*Article* **Al1**−**xScxN Thin Films at High Temperatures: Sc-Dependent Instability and Anomalous Thermal Expansion**

**Niklas Wolff 1,\*, Md Redwanul Islam <sup>1</sup> , Lutz Kirste <sup>2</sup> , Simon Fichtner 1,3, Fabian Lofink <sup>3</sup> , Agne Žukauskait ˙ e˙ 2,\* ,† and Lorenz Kienle 1,\***


**Abstract:** Ferroelectric thin films of wurtzite-type aluminum scandium nitride (Al1−xScxN) are promising candidates for non-volatile memory applications and high-temperature sensors due to their outstanding functional and thermal stability exceeding most other ferroelectric thin film materials. In this work, the thermal expansion along with the temperature stability and its interrelated effects have been investigated for Al1−xScxN thin films on sapphire Al2O<sup>3</sup> (0001) with Sc concentrations *x* (*x* = 0, 0.09, 0.23, 0.32, 0.40) using in situ X-ray diffraction analyses up to 1100 ◦C. The selected Al1−xScxN thin films were grown with epitaxial and fiber textured microstructures of high crystal quality, dependent on the choice of growth template, e.g., epitaxial on Al2O<sup>3</sup> (0001) and fiber texture on Mo(110)/AlN(0001)/Si(100). The presented studies expose an anomalous regime of thermal expansion at high temperatures >~600 ◦C, which is described as an isotropic expansion of *a* and *c* lattice parameters during annealing. The collected high-temperature data suggest differentiation of the observed thermal expansion behavior into defect-coupled intrinsic and oxygen-impurity-coupled extrinsic contributions. In our hypothesis, intrinsic effects are denoted to the thermal activation, migration and curing of defect structures in the material, whereas extrinsic effects describe the interaction of available oxygen species with these activated defect structures. Their interaction is the dominant process at high temperatures >800 ◦C resulting in the stabilization of larger modifications of the unit cell parameters than under exclusion of oxygen. The described phenomena are relevant for manufacturing and operation of new Al1−xScxN-based devices, e.g., in the fields of high-temperature resistant memory or power electronic applications.

**Keywords:** aluminum scandium nitride; thermal stability; structure analysis; X-ray diffraction; ferroelectrics
