam originates from a pure material that emits a stream of atoms towards the source. Several sources are usually available within the machine, and they can be turned on and shut off with a series of shutters. Shooting beams of atoms isn't possible unless a high level of vacuum is achieved. MBE machines have some of the highest vacuum ratings in a given laboratory. Because of this, they are far more expensive and difficult to maintain compared to a relatively low-vacuum electron microscope.
These stringent demands mean that MBE is a slow, time-consuming process. It is not ideally suited for mass production. However, it is the only process that can create high-quality thin-film layers of semiconductor crystal. Almost all quantum wells are now created with MBE, and it is also one of the only processes that can create high-quality quantum dots.
One important part of the MBE process is its ability to monitor the growth of the crystal layers with high-energy electron diffraction. This diffractor is built right into the machine, and allows users to monitor the growth of each crystal layer!
MBE offers unprecedented control of epitaxial growth that is not possible with electroplating and chemical vapor deposition. Its only drawbacks are high cost and long production times. Right now, most MBE machines are only used in research laboratories.