Aluminum nitride ( AlN ) is a solid nitride of aluminum . It has a high thermal conductivity of up to 321 W/(m K) [5] and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potential application in optoelectronics operating at deep ultraviolet frequencies .
History and physical properties
AlN was first synthesized in 1877.
AlN in the pure (undoped) state has an electrical conductivity of 10 -11 -10 -13 -1 cm -1 increasing to 10, -5 -10 -6 -1 cm -1 when poured . [9] Electrical breakdown occurs in the region of 1.2–1.8 × 10 5 V/mm (dielectric strength). It has been speculated that the cubic zinc mixture phase of AlN(zb-AlN) may exhibit superconductivity at high pressure.
AlN has a high thermal conductivity, the high-quality MOCVD-grown AlN single crystal has an intrinsic thermal conductivity of 321 W/(m K), which is consistent with first-principles calculations. [5] For electrically insulating ceramics, it is 70–210 W/(m K) for polycrystalline materials, and as high as 285 W/(m K) for single crystals). [9]
stability and chemical properties
Aluminum nitride is stable at high temperatures in an inert atmosphere and melts at about 2200 °C. In vacuum, AlN decomposes at ~1800 °C. In 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. Aluminum nitride is stable in hydrogen and carbon-dioxide atmospheres up to 980 °C. [11 1]
The material dissolves slowly in mineral acid through grain-boundary attack and in strong alkali through attack on aluminum-nitride grain. The material hydrolyzes slowly in water. Aluminum nitride is resistant to attack by most molten salts, including chloride and cryolite.
Production
AlN is synthesized by reduction of carbothermal aluminum oxide gas in the presence of nitrogen or ammonia or by direct nitridation of aluminum. Producing dense techno-grade materials requires the use of sintering aids, such as Y2O3 or CaO , and hot pressing.
Application
Epitaxially grown thin film crystalline aluminum nitride used for surface acoustic wave sensors (SAWs) deposited on silicon wafers due to the piezoelectric properties of LNs. One application is an RF filter, widely used in mobile phones, [14] called a thin film bulk acoustic resonator (FBAR). It is a MEMS device that uses aluminum nitride sandwiched between two metal layers. [15]
ALNs are also used to make piezoelectric micromachined ultrasound transducers, which emit and receive ultrasound and which can be used for in-air rangefinding at distances of up to a meter. Metallization methods are available to allow the use of AlN in electronic applications similar to alumina and beryllium oxide. AlN nanotubes in the form of inorganic quasi-one-dimensional nanotubes, which are isoelectronic with carbon nanotubes, have been suggested as chemical sensors for toxic gases.
There is currently much research into developing light emitting diodes to operate in the ultraviolet using gallium nitride based semiconductors and using the alloy aluminum gallium nitride, wavelengths as low as 250 nm have been achieved. In 2006, an inefficient LN LED emission at 210 nm was reported. [20]
Applications of AlN include
- opto-electronics,
- dielectric layers in optical storage media,
- electronic substrates, chip carriers where high thermal conductivity is required,
- military applications,
- As a crucible for growing crystals of gallium arsenide,
- Steel and semiconductor manufacturing.
