**5. Conclusions**

**5. Conclusions** The thermal stability and temperature-induced effects of AlScN thin film samples with different microstructures based on the growth template were investigated by in situ XRD. The degradation of the crystalline quality and a remnant lattice expansion in the *c*direction of up to 0.5% for epitaxial Al0.60Sc0.40N(0001)/Al2O3(0001) were observed as a function of Sc concentration. There is first evidence that the remnant expansion is related to the activation of intrinsic defects and the films' oxygen affinity at elevated temperatures, which are accounted for as intrinsic and extrinsic sources of anomalous thermal expansion. The understanding of the exact details of these phenomena provides opportunity for further investigations on the exact type of defect structures using more advanced methods, such as positron annihilation spectroscopy [48–50]. The detailed understanding of the intrinsic defect structures and temperature activation effects is of high importance for the integration of sputtered AlScN layers into sandwich structures, e.g., for The thermal stability and temperature-induced effects of AlScN thin film samples with different microstructures based on the growth template were investigated by in situ XRD. The degradation of the crystalline quality and a remnant lattice expansion in the *c*-direction of up to 0.5% for epitaxial Al0.60Sc0.40N(0001)/Al2O3(0001) were observed as a function of Sc concentration. There is first evidence that the remnant expansion is related to the activation of intrinsic defects and the films' oxygen affinity at elevated temperatures, which are accounted for as intrinsic and extrinsic sources of anomalous thermal expansion. The understanding of the exact details of these phenomena provides opportunity forfurther investigations on the exact type of defect structures using more advanced methods, such as positron annihilation spectroscopy [48–50]. The detailed understanding of the intrinsic defect structures and temperature activation effects is of high importance for the integration of sputtered AlScN layers into sandwich structures, e.g., for ferroelectric field-effect transistor memory capacitors or ferroelectric tunnel junctions. That is especially

relevant with respect to downscaling of the layer thickness and high-temperature operation, where oxidation of the functional layer has to be prohibited and an anomalously high expansion of the AlScN crystal lattice puts increased stresses on any neighboring crystalline layers to avoid device failure via crack formation or delamination. In conclusion, our investigation emphasizes the requirement of a low density of material defects in AlScN thin films when operating at high temperatures and the benefit of integration with a protecting top layer, e.g., a temperature-resistant electrode such as Mo or NbN.

**Author Contributions:** Conceptualization, N.W. and A.Ž.; methodology, M.R.I. and L.K. (Lutz Kirste); validation of experiments, A.Ž., S.F., L.K. (Lutz Kirste) and L.K. (Lorenz Kienle); formal analysis, N.W., M.R.I. and L.K. (Lutz Kirste); investigation, N.W., M.R.I. and L.K. (Lutz Kirste); data curation, N.W., M.R.I. and L.K. (Lutz Kirste); writing—original draft preparation, N.W.; writing—review and editing, A.Ž., L.K. (Lutz Kirste), S.F. and L.K. (Lorenz Kienle); visualization, N.W.; supervision, A.Ž. and S.F.; project administration, F.L. and L.K. (Lorenz Kienle); funding acquisition, S.F., A.Ž. and L.K. (Lorenz Kienle). All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the project 'ForMikro-SALSA' (grant no. 16ES1053) from the Federal Ministry of Education and Research (BMBF) and the DFG under the scheme of the Collaborative Research Center (CRC) 1261/A6.

**Data Availability Statement:** Original XRD data are available from the corresponding authors upon reasonable request.

**Acknowledgments:** The authors would like to thank Yuan Lu for his contribution to providing AlScN/Al2O<sup>3</sup> , Fredrik Eriksson for his contribution to performing in situ XRD annealing experiments on AlScN/Al2O<sup>3</sup> samples and Maximilian Kessel for enthusiastic discussions on the presented topics of this paper. Financial support by the Fraunhofer Society is gratefully acknowledged.

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