*2.2. X-ray Diffraction Experiments*

The temperature-dependent microstructural changes in all thin film samples were investigated via XRD experiments on as-deposited samples as well as during and after in situ experiments. The structural properties of the as-grown epitaxial films (#2) were analyzed on a PANalytical X'Pert Pro MRD diffractometer equipped with a multi-layer mirror and a 2 × Ge 220 monochromator, providing Cu-Kα<sup>1</sup> radiation (λ = 154.06 pm). XRD experiments on the sample sets #1 and #3 were conducted on a Rigaku SmartLab diffractometer operated under similar conditions. To characterize the as-deposited and annealed samples, phase analyses and mosaic tilt analyses were performed by recording 2θ/θ-scans and rocking curve ω-scans of the AlScN 0002-reflection.

To study effects of high-temperature annealing in (#2) Al1−xScxN/Al2O<sup>3</sup> films as a function of Sc concentration, we performed in situ XRD measurements in the temperature range of 30–1000 ◦C. The experiments were conducted at a base pressure of <sup>∼</sup><sup>6</sup> <sup>×</sup> <sup>10</sup>−<sup>3</sup> mbar achieved inside a graphite dome heating furnace (Anton Paar, Ostfildern-Scharnhausen, GermanyDHS 1100 Domed Hot Stage) placed in a PANalytical Empyrean diffractometer equipped with a PIXcel hybrid detector (Malvern Panbalytical, Kassel, Germany). 2θ/θ and ω/2θ diffraction scans were recorded after 20 min hold time at selected temperatures for approximately 60 min of dwell time. The heating rate was set to 150 ◦C/min. Lattice parameters were estimated from scans of the symmetrical 0004-reflection and asymmetric 10–15-reflection with shallow (−) and steep (+) angles of incidence ω, similar to the technique described by Herres et al. [30]. For the analysis of sample set #1 and #3, simple 2θ/θ-scans were conducted during in situ heating experiments under similar conditions using an AntonPaar (DHS 1100) hot stage placed on the Rigaku diffractometer and as described in [11].

In addition to the in situ examinations, we also conducted an ex situ annealing experiment to compare the high-temperature behavior of samples exposed to and protected from oxygen-contaminated environments during high-temperature annealing. Here, a Al0.73Sc0.27N/Mo(110)/AlN(0001)/Si(001) sample was placed inside a self-built tube furnace keeping a quartz tube under vacuum (3–4 <sup>×</sup> <sup>10</sup>−<sup>7</sup> mbar). Initially, the quartz tube was annealed without the sample to 800 ◦C together with Ti getter material to reduce the amount of adsorbed oxygen on the tube walls. The actual annealing experiment was started at a base pressure of 1.2 <sup>×</sup> <sup>10</sup>−<sup>6</sup> mbar and the temperature was increased to 800 ◦C in steps of 150 ◦C and cooled down to room temperature in a single step.
