Molecular-beam epitaxy
Molecular beam epitaxy (MBE), 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.
Method
Molecular beam epitaxy takes place in high vacuum or ultra high vacuum (10−8 Pa). The most important aspect of MBE is the slow deposition rate (typically less than 1000 nm per hour), which allows the films to grow epitaxially. The slow deposition rates require proportionally better vacuum to achieve the same impurity levels as other deposition techniques.
In solid-source MBE, ultra-pure elements such as gallium and arsenic are heated in separate quasi-knudsen effusion cells until they begin to slowly sublimate. 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, 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 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 kelvins (−196 degrees Celsius). 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.
ATG instability
The ATG (Asaro-Tiller-Grinfeld) 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 latterally. The critical height depends on Young 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-Krastanov for ATG.
See also
 Alfred Y. Cho
Colin P Flynn
Arthur Gossard
Herbert Kroemer
Ben G. Streetman
Solar cell
Wetting layer
High Electron Mobility Transistor
Heterojunction Bipolar Transistor
Quantum cascade laser
External links
Silicon and germanium nanowires by molecular beam epitaxy
University of Texas MBE group
Physics of Thin Films: Molecular Beam Epitaxy (class notes)
|
|
|
|
This section is sponsored by:
Limited Offer
How About Take 3 Seconds to Enter Postal Address to Claim Your Free Laptop Now while you are looking for Molecular Beam Epitaxy?
www.RewardLaptop.com
Laptops
Find a great deal on a Laptop Computer by browsing our Laptop Computer listings.
www.LaptopComputersInfo.com
Top Laptops Info. All in One.
Find great deals and info on Laptops. Search smart for the right stuff with OXY.
www.oxysearch.com
Used Laptops start @ $177
Excellent Used Notebooks Low Prices All Brands, XP, Super Fast WIFI -Free Shipping!
www.notebookchoice.com
laptops
Don't just search for laptops, find results.
www.ask.com
laptops
Search for laptops here.
www.aywoh.com
Laptop
Laptop At Huge Discount! Free Shipping On Laptop.
www.insights1.com
Best Rated Laptops
Looking To Buy A Laptop? Which Are The Best Laptop Brands?
BestRatedLaptops.info
|
