Epitaxy
Encyclopedia
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 overlayer is called an epitaxial film or epitaxial layer. The term epitaxy comes from the Greek
roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to arrange upon". For most technological applications, it is desired that the deposited material form a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal structure (single-domain epitaxy).
Epitaxial films may be grown from gas
eous or liquid
precursors. Because the substrate acts as a seed crystal
, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal. If the overlayer either forms a random orientation with respect to the substrate or does not form an ordered overlayer, this is termed non-epitaxial growth. If an epitaxial film is deposited on a substrate of the same composition, the process is called homoepitaxy; otherwise it is called heteroepitaxy.
Homoepitaxy is a kind of epitaxy performed with only one material. In homoepitaxy, a crystal
line film is grown on a substrate or film of the same material. This technology is used to grow a film which is more pure than the substrate and to fabricate layers having different doping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".
Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystal
line film grows on a crystalline substrate or film of a different material. This technology is often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. Examples include gallium nitride on sapphire
or aluminium gallium indium phosphide
on gallium arsenide .
Heterotopotaxy is a process similar to heteroepitaxy except for the fact that thin film growth is not limited to two dimensional growth. Here the substrate is similar only in structure to the thin film material.
Epitaxy is used in silicon
-based manufacturing processes for BJTs and modern CMOS
, but it is particularly important for compound semiconductor
s such as gallium arsenide. Manufacturing issues include control of the amount and uniformity of the deposition's resistivity and thickness, the cleanliness and purity of the surface and the chamber atmosphere, the prevention of the typically much more highly doped substrate wafer's diffusion of dopant to the new layers, imperfections of the growth process, and protecting the surfaces during the manufacture and handling.
and in semiconductor fabrication
. Indeed, epitaxy is the only affordable method of high quality crystal growth for many semiconductor materials, including technologically important materials as silicon-germanium, gallium nitride, gallium arsenide, indium phosphide and graphene
.
Epitaxy is also used to grow layers of pre-doped
silicon
on the polished sides of silicon wafers
, before they are processed into semiconductor
devices. This is typical of power devices, such as those used in pacemakers
, vending machine
controllers, automobile computers, etc.
. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductor
s. Solid-phase epitaxy is used primarily for crystal-damage healing.
and hydrogen
at approximately 1200 °C:
This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching
) may occur if too much hydrogen chloride
byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:
Silicon VPE may also use silane
, dichlorosilane
, and trichlorosilane
source gases. For instance, the silane reaction occurs at 650 °C in this way:
This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide
.
VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE.
The method is mainly used for the growth of compound semiconductors; uniform and high quality layers can be produced.
A typical example for the liquid phase epitaxy method is the growth of ternary and quaternary III-V compounds on gallium arsenide (GaAs) substrates. As a solvent quite often gallium is used in this case. Another frequently used substrate is indium phosphide (InP). Other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, and to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar.
In MBE, a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum
(10−8 Pa
; practically free space) to the substrate, where they condense. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing III-V semiconductor crystals.
, phosphine
or diborane
. The concentration of impurity in the gas phase determines its concentration in the deposited film. As in CVD, impurities change the deposition rate.
Additionally, the high temperatures at which CVD is performed may allow dopants to diffuse
into the growing layer from other layers in the wafer ("out-diffusion"). Also, dopants in the source gas, liberated by evaporation or wet etching of the surface, may diffuse into the epitaxial layer ("autodoping"). The dopant profiles of underlying layers change as well, however not as significantly.
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
roots epi, meaning "above", and taxis, meaning "in ordered manner". It can be translated "to arrange upon". For most technological applications, it is desired that the deposited material form a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal structure (single-domain epitaxy).
Epitaxial films may be grown from gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
eous or liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
precursors. Because the substrate acts as a seed crystal
Seed crystal
A seed crystal is a small piece of single crystal/polycrystal material from which a large crystal of the same material typically is to be grown...
, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal. If the overlayer either forms a random orientation with respect to the substrate or does not form an ordered overlayer, this is termed non-epitaxial growth. If an epitaxial film is deposited on a substrate of the same composition, the process is called homoepitaxy; otherwise it is called heteroepitaxy.
Homoepitaxy is a kind of epitaxy performed with only one material. In homoepitaxy, a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line film is grown on a substrate or film of the same material. This technology is used to grow a film which is more pure than the substrate and to fabricate layers having different doping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".
Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line film grows on a crystalline substrate or film of a different material. This technology is often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. Examples include gallium nitride on sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
or aluminium gallium indium phosphide
Aluminium gallium indium phosphide
Aluminium gallium indium phosphide is a semiconductor material.AlGaInP is used in manufacture of light-emitting diodes of high-brightness red, orange, green, and yellow color, to form the heterostructure emitting light...
on gallium arsenide .
Heterotopotaxy is a process similar to heteroepitaxy except for the fact that thin film growth is not limited to two dimensional growth. Here the substrate is similar only in structure to the thin film material.
Epitaxy is used in silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
-based manufacturing processes for BJTs and modern CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
, but it is particularly important for compound semiconductor
Compound semiconductor
A compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
s such as gallium arsenide. Manufacturing issues include control of the amount and uniformity of the deposition's resistivity and thickness, the cleanliness and purity of the surface and the chamber atmosphere, the prevention of the typically much more highly doped substrate wafer's diffusion of dopant to the new layers, imperfections of the growth process, and protecting the surfaces during the manufacture and handling.
Applications
Epitaxy is used in nanotechnologyNanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
and in semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
. Indeed, epitaxy is the only affordable method of high quality crystal growth for many semiconductor materials, including technologically important materials as silicon-germanium, gallium nitride, gallium arsenide, indium phosphide and graphene
Graphene
Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. The term graphene was coined as a combination of graphite and the suffix -ene by Hanns-Peter Boehm, who described single-layer...
.
Epitaxy is also used to grow layers of pre-doped
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
on the polished sides of silicon wafers
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
, before they are processed into 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. This is typical of power devices, such as those used in pacemakers
Artificial pacemaker
A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...
, vending machine
Vending machine
A vending machine is a machine which dispenses items such as snacks, beverages, alcohol, cigarettes, lottery tickets, consumer products and even gold and gems to customers automatically, after the customer inserts currency or credit into the machine....
controllers, automobile computers, etc.
Methods
Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor depositionChemical 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...
. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductor
Compound semiconductor
A compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
s. Solid-phase epitaxy is used primarily for crystal-damage healing.
Vapor-phase
Silicon is most commonly deposited by dosing with silicon tetrachlorideSilicon tetrachloride
Silicon tetrachloride is the inorganic compound with the formula SiCl4. It is a colourless volatile liquid that fumes in air. It is used to produce high purity silicon and silica for commercial applications.-Preparation:...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
at approximately 1200 °C:
- SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g)
This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching
Etching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
) may occur if too much hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:
- SiCl4(g) + Si(s) ↔ 2SiCl2(g)
Silicon VPE may also use silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
, dichlorosilane
Dichlorosilane
Dichlorosilane , or DCS as it is commonly known, is usually mixed with ammonia in LPCVD chambers to grow silicon nitride in semiconductor processing.A higher concentration of DCS:NH3 Dichlorosilane (H2SiCl2), or DCS as it is commonly known, is usually mixed with ammonia (NH3) in LPCVD chambers to...
, and trichlorosilane
Trichlorosilane
Trichlorosilane is a chemical compound containing silicon, hydrogen, and chlorine. At high temperatures, it decomposes to produce silicon, and therefore purified trichlorosilane is the principal source of ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to produce...
source gases. For instance, the silane reaction occurs at 650 °C in this way:
- SiH4 → Si + 2H2
This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
.
VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE.
Liquid-phase
Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid substrates. This happens at temperatures well below the melting point of the deposited semiconductor. The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition, the deposition of the semiconductor crystal on the substrate is relatively fast and uniform. Typical deposition rates for monocrystalline films range from 0.1 to 1 μm/minute. The equilibrium conditions depend very much on the temperature and on the concentration of the dissolved semiconductor in the melt. The growth of the layer from the liquid phase can be controlled by a forced cooling of the melt. Impurity introduction can be strongly reduced. Doping can be achieved by the addition of dopants to the melt.The method is mainly used for the growth of compound semiconductors; uniform and high quality layers can be produced.
A typical example for the liquid phase epitaxy method is the growth of ternary and quaternary III-V compounds on gallium arsenide (GaAs) substrates. As a solvent quite often gallium is used in this case. Another frequently used substrate is indium phosphide (InP). Other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, and to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar.
Solid-phase
Solid Phase Epitaxy (SPE) is a transition between the amorphous and crystalline phases of a material. It is usually done by first depositing a film of amorphous material on a crystalline substrate. The substrate is then heated to crystallize the film. The single crystal substrate serves as a template for crystal growth. The annealing step used to recrystallize or heal silicon layers amorphized during ion implantation is also considered one type of Solid Phase Epitaxy. The Impurity segregation and redistribution at the growing crystal-amorphus layer interface during this process is used to incorporate low-solubility dopants in metals and Silicon.Molecular-beam
In MBE, a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
(10−8 Pa
Pascal (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...
; practically free space) to the substrate, where they condense. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing III-V semiconductor crystals.
Doping
An epitaxial layer can be doped during deposition by adding impurities to the source gas, such as arsineArsine
Arsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
, phosphine
Phosphine
Phosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...
or diborane
Diborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
. The concentration of impurity in the gas phase determines its concentration in the deposited film. As in CVD, impurities change the deposition rate.
Additionally, the high temperatures at which CVD is performed may allow dopants to diffuse
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
into the growing layer from other layers in the wafer ("out-diffusion"). Also, dopants in the source gas, liberated by evaporation or wet etching of the surface, may diffuse into the epitaxial layer ("autodoping"). The dopant profiles of underlying layers change as well, however not as significantly.
See also
- Island growthIsland growthIsland growth is a physical model of deposited film growth and chemical vapor deposition.Consider a situation where atoms are being deposited onto a flat surface at a very slow rate. The first atom deposited undergoes a random walk on the surface. Eventually a second atom is deposited and can be...
- Atomic layer epitaxyAtomic layer epitaxyAtomic layer epitaxy or Atomic Layer Chemical Vapor Deposition , now more generally called Atomic Layer Deposition , is a specialized form of epitaxy that typically deposit alternating monolayers of two elements onto a substrate. The crystal lattice structure achieved is thin, uniform, and...
- Topotaxy
- EpiwaferEpiwaferAn epiwafer is a wafer of semiconducting material made by epitaxial growth for use in making microelectronic devices such as light-emitting diodes...
- Exchange biasExchange biasExchange bias or exchange anisotropy occurs in bilayers of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film...
- HeterojunctionHeterojunctionA heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
- Nano-RAMNano-RAMNano-RAM is a proprietary computer memory technology from the company Nantero. It is a type of nonvolatile random access memory based on the mechanical position of carbon nanotubes deposited on a chip-like substrate. In theory, the small size of the nanotubes allows for very high density memories...
- 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...
- Selective area epitaxySelective area epitaxySelective area epitaxy is the local growth of epitaxial layer through a patterned dielectric mask deposited on a semiconductor substrate....
- Silicon on sapphireSilicon on sapphireSilicon on sapphire is a hetero-epitaxial process for integrated circuit manufacturing that consists of a thin layer of silicon grown on a sapphire wafer. SOS is part of the Silicon on Insulator family of CMOS technologies...
- Single event upsetSingle event upsetA single event upset is a change of state caused by ions or electro-magnetic radiation striking a sensitive node in a micro-electronic device, such as in a microprocessor, semiconductor memory, or power transistors. The state change is a result of the free charge created by ionization in or close...
- VCSELVCSELThe vertical-cavity surface-emitting laser, or VCSEL , is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a...
- Wake Shield FacilityWake Shield Facility]Wake Shield Facility is an experimental science platform that was placed in low-earth orbit by the Space Shuttle. It is a 3.7 meter diameter, free-flying stainless steel disk....
- Zhores Ivanovich AlferovZhores Ivanovich AlferovZhores Ivanovich Alferov is a Soviet and Russian physicist and academic who contributed significantly to the creation of modern heterostructure physics and electronics. He is an inventor of the heterotransistor and the winner of 2000 Nobel Prize in Physics. He is also a Russian politician and has...
External links
- epitaxy.net: a central forum for the epitaxy-communities
- Deposition processes