**4. Conclusions**

The vibrational structure of AlScN pseudobinary alloys in the thin-film form with various Sc content grown on sapphire substrates was investigated in this work by means of Raman spectroscopy. The films were confirmed to exhibit the wurtzite structure with the c-axis orientation facilitated by the columnar growth and the pebble-like surface, regardless of Sc content. Apart from the modes expected for the AlN-based wurtzite lattice, the Raman spectra of the AlScN films exhibited an enhanced phonon density of states and the long-time neglected second-order phonon modes. The one-phonon modes were used to analyse the defect density, which rose by almost two orders of magnitude in the case of AlScN with the highest Sc content examined. The comparison between the phonon

correlation length and the average grain size showed the dominant role of the point defects originating largely due to the alloying. The temperature-dependent Raman measurements enabled the temperature coefficients for the E2(high) mode to be determined, which can be used for the temperature monitoring in the AlScN-based devices. Using the nearexcitation tunable notch filter, the low-frequency Raman spectra were recorded for the first time, demonstrating the Raman-active bands between 100 and 150 cm−<sup>1</sup> attributed to the acoustic phonons confined in the spherical nanoparticles. We showed that Raman spectroscopy allows multifaceted material characterisation in the field of III-V compounds and their alloys, owing to the robust vibrational properties of the condensed matter. This work enables further studies of the vibrational properties of Al1−*x*Sc*x*N alloys concerning the dependence of pressure and temperature in broader ranges.

**Author Contributions:** Conceptualisation, methodology, D.S.; resources, A.Z.; validation, formal analysis, D.S.; writing—review and editing, D.S., J.P., S.R. and A.Z.; project administration; funding acquisition, S.R. and M.M. All authors read and agreed to the published version of the manuscript.

**Funding:** This project was performed within the COMET Centre ASSIC Austrian Smart Systems Integration Research Center, which is funded by BMK, BMDW, and the Austrian provinces of Carinthia and Styria, within the framework of COMET—Competence Centers for Excellent Technologies. The COMET programme is run by FFG.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Acknowledgments:** The authors would like to thank Vladimir Pashchenko, Yuriy Azhniuk, and Volodymyr Dzhagan for fruitful discussions on the presented topics of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
