Aluminium nitride

Aluminium nitride (AlN) is a nitride of aluminium. Its wurtzite phase (w-AlN) is an extremely wide bandgap (6.2 eV) semiconductor material which has potential application for deep ultraviolet optoelectronics.

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History

AlN was first synthesised in 1877, but it was not until the middle of the 1980s that its potential for application in microelectronics was realised due to its relative high thermal conductivity for an electrical insulating ceramic (70-210 W•m⁻¹•K⁻¹ for polycrystalline material, and as high as 275 W•m⁻¹•K⁻¹ for single crystals).

Chemical description and properties

Aluminium nitride is a (mostly) covalently bonded material, and has a hexagonal crystal structure which is isomorphic with one of the polytypes of zinc sulfide known as wurtzite. The space group for this structure is P6_3mc.

The material is stable at very high temperatures in inert atmospheres. In air, surface oxidation occurs above 700°C, and even at room temperature, surface oxide layers of 5-10 nm 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 acids through grain boundary attack, and in strong alkalis through attack on the aluminum nitride grains. The material hydrolyzes slowly in water. Aluminium nitride is resistant to attack from most molten salts including chlorides and cryolite.

Manufacture

AlN is synthesised by carbothermal reduction of alumina or by direct nitridation of aluminium. The use of sintering aids and hot pressing is required to produce a dense technical grade material.

Applications

Metallization methods are available to allow AlN to be used in electronics applications similar to those of alumina and BeO.

Currently there is much research into developing light-emitting diodes to operate in the ultraviolet using the gallium nitride based semiconductors and, using the alloy aluminium gallium nitride, wavelengths as short as 250 nm have been reported. In May 2006 an inefficient LED emission at 210 nm was reported [1]. The bandgap of single crystal AlN has been measured (using vacuum UV reflectivity) at 6.2 eV. This allows a wavelength of around 200 nm to be achieved, in principle. However, there are many difficulties to be overcome if such emitters are to become a commercial reality.

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.

Epitaxially grown crystalline aluminum nitride is also used for surface acoustic wave sensors (SAW's) deposited on silicon wafers because of the AlN's piezoelectric properties. Very few places can reliably fabricate these thin films. Agilent after more than a decade of research now has a RF filter used in mobile phone called the FBAR. This technology is closely associated with engineers working in the MEMS field.

See also

• Boron nitride
• Aluminium phosphide
• Indium nitride

References

Academic

• Ioffe data archive
• Electronic Structure of AlN
• MSDS (University of Oxford)

Commercial suppliers

• Ceramic Substrates and Components Ltd
• Goodfellow
• NanoAmor's AlN Nanoparticles