**4. Conclusions**

In summary, a thin film, c−axis textured AlScN was electrically tested in a metal– ferroelectric–metal capacitor for polarization hysteresis, retention, and fatigue up to 400 ◦C. The leakage current density increased by four orders of magnitude between RT and 400 ◦C, which revealed the importance of compensating for the leakage contribution to polarization, especially as temperature increased. While a strong E<sup>c</sup> dependence on temperature (4.5 kV·cm−<sup>1</sup> ·K −1 ) led to a 60% reduction at 400 ◦C compared to RT, the P<sup>r</sup> remained stable over all temperatures. As an important characteristic for NVM, in situ polarization retention testing up to 1000 s revealed negligible (<2%) polarization loss for either the metal or the nitrogen polar state at all temperatures tested up to 400 ◦C. On average, the highest endurance before complete failure occurred at 200 ◦C with 2.7 <sup>×</sup> <sup>10</sup><sup>5</sup> switching cycles; however, the reduction in P<sup>r</sup> versus cycles was lowest at 100 ◦C. Our results showing >10<sup>5</sup> switching cycles are encouraging for using ferroelectric AlScN in HOT−NVM applications where <10<sup>3</sup> switching cycles would be required. Future work on the fatigue mechanisms at different temperatures and expanding the retention duration time are vital avenues for understanding the intrinsic limitations of AlScN.

**Author Contributions:** Conceptualization, D.D., B.H. and G.B.; methodology, D.D.; validation, D.D., B.H. and G.B.; formal analysis, D.D.; investigation, D.D.; resources, K.Y., A.Z. and B.H.; data curation, D.D.; writing—original draft preparation, D.D., B.H. and G.B.; writing—review and editing, D.D., B.H. and G.B.; supervision, K.Y., A.Z., B.H. and G.B.; project administration; funding acquisition, A.Z., B.H. and G.B. All authors read and agreed to the published version of the manuscript.

**Funding:** This research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-21-2-0210. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the US Government. The US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein. This work was coauthored by the Colorado School of Mines and the National Renewable Energy Laboratory, operated by the Alliance for Sustainable Energy, LLC, for the US Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding was provided by the DARPA Tunable Ferroelectric Nitrides (TUFEN) program (DARPA-PA-19-04-03) as a part of Development and Exploration of Ferroelectric Nitride Semiconductors (DEFENSE) project (diffraction, microscopy, and electrical characterization), and by Office of Science (SC), Office of Basic Energy Sciences (BES) as part of the Early Career Award "Kinetic Synthesis of Metastable Nitrides" (material synthesis and in situ monitoring).

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

**Acknowledgments:** The authors also express their appreciation to Wanlin Zhu and Susan Trolier-McKinstry of the Pennsylvania State University for providing Pt/TiO2/SiO2/Si substrates. We also thank Jeff Alleman for assistance with setting up the sputter system at NREL.

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