**2. Materials and Methods**

*2.1. Thin Film Samples*

Three different sets of Al1−xScxN thin film samples summarized in Table 1 were observed:


**Table 1.** Summary of all thin film samples investigated in this study.

(#1) Thin films of Al0.73Sc0.27N (thickness 400 nm) were deposited on high-temperaturestable metal electrode Mo(110)/AlN(0001)/Si(001) templates using an Oerlikon MSQ200 multi-source pulsed-DC sputter chamber and previously reported processes [26,27]. These films show a typical highly *c*-axis-oriented fiber texture (the 0002 XRD rocking curve (XRC) full-width at half-maxima (FWHM) is ~1.6◦ ) with a small number of misoriented grains [26]. Surface capping of the films with a 100 nm layer of amorphous SiN<sup>x</sup> was applied to protect the film from oxidation during high-temperature treatment.

(#2) Epitaxial 1 µm thick films of Al1−xScxN (x = 0, 0.09, 0.23, 0.32, 0.40) with *c*-axisoriented columnar growth were deposited onto sapphire Al2O3(0001) wafer substrates (diameter: 100 × 100 mm) by reactive pulsed-DC magnetron sputter epitaxy (MSE). For the depositions, an Evatec cluster sputter tool with a base pressure of ~5 <sup>×</sup> <sup>10</sup>−<sup>6</sup> Pa in a co-sputtering configuration of 99.9995% pure Al and 99.99% pure Sc targets (diameter: 100 mm) was used running a process described in previous studies [17,28]. The FWHMs of XRC scans for the AlScN 0002-reflection are in the range of 0.9 and 1.6◦ for *x* = 0 and *x* = 0.40, respectively.

(#3) Epitaxial thin films of Al0.6Sc0.4N (thickness of 400 nm) with *c*-axis-oriented columnar growth on a template of Mo(110)/AlN(0001)/Al2O3(0001). The films were deposited according to the procedure described in [29] and capped with a 100 nm thick layer of SiN<sup>x</sup> for passivation during annealing experiments.
