Al1−xHfxN Thin Films with Enhanced Piezoelectric Responses for GHz Surface Acoustic Wave Devices

ABSTRACT

Ternary compounds obtained by alloying wurtzite AlN with transition metals have emerged as promising materials with significantly enhanced piezoelectric characteristics relative to binary AlN. The increased electromechanical coupling in these compounds boosts the performance of high‐frequency acoustic devices. To date, progress has largely focused on aluminum scandium nitride (), which, while widely deployed, relies on scandium, a costly and critical element that is challenging to refine. Here, we investigate aluminum hafnium nitride () as a more economical and accessible alternative to . Using reactive co‐sputtering on both Si and ‐ substrates, we demonstrate wurtzite thin films () with strong ‐axis texture and nearly isotropic lattice expansion upon Hf incorporation. X‐ray absorption spectroscopy indicates cross‐gap hybridization between N 2 and Hf 5 states, which can enhance the Born effective charge and, thereby, the piezoelectric response. The incorporation of Hf leads to a multifold enhancement in the piezoelectric coefficient, , relative to AlN, despite increasing structural disorder in these  thin films. Building on this increased electromechanical coupling, we present functional GHz surface acoustic wave (SAW) resonators that exhibit enhanced performance, as well as efficient excitation of bulk acoustic waves with low propagation losses. These results establish as a promising platform for next‐generation high‐frequency electromechanical devices, with prospects for further piezoelectric enhancements through improved epitaxy.