*3.1. Film Microstructure*

The XRD scans of the Al1−*x*Sc*x*N (*x* = 0, 0.14, 0.23, 0.32, 0.41) films grown on the c-plane Al2O<sup>3</sup> substrates show the 000*l* (*l* = 2, 4) reflections of the nitride and 000*l* (*l* = 6, 9) reflections of the oxide compounds (Figure 1). The lack of additional peaks assigned to the AlScN and the reflections stemming from the c-plane suggest that the films are highly c-axis-oriented. Pole figure measurements confirm in-plane oriented growth of AlScN with the epitaxial relationship defined as [10-10]AlScN//[11-20]sapphire and (0001)AlScN//(0001)sapphire [21]. Provided that the film thickness values are similar, the variation in the peak intensity indicates different amounts of the diffracting domains, and the variation in the peak linewidths suggests their diverging size distributions (Figure 1, inset). The trends for the peak position, which are dependent on the Sc concentration and the thermal strain, are in agreement with the previously reported ones [20,25,26]. The alterations in the peak positions and linewidths of the (0004) reflection peak mirror the behaviour observed for the (0002) one scaled due to the higher 2*θ* angles, which suggests good uniformity for the long-range order. The peak intensity is continuously reduced for higher amounts of Sc, which can be related to the size reduction of crystalline domains. No peak solely related to the rock-salt ScN or AlScN phase, which might have been expected in the alloy phase diagram [8], was observed in the diffractograms, confirming the dominating wurtzite phase in the pseudobinary AlScN alloy.

**Figure 1.** XRD symmetric *θ*/2*θ* scans of AlScN films with various Sc amounts. Inset: the 2*θ* range in the vicinity of the (0002) peak of the AlScN alloy.

Figure 2 shows AFM micrographs of the films, revealing the pebble-like morphology characteristic of the AlN films grown via magnetron sputtering [27,28]. The AFM results revealed a less random arrangement of the grains being seemingly clustered into short chains of hemispherical droplets, which resembled the surface of AlN after the annealing

process [29] and thus indicated the relation to the alteration of the adatom surface mobility with adding more Sc. When applying the structure zone model [30], the worm-like surface texture is characteristic for the low adatom mobility [31], which is caused by the different kinetic energies of co-sputtered species. The surface of the Al0.59Sc0.41N film reveals the presence of larger grains located at the nodes of the worm-like surface. These specific grains may be attributed to the abnormally oriented grains of AlScN observed for the cases of high Sc concentration [21,32]. The surface roughness of the films decreased when alloying to more Sc atoms and reached its minimum value for *x* = 0.32 of the scandium composition. The following increase in the roughness values for the film alloyed with the highest Sc amount was related to the bright protrusions visible in the image. Nevertheless, the roughness values below 2 nm revealed that AlScN films were largely smooth and exhibited no surface structures to be assigned to other crystalline phases. Thus, the investigation of the film microstructure showed that the films of the pseudobinary AlScN alloys on Al2O<sup>3</sup> consisted of one crystalline wurtzite phase, which was oriented along the c-axis, indicating columnar growth.

**Figure 2.** Surface morphology of the thin films with the following Sc content, *x* = (**a**) 0 (AlN), (**b**) 0.14, (**c**) 0.32, (**d**) 0.41 obtained via AFM. The sample with *x* = 0.23 is not shown. The false colour scale is common for all images. The rms roughness, *Sq*, values is indicated on the corresponding images. Insets: complimentary 1 <sup>×</sup> <sup>1</sup> <sup>µ</sup>m<sup>2</sup> AFM images.
