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Al$_{1-x}$Hf$_{x}$N Thin Films with Enhanced Piezoelectric Responses for GHz Surface Acoustic Wave Devices

Laura I. Wagner, Verena Streibel, Esperanza Luna, Katarina S. Flashar, Walid Anders, Nicole Volkmer, Doreen Steffen, Frans Munnik, Tsedenia A. Zewdie, Saswati Santra, Ian D. Sharp, Mingyun Yuan

TL;DR

This work introduces $Al_{1-x}Hf_xN$ as a CMOS-compatible alternative to $Al_{1-x}Sc_xN$ for high-frequency piezoelectric applications. By reactive magnetron co-sputtering, phase-pure wurtzite films with $x$ up to 0.17 are grown on $Si$ and $c$-plane $Al_2O_3$, exhibiting nearly isotropic lattice expansion with $c/a\approx1.6$. Electronic-structure probes reveal cross-gap $N\,2p$–$Hf\,5d$ hybridization, increasing the Born effective charge and driving a near twofold enhancement of $d_{33}$, despite growing structural disorder. GHz SAW resonators and low-loss BAWs demonstrate improved electromechanical performance relative to AlN, highlighting $Al_{1-x}Hf_xN$ as a scalable, CMOS-friendly platform for next-generation RF piezoelectric devices, with further improvements anticipated through epitaxial optimization.

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. 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.

Al$_{1-x}$Hf$_{x}$N Thin Films with Enhanced Piezoelectric Responses for GHz Surface Acoustic Wave Devices

TL;DR

This work introduces as a CMOS-compatible alternative to for high-frequency piezoelectric applications. By reactive magnetron co-sputtering, phase-pure wurtzite films with up to 0.17 are grown on and -plane , exhibiting nearly isotropic lattice expansion with . Electronic-structure probes reveal cross-gap – hybridization, increasing the Born effective charge and driving a near twofold enhancement of , despite growing structural disorder. GHz SAW resonators and low-loss BAWs demonstrate improved electromechanical performance relative to AlN, highlighting as a scalable, CMOS-friendly platform for next-generation RF piezoelectric devices, with further improvements anticipated through epitaxial optimization.

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. So far, progress has largely focused on AlScN, which is costly and poorly compatible with complementary metal-oxide-semiconductor (CMOS) technologies. Here, we investigate aluminum hafnium nitride (AlHfN) as a scalable and potentially CMOS-compatible alternative to AlScN. Using reactive co-sputtering on both Si and sapphire substrates, we demonstrate wurtzite AlHfN 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. Correspondingly, we observe a nearly two-fold enhancement in the piezoelectric coefficient, , relative to AlN, despite increasing structural disorder in AlHfN. Building on this finding, we demonstrate AlHfN 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 AlHfN as a promising platform for next-generation high-frequency electromechanical devices, with prospects for further piezoelectric enhancements through improved epitaxy.

Paper Structure

This paper contains 8 sections, 1 equation, 9 figures.

Table of Contents

  1. Introduction
  2. Results and Discussion
  3. Synthesis of Al_1-xHf_xN thin films
  4. Crystallographic and microstructural analysis of Al_1-xHf_xN
  5. Electronic structure and piezoelectric properties
  6. Surface acoustic wave and bulk acoustic wave excitation
  7. Conclusion
  8. Experimental Section

Figures (9)

  • Figure 1: Synthesis of $\ce{Al_{1-x}Hf_{x}N}$ thin films and analysis of their elemental compositions. (a) Schematic illustration of reactive magnetron co-sputtering of $\ce{Al_{1-x}Hf_{x}N}$ using two metallic targets. (b) Hf cation fraction as a function of Hf sputter power in $\ce{Al_{1-x}Hf_{x}N}$ thin films deposited for 60 min (squares) and 180 min (circles), as determined by XPS (filled symbols) and ERDA (open squares). (c) ERDA elemental composition depth profiles measured on samples sputtered with 0 W (upper panel) and 30 W (lower panel) applied to the Hf sputter target.
  • Figure 2: Lattice parameters of Al_1-xHf_xN. (a) Lattice constants, $c$ (upper panel) and $a$ (lower panel), of $\ce{Al_{1-x}Hf_{x}N}$ thin films deposited for 60 min on Si as a function of Hf cation fraction. For reference, the corresponding lattice constants of bulk AlN (stars) are also provided Ott.1924. (b) Lattice parameter ratio $c/a$ for the thin films deposited for 60 min on Si (inverted triangles) and for thicker films deposited for 180 min on c-plane sapphire (squares).
  • Figure 3: Structures of sputtered Al_1-xHf_xN thin films. (a) HRXRD (out-of-plane) $2\theta/\omega$ scans of Al_1-xHf_xN thin films on $c$-plane sapphire displayed on a logarithmic intensity scale. The vertical gray reference lines indicate the diffraction pattern of wurtzite AlN, labeled with their Miller-Bravais indices Ott.1924. Stars mark the $(0006)$ and $(0009)$ peaks of sapphire. (b) Rocking curve scans on the $(0002)$ reflection of wurtzite Al_1-xHf_xN. In-plane pole figures on a logarithmic intensity scale of the $(10\overline{1}0)$ diffraction peak of (c) AlN, (d)Al_0.94Hf_0.06N, and (e)Al_0.88Hf_0.12N.
  • Figure 4: Microstructure of sputtered Al_1-xHf_xN thin films. Representative (a) HAADF-STEM and (b) HRTEM images of 340 nm thick Al_0.88Hf_0.12N films grown on $c$-plane sapphire, along with (c), FFT images from the selected areas indicated by the corresponding boxes i-v.
  • Figure 5: Electronic structure of Al_1-xHf_xN. Normalized X-ray absorption spectra of AlN and Al_0.88Hf_0.12N thin films at the (a) Al L$_{2,3}$- and (b) N K-edge, collected via total fluorescence yield (TFY) and total electron yield (TEY), respectively, as a function of photon energy (P.E.). The corresponding differences between the Al_0.88Hf_0.12N and the binary AlN reference spectra are given in the lower panels. (c) Valence band spectra measured by XPS as a function of binding energy (B.E.)
  • ...and 4 more figures