*Article* **Raman Spectroscopy and Spectral Signatures of AlScN/Al2O<sup>3</sup>**

**Dmytro Solonenko 1,\* , Agne˙ Zukauskait ˘ e˙ 2,† , Julian Pilz <sup>1</sup> , Mohssen Moridi <sup>1</sup> and Sarah Risquez <sup>1</sup>**


**Abstract:** III-V solid solutions are sensitive to growth conditions due to their stochastic nature. The highly crystalline thin films require a profound understanding of the material properties and reliable means of their determination. In this work, we have investigated the Raman spectral fingerprint of Al1−*x*Sc*x*N thin films with Sc concentrations *<sup>x</sup>* = 0, 0.14, 0.17, 0.23, 0.32, and 0.41, grown on Al2O3(0001) substrates. The spectra show softening and broadening of the modes related to the dominant wurtzite phase with increasing Sc content, in agreement with the corresponding XRD results. We investigated the primary scattering mechanism responsible for the immense modes' linewidths by comparing the average grain sizes to the phonon correlation length, indicating that alloying augments the point defect density. The low-frequency Raman bands were attributed to the confined spherical acoustic modes in the co-forming ScN nanoparticles. Temperature-dependent Raman measurements enabled the temperature coefficient of the E2(high) mode to be determined for all Sc concentrations for the precise temperature monitoring in AlScN-based devices.

**Keywords:** aluminium scandium nitride; piezoelectric films; Raman spectroscopy; alloy scattering; temperature coefficient
