Molecular beam epitaxy is one of several methods of depositing single crystals.
It was invented in the late 1960s at Bell Telephone Laboratories by J. R.
Arthur and Alfred Y. Cho. MBE is widely used in the manufacture of semiconductor
devices, including transistors for cellular phones and WiFi.
Method Molecular beam epitaxy takes place in
high vacuum or ultra-high vacuum. The most important aspect of MBE is the
deposition rate that allows the films to grow epitaxially. These deposition rates
require proportionally better vacuum to achieve the same impurity levels as
other deposition techniques. The absence of carrier gases as well as the ultra
high vacuum environment result in the highest achievable purity of the grown
films. In solid-source MBE, elements such as
gallium and arsenic, in ultra-pure form, are heated in separate quasi-Knudsen
effusion cells until they begin to slowly sublime. The gaseous elements
then condense on the wafer, where they may react with each other. In the
example of gallium and arsenic, single-crystal gallium arsenide is
formed. The term "beam" means that evaporated atoms do not interact with
each other or vacuum chamber gases until they reach the wafer, due to the long
mean free paths of the atoms. During operation, 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 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.
In systems where the substrate needs to be cooled, the ultra-high vacuum
environment within the growth chamber is maintained by a system of cryopumps, and
cryopanels, chilled using liquid nitrogen or cold nitrogen gas to a
temperature close to 77 Kelvin. Cryogenic temperatures act as a sink for
impurities in the vacuum, 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 Celsius 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. Lately molecular beam epitaxy has been
used to deposit oxide materials for advanced electronic, magnetic and
optical applications. For these purposes, MBE systems have to be
modified to incorporate oxygen sources. ATG instability
The ATG instability, also known as the 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 laterally. The critical
height depends on Young's moduli, mismatch size, and surface tensions.
Some applications for this instability have been researched, such as the
self-assembly of quantum dots. This community uses the name of
Stranski–Krastanow growth for ATG. See also
Pulsed laser deposition Metalorganic vapour phase epitaxy
Colin P. Flynn Arthur Gossard
High electron mobility transistor Heterojunction bipolar transistor
Herbert Kroemer Quantum cascade laser
Solar cell Ben G. Streetman
Wetting layer Notes
^ Cho, A. Y.; Arthur, J. R.; Jr. "Molecular beam epitaxy". Prog. Solid
State Chem. 10: 157–192. doi:10.1016/0079-6786(75)90005-9.
^ J. Cheng, V.K. Lazarov, et al, J. of Vacuum Science & Technology B, 27,
148(2009). References
Jaeger, Richard C.. "Film Deposition". Introduction to Microelectronic
Fabrication. Upper Saddle River: Prentice Hall. ISBN 0-201-44494-1.
McCray, W.P.. "MBE Deserves a Place in the History Books". Nature
Nanotechnology 2: 259–261. Bibcode:2007NatNa...2..259M.
doi:10.1038/nnano.2007.121. Shchukin, Vitaliy A.; Dieter Bimberg.
"Spontaneous ordering of nanostructures on crystal surfaces". Reviews of Modern
Physics 71: 1125–1171. Bibcode:1999RvMP...71.1125S.
doi:10.1103/RevModPhys.71.1125. Stangl, J.; V. Holý; G. Bauer.
"Structural properties of self-organized semiconductor nanostructures". Reviews
of Modern Physics 76: 725–783. Bibcode:2004RvMP...76..725S.
doi:10.1103/RevModPhys.76.725. Further reading
Frigeri, P.; Seravalli, L.; Trevisi, G.; Franchi, S.. "3.12: Molecular Beam
Epitaxy: An Overview". In Pallab Bhattacharya, Roberto Fornari, and
Hiroshi Kamimura. Comprehensive Semiconductor Science and Technology 3.
Amsterdam: Elsevier. pp. 480–522. doi:10.1016/B978-0-44-453153-7.00099-7.
ISBN 9780444531537. External links
Silicon and germanium nanowires by molecular beam epitaxy
University of Texas MBE group Physics of Thin Films: Molecular Beam
Epitaxy