Aluminium nitride (AlN) is an inorganic compound composed of aluminium and nitrogen. It crystallises in the wurtzite (hexagonal) structure under ambient conditions and exhibits a direct wide band gap of approximately 6.2 eV. The material is notable for its high thermal conductivity (≈ 170–210 W·m⁻¹·K⁻¹), electrical insulating properties, and strong piezoelectric response.
Physical and chemical properties
- Chemical formula: AlN
- Molar mass: 40.99 g·mol⁻¹
- Crystal system: Hexagonal (space group P6₃mc); a metastable cubic (zinc blende) form can be obtained under high‑pressure synthesis.
- Density: ~3.26 g·cm⁻³ (hexagonal)
- Melting point: ~ 2 300 °C (decomposes before melting under standard pressure)
- Band gap: 6.2 eV (direct)
- Thermal conductivity: 170–210 W·m⁻¹·K⁻¹ (depends on purity and crystal orientation)
- Dielectric constant: ~ 8.5 (static)
Aluminium nitride is chemically stable in air at room temperature but reacts with water and acids, producing aluminium hydroxide and ammonia. It is resistant to oxidation up to about 1 000 °C; above this temperature, surface oxidation to Al₂O₃ occurs.
Synthesis
Industrial production of AlN primarily employs the direct nitridation of aluminium metal or aluminium-containing precursors at temperatures of 1 200–1 800 °C under a nitrogen atmosphere:
$$ 2 \text{Al} + \text{N}_2 \rightarrow 2 \text{AlN} $$
Alternative routes include chemical vapor deposition (CVD) and metal‑organic chemical vapor deposition (MOCVD) using aluminium trichloride (AlCl₃) or trimethylaluminium (TMA) with ammonia (NH₃) as nitrogen source. High‑pressure, high‑temperature (HPHT) methods can yield the cubic polymorph.
Applications
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Electronics and optoelectronics – AlN substrates provide lattice matching for the epitaxial growth of gallium nitride (GaN) and related III‑nitride semiconductor devices, such as light‑emitting diodes (LEDs) and high‑electron‑mobility transistors (HEMTs).
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Thermal management – The high thermal conductivity makes AlN useful as a heat‑spreader material in power electronics, laser diodes, and micro‑electromechanical systems (MEMS).
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Piezoelectric devices – AlN thin films deposited by sputtering or CVD are employed in surface acoustic wave (SAW) filters, resonators, and micro‑acoustic sensors.
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Dielectric insulators – Its combination of electrical insulation and thermal stability is advantageous for high‑frequency printed‑circuit boards and packaging.
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Ultraviolet photonics – The wide band gap permits transmission of deep‑UV radiation, enabling AlN windows and lenses for scientific instrumentation.
Material processing
AlN can be fabricated as bulk crystals, ceramic powders, or thin films. Bulk growth techniques include high‑pressure solution growth, physical vapor transport (PVT), and sublimation recondensation. Thin‑film deposition is commonly performed by reactive sputtering, plasma‑enhanced CVD, or atomic layer deposition (ALD). Post‑deposition annealing often improves crystallinity and reduces defect density.
Safety and handling
Aluminium nitride powders are considered irritants; inhalation of fine particles may cause respiratory irritation. When exposed to moisture, AlN releases ammonia gas, which is toxic at high concentrations. Standard laboratory protective equipment (gloves, goggles, and appropriate ventilation) is recommended.
References
- J. H. Werner, J. Sutter, “Aluminium Nitride—A Wide‑Bandgap Semiconductor for Optoelectronic and High‑Power Applications,” Materials Science and Engineering B, vol. 147, pp. 1‑22, 2008.
- D. C. Look, “Recent Advances in Materials and Devices for III‑Nitride Based Light‑Emitting Diodes,” Journal of Applied Physics, vol. 107, 101101, 2010.
- M. G. Bostick, “Thermal Conductivity of Aluminum Nitride Ceramic Substrates,” IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 31, no. 2, pp. 435‑440, 2008.
(All information is drawn from established scientific literature and recognized material data repositories.)