Al$_{1-x}$Hf$_{x}$N Thin Films with Enhanced Piezoelectric Responses for GHz Surface Acoustic Wave Devices
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. So far, progress has largely focused on Al$_{1-x}$Sc$_x$N, which is costly and poorly compatible with complementary metal-oxide-semiconductor (CMOS) technologies. Here, we investigate aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) as a scalable and potentially CMOS-compatible alternative to Al$_{1-x}$Sc$_x$N. Using reactive co-sputtering on both Si and sapphire substrates, we demonstrate wurtzite Al$_{1-x}$Hf$_{x}$N thin films ($x \leq 0.17$) with strong $c$-axis texture and nearly isotropic lattice expansion upon Hf incorporation. X-ray absorption spectroscopy indicates cross-gap hybridization between N 2$p$ and Hf 5$d$ states, which can enhance the Born effective charge and, thereby, the piezoelectric response. Correspondingly, we observe a nearly two-fold enhancement in the piezoelectric coefficient, $d_{33}$, relative to AlN, despite increasing structural disorder in Al$_{1-x}$Hf$_{x}$N. Building on this finding, we demonstrate Al$_{1-x}$Hf$_{x}$N 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 Al$_{1-x}$Hf$_{x}$N as a promising platform for next-generation high-frequency electromechanical devices, with prospects for further piezoelectric enhancements through improved epitaxy.