Wurtzite-Type Ferroelectricity

• Wurtzite-type ferroelectricity is characterized by switchable spontaneous polarization along the c-axis in noncentrosymmetric wurtzite lattices, distinguishing it from conventional perovskite mechanisms.
• The material’s behavior is finely tuned by composition, strain, and local chemical order, which affect coercive fields, domain kinetics, and the energy landscape of polarization reversal.
• Advances in alloying with elements like Sc, Gd, and Hf, as well as nanoscale and 2D implementations, enable optimized ferroelectric performance and broaden operational regimes including cryogenic conditions.

Wurtzite-type ferroelectricity denotes switchable spontaneous polarization in polar wurtzite and wurtzite-like lattices, most prominently in AlN-based and ZnO-based systems, where reversal occurs between two antiparallel orientations of the same noncentrosymmetric framework rather than by the canonical perovskite mechanism of B-site off-centering. In this class, the polar semiconductor lattice itself is the source of bistability, typically along the crystallographic $c$-axis, and alloying, strain, defects, local chemical order, and domain-wall topology determine whether reversal occurs below dielectric breakdown (Fichtner et al., 2018, Song et al., 25 Mar 2025). Since the first clear observation of ferroelectricity in AlScN, the subject has expanded to single-crystalline III-nitride films, ZnO-based alloys, rare-earth and heterovalent AlN alloys, defective wurtzites, multilayers, nanowires, and 2D wurtzite-type polymorphs, while also developing a distinct switching physics involving layered-hexagonal intermediates, sequential columnar reversal, ultrasharp domain walls, proximity effects, and cryogenic robustness (Fichtner et al., 2020, Lee et al., 2024, Bernstein et al., 11 Mar 2025, Jiang et al., 9 Feb 2026).

1. Crystallographic basis and defining concepts

In the AlN-derived case, the parent polar structure is wurtzite $P6_3mc$, which is polar along the unique $[0001]$ direction. The internal parameter $u$ describes the relative position of the metal–N bond along the $c$-axis, and the two polarization orientations correspond to the two sides of the wurtzite double well. In the switching picture established for AlScN, the nonpolar layered-hexagonal structure is the transition state at $u = \frac{1}{2}$ (Fichtner et al., 2018).

This mechanism is explicitly distinct from conventional oxide ferroelectrics. In ultrathin Al$_{0.68}$Sc$_{0.32}$N, polarization is described as arising from ionic charge separation in a non-centrosymmetric lattice rather than from the usual displacement of B-site cations in perovskite oxides or vacancy ordering in fluorite oxides. The same work emphasizes that wurtzite ferroelectrics can gain polarization as lattice anisotropy increases on cooling, in contrast to oxide ferroelectrics that often become less favorable for switching at low temperature (Song et al., 25 Mar 2025).

A broader structural interpretation treats wurtzite ferroelectrics as a distinct class of five-coordination ferroelectrics. In that description, a fifth anion is counted as coordinated if the smallest angle it forms with existing bonds is greater than $65^\circ$, making the wurtzite state a polar 5C arrangement rather than a simple tetrahedral 4C analogue. The same framework links wurtzite stability relative to zinc blende to ionicity and $c$-axis compression, with the ideal tetrahedral ratio $P6_3mc$0 as a reference and stable wurtzite compounds lying in the compressed regime $P6_3mc$1 (Zhou et al., 2024).

2. Establishment of the materials class

The initial experimental consolidation of the field occurred in AlScN. Ferroelectric switching in Al$P6_3mc$2Sc$P6_3mc$3N established the first clear observation of ferroelectricity in the wurtzite crystal structure and yielded a combination of coercive fields of $P6_3mc$4–$P6_3mc$5 and remnant polarizations of $P6_3mc$6–$P6_3mc$7. Ferroelectricity was reported from $P6_3mc$8 down to $P6_3mc$9, with coercive field largely independent of thickness between $[0001]$0 and $[0001]$1, and switching was interpreted as a domain-wall-motion-limited process progressing from the electrode interfaces (Fichtner et al., 2020).

A complementary early study framed AlScN as the first unambiguous ferroelectric switching in a III-V semiconductor-based material. Ferroelectric switching was first observed at $[0001]$2; the coercive field exceeded $[0001]$3 at $[0001]$4, decreased systematically with Sc content, and fell below $[0001]$5 at $[0001]$6. The remnant polarization was about $[0001]$7 at $[0001]$8, residual tensile stress reduced the coercive field, and the polarization survived thermal treatment to at least $[0001]$9, establishing $u$0 as a lower bound for the paraelectric transition temperature (Fichtner et al., 2018).

The class was subsequently verified in a single-crystalline, industrially relevant growth route. In MOCVD-grown single-crystalline Al$u$1Sc$u$2N, a $u$3 film exhibited a coercive field of $u$4 at $u$5, PUND-corrected remanent polarization values of $u$6 and $u$7 for negative and positive branches, and an average $u$8. Annular bright-field STEM directly resolved unit-cell-level polarization inversion from a fully M-polar as-grown state to N-polar regions after cycling (Wolff et al., 2023).

3. Switching pathways and kinetic regimes

The homogeneous reversal picture in wurtzites is no longer limited to a single collective route. In Al$u$9Gd$c$0N, first-principles calculations using the modern theory of polarization and SS-NEB showed that the minimum-energy pathway changes from a collective to an individual switching process at a rare-earth fraction $c$1–$c$2. Experimentally, ferroelectric switching was then observed at room temperature for $c$3, strongly supporting a composition-driven crossover in the atomic-scale reversal mode (Lee et al., 2024).

A more detailed atomistic mechanism was reported for bulk AlN. Large-scale molecular dynamics and Monte Carlo simulations found that the critical nucleus is a single aluminum ion that breaks its bond with one nitrogen and bonds to another nitrogen, triggering a cascade that flips only atoms directly in the same column. The resulting picture is “fast 1D single columns of atoms propagating from a slow-moving 2D fractal-like domain wall.” Merz-law fits yielded activation fields of about $c$4 at $c$5 and $c$6 at $c$7 for in-plane switching, versus about $c$8 at both temperatures for out-of-plane switching. The same work estimated a fractal dimension of $c$9 from Monte Carlo simulations and $u = \frac{1}{2}$0 from experimental images, and argued that the convex-domain assumptions of the KAI model therefore break down in wurtzite AlN (Behrendt et al., 2024).

In Zn$u = \frac{1}{2}$1Mg$u = \frac{1}{2}$2O, local disorder produces yet another pathway. NEB calculations showed that pure ZnO switches collectively through a nonpolar hexagonal boron nitride-like intermediate, whereas at $u = \frac{1}{2}$3 Mg the pathway becomes sequential and passes through an anti-polar intermediate. The headline result was that local strain fluctuations can reduce local barriers to ferroelectric switching by more than $u = \frac{1}{2}$4, with coercive-field reductions of up to $u = \frac{1}{2}$5, and that engineered ZnO/(Zn,Mg)O/ZnO heterostructures featuring built-in interfacial strain gradients indeed switch experimentally (Baksa et al., 2023).

These results collectively displace a simple “uniform bulk reversal” picture. In wurtzite ferroelectrics, collective switching, individual switching, sequential columnar switching, anti-polar intermediates, and domain-wall-controlled kinetics all appear as material- and microstructure-dependent realizations of the same polar lattice instability.

4. Microscopic control parameters beyond average composition

Composition and stress were the earliest practical control variables. In AlScN, increasing Sc content and tensile strain both push the structure toward the nonpolar layered-hexagonal limit and reduce the switching barrier. For Al$u = \frac{1}{2}$6Sc$u = \frac{1}{2}$7N, changing the residual stress from about $u = \frac{1}{2}$8 to $u = \frac{1}{2}$9 reduced the coercive field by more than $_{0.68}$0 (Fichtner et al., 2018).

Later work showed that the global $_{0.68}$1 ratio is not a sufficient descriptor. In combinatorial Al$_{0.68}$2Sc$_{0.68}$3N thin-film libraries, the pure wurtzite phase retained an almost composition-invariant $_{0.68}$4 ratio, varying only from about $_{0.68}$5 to $_{0.68}$6 and then down to about $_{0.68}$7, yet both coercive field and spontaneous or remanent polarization still decreased strongly with increasing Sc content. The chemical interpretation was that Sc–N bonds are more ionic and less directional than Al–N bonds; DFT-supported Born effective charges of $_{0.68}$8 for Al and $_{0.68}$9 for Sc, together with average Bader charges of $_{0.32}$0 for Al and $_{0.32}$1 for Sc, were used to argue that bond ionicity rather than tetrahedral distortion alone controls the macroscopic ferroelectric response (Yazawa et al., 2022).

An analogous refinement appeared in Mg-doped ZnO. The argument that Mg simply softens the ionic potential energy surface was described as overly simplified. Under clamped strain, even Zn$_{0.32}$2Mg$_{0.32}$3O retained $_{0.32}$4, but when strain relaxation was allowed the barrier collapsed, with $_{0.32}$5 for Zn$_{0.32}$6Mg$_{0.32}$7O, comparable to PbTiO$_{0.32}$8. The enabling factor was that MgO has a dynamically stable hexagonal reference phase much lower in energy than wurtzite MgO, so switchability emerges from the interplay of alloy chemistry and mechanical boundary conditions rather than from uniform softening alone (Huang et al., 2022).

A further step was the recognition of local chemical order as an independent microscopic variable. In wurtzite ScAlN, first-principles canonical Monte Carlo sampling combined with DFT showed that the alloy relaxes toward thermodynamically favored short-range order rather than remaining random. This SRO is strongly anisotropic: in-plane Sc–N–Sc motifs are suppressed, while columnar mixed-cation chains along the polar $_{0.32}$9-axis are enhanced. Relative to random-alloy structures, SRO systematically increases the intrinsic switching barrier across a broad composition range, by as much as about $65^\circ$0. In a controlled motif comparison, a cross-plane Sc–N–Sc motif was lower in energy than an in-plane Sc–N–Sc motif by $65^\circ$1, while a columnar Sc–N–Al–N–Sc motif was lower by $65^\circ$2. For Sc$65^\circ$3Al$65^\circ$4N, the switching barrier showed $65^\circ$5 against in-plane Sc–N–Sc population, $65^\circ$6 against columnar Sc–N–Al–N–Sc population, and $65^\circ$7 for a two-descriptor linear model using both motif populations (Chen et al., 16 Jun 2026).

A recurring misconception is therefore that average composition, average strain, or average $65^\circ$8 alone determine switching. The combined evidence instead points to a multiscale control hierarchy in which bond ionicity, strain relaxation, and anisotropic local order can independently reshape the switching barrier at fixed nominal composition.

5. Domains, domain walls, and interfacial switching physics

Real switching in wurtzite ferroelectrics is strongly domain-mediated. In single-crystalline MOCVD-grown Al$65^\circ$9Sc$c$0N on M-polar GaN, ABF-STEM showed N-polar domains nucleating near the top Pt/SiN interface and extending toward the substrate as predominantly cone-like features. These domains were approximately $c$1–$c$2 wide, began $c$3–$c$4 away from the GaN interface, and could extend to the middle of the film. The polarity transition occurred across roughly $c$5 unit cells, with an estimated metal-sublattice shift of $c$6, and the domain-wall thickness had an upper bound of about $c$7, while possibly being atomically sharp (Wolff et al., 2023).

At the atomic scale, domain walls can host distinct structural phases. In ferroelectric ScGaN, transmission electron microscopy and theory identified neutral vertical domain walls and a charged horizontal domain wall with a buckled 2D hexagonal phase. The horizontal wall contains dangling bonds that generate metallic-like mid-gap states within the forbidden band. The paper estimated a bound charge $c$8 for the antipolar geometry, and a dangling-bond electron density of about $c$9, arguing for a universal charge-compensation mechanism in tetrahedral ferroelectrics. Conductive AFM further demonstrated reconfigurable wall conductivity (Wang et al., 2023).

Interfacial coupling has introduced the notion of proximity ferroelectricity. In Landau-Ginzburg-Devonshire modeling of Al$P6_3mc$00Sc$P6_3mc$01N/AlN bilayers and trilayers under a sharp biased tip, a nominally unswitchable polar layer such as AlN becomes practically switchable when placed in direct contact with a switchable ferroelectric layer. The proposed mechanism is a depolarizing electric field generated by polarization discontinuity and relative thickness, which renormalizes the double-well potential and lowers the steepness of the switching barrier in the otherwise unswitchable layer. In the probe geometry, only two collective regimes were found: proximity switching and proximity suppression (Eliseev et al., 10 Jun 2025).

The microscopic origin of proximity switching remains an active point of interpretation. A later first-principles and machine-learning molecular-dynamics study of AlN/AlScN proposed an alternative divide-and-conquer mechanism centered on high-Miller-index $P6_3mc$02 domain walls. In that picture, Sc lowers the formation energy of weakly charged high-index walls and promotes nucleus formation, while pristine AlN provides a low-pinning medium for subsequent wall motion. High-index walls such as $P6_3mc$03 and $P6_3mc$04 were reported to have formation energies of about $P6_3mc$05 and migration barriers below $P6_3mc$06, whereas conventional charge-neutral vertical walls have migration barriers above $P6_3mc$07. This study therefore shifted emphasis from homogeneous bulk softening to domain-wall topology and propagation kinetics (Ke et al., 28 Apr 2026).

6. Expanded chemistries, reduced dimensions, and operating regimes

The chemistry of wurtzite ferroelectrics has expanded well beyond Sc alloying. In Al$P6_3mc$08Gd$P6_3mc$09N, room-temperature ferroelectric switching was observed for $P6_3mc$10, with a large remanent polarization exceeding $P6_3mc$11 and switching current peaks near $P6_3mc$12 in Al$P6_3mc$13Gd$P6_3mc$14N at $P6_3mc$15; this was also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element (Lee et al., 2024). In Al$P6_3mc$16Hf$P6_3mc$17N, heterovalent tetravalent substitution produced distinct switching current peaks for $P6_3mc$18 and $P6_3mc$19, coercive fields of about $P6_3mc$20 and $P6_3mc$21, and a sign reversal of $P6_3mc$22 with magnitude about $P6_3mc$23 at $P6_3mc$24, demonstrating that ferroelectricity in AlN-based wurtzites is not limited to trivalent alloying (Bernstein et al., 11 Mar 2025).

Defect-ordered and vacancy-enabled variants extend the energy-landscape possibilities. Cation-vacancy ordered $P6_3mc$25-Al$P6_3mc$26S$P6_3mc$27 was predicted to exhibit a uniaxial quadruple-well sequence $P6_3mc$28, with a switching barrier of $P6_3mc$29, much lower than conventional wurtzite ferroelectrics. Biaxial compressive strain and Ga doping lower the switching barriers by up to $P6_3mc$30, suggesting a vacancy-enabled route to lower-coercive-field switching and multi-valued polarization states (Shimomura et al., 2024).

Reduced-dimensional realizations have also emerged. In ScAlN/GaN nanowires on Si(111), a highly ordered wurtzite phase was observed to persist near the ScAlN/GaN interface at Sc contents around $P6_3mc$31, extending at least $P6_3mc$32 from the interface and beyond what is usually possible in conventional films. The same work reported the first evidence of ferroelectric switching in ScAlN nanowires, with a phase difference approaching $P6_3mc$33 in PFM after electrical poling, establishing what it described as the first wurtzite-phase ferroelectric nanowires (Wang et al., 2024).

The operating window has broadened toward cryogenic conditions. In $P6_3mc$34 Al$P6_3mc$35Sc$P6_3mc$36N, the saturated remanent polarization increased from about $P6_3mc$37–$P6_3mc$38 at room temperature to $P6_3mc$39–$P6_3mc$40 at $P6_3mc$41–$P6_3mc$42, with a giant remnant polarization exceeding $P6_3mc$43, breakdown field $P6_3mc$44 at $P6_3mc$45, and endurance beyond $P6_3mc$46 polarization-reversal cycles (Song et al., 25 Mar 2025). A separate study of $P6_3mc$47 Al$P6_3mc$48Sc$P6_3mc$49N reported the lowest ferroelectric switching temperature for wurtzite ferroelectrics, showing switching at $P6_3mc$50 with $P6_3mc$51 and a temperature-dependent fatigue crossover from hard breakdown to ferroelectricity loss (Wang et al., 14 Apr 2025).

The wurtzite-type concept has also entered 2D and multiferroic territory. WZ′-α-In$P6_3mc$52Se$P6_3mc$53 was experimentally realized as a centimeter-scale continuous film directly on SiO$P6_3mc$54/Si, with a Curie temperature exceeding $P6_3mc$55, a tunable bandgap of $P6_3mc$56–$P6_3mc$57 modulated by charged domain walls, and a large optical absorption coefficient of $P6_3mc$58. Under illumination, synapse devices based on this phase reached a recognition accuracy of $P6_3mc$59 in a supervised pattern classification task (Jiang et al., 9 Feb 2026). In orthorhombic wurtzite-type nitrides, MnSiN$P6_3mc$60 and MnGeN$P6_3mc$61 were proposed as aristotypes of a Type-1 multiferroic family with reversal barriers of $P6_3mc$62 and $P6_3mc$63, robust G-type antiferromagnetism at room temperature, and altermagnetic spin splitting that reverses sign upon polarization switching (Baksa et al., 8 Sep 2025).

Taken together, these developments show that wurtzite-type ferroelectricity is no longer restricted to a single alloy system or a single switching narrative. It spans bulk nitrides, oxide-derived wurtzites, defect-ordered chalcogenides, multilayers, nanowires, and 2D films; it supports cryogenic operation, sharp and electronically active domain walls, and proximity-induced switching; and it is increasingly understood through a hierarchy of descriptors that includes coordination geometry, ionicity, strain relaxation, local motif topology, and domain-wall energetics.