**2. Experimental**

### *2.1. Sample Preparation*

AlN and AlScN thin films are prepared by DC reactive magnetron sputtering in an Evatec Clusterline multichamber sputtering system. AlN films are sputter deposited in a gas plasma mixture of argon (Ar) and nitrogen (N2) at a temperature of 300 ◦C, while AlScN films are prepared by cosputtering of Sc and Al targets in a pure N<sup>2</sup> plasma system. The detailed process parameters are listed in Table 1. In this work, (100) Si, oxidized Si and poly-Si are used as substrates to grow AlN and AlScN films. All three substrates have a smooth surface on the front side (<2 nm RMS (root mean square)). Prior to the deposition of piezoelectric layers, all substrates are heated to 200 ◦C for 30 s for degassing and then cleaned by a soft Ar plasma surface etching (300 W for 40 s) in vacuum. In the first part of this study, AlN as well as AlScN films with a thickness of 1 µm are deposited directly on three different substrates, respectively. In the second part, in order to investigate the effect of AlN seed layer on the crystalline orientation and piezoelectric response of AlScN thin films, a 20 nm AlN layer is first deposited on substrates, followed by a 500 nm AlScN layer after a vacuum break. For the characterization of the piezoelectric coefficient, the AlScN film has to be sandwiched between bottom and top electrodes [37]. Therefore, a stack of four layers consisting of bottom electrode, 20 nm AlN, 500 nm AlScN, and top electrode is deposited on an oxidized Si wafer. Here, 20 nm Ti/100 nm Pt and 200 nm Mo are used as bottom and top electrodes, respectively. All samples in this work are 8-inch wafer-scale in size.

**Table 1.** Sputter deposition parameters of AlN and Al0.73Sc0.27N thin films.

