Synthesis and the Structural, Chemical, and Electrical Characterization of Al₁₋ₓ ScₓN Solid-Solutions for Piezoelectric and Ferroelectric Thin Film Applications

Abstract

Piezoelectric and ferroelectric thin-film materials are integral to a wide range of modern electronic systems, enabling the functionality of RF filters, LIDAR systems, gyroscopes, micromachined ultrasonic transducers, computer memory, and device switches. As devices shrink and performance demands rise, there is growing interest in enhancing the piezoelectric and ferroelectric properties of material’s while maintaining compatibility with silicon and CMOS processes. Aluminum nitride (AlN), a CMOScompatible, wide bandgap material, is widely used in resonators and filters due to its strong piezoelectric response, thermal stability, and low dielectric and acoustic losses, which together enable high electromechanical coupling and quality factor. Substituting scandium (Sc) into the AlN lattice to form Al₁-ₓScₓN increases lattice distortion and internal strain sensitivity, enhancing the longitudinal piezoelectric coefficient (d₃₃) and effective electromechanical coupling (kₜ,eff²) by up to 400%. However, Sc substitution also introduces a structural trade-off, as increasing content beyond a critical threshold (x ≈ 0.43) induces a phase transition from the polar wurtzite phase to a non-polar cubic structure, diminishing piezoelectric performance.

The two major aims of this thesis defense are to understand how Sc target power influences the composition and crystallinity of co-sputtered AlScN films and to examine how sputtering mode affects the ferroelectric behavior of Al₇₅Sc₂₅N. Increasing Sc target power from 100 W to 150 W raises the Sc content from 17 at% to 27 at% and results in greater oxygen uptake and reduced c-axis crystallinity. Separately, by holding all deposition parameters constant and varying the sputtering mode (DC, RF, and mixed RF+DC), this work demonstrates that deposition energetics strongly influence ferroelectric response - RF sputtering yields strong polarization switching, while DC sputtering results in films with primarily dielectric behavior. These findings provide a foundation for optimizing AlScN thin films for next-generation piezoelectric and ferroelectric MEMS devices.