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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 (1 to 300 nm per minute), which allows the films to grow epitaxially. However, the slow deposition rates require proportionally better vacuum in order 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" 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 mean free paths of the beams.
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). 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 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 (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.
Some applications for this instability have been searched, such as the self assembly of quantum dots. This community uses the name of Stranski-Krastanov for ATG.
J. Stangl, V. Holý & G. Bauer, Rev. Mod. Phys.76, 725 (2004) (59 pages).
Rev. Mod. Phys. 71, 001125 (1999) (47 pages)
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Molecular-beam_epitaxy". A list of authors is available in Wikipedia.|