Molecular beam epitaxy
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Molecular-beam epitaxy (MBE) is an epitaxy method for
thin-film deposition A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ap ...
of
single crystal In materials science, a single crystal (or single-crystal solid 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.RIWD. "Re ...
s. MBE is widely used in the manufacture of
semiconductor devices A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity l ...
, including
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s, and it is considered one of the fundamental tools for the development of
nanotechnologies Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal o ...
. MBE is used to fabricate
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
s and
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s (MOS
field-effect transistors The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of Electric current, current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''dra ...
) at
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
frequencies, and to manufacture the
lasers A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...
used to read
optical discs In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of Compact disk#Physical details, pits and lands on a special material, often aluminum, ...
(such as
CDs The compact disc (CD) is a digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then released in Octo ...
and
DVDs The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kind ...
).


History

Original ideas of MBE process were first established by Günther. Films he deposited were not epitaxial, but were deposited on glass substrates. With the development of vacuum technology, MBE process was demonstrated by Davey and Pankey who succeeded in growing GaAs epitaxial films on
single crystal In materials science, a single crystal (or single-crystal solid 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.RIWD. "Re ...
GaAs substrates using Günther's method. Major subsequent development of MBE films was enabled by J.R. Arthur's investigations of kinetic behavior of growth mechanisms and
Alfred Y. Cho Alfred Yi Cho (; born July 10, 1937) is a Chinese-American electrical engineer, inventor, and optical engineer. He is the Adjunct Vice President of Semiconductor Research at Alcatel-Lucent's Bell Labs. He is known as the "father of molecular be ...
's
in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
observation of MBE process using RHEED in the late 1960s.


Method

Molecular-beam epitaxy takes place in high vacuum or
ultra-high vacuum Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...
(10−8–10−12 
Torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one "millimeter of mercury ...
). The most important aspect of MBE is the deposition rate (typically less than 3,000 nm per hour) that allows the films to grow
epitaxially Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epit ...
. These deposition rates require proportionally better vacuum to achieve the same
impurity In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear thermodynam ...
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 Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
and
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
, in ultra-pure form, are heated in separate quasi-
Knudsen effusion cell In crystal growth, a Knudsen cell is an effusion evaporator source for relatively low partial pressure elementary sources (e.g. Ga, Al, Hg, As). Because it is easy to control the temperature of the evaporating material in Knudsen cells, they are c ...
s or electron-beam evaporators until they begin to slowly sublime. The gaseous elements then
condense Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
on the wafer, where they may react with each other. In the example of gallium and arsenic, single-crystal
gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
is formed. When evaporation sources such as copper or gold are used, the gaseous elements impinging on the surface may be
adsorbed Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
(after a time window where the impinging atoms will hop around the surface) or reflected. Atoms on the surface may also desorb. Controlling the temperature of the source will control the rate of material impinging on the substrate surface and the temperature of the substrate will affect the rate of hopping or desorption. 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 In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...
s of the atoms. During operation,
reflection high-energy electron diffraction Reflection high-energy electron diffraction (RHEED) is a analytical technique, technique used to characterize the surface of crystalline materials. RHEED systems gather information only from the surface layer of the sample, which distinguishes RHEE ...
(RHEED) is often used for monitoring the growth of the crystal layers. A computer controls shutters in front of each
furnace A furnace is a structure in which heat is produced with the help of combustion. Furnace may also refer to: Appliances Buildings * Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings * Boiler, used t ...
, 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 A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...
s or even
quantum dot Quantum dots (QDs) are semiconductor particles a few nanometres in size, having light, optical and electronics, electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanote ...
s. Such layers are now a critical part of many modern
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
devices, including
semiconductor laser The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
s and
light-emitting diode A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
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 A cryopump or a "cryogenic pump" is a vacuum pump that traps gases and vapours by condensing them on a cold surface, but are only effective on some gases. The effectiveness depends on the freezing and boiling points of the gas relative to the cryop ...
s and cryopanels, chilled using
liquid nitrogen Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
or cold nitrogen gas to a temperature close to 77 
kelvin The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
s (−196 
degree Celsius Degree may refer to: As a unit of measurement * Degree (angle), a unit of angle measurement ** Degree of geographical latitude ** Degree of geographical longitude * Degree symbol (°), a notation used in science, engineering, and mathematics ...
). Cold surfaces 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 (MBE) is also used for the deposition of some types of
organic semiconductor Organic semiconductors are solids whose building blocks are pi-bonded molecules or polymers made up by carbon and hydrogen atoms and – at times – heteroatoms such as nitrogen, sulfur and oxygen. They exist in the form of molecular crystals or ...
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 Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substra ...
. MBE systems can also be modified according to need. Oxygen sources, for example, can be incorporated for depositing oxide materials for advanced electronic, magnetic and optical applications, as well as for fundamental research. Here, a molecular beam of an oxidant is used to achieve the desired oxidation state of a multicomponent oxide.


Quantum nanostructures

One of the most accomplished achievements of molecular-beam epitaxy is the nano-structures that permitted the formation of atomically flat and abrupt hetero-interfaces. Such structures have played an unprecedented role in expanding the knowledge of physics and electronics. Most recently the construction of
nanowires A nanowire is a nanostructure in the form of a wire with the diameter of the order of a nanometre (10−9 metres). More generally, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less ...
and quantum structures built within them that allow information processing and the possible integration with on-chip applications for quantum communication and computing. These heterostructure
nanowire lasers Semiconductor nanowire lasers are nano-scaled lasers that can be embedded on chips and constitute an advance for computing and information processing applications. Nanowire lasers are coherent light sources (single mode optical waveguides) as any ...
are only possible to build using advance MBE techniques, allowing monolithical integration on silicon and picosecond signal processing.Mayer, B., et al
"Long-term mutual phase locking of picosecond pulse pairs generated by a semiconductor nanowire laser"
Nature Communications 8 (2017): 15521.


Asaro–Tiller–Grinfeld instability

The Asaro–Tiller–Grinfeld (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 the
Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
, mismatch size, and surface tension. Some applications for this instability have been researched, such as the
self-assembly Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the ...
of quantum dots. This community uses the name of
Stranski–Krastanov growth Stranski–Krastanov growth (SK growth, also Stransky–Krastanov or Stranski–Krastanow) is one of the three primary modes by which thin films grow epitaxially at a crystal surface or interface. Also known as 'layer-plus-island growth', the SK mo ...
for ATG.


See also

*
Pulsed laser deposition Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the ...
*
Metalorganic vapour phase epitaxy Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. ...
*
Colin P. Flynn C. Peter Flynn (August 18, 1935 – October 27, 2011) was a Professor of Physics and of Materials Science at the University of Illinois at Urbana-Champaign. Professor Flynn was on the faculty of the University of Illinois from 1960 until 2011 and s ...
* Arthur Gossard *
High-electron-mobility transistor A high-electron-mobility transistor (HEMT), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heterojunction) ...
(HEMT) *
Heterojunction bipolar transistor The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it can handle s ...
*
Herbert Kroemer Herbert Kroemer (; born August 25, 1928) is a German-American physicist who, along with Zhores Alferov, received the Nobel Prize in Physics in 2000 for "developing semiconductor heterostructures used in high-speed- and opto-electronics". Kroemer ...
*
Quantum cascade laser Quantum-cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jérôme Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, ...
*
Solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.
*
Ben G. Streetman Ben G. Streetman is the former Dean of the Cockrell School 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 Microe ...
* Wetting layer * Thermal Laser Epitaxy


Notes


References

* * * *


Further reading

* {{Cite book , last1 = Frigeri , first1 = P. , last2 = Seravalli , first2 = L. , last3 = Trevisi , first3 = G. , last4 = Franchi , first4 = S. , chapter = 3.12: Molecular Beam Epitaxy: An Overview , doi = 10.1016/B978-0-44-453153-7.00099-7 , title = Comprehensive Semiconductor Science and Technology , pages = 480–522 , year = 2011 , isbn = 9780444531537 , volume = 3, publisher = Elsevier, editor1= Pallab Bhattacharya , editor2=Roberto Fornari , editor3=Hiroshi Kamimura, place = Amsterdam


External links


Silicon and germanium nanowires by molecular beam epitaxy

University of Texas MBE group (Primer on MBE growth)



CrystalXE: A specialized software in epitaxy
Semiconductor growth Thin film deposition