Properties of higher-order surface acoustic wave modes in Al1−xScxN/sapphire structures

Abstract

In this work, surface acoustic wave (SAW) modes and their dependence on propagation directions in epitaxial Al0.68Sc0.32N(0001) films on Al2O3(0001) substrates were studied using numerical and experimental methods. In order to find optimal propagation directions for higher-order SAW modes, phase velocity dispersion branches of Al0.68Sc0.32N on Al2O3 with Pt mass loading were computed for the propagation directions <112¯0> and <11¯00> with respect to the substrate. Experimental investigations of phase velocities and electromechanical coupling were performed for comparison with the numerical results. Simulations carried out with the finite element method and a Green function approach allowed identification of each wave type, including Rayleigh, Sezawa, and shear-horizontal wave modes. For the propagation direction <11¯00>, significantly increased wave guidance of the Sezawa mode compared to other directions was observed, resulting in enhanced electromechanical coupling (keff2=1.6%) and phase velocity (vphase=6km/s). We demonstrated that selecting wave propagation in <11¯00> with high mass density electrodes results in increased electromechanical coupling without significant reduction in phase velocities for the Sezawa wave mode. An improved combination of metallization, Sc concentration x, and SAW propagation direction is suggested that exhibits both high electromechanical coupling (keff2>6%) and high velocity (vphase=5.5km/s) for the Sezawa mode.

Growth Optimization of Nonpolar Al0.7Sc0.3N(112¯0)/Al2O3(11¯02) Thin Films Using Reactive Magnetron Sputter Epitaxy

“…It would also be interesting to see the effect of the morphological anisotropy on the propagation of SAW waves along different propagation directions. Some preliminary work [ 54 ] has already been carried out on films grown within the group in studying the influence of propagation direction on the SAW resonator characteristics. That study demonstrated that there is no significant influence of propagation direction for AlScN(0001) films grown on sapphire(0001) due to the isotropic nature of the films.…”

Laser Ultrasound Investigations of AlScN(0001) and AlScN(11-20) Thin Films Prepared by Magnetron Sputter Epitaxy on Sapphire Substrates

“…If the minimal achievable SAW wavelength is larger or of the order of the film thickness, such that the SAW displacement field penetrates deeply into the substrate, the elastic properties of the substrate as well as its orientation (e.g., off-cut angles) have to be known to a high precision. On the other hand, if the frequency range can be considerably extended to higher values, higher-order guided acoustic modes can be used in addition to the lowest SAW to gain information about the elastic moduli of the film [ 11 ].…”

“…The sensitivity analysis in [ 17 ] confirms that the piezoelectric constants have a very small influence on the SAW dispersion curves if compared to the elastic constants. However, depending on the required accuracy in the determination of the elastic moduli, auxiliary measurements with an alternative technique, such as the resonator method [ 10 , 11 ] will be needed.…”

“…In the past, elastic constants of AlScN films have been determined with Brillouin scattering [ 7 , 8 , 9 ] and via electrical excitation of acoustic vibrations, especially with the help of SAW resonators (called resonator method in the following) [ 10 , 11 ] or delay lines [ 12 ] (for a review see for example [ 10 ]). However, preparing SAW or other types of resonators requires advanced fabrication capabilities to obtain the full dispersion curves from multiple test structures (phase velocity as a function of frequency).…”