Effect of scandium composition on the phonon scattering lifetime of aluminum scandium nitride acoustic wave resonators

In this work, we analyze the anharmonic phonon scattering time to project the acoustic loss limits in thin-film aluminum scandium nitride (Al1-xScxN) micromechanical resonators. Although electromechanical coupling coefficients have been improved significantly with Sc alloying, making Al1-xScxN a promising material for wideband acoustic filters, lower quality factor (Q) have been observed when increasing Sc content in Al1-xScxN. This has often been attributed to poor crystallinity and surface roughness of the film. To investigate the intrinsic limiting factors of Q in Al1-xScxN resonators, we summarize the figure of merits from the experimental works of Al1-xScxN resonators from multiple research and industry groups in the past decades. We classify the major phonon scattering mechanisms present in Al1-xScxN lattice and their theoretical expressions. The results suggest that while both AlN and Al1-xScxN phonon lifetimes can be limited by grain boundary scattering, which depends on the crystal microstructure, as well as the film thickness, Al0.7Sc0.3N suffers from a decrease of phonon lifetime by up to four orders of magnitude due to alloy scattering, conceivably responsible for the lower Q compared to AlN acoustic resonators.