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Molecular Beam EpitaxyMolecular beam epitaxy, also called MBE, is the deposition of one or more pure materials onto a single crystal wafer, one layer of atoms at a time, under ultra-high vacuum, forming a perfect crystal. In solid-source MBE, ultra-pure elements such as gallium and arsenic are heated in separate furnaces until they each slowly begin to evaporate. The evaporated elements (although strictly speaking the arsenic in this case is actually in a molecular form) condense on the wafer, where they react with each other, forming, in this case, gallium arsenide. The term "beam" simply means that evaporated atoms do not meet each other or any other gases until they reach the wafer. A computer controls shutters in front of each furnace, allowing precise control of the thickness of each layer, down to a single layer of atoms. Intricate structures of layers of different materials may be fabricated this way. Such control has allowed the development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots. Such layers are now a critical part of many modern semiconductor devices, including semiconductor lasers and light emitting diodes. During operation, RHEED (Reflection High Energy Electron Diffraction) is often used for monitoring the growth of the crystal layers. The ultra-high vacuum environment within the growth chamber is maintained by a system of cryopumps, and cryopanels, chilled using liquid nitrogen to a temperature of 77 Kelvin (or 77 degrees Celsius above absolute zero). The wafers on which the crystals are grown are mounted on a rotating platter which can be heated to several hundred degrees C during operation. Molecular beam epitaxy is also used for the deposition of some types of organic semiconductors. In this case, molecules, rather than atoms, are evaporated and deposited onto the wafer. Other variations include gas-source MBE, which resembles chemical vapor deposition but in vacuum. The term "molecular" beam is used for historical reasons even when atoms, rather than molecules, are being evaporated. External links
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