*Article* **Growth of Highly c-Axis Oriented AlScN Films on Commercial Substrates**

**Jingxiang Su 1,\* , Simon Fichtner 1,2, Muhammad Zubair Ghori <sup>1</sup> , Niklas Wolff <sup>2</sup> , Md. Redwanul Islam <sup>2</sup> , Andriy Lotnyk <sup>3</sup> , Dirk Kaden <sup>1</sup> , Florian Niekiel <sup>1</sup> , Lorenz Kienle <sup>2</sup> , Bernhard Wagner <sup>1</sup> and Fabian Lofink 1,\***

> <sup>1</sup> Fraunhofer Institute for Silicon Technology ISIT, Fraunhoferstrasse 1, 25524 Itzehoe, Germany; simon.fichtner@isit.fraunhofer.de (S.F.); muhammad.zubair.ghori@isit.fraunhofer.de (M.Z.G.); dirk.kaden@isit.fraunhofer.de (D.K.); florian.niekiel@isit.fraunhofer.de (F.N.); beha.wagner@gmail.com (B.W.)

> 2 Institute for Material Science, Kiel University, Kaiserstr. 2, 24143 Kiel, Germany; niwo@tf.uni-kiel.de (N.W.); mdis@tf.uni-kiel.de (M.R.I.); lk@tf.uni-kiel.de (L.K.)


**Abstract:** In this work, we present a method for growing highly *c*-axis oriented aluminum scandium nitride (AlScN) thin films on (100) silicon (Si), silicon dioxide (SiO2) and epitaxial polysilicon (poly-Si) substrates using a substrate independent approach. The presented method offers great advantages in applications such as piezoelectric thin-film-based surface acoustic wave devices where a metallic seed layer cannot be used. The approach relies on a thin AlN layer to establish a wurtzite nucleation layer for the growth of *w*-AlScN films. Both AlScN thin film and seed layer AlN are prepared by DC reactive magnetron sputtering process where a Sc concentration of 27% is used throughout this study. The crystal quality of (0002) orientation of Al0.73Sc0.27N films on all three substrates is significantly improved by introducing a 20 nm AlN seed layer. Although AlN has a smaller capacitance than AlScN, limiting the charge stored on the electrode plates, the combined piezoelectric coefficient d33,*f* with 500 nm AlScN is only slightly reduced by about 4.5% in the presence of the seed layer.

**Keywords:** aluminium scandium nitride; piezoelectric thin films; MEMS; non-metallic substrates
