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

**Elena A. Mayer <sup>1</sup> , Olga Rogall <sup>1</sup> , Anli Ding <sup>2</sup> , Akash Nair <sup>2</sup> , Agne Žukauskait ˙ e˙ 2,3,\* , Pavel D. Pupyrev <sup>1</sup> , Alexey M. Lomonosov <sup>4</sup> and Andreas P. Mayer 1,\***


**Abstract:** The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al2O<sup>3</sup> systems. Polar and non-polar Al0.77Sc0.23N thin films were prepared by magnetron sputter epitaxy on Al2O<sup>3</sup> substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system's anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

**Keywords:** laser ultrasound; AlScN; surface acoustic waves; magnetron sputter epitaxy; elastic properties; thin films
