Aluminum nitride (AlN) has recently attracted considerable attention because of its ability to exhibit a high phase velocity, high stiffness coefficients, and a relatively low dielectric constant. Moreover, the compatibility of complementary metal–oxide semiconductor (CMOS) fabrication to form monolithic devices has allowed for easy fabrication processes. 

In addition, compared with materials such as PZT, its piezoelectric properties are weak; therefore, recent AlN developments include enhancement its piezoelectric properties, such as through doping with various metals, such as Sc, V, Ti, Ta, Mg, and others. Among these metals, Sc stands out, and up to 40% of doped cases of Sc-doped AlN have been studied; the piezoelectric properties have been found to increase by up to five times for the case of a 40% Sc-doping ratio. In addition, other research areas related to integrated photonics have shown excellent performance in terms of optical and pyroelectric properties.

The most critical design aspect of a piezoelectric device is its crystal quality, the figure below shows the crystal structure of aluminum nitride.

Figure 1. Crystal Structure of Aluminum Nitride.^[1]

AlN belongs to the III–V family because it forms a hexagonally packed wurtzite structure, as shown in Fig.(a). The lattice parameters for the a- and c-axes were approximately 3.110 and 4.978?, respectively. Fig.(b) shows the crystalline structure of AlN. The tetrahedral structure consists of each Al atom bonded to four N atoms, three of which are B1 bonds and one is a B2 bond. The maximum piezoelectric response d33 was measured to be approximately 5.5 pC/N and was obtained in the longitudinal direction. In addition, a high stiffness C33 was measured at 3.67e11 N/m2 . The intrinsic Q factor of AlN and its compatibility with CMOS fabrication processes are important design features of bulk acoustic wave (BAW) resonators and filters. 

Reference

[1] S. T. Haider, M. A. Shah, D. -G. Lee and S. Hur, "A Review of the Recent Applications of Aluminum Nitride-Based Piezoelectric Devices," in IEEE Access, vol. 11, pp. 58779-58795, 2023, doi: 10.1109/ACCESS.2023.3276716.