Aluminium nitride (AlN) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) 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 1862 by F. Briegleb and A. Geuther. AlN, in the pure (undoped) state has an electrical conductivity of 10−11–10−13 Ω−1⋅cm−1, rising to 10−5–10−6 Ω−1⋅cm−1 when doped. Electrical breakdown occurs at a field of 1.2–1.8 V/mm (dielectric strength). The material exists primarily in the hexagonal wurtzite crystal structure, but also has a metastable cubic zincblende 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 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 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 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 (Psp: AlN 0.081 C/m2 > InN 0.032 C/m2 > GaN 0.029 C/m2). 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, 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, 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