Aluminium nitride

Aluminium nitride (AlN) is a solid nitride of aluminium, which was first synthesized in 1862 by F. Briegleb and A. Geuther.

AlN is a wide-bandgap semiconductor composed of aluminium and nitrogen. It crystallizes predominantly in the wurtzite structure and exhibits a direct band gap of approximately 6 eV at room temperature The exceptionally wide bandgap enables applications in deep-ultraviolet optoelectronics, while the material's thermal conductivity 321 W/(m·K), and strong polarization effects make it an important buffer and template material for III-nitride quantum heterostructures used in high-power and high-frequency electronic devices.

Structures and physical properties

AlN exists primarily in the hexagonal wurtzite crystal structure, which plays a central role in determining its physical properties. In this structure, aluminium and nitrogen atoms alternate along the crystallographic c-axis, with each atom tetrahedrally coordinated to four atoms of the opposite species adopting a sp3-like bonding and an in-plane constant of 3.11Å.

In addition to the thermodynamically stable wurtzite phase, AlN can also form a metastable cubic zincblende phase, typically realized in thin films grown under non-equilibrium conditions. It is predicted that the cubic phase of AlN (zb-AlN) can exhibit superconductivity at high pressures. At sufficiently high pressures, a phase transition to a rocksalt structure has been reported.

The lattice parameters and the excess of the ground state energies for different AlN crystal structures at zero pressure are listed in the table below:

In the pure (undoped) state, AlN exhibits very low electrical conductivity, typically in the range of 10−11–10−13 Ω−1⋅cm−1, which can increase to 10−5–10−6 Ω−1⋅cm−1 upon intentional doping. Electrical breakdown occurs at high electric fields on the order of 1×105 V/mm (dielectric strength), reflecting the material's wide band gap and strong chemical bonding.

One of the most distinctive intrinsic properties of wurtzite AlN is its spontaneous polarization, which arises from the non-centrosymmetric nature of the crystal structure and the strong ionic character of the Al-N bonds.