Molecular beam epitaxy
Encyclopedia
Molecular beam epitaxy
(MBE) is one of several methods of depositing single crystal
s. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho
.
). 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, elements such as gallium
and arsenic
, in ultra-pure form, 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 path
s of the atoms.
During operation, reflection high energy electron diffraction (RHEED) is often used for monitoring the growth of the crystal layers. A computer controls shutter
s 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 well
s or even quantum dot
s. Such layers are now a critical part of many modern semiconductor
devices, including semiconductor lasers and light-emitting diode
s.
In systems where the substrate needs to be cooled, the ultra-high vacuum environment within the growth chamber is maintained by a system of cryopump
s, and cryopanels, chilled using liquid nitrogen
or cold nitrogen gas to a temperature close to 77 Kelvin
(−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 semiconductor
s. 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
.
, mismatch size, and surface tensions.
Some applications for this instability have been researched, such as the self-assembly of quantum dot. This community uses the name of Stranski–Krastanov growth for ATG.
Epitaxy
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where the overlayer is in registry with the substrate. In other words, there must be one or more preferred orientations of the overlayer with respect to the substrate for this to be termed epitaxial growth. The...
(MBE) is one of several methods of depositing single crystal
Single crystal
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries...
s. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho
Alfred Y. Cho
Alfred Yi Cho is the Adjunct Vice President of Semiconductor Research at Alcatel-Lucent's Bell Labs. He is known as the "father of molecular beam epitaxy"; a technique he developed at that facility in the late 1960s. He is also the co-inventor, with Federico Capasso of quantum cascade lasers at...
.
Method
Molecular beam epitaxy takes place in high vacuum or ultra-high vacuum (10−8 PaPascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
). The most important aspect of MBE is the slow deposition rate
Vacuum deposition
Vacuum deposition is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub-atmospheric pressure on a solid surface. The layers may be as thin as one atom to millimeters thick . There may be multiple layers of different materials...
(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
Impurity
Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.Impurities are either naturally occurring or added during synthesis of a chemical or commercial product...
levels as other deposition techniques.
In solid-source MBE, elements such as gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
and arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, in ultra-pure form, are heated in separate quasi-Knudsen effusion cells
Knudsen Cell
In crystal growth, Knudsen Cells are often used as sources evaporators for relatively low partial pressure elementary sources...
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 path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
s of the atoms.
During operation, reflection high energy electron diffraction (RHEED) is often used for monitoring the growth of the crystal layers. A computer controls shutter
Shutter
Shutter may refer to:Windows:*Window shutter, a solid window covering used to block light and winds*Plantation Shutters/Jigsaw Shutters, interior wooden louvred shutters to control heat, light and privacy...
s in front of each furnace
Furnace
A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...
, 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 well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
s or even quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
s. Such layers are now a critical part of many modern semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
devices, including semiconductor lasers and light-emitting diode
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
s.
In systems where the substrate needs to be cooled, the ultra-high vacuum environment within the growth chamber is maintained by a system of cryopump
Cryopump
A cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface. They are only effective on some gases, depending on the freezing and boiling points of the gas relative to the cryopump's temperature...
s, and cryopanels, chilled using liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...
or cold nitrogen gas to a temperature close to 77 Kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(−196 degrees Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
). 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 semiconductor
Organic semiconductor
An organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
s. In this case, molecules, rather than atoms, are evaporated and deposited onto the wafer. Other variations include gas-source MBE
Chemical beam epitaxy
Chemical beam epitaxy forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically...
, which resembles chemical vapor deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
.
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 laterally. The critical height depends on Young's moduliYoung's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
, mismatch size, and surface tensions.
Some applications for this instability have been researched, such as the self-assembly of quantum dot. This community uses the name of Stranski–Krastanov growth for ATG.
See also
- Metalorganic vapour phase epitaxyMetalorganic vapour phase epitaxyMetalorganic vapour phase epitaxy , also known as organometallic vapour phase epitaxy or metalorganic chemical vapour deposition , is a chemical vapour deposition method of epitaxial growth of materials, especially compound semiconductors, from the surface reaction of organic compounds or...
- Colin P. Flynn
- Arthur GossardArthur GossardArthur C. Gossard is a professor of Materials and Electrical Engineering at the University of California, Santa Barbara. His research is related to molecular beam epitaxy . He has a doctorate from UC Berkeley. He is a member of the United States National Academy of Engineering.-External links:*...
- High electron mobility transistor (HEMT)
- Heterojunction bipolar transistorHeterojunction bipolar transistorThe heterojunction bipolar transistor is a type of bipolar junction transistor which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it that can handle signals of very high frequencies, up to several hundred...
- Herbert KroemerHerbert KroemerHerbert Kroemer , a professor of electrical and computer engineering at the University of California, Santa Barbara, received his Ph.D. in theoretical physics in 1952 from the University of Göttingen, Germany, with a dissertation on hot electron effects in the then-new transistor, setting the stage...
- Quantum cascade laserQuantum cascade laserQuantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.Unlike typical...
- Solar cellSolar cellA solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
- Ben G. StreetmanBen G. StreetmanBen G. Streetman is the former Dean of the College of Engineering at the University of Texas at Austin. He earned a Ph.D in electrical engineering from Texas in 1966, and became a professor there in 1982. He founded the university's Microelectronics Research Center and holds the Dula D...
- Wetting layerWetting layerIn experimental physics, a wetting layer is an initial layer of atoms that is epitaxially grown on a surface upon which self-assembled quantum dots or thin films are created. The atoms composing a wetting layer can be semimetallic elements/compounds or metallic alloys...