**1. Introduction**

Thin films of scandium-substituted aluminum nitride (Al1−xScxN or 'AlScN') are a pioneering new class of ferroelectric materials with wurtzite-type structure and unidirectional polarization reversal resulting in square-like hysteresis loops [1–5]. Their ultra-stable remnant polarization and high coercive fields promise ferroelectric field-effect transistorbased non-volatile memory devices maintaining large memory windows combined with high access speed, high endurance and low energy consumption [6]. In this respect, the integration of AlScN on Si has been demonstrated recently for ferroelectric field-effect transistors as well as GaN technology-based structures, e.g., for high electron mobility transistors [7–10] with integrated memory functions. Further, thin films of AlScN are potential candidates for high-temperature actuation and sensing applications in harsh environments (T > 500 ◦C) or for high-temperature non-volatile memory due to remarkable temperature

**Citation:** Wolff, N.; Islam, M.R.; Kirste, L.; Fichtner, S.; Lofink, F.; Žukauskaite, A.; Kienle, L. Al ˙ <sup>1</sup>−xScxN Thin Films at High Temperatures: Sc-Dependent Instability and Anomalous Thermal Expansion. *Micromachines* **2022**, *13*, 1282. https://doi.org/10.3390/ mi13081282

Academic Editor: Niall Tait

Received: 22 June 2022 Accepted: 4 August 2022 Published: 8 August 2022

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stability of the wurtzite-type structure and its piezo/ferroelectric properties at least up to 1100 ◦C [11].

In contrast, many conventional oxide-based piezoelectric and especially ferroelectric thin film materials such as Pb1−xZrxTiO<sup>3</sup> or BaTiO<sup>3</sup> are limited in their usable temperature range either by phase transitions (relatively low Curie temperatures), which describe the transition from the pyroelectric and thus ferroelectric state to a paraelectric state, or chemical decomposition [12,13]. Numerous detailed studies on temperature-dependent physical properties, e.g., the change in coercive fields [14,15], the pyroelectric coefficient [16], thermo-electro-acoustic coupling and thermal expansion [17–19], have been conducted to date, but only moderate temperatures of max. 400 ◦C were investigated. Recently, we also reported on in situ and ex situ high-temperature investigations, addressing the structural stability of Al1−xScxN (x~0.3) thin films [11,20]. However, depending on the annealing conditions, thin film stack and deposition conditions, both degradation of crystal quality and improvement after annealing were observed. Thereafter, the overall picture of the thermal stability of AlScN thin films in the high-temperature regime remains diffuse and a discussion of the influence of the Sc content is still missing.

Structurally, the substitution of Al atoms by Sc atoms into the AlN<sup>4</sup> tetrahedra distorts the (Al, Sc)-N bond lengths and bond angles of the three bonds forming the basal plane of the new tetrahedral *M*N<sup>4</sup> (*M* = Al, Sc) unit. As a result, the in-plane lattice parameter *a* and out-of-plane lattice parameter *c* change anisotropically with increasing concentration of Sc [19]. Statistical calculations on supercell models support this relationship showing that Sc increases disorder and symmetry breaking, establishing a tilt of the *M*N<sup>4</sup> tetrahedra [21]. In consequence, a pronounced elastic softening along the *c*-axis is observed [17] which couples to the evolution of the internal displacement parameter *u* of the cation–anion distance described by the product *uc*. The compromise between the ideal tetrahedral coordination of Al (*u* = 3/8) and the favored octahedral coordination of Sc (*u* = 1/2) results in the displacement of Sc in the tetrahedra. With increasing Sc concentration, this displacement approaches a theoretical layered hexagonal phase with *u = 0.5* promoting phase transition [21]. The flattened energy landscape with increasing Sc concentration [22] also improves the materials' piezoelectric response up to *d*33~27 pC/N at *x* = 0.43 [23] and promotes ferroelectric switching in high electric fields [1]. In conclusion, the substitution of Al by Sc in AlN in tetrahedral positions destabilizes the wurtzite-type crystal structure, resulting in a phase transformation above *x* > 0.46 to a non-polar cubic AlScN exhibiting rocksalt-type structure with octahedral coordination [24,25].

With this contribution, we extend our previous work on Al0.73Sc0.27N thin films on templates of Mo(110)/AlN(0001)/Si(001) [11] and match our new results to high-temperature data of Al1−xScxN(0001)/Al2O3(0001) to provide a joint perspective on AlScN films subjected to the high-temperature regime. In the latter systems, the Sc content was varied from Sc *x* = 0.0 to 0.40 to study Sc content-dependent thermal expansion behavior and high-temperature stability. The discussed results are based on the combination of X-ray diffraction techniques and in situ annealing experiments. Our measurements reveal the increasing structural destabilization of AlScN with increasing Sc content. Further, the demonstration of thermally activated effects is described, which drive an unexpected volume expansion at intermediate and high temperatures exceeding 550 ◦C. The high-temperature branch of thermal expansion is separated into intrinsic and extrinsic contributions related to oxygen impurities resulting in an irreversible change in the lattice parameters after annealing. Our results could have implications for the integration of AlScN thin films into metal–ferroelectric–metal capacitors for actuator, sensor or computing structures designed for high-temperature operation exceeding 500 ◦C up to the materials' Curie temperature of >1100 ◦C.
