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ABSTRACT Ternary nitride alloys based on wurtzite AlN are a promising platform to realize functional materials, particularly ferroelectrics and optical emitters, that can smoothly integrate with conventional microelectronics. Here, a strategic design is presented to enable multifunctional materials by substituting multiple elements into AlN to create quaternary nitride alloys. By combining computational predictions and combinatorial thin film synthesis, the phase diagram of these quaternary Al–Sc–Gd–N alloys (or pseudo‐ternary heterostructural AlN–ScN–GdN alloys) is successfully predicted as a function of effective temperature, and we experimentally grow thin films for the first time. It is revealed that crystallizes in a wurtzite‐derived structure for , consistent with the calculated phase diagram. The computational investigation explores whether co‐substitution induces cooperative effects on these alloys' piezoelectric and ferroelectric properties, finding that it is beneficial for reducing the polarization switching barrier. We calculate that thin films should display ferroelectric switching. This is supported by our experimental measurements of a high optical bandgap, enhanced piezoelectric coefficient, and a change in the calculated polarization switching mechanism, and we achieve preliminary ferroelectric switching that experimentally realizes the prediction. Overall, our work sets the foundation toward quaternary wurtzite‐nitride‐based multifunctional materials, including piezoelectrics, ferroelectrics, and possibly even multiferroics. 

Abstract Ferroelectricity was long considered incompatible with the wurtzite structure, but the recent discovery of switchable polarization in wurtzite alloys has renewed interest in these materials for integrated electronic and memory applications. The development of wurtzite ferroelectrics faces significant technological challenges, which can be addressed through a fundamental physical understanding of their dielectric and ferroelectric properties. This article focuses on the physics that govern the polarization switching behavior, emphasizing the atomic- and meso-scale (domain) mechanisms involved in the transition between polarization states. A distinguishing feature of this article is a deep dive into the role of intrinsic and extrinsic defects—an area that has received limited attention in prior reviews, but is increasingly recognized as central to polarization switching, coercive fields, leakage, and fatigue. We highlight how defect behavior evolves during processing and electrical cycling, often contributing to long-term degradation. We also introduce powerful first-principles defect calculations, common in semiconductors but not yet widespread in ferroelectrics, as tools to understand and design materials. By integrating recent theoretical and experimental insights, we aim to provide a framework for advancing wurtzite ferroelectrics. Graphical abstract 

Abstract Wurtzite ferroelectric materials are promising candidates for energy‐efficient memory technologies, particularly for applications requiring high operating temperatures. Asymmetric wake‐up behaviors, in which the polarization reversal depends both on polarity and cycle number for the first few dozen cycles, must be better understood for reliable device operation. Here, the detailed analysis of the asymmetric wake‐up behavior of thin film Al_0.94B_0.06N was performed combining time‐resolved switching measurements with Rayleigh analysis, piezoelectric measurements, and etching experiments of progressively switched samples. The analysis shows that the gradual opening of the polarization hysteresis loops associated with wake‐up is driven by a gradual increase in the domain‐wall density and/or domain‐wall mobility with electric field cycle to the polarity opposite to the growth polarity. The insights of this discovery will help to guide interface and polarity design in the eventual deployment of reliable devices based on these materials. 

Characteristic electrical transport analysis of textured ferroelectric wurtzite Al1−xHfxN (x = 0.00,  0.07,  0.14,  0.28,  0.42) thin films was performed using temperature-dependent leakage current measurements on Mo/Al1−xHfxN/Mo/SiC capacitors. A unified analysis couples (i) slope-magnitude comparisons against the theoretical forms for conduction mechanisms of Poole–Frenkel, Schottky, and fixed-range hopping, (ii) extractions of effective energy barriers via Arrhenius fits with variable applied E-field when applicable, and (iii) low-field ohmic fits. Low-field response is ohmic for all x, while compositions with increasing x show an increase in the E-field value at which ohmic-to-non-ohmic onset occurs. The unified analysis shows a trend of high-field electrical transport shifting from interface limited (Schottky) to bulk limited (Poole–Frenkel) conduction with increasing x. A resulting conduction mechanism map illustrates this gradual composition-driven change from interface to bulk limited. This is consistent with HfAl acting as deep trap states, though mechanistic details require additional investigation. 

Wurtzite ferroelectrics (FEs) are promising candidates for next-generation memory and computing devices due to their compatibility with semiconductor processing. However, their adoption is limited by large coercive electric fields (Ec), often approaching the dielectric breakdown field (Eb), raising concerns about energy efficiency and device reliability. The discovery of wurtzite FEs has also been constrained by a narrow chemical design space, with few known examples to date. Ferroelectricity in AlN-based alloys, particularly Al1−xMxN with trivalent M3+ cations, has been a focal point of recent work. Building on studies of co-alloyed AlN for enhanced piezoelectricity, we computationally investigate ferroelectricity in Al1−x(M1,M2)xN alloys, where M12+ and M24+ are non-trivalent cations. Using density functional theory, solid-state nudged elastic band method, and structural analysis, we predict switchable polarization in Al1−x(Mg,Hf)xN. Compared to the prototypical Al1−xScxN, this co-alloy exhibits a more rapid decrease in both the switching barrier and bandgap with increasing x, suggesting a simultaneous reduction in Ec and Eb. This reduction in Ec is attributed to enhanced structural distortions introduced by co-alloying. By using bandgap and distortion as design metrics, we identify several other promising M1–M2 combinations, and highlight Al1−x(Ca,Si)xN as a strong candidate for experimental validation. Our work introduces a co-alloying strategy to access new wurtzite FEs and expands the design space to include earth-abundant elements. 

Thin films of aluminum hafnium nitride (Al1−xHfxN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard x-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d33,f,meas measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent–and not just trivalent–alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices. 

Highly alloyed (Al,Gd)N is of potential interest in a variety of applications, including neutron detection and in devices such as non-volatile memory. Gd has been shown to have very low equilibrium solubility in AlN at room temperature; however, non-equilibrium deposition techniques such as sputtering are able to deposit thin films, which incorporate large amounts of Gd. Here, we characterize a highly-alloyed (Al,Gd)N combinatorial thin film grown by RF sputtering on a GaN substrate, looking for any evidence of chemical or phase segregation or structural disorder in the films. Compositions with between 13% and 32% Gd (on a cation basis) were studied. No evidence was found for chemical or phase segregation in any studied composition. Higher degrees of Gd incorporation led to greater structural disorder in the film and a tendency toward amorphization; however, electron diffraction shows that the film does not become fully amorphous at any of the studied compositions, instead retaining textured local order even at 32% Gd. Electron energy loss spectra suggest that the material retains a locally wurtzite-like tetrahedral bonding environment at all studied compositions. 

The 2019 report of ferroelectricity in (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] broke a long-standing tradition of considering AlN the textbook example of a polar but non-ferroelectric material. Combined with the recent emergence of ferroelectricity in HfO2-based fluorites [Böscke et al., Appl. Phys. Lett. 99, 102903 (2011)], these unexpected discoveries have reinvigorated studies of integrated ferroelectrics, with teams racing to understand the fundamentals and/or deploy these new materials—or, more correctly, attractive new capabilities of old materials—in commercial devices. The five years since the seminal report of ferroelectric (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] have been particularly exciting, and several aspects of recent advances have already been covered in recent review articles [Jena et al., Jpn. J. Appl. Phys. 58, SC0801 (2019); Wang et al., Appl. Phys. Lett. 124, 150501 (2024); Kim et al., Nat. Nanotechnol. 18, 422–441 (2023); and F. Yang, Adv. Electron. Mater. 11, 2400279 (2024)]. We focus here on how the ferroelectric wurtzites have made the field rethink domain walls and the polarization reversal process—including the very character of spontaneous polarization itself—beyond the classic understanding that was based primarily around perovskite oxides and extended to other chemistries with various caveats. The tetrahedral and highly covalent bonding of AlN along with the correspondingly large bandgap lead to fundamental differences in doping/alloying, defect compensation, and charge distribution when compared to the classic ferroelectric systems; combined with the unipolar symmetry of the wurtzite structure, the result is a class of ferroelectrics that are both familiar and puzzling, with characteristics that seem to be perfectly enabling and simultaneously nonstarters for modern integrated devices. The goal of this review is to (relatively) quickly bring the reader up to speed on the current—at least as of early 2025—understanding of domains and defects in wurtzite ferroelectrics, covering the most relevant work on the fundamental science of these materials as well as some of the most exciting work in early demonstrations of device structures. 

Advances in wurtzite nitride ferroelectrics of Al1−xMxN (M = Sc or B) have led to novel capabilities, which must be integrated into existing fabrication processes. In the case of electronics operating >200°C, a movement toward SiC-based platforms enables better performance over conventional Si, primarily due to the higher bandgap and lower intrinsic carrier concentration of SiC. Hence, the challenge is to develop a deposition process to integrate Al1−xMxN ferroelectrics with elevated temperature-compatible materials for high temperature electronics such as the non-volatile memory component. We demonstrate epitaxial AlMN/Mo/SiC heterostructures, which provides both the crystalline and surface features that promote high-quality ferroelectric nitride film growth. Omega scans of the Mo (110) reflection exhibit a full width at half max of <0.02° (40 arc sec) and the (0002) peak of the subsequently grown nitride film had a value of 1.1° for 160 nm thick Al0.7Sc0.3N and 1.3° for 400 nm thick Al0.94B0.06N. The crystallographic relationships found between the AlMN, Mo, and SiC layers indicate an advancement in sputter deposition of epitaxial films. Ferroelectric switching is also shown at 400 °C in both samples via polarization-electric field hysteresis and pulsed measurements, which exhibited Pr values >100 μC cm−2 and Ec between 3 and 4 MV cm−1, despite a large presence of oxygen in both AlMN films ranging between 4 and 5 at.%, revealed by compositional analysis. This study demonstrates the process for synthesizing high-crystal quality ferroelectric nitride films, which can be used in extremely high temperature applications. 

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors, such as Si, SiC, and III–Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al1−xGdxN alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction x with x > 0.10–0.15. Experimentally, ferroelectric switching is observed at room temperature in Al1−xGdxN films with x > 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously [Lee et al., Sci. Adv. 10, eadl0848 (2024)]. This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials.