Aluminium nitride (
Al N) is a solid
nitride
In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...
of
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
. It has a high
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
of up to 321 W/(m·K)
and is an electrical insulator. Its
wurtzite
Wurtzite is a zinc and iron sulfide mineral with the chemical formula , a less frequently encountered Polymorphism (materials science), structural polymorph form of sphalerite. The iron content is variable up to eight percent.Palache, Charles, Har ...
phase (w-AlN) has a
band gap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
of ~6 eV at room temperature and has a potential application in
optoelectronic
Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiatio ...
s operating at
deep ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
frequencies.
History and physical properties
AlN was first synthesized in 1862 by F. Briegleb and A. Geuther.
AlN, in the pure (undoped) state has an
electrical conductivity
Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...
of 10
−11–10
−13 Ω
−1⋅cm
−1, rising to 10
−5–10
−6 Ω
−1⋅cm
−1 when doped.
Electrical breakdown
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating mate ...
occurs at a field of 1.2–1.8 V/mm (
dielectric strength
In physics, the term dielectric strength has the following meanings:
*for a pure electrically insulating material, the maximum electric field that the material can withstand under ideal conditions without undergoing electrical breakdown and becom ...
).
The material exists primarily in the hexagonal
wurtzite
Wurtzite is a zinc and iron sulfide mineral with the chemical formula , a less frequently encountered Polymorphism (materials science), structural polymorph form of sphalerite. The iron content is variable up to eight percent.Palache, Charles, Har ...
crystal structure, but also has a metastable cubic
zincblende
Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in sedimentary exhalative, Mississippi-Va ...
phase, which is synthesized primarily in the form of thin films. It is predicted that the cubic phase of AlN (zb-AlN) can exhibit
superconductivity
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
at high pressures. In AlN wurtzite crystal structure, Al and N alternate along the c-axis, and each bond is tetrahedrally coordinated with four atoms per unit cell.
One of the unique intrinsic properties of
wurtzite
Wurtzite is a zinc and iron sulfide mineral with the chemical formula , a less frequently encountered Polymorphism (materials science), structural polymorph form of sphalerite. The iron content is variable up to eight percent.Palache, Charles, Har ...
AlN is its spontaneous polarization. The origin of spontaneous polarization is the strong ionic character of chemical bonds in wurtzite AlN due to the large difference in
electronegativity
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
between aluminum and nitrogen atoms. Furthermore, the non-centrosymmetric wurtzite crystal structure gives rise to a net polarization along the c-axis. Compared with other III-nitride materials, AlN has a larger spontaneous polarization due to the higher nonideality of its crystal structure (P
sp: AlN 0.081 C/m
2 > InN 0.032 C/m
2 > GaN 0.029 C/m
2).
Moreover, the piezoelectric nature of AlN gives rise to internal piezoelectric polarization charges under strain. These polarization effects can be utilized to induce a high density of free carriers at III-nitride semiconductor heterostructure interfaces completely dispensing with the need of intentional doping. Owing to the broken inversion symmetry along the polar direction, AlN thin film can be grown on either metal-polar or nitrogen-polar faces. Their bulk and surface properties depend significantly on this choice. The polarization effect is currently under investigation for both polarities.
Critical spontaneous and piezoelectric polarization constants for AlN are listed in the table below:
AlN has high
thermal conductivity
The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
, high-quality MOCVD-grown AlN single crystal has an intrinsic thermal conductivity of 321 W/(m·K), consistent with a first-principle calculation.
For an electrically insulating
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
, it is 70–210 W/(m·K) for polycrystalline material, and as high as 285 W/(m·K) for single crystals).
AlN is one of the few materials that have both a wide and direct bandgap (almost as twice as SiC and GaN) and large thermal conductivity. This is due to its small atomic mass, strong interatomic bonds, and simple crystal structure. This property makes AlN attractive for application in high speed and high power communication networks. Many devices handle and manipulate large amounts of energy in small volumes and at high speeds, so due to the electrically insulating nature and high thermal conductivity of AlN, it becomes a potential material for high-power power electronics. Among group III-nitride materials, AlN has higher thermal conductivity compared to
gallium nitride
Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it ...
(GaN). Therefore, AlN is more advantageous than GaN in terms of heat dissipation in many power and radio frequency electronic devices.
Thermal expansivity is another critical property for high temperature applications. The calculated thermal expansion coefficients of AlN at 300 K are 4.2×10
-6 K
-1along a-axis and 5.3×10
-6 K
-1 along c-axis.
Stability and chemical properties
Aluminium nitride is stable at high temperatures in inert atmospheres and melts at about 2200 °C. In a vacuum, AlN decomposes at ~1800 °C. In the air, surface oxidation occurs above 700 °C, and even at room temperature, surface oxide layers of 5–10 nm thickness have been detected. This oxide layer protects the material up to 1370 °C. Above this temperature bulk oxidation occurs. Aluminium nitride is stable in hydrogen and carbon-dioxide atmospheres up to 980 °C.
The material dissolves slowly in
mineral acid
A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water.
Cha ...
s through
grain-boundary attack and in strong
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
es through attack on the aluminium-nitride grains. The material hydrolyzes slowly in water. Aluminium nitride is resistant to attack from most molten salts, including
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
s and
cryolite
Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987.
History
Cryolite was first described in 1798 by Danish vete ...
.
Aluminium nitride can be patterned with a Cl
2-based
reactive ion etch
Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The pl ...
.
Manufacture
AlN is synthesized by the
carbothermal reduction
Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are appl ...
of
aluminium oxide in the presence of gaseous nitrogen or ammonia or by direct nitridation of aluminium. The use of
sintering
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
aids, such as Y
2O
3 or CaO, and hot pressing is required to produce a dense technical-grade material.
Applications
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 ...
grown
thin film
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 ...
crystalline aluminium nitride is used for
surface acoustic wave
A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the material, such that they are confined to a depth of abou ...
sensors (SAWs) deposited on silicon
wafers
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
because of AlN's
piezoelectric
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied Stress (mechanics), mechanical s ...
properties. One application is an
RF filter, widely used in mobile phones, which is called a
thin-film bulk acoustic resonator A thin-film bulk acoustic resonator (FBAR or TFBAR) is a device consisting of a piezoelectric material manufactured by thin film methods between two conductive – typically metallic – electrodes and acoustically isolated from the surrounding medi ...
(FBAR). This is a
MEMS
Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
device that uses aluminium nitride sandwiched between two metal layers.
AlN is also used to build piezoelectric micromachined ultrasound transducers, which emit and receive ultrasound and which can be used for in-air rangefinding over distances of up to a meter.
Metallization methods are available to allow AlN to be used in electronics applications similar to those of alumina and
beryllium oxide
Beryllium oxide (BeO), also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and exceeds that of m ...
. AlN nanotubes as inorganic quasi-one-dimensional nanotubes, which are isoelectronic with carbon nanotubes, have been suggested as chemical sensors for toxic gases.
Currently there is much research into developing
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 to operate in the ultraviolet using
gallium nitride
Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it ...
based semiconductors and, using the alloy
aluminium gallium nitride Aluminium gallium nitride (AlGaN) is a semiconductor material. It is any alloy of aluminium nitride and gallium nitride.
The bandgap of AlxGa1−xN can be tailored from 3.4eV (xAl=0) to 6.2eV (xAl=1).
AlGaN is used to manufacture light-emitting d ...
, wavelengths as short as 250 nm have been achieved. In 2006, an inefficient AlN
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
emission at 210 nm was reported.
AlN-based
high electron mobility transistors (HEMTs) have attracted a high level of attention due to AlN’s superior properties, such as better thermal management, reduced buffer leakage, and excellent integration for all nitride electronics. AlN buffer layer is a critical building block for AlN-based HEMTs, and it has been grown by using MOCVD or MBE on different substrates. Building on top of AlN buffer, n-channel devices with
2D electron gas (2DEG) and p-channel devices with 2D hole gas (2DHG) have been demonstrated. The combination of high-density 2DEG and 2DHG on the same semiconductor platform makes it a potential candidate for CMOS devices.
Among the applications of AlN are
* opto-electronics,
* dielectric layers in optical storage media,
* electronic substrates, chip carriers where high thermal conductivity is essential,
* military applications,
* as a
crucible
A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands te ...
to grow crystals of
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 ...
,
*
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
and
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 ...
manufacturing.
See also
*
Boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...
*
Aluminium phosphide
Aluminium phosphide is a highly toxic inorganic compound with the chemical formula AlP, used as a wide band gap semiconductor and a fumigant. This colorless solid is generally sold as a grey-green-yellow powder due to the presence of impurities ...
*
Indium nitride
Indium nitride () is a narrow gap, small bandgap semiconductor material which has potential application in solar cells and high speed electronics.
The bandgap of InN has now been established as ~0.7 eV depending on temperature (the obsolete value ...
*
Aluminium oxynitride
Aluminium oxynitride (marketed under the name ALON by Surmet Corporation) is a transparent ceramic composed of aluminium, oxygen and nitrogen. ALON is optically transparent (≥ 80%) in the near-ultraviolet, visible, and midwave-infrared re ...
*
Titanium aluminium nitride
Titanium aluminium nitride (TiAlN) or aluminium titanium nitride (AlTiN; for aluminium contents higher than 50%) is a group of metastable hard coatings consisting of nitrogen and the metallic elements aluminium and titanium. Four important compos ...
, TiAlN or AlTiN
References
Cited sources
*
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Nitrides
Aluminium compounds
Piezoelectric materials
III-V semiconductors
Light-emitting diode materials
III-V compounds
Wurtzite structure type