*2.2. Characterization*

Film composition was determined by energy-dispersive spectrometry (EDS) of X-ray (Octane Elect Plus, EDAX, Pleasanton, CA, USA) using 10 kV accelerating voltage and the aluminum and scandium Kα lines for quantification.

Film stress measurements were performed using the wafer curvature method based on Stoney's equation. The measurements were carried out with a surface profiler, P15-Ls (KLA Tencor, Milpitas, CA, USA).

The XRD characterizations were carried out in a Bruker D8 Discover diffractometer equipped with a Göbel mirror for Cu-Ka parallel beam geometry and a 1D LynxEyeTM XET semiconductor detector (Bruker AXS, Karlsruhe, Germany). θ–2θ scans in 1D detector mode were used to characterize lattice parameters of the wurtzite structure. Film quality in terms of crystallographic orientation was investigated by evaluating the (002) rocking curve FWHM in 0D detector mode. For calculation of the wurtzite lattice parameter c, the (002) peaks in θ–2θ scans were used. Peak fitting including a Cu-Kα1/α<sup>2</sup> doublet correction followed by a correction for instrumental 2θ alignment errors was carried out to extract the exact 2θ peak positions from the raw data.

The piezoresponse force microscopy (PFM) measurements were carried out using an AFM NX 20 from Park Systems (Suwon, Republic of Korea). Silicon wafers coated with a Ti/Pt seed layer and bottom electrode for grounding connection were used as the substrate for AlScN layers. An ac voltage with an amplitude of 10 V and 17 kHz frequency was applied to a platinum-coated AFM tip (Spark 350 Pt) to measure PFM amplitude and phase of inverse piezoelectric effect in off-resonance mode.

The piezoelectric and ferroelectric characterization was performed using a doublebeam laser interferometer (DBLI) of aixACCT systems GmbH (Aachen, Germany). This method uses laser interferometry to measure surface displacement of the sample depending on applied electrical voltage. By using a double-beam configuration, substrate bending can be considered. The effective piezoelectric coefficient d33,f of the film material can be calculated by measuring at different electrode pad sizes and correcting using a geometric factor f(r), which is a function of the ratio of the pad size to substrate thickness and substrate Poisson's ratio. Besides d33,f, this also allows calculation of transverse effective piezoelectric coefficient e31,f. The method is described in further detail in [16].
