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| − | [[Image:Molecular beam epitaxy pnl.png|thumb|right|300px|The Molecular Beam Epitaxy System in the William R. Wiley Environmental Molecular Sciences Laboratory is used to grow and characterize thin crystalline films of oxides and ceramics to understand in detail the chemistry that occurs on oxides and ceramic surfaces.]]
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| − | '''Molecular beam [[:Wikipedia:epitaxy|epitaxy]]''' (MBE), is one of several methods of [[:Wikipedia:thin-film deposition|depositing]] [[:Wikipedia:single crystal|single crystal]]s. It was invented in the late 1960s at [[:Wikipedia:Bell Telephone Laboratories|Bell Telephone Laboratories]] by J. R. Arthur and [[:Wikipedia:Alfred Y. Cho|Alfred Y. Cho]].
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| − | ==Method==
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| − | Molecular Beam Epitaxy takes place in [[:Wikipedia:Vacuum#Quality|high vacuum]] or [[:Wikipedia:ultra high vacuum|ultra high vacuum]] (10<sup>-8</sup> [[:Wikipedia:pascal (unit)|Pa]]). The most important aspect of MBE is the slow deposition rate (1 to 300 nm per minute), which allows the films to grow [[:Wikipedia:epitaxially|epitaxially]]. However, the slow deposition rates require proportionally better vacuum in order to achieve the same impurity levels as other deposition techniques.
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| − | In solid-source MBE, ultra-pure elements such as [[:Wikipedia:gallium|gallium]] and [[:Wikipedia:arsenic|arsenic]] are heated in separate quasi-[[:Wikipedia:Knudsen Cell|knudsen effusion cells]] until they begin to slowly [[:Wikipedia:sublimate|sublimate]]. The gaseous elements then [[:Wikipedia:evaporative deposition|condense]] on the wafer, where they may react with each other. In the example of gallium and arsenic, single-crystal [[:Wikipedia:gallium arsenide|gallium arsenide]] is formed. The term "beam" simply means that evaporated atoms do not interact with each other or any other vacuum chamber gases until they reach the wafer, due to the long [[:Wikipedia:mean free path|mean free path]]s of the beams.
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| − | During operation, [[:Wikipedia:RHEED|RHEED]] (Reflection High Energy Electron Diffraction) is often used for monitoring the growth of the crystal layers. 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 [[:Wikipedia:quantum well|quantum well]]s or even [[:Wikipedia:quantum dots|quantum dots]]. Such layers are now a critical part of many modern [[:Wikipedia:semiconductor|semiconductor]] devices, including [[:Wikipedia:semiconductor laser|semiconductor laser]]s and [[:Wikipedia:light-emitting diode|light-emitting diode]]s.
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| − | In systems where the substrate needs to be cooled, the ultra-high vacuum environment within the growth chamber is maintained by a system of [[:Wikipedia:cryopump|cryopump]]s, and cryopanels, chilled using [[:Wikipedia:liquid nitrogen|liquid nitrogen]] or cold nitrogen gas to a temperature close to 77 [[:Wikipedia:kelvin|kelvin]]s (−196 degrees [[:Wikipedia:Celsius|Celsius]]). However, cryogenic temperatures act as a sink for impurities in the vacuum, and so vacuum levels need to be several orders of magnitude better to deposit films under these conditions. In other systems, the wafers on which the crystals are grown may be mounted on a rotating platter which can be heated to several hundred degrees [[:Wikipedia:Celsius|Celsius]] during operation.
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| − | Molecular beam epitaxy is also used for the deposition of some types of [[:Wikipedia:organic semiconductor|organic semiconductor]]s. In this case, molecules, rather than atoms, are evaporated and deposited onto the wafer. Other variations include [[:Wikipedia:chemical beam epitaxy|gas-source MBE]], which resembles [[:Wikipedia:chemical vapor deposition|chemical vapor deposition]].
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| − | ==ATG instability==
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| − | '''ATG''' (Asaro-Tiller-Grinfeld) instability, formerly known as Grinfeld instability, is an elastic instability often encountered during molecular beam epitaxy. If there is a mismatch between the lattice sizes of the growing film and the supporting crystal, elastic energy will be accumulated in the growing film. At some critical height, the free energy of the film can be lowered if the film breaks into isolated islands, where the tension can be relaxed latterally. The critical height depends on Young moduli, mismatch size, and surface tensions.
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| − | Some applications for this instability have been searched, such as the self assembly of [[:Wikipedia:quantum dots|quantum dots]]. This community uses the name of Stranski-Krastanov for ATG.
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| − | == References ==
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| − | # Structural properties of self-organized semiconductor nanostructures
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| − | J. Stangl, V. Holý & G. Bauer, Rev. Mod. Phys.'''76''', 725 (2004) (59 pages).
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| − | # Spontaneous ordering of nanostructures on crystal surfaces, Vitaliy A. Shchukin and Dieter Bimberg,
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| − | Rev. Mod. Phys. '''71''', 001125 (1999) (47 pages)
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| − | # {{cite book |last=Jaeger |first=Richard C. |title=Introduction to Microelectronic Fabrication |year=2002 |publisher=Prentice Hall |location=Upper Saddle River |id=ISBN 0-201-44494-7 |chapter=Film Deposition}}
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| − | ==See also==
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| − | *[[:Wikipedia:Alfred Y. Cho|Alfred Y. Cho]]
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| − | *[[:Wikipedia:Colin P Flynn|Colin P Flynn]]
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| − | *[[:Wikipedia:Arthur Gossard|Arthur Gossard]]
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| − | *[[:Wikipedia:Herbert Kroemer|Herbert Kroemer]]
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| − | *[[:Wikipedia:Ben G. Streetman|Ben G. Streetman]]
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| − | *[[:Wikipedia:Solar cell|Solar cell]]
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| − | *[[:Wikipedia:Wetting layer|Wetting layer]]
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| − | *[[:Wikipedia:HEMT|High Electron Mobility Transistor|]]
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| − | *[[:Wikipedia:Heterojunction Bipolar Transistor|Heterojunction Bipolar Transistor]]
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| − | *[[:Wikipedia:Quantum cascade laser|Quantum cascade laser]]
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| − | == External links ==
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| − | *[http://www.micro.uiuc.edu/mbe/biografs(better).html The MBE Group - University of Illinois]
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| − | *[http://www.veeco.com/learning/learning_vaporelements.asp Vapor pressure curves of the elements]
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| − | *[http://www.ece.utexas.edu/projects/ece/mrc/groups/street_mbe/mbechapter.html University of Texas MBE group]
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| − | *[http://www.uccs.edu/~tchriste/courses/PHYS549/549lectures/mbe.html Physics of Thin Films: Molecular Beam Epitaxy (class notes)]
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| − | *[http://www.semiconductor-today.com Semiconductor Today: Online resource covering compound semiconductor processing]
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