Aluminum scandium nitride (Al_1−xSc_xN) film has drawn considerable attention owing to its enhanced piezoelectric response for micro-electromechanical system (MEMS) applications. Understanding the fundamentals of piezoelectricity would require a precise characterization of the piezoelectric coefficient, which is also crucial for MEMS device design. In this study, we proposed an in situ method based on a synchrotron X-ray diffraction (XRD) system to characterize the longitudinal piezoelectric constant d₃₃ of Al_1−xSc_xN film. The measurement results quantitatively demonstrated the piezoelectric effect of Al_1−xSc_xN films by lattice spacing variation upon applied external voltage. The as-extracted d₃₃ had a reasonable accuracy compared with the conventional high over-tone bulk acoustic resonators (HBAR) devices and Berlincourt methods. It was also found that the substrate clamping effect, leading to underestimation of d₃₃ from in situ synchrotron XRD measurement while overestimation using Berlincourt method, should be thoroughly corrected in the data extraction process. The d₃₃ of AlN and Al_0.9Sc_0.1N obtained by synchronous XRD method were 4.76 pC/N and 7.79 pC/N, respectively, matching well with traditional HBAR and Berlincourt methods. Our findings prove the in situ synchrotron XRD measurement as an effective method for precise piezoelectric coefficient d₃₃ characterization.

The piezoelectric and elastic properties are critical for the performance of AlN-based 5G RF filters. The improvement of the piezoelectric response in AlN is often accompanied by lattice softening, which compromises the elastic modulus and sound velocities. Optimizing both the piezoelectric and elastic properties simultaneously is both challenging and practically desirable. In this work, 117 X_0.125Y_0.125Al_0.75N compounds were studied with the high-throughput first-principles calculation. B_0.125Er_0.125Al_0.75N, Mg_0.125Ti_0.125Al_0.75N, and Be_0.125Ce_0.125Al_0.75N were found to have both high C₃₃ (>249.592 GPa) and high e₃₃ (>1.869 C/m²). The COMSOL Multiphysics simulation showed that most of the quality factor (Q_r) values and the effective coupling coefficient (K_eff²) of the resonators made with these three materials were higher than those with Sc_0.25AlN with the exception of the K_eff² of Be_0.125Ce_0.125AlN, which was lower due to the higher permittivity. This result demonstrates that double-element doping of AlN is an effective strategy to enhance the piezoelectric strain constant without softening the lattice. A large e₃₃ can be achieved with doping elements having d-/f- electrons and large internal atomic coordinate changes of du/dε. The doping elements–nitrogen bond with a smaller electronegativity difference (ΔEd) leads to a larger elastic constant C₃₃.