**1. Introduction**

Nonvolatile memories, which do not require a permanent/frequent voltage supplied to maintain the bit state, are important for storing information indefinitely and reliably. The concept of high−operating−temperature nonvolatile memory (HOT−NVM) has been an elusive capability. Current computational and data storage limitations in harsh environments either require on−board cooling or locating sensors/computations away from the heat source. These harsh environments for electronics are a focus of the NASA HOTTech project and can be found within jet turbines, within deep−well drilling, and on the surface of Venus [1].

Nonvolatile random−access memory (RAM) based on FLASH, magnetic, phase−change, and resistive mechanisms degrade quickly even at moderate temperatures (<200 ◦C) [2–5]. Microelectromechanical (MEM) and nanogap resistance switching (NGS) offer promise for elevated temperatures NVM, but there are downsides with moving parts and operating in various atmospheres [6,7]. Ferroelectric technology based on perovskite or fluorite structures (e.g., Pb(Zr,Ti)O<sup>3</sup> or (Hf,Zr,Si)O2) is currently limited to temperatures < 200 ◦C [8,9]. This is a result of a destabilized polar structure, chemical instabilities, and increased domain wall mobility in Pb(Zr,Ti)O<sup>3</sup> [10,11], whereas increased pyroelectric contributions and depolarization fields plague (Hf,Zr)O<sup>2</sup> compositions when increasing temperature [12,13]. Here, we demonstrate promising initial results when using ferroelectric Al0.7Sc0.3N to meet the HOT−NVM demands.

**Citation:** Drury, D.; Yazawa, K.; Zakutayev, A.; Hanrahan, B.; Brennecka, G. High-Temperature Ferroelectric Behavior of Al0.7Sc0.3N. *Micromachines* **2022**, *13*, 887. https:// doi.org/10.3390/mi13060887

Academic Editor: Faisal Mohd-Yasin

Received: 12 May 2022 Accepted: 30 May 2022 Published: 31 May 2022

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Since its discovery in 2019 as the first wurtzite nitride ferroelectric, Al1−xScxN (AlScN) has received much attention from both the ferroelectric and piezoelectric communities [14]. The high remanent polarization (Pr) associated with the ideal wurtzite structure is an attractive property for increasing either device density or extracted charge density. Previous studies on AlN−based ferroelectrics reported the high coercive field (Ec) necessary to switch between the two polarization states. While this high E<sup>c</sup> is problematic for integration with low−voltage devices, it can be valuable for retention characteristics as temperatures increase [8]. The E<sup>c</sup> of AlScN can be tuned through various modifications such as composition [14–16], stress [17], epitaxial alignment [18], and temperature [19–22]. In addition to thickness scaling, these trends are important for integrating AlScN into a high−temperature chip to reduce the operating voltages.

To assess polarization stability for a nonvolatile memory, Islam et al. poled the sample at room temperature (RT), baked the package at 1100 ◦C, and remeasured the P<sup>r</sup> once cooled back to RT [23]. While promising for an NVM since the polarization state is maintained after exposure to extreme temperatures, these ex situ results did not address the P<sup>r</sup> or switching behavior at elevated temperatures. Liu et al. focused on the retention state at RT for an AlScN ferroelectric−gated field effect transistor (FET) device by monitoring changes in the resistance state over time [24]. Furthermore, an NVM must endure switching cycles without compromising the P<sup>r</sup> or E<sup>c</sup> values, which can impact deciphering between either polarization state, and the device must avoid electrical shorts. Two recent reports have noted that AlScN has low endurance, a measure of repeated bit flips, at <10<sup>5</sup> cycles before dielectric failure, which may be limited by the slim margin between coercive and breakdown fields (Ebd) for the samples, but a true mechanistic understanding of fatigue in AlScN is still lacking [25,26]. Thus, these reports do not represent a fundamental limitation of the ferroelectric switching endurance. In this article, we report device relevant behavior (i.e., leakage, retention, and fatigue) between 23 and 400 ◦C to introduce AlScN as a candidate material for HOT−NVM.
