Perspectives and progress on wurtzite ferroelectrics: Synthesis, characterization, theory, and device applications

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of (cid:1) 135 l C/cm 2 charge in Al 1 (cid:3) x B x N via electron microscopy, Al 1 (cid:3) x B x N ferroelectric domain patterns poled down to 400nm in width via scanning probe microscopy, and full polarization retention after over 1000h of 200 (cid:4) C baking and a 2 (cid:5) enhancement relative to ZnO in the nonlinear optical response of Zn 1 (cid:3) x Mg x O. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.


I. INTRODUCTION
Wurtzite semiconductors, including zinc oxide (ZnO) and aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and their alloys, are critical components of numerous electronic, photonic, and acoustic systems.2][3][4][5] The impact of these technologies is further enhanced by the fact that wurtzite semiconductors are the leading candidates for high temperature, harsh environment electronics.Consequently, their monolithic integration efforts with silicon complementary metal-oxide-semiconductor (CMOS) integrated circuits have increased in recent years to improve on-chip power delivery and switching speeds.For example, epitaxial GaN integration on 300 mm diameter silicon wafers has been demonstrated, providing monolithic integration opportunities for GaN based technologies such as compact millimeter wave and radio frequency (RF) integrated circuits. 6,7he technological relevance of wurtzite semiconductors is a result of strong metal-anion bonds, direct bandgaps from the infrared to ultraviolet, and tunable lattice parameters from cation alloying [e.g., aluminum indium nitride (Al 1Àx In x N)].However, the physical property that most distinguishes wurtzites from other technologically relevant semiconductors is the spontaneous polarization that arises in the wurtzite unit cell along the crystallographic c axis.Unlike silicon and gallium arsenide (GaAs) based heterostructures, chemical doping is not needed to generate mobile charge carriers in polar nitride heterostructures.Heterojunction polarization discontinuities lead to confined electron channels [e.g., two-dimensional electron gases (2DEGs)] 8-10 with high mobility and more recently hole channels. 11hese channels serve as the backbone of nitride high electron mobility transistors (HEMTs), for power amplifiers in RF integrated circuits.All of these aspects allow nitrides in devices with high operational temperatures, efficient light emission and low optical losses, strong electromechanical coupling, high breakdown voltage, and ultrahigh frequency operation.
For more than one century, wurtzite crystals were understood to be piezoelectric and pyroelectric, but not ferroelectric.Namely, dielectric breakdown was expected (and observed) before field-induced polarization reversal because the energy barrier was too large.However, alloying approaches were discovered in the late 2000s that boosted the piezoelectric response of AlN by nearly 4X through Sc substitution for Al. 12 In 2019, it was shown that the high-Sc content aluminum scandium nitride (Al 1Àx Sc x N) alloys were also ferroelectric. 13 similar notion for distorting the bonding environment to lower the energy barrier for polarization reversal was manifest in aluminum boron nitride (Al 1Àx B x N), where boron nitride (BN) itself is stable in the layered hexagonal phase, which was predicted to be the transient, intermediate crystal structure in polarization reversal for ferroelectric wurtzites.14 Accordingly, ferroelectricity was experimentally observed in newer wurtzite ternary alloys such as Al 1Àx B x N 15 and zinc magnesium oxide (Zn 1Àx Mg x O) in 2021.16 The ability to engineer ferroelectricity and nonlinear property responses in the wurtzite crystal structure comes with distinct technological advantages, if structure-property relationships and associated tradeoffs in processing and physical properties are established.For example, Al 1Àx Sc x N boasts ferroelectricity and enhanced piezoelectric, dielectric, and non-linear optical responses, [17][18][19][20][21] but the quality factor at resonance in acoustic devices is degraded in part due to the ionic nature of the scandium-nitrogen bond.In addition, the thermal conductivity decreases and optical losses of AlN increase when alloyed with scandium.21,22 These examples highlight significant opportunities to engineer and improve desired physical properties in wurtzite ferroelectrics and tailor their application space.These potential opportunities include, but are not limited to, the design of new materials that accommodate symmetry mismatched constituent compounds, synthesis strategies that facilitate formation of thermodynamically metastable phases, characterization techniques that analyze these materials in extreme conditions, and new device designs that incorporate these new physical properties.
This perspective discusses recent advancements in wurtzite ferroelectric materials, with a focus on Al 1Àx B x N and Zn 1Àx Mg x O and a comparison to Al 1Àx Sc x N highlighting commonalities and differences.Progress in the synthesis, theory and design, characterization, and device fabrication of wurtzite ferroelectrics is presented.This perspective also provides a broad overview of applications for wurtzite ferroelectrics and an outlook on integrating their desired physical properties into next generation electronic, photonic, and acoustic technologies.While perspective articles involving wurtzite ferroelectrics have been written recently 23,24 this perspective differs by providing insight into challenges and opportunities for understanding and reducing leakage current densities at large electric fields; from synthesis via physical vapor deposition (PVD) to metal-organic chemical vapor deposition (MOCVD) and complementary characterization methods such as scanning transmission electron microscopy (STEM).It also discusses how those challenges and opportunities are linked to new devices that could leverage this emergent ferroelectric behavior.

II. THIN FILM DEPOSITION
Al 1Àx Sc x N was the first experimentally demonstrated ferroelectric nitride material in 2019; the films were deposited by reactive magnetron sputtering.A key feature to unveiling the ferroelectric behavior was the ability to reduce leakage current densities at electric fields above 3-5 MV/cm and reduce the coercive field below the breakdown field in this range.Since 2019, sputtering has become a popular technique to explore new ferroelectric wurtzites due to the ability to tailor energetics and generate insulating nitride compounds in a cost-effective, scalable deposition platform.Changes to adatom kinetic energy allow for engineering of point defect densities and microstructure, which in turn affect film stoichiometry and electrical resistivity.Wurtzite ferroelectrics have been predicated on alloying the binary semiconductors of AlN and ZnO on the cation sites to induce distortions in local bonding environments, pushing the system toward a crystallographic phase transformation to facilitate polarization reversal.Many such alloys are metastable in nature, which favors deposition techniques that proceed out of equilibrium to kinetically stabilize these materials.For example, Mg alloyed ZnO has a thermodynamic solubility limit of $22% Mg 25 in the wurtzite structure at the eutectic point near 1800 C and the solubility decreases significantly with decreasing temperature.Nevertheless, sputtered Zn 1Àx Mg x O alloys deposited at room temperature have achieved upward of 37% Mg in the wurtzite structure.Tailored energetics, tuned synthesis parameters, and low substrate temperatures can extend solubility limits well into metastable composition space.][28] Sputtering and MBE are not the most commonly utilized deposition techniques for active semiconductor device layers in the semiconductor industry.MOCVD and a subset of MOCVD, atomic layer deposition (ALD), are utilized due to cost-effectiveness when depositing on 300 mm substrates and conformal deposition.For integrating a memory layer directly on a logic transistor, the space around fin field effect transistors (FinFETs) and future nanoribbon FETs is very limited ($2 nm) and requires conformal deposition.Hence, ALD and/or CVD will be the ultimate deposition technique for that specific application.While there are no known demonstrations of ferroelectricity in CVD and ALD-prepared wurtzites to date, the parent compounds of AlN and ZnO have been investigated and compositions including those that have been shown to be ferroelectric when grown by other techniques.For example, plasma-enhanced ALD processes exist for synthesis of both polycrystalline 29,30 and epitaxial AlN. 31 There are also reports of ALD-prepared AlN films that are strongly (0001) oriented, with respectable piezoelectric coefficients, low carbon and oxygen impurity levels, and low leakage under fields approaching 8 MV/cm, which suggests that through process development, ferroelectric AlN-based wurtzites are feasible. 324][35][36] Future work involving high purity precursor development with higher vapor pressures will be crucial to expedited progress.The high temperature, plasma-free growth of CVD presents unique opportunities and challenges for synthesis of metastable materials like Al 1Àx Sc x N. The ability to understand processing-property relationships and engineer microstructure and defect formation and prevent phase separation across all deposition techniques from PVD to CVD will enable the largest opportunities for heterostructure design, targeted applications, and technological implementation for nitride ferroelectrics.Figure 1 shows a general landscape of temperature-energy space for the synthesis techniques commonly utilized for wurtzite semiconductors.

III. ELECTRON MICROSCOPY
To guide synthesis routes of the expanding wurtzite ferroelectric family, STEM and the corresponding elemental distribution by energy dispersive spectroscopy (EDS) offer the ability to observe structural and chemical changes at atomic resolution.These techniques build on recent advances in the field of electron microscopy and serve as key methods to understanding the structure-property relationship in wurtzite ferroelectrics.STEM has been extensively used to characterize wurtzite materials, providing information about inversion domain boundaries (IDB), interphases, strain, phase identification, and defects, [37][38][39][40][41][42] [see Fig. 2(a)].After ferroelectricity was first experimentally demonstrated in Al 1Àx Sc x N, 13 (S)TEM studies of wurtzite ferroelectrics have been primarily focused on determining the effect of the dopants such as Sc and B in AlN and Mg in ZnO on the local structural characteristics.Investigating grain growth and texture, [43][44][45][46][47][48] phase separation and distributions, [49][50][51][52] strain, 53 and interfaces 54 have helped identify proper deposition conditions to produce uniform c-axis textured films, avoid dopant segregation, and identify phase inhomogeneities and phase separation that can be detrimental to the ferroelectric properties.For example, a detailed study of the abnormal grain growth as a function of Sc content 55 in magnetron sputtered Al 1Àx Sc x N (x¼ 0-0.43) demonstrated the segregation of Sc to the columnar grain boundaries [see Fig. 2(b)].This showed that abnormally oriented grains (AOGs) do not nucleate at the substrate interface and provided insight into how certain deposition parameters may promote AOG formation.
Furthermore, STEM capabilities can be extended to study ferroelectric domain distributions, domain boundaries, polarization, and switching mechanisms at the nanoscale.Moreover, the local spontaneous polarization, at the unit-cell scale level, can be mapped and quantified 56 by locating the atomic columns and applying the modern theory of polarization. 57,58Polarization values can even be obtained for binary wurtzite structures, such as AlN, 59 where ferroelectric switching was previously inaccessible due to the material dielectric breakdown. 60his is extremely timely as the measured remanent polarization values for wurtzites are in general agreement with those from first principles calculations following the modern theory of polarization and utilizing the nonpolar layered hexagonal structure as in Ref. 61.
In Al 0.94 B 0.06 N, studies have demonstrated a good agreement between the calculated structure-based polarization via STEM (140 6 14 lC cm À2 ) and the remanent polarization (138 6 1 lC cm À2 ) measured via hysteresis loops, 59 where a vector map for individual unit-cell polarization can be attained [see Fig. 2(c)].General characterization of the polarization direction can be also obtained by observing the structure along the [110] zone axis.For example, atomic resolution annular bright field (ABF) STEM was utilized to show that sputter deposited Al 1Àx Sc x N changes from metal polar orientation to nitrogen polar after 40 nm thickness when grown on metal polar GaN substrates, indicating the propensity for sputter deposited films to adopt a nitrogen polar orientation. 53Qualitative polarization mapping for large fields of view at low magnification 54 is also feasible, by exploiting a preferential electron scattering along the polar direction in ferroelectric materials, arising from dynamical scattering, which breaks Friedel's laws.These methodologies have been used to determine local polarization after ex situ electrical switching experiments 59,62 [see Fig. 2(d)] and may be further applied to evaluate the role of defects, elemental segregation, and interphases on the local polarization.It should be noted, however, that sample preparation techniques should be identified which do not alter the polarization, as can occur during focused-ion-beam preparation.Furthermore, careful consideration of the experimental conditions is necessary to disentangle polarization response, sample thickness, and sample tilts. 21,63witching in wurtzite ferroelectrics occurs as domains toggle between a local metal-or anion-polar state.In the case of Al 1Àx B x N, FIG. 1.Typical growth temperatures for wurtzite semiconductors vs average adatom energy for various deposition techniques.These numbers do not take into account tailored processes (e.g., thermalization in high-pressure sputtering which lowers average energy or the plasma electron temperature in plasma-assisted MBE), the Maxwellian vs Gaussian energy distribution differences, and other factors such as strain and chemical potentials that affect the overall ability to obtain wurtzite films with c-axis orientation.For MBE, CVD, and ALD techniques, the principal adatom energy is thermal (k b T) and in sputtering, it is due to the kinetic energy of ejected atoms from the cathode.
Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/apl this is in response to aluminum/boron nitride rings "puckering" in one direction or another. 59With this toggling of the polar state, ferroelectric domain walls in wurtzite materials have a structure analogous to the inversion domains long studied in these materials, [64][65][66][67] but in some cases also have a horizontal component, 68 which poses significant opportunities to investigate the possibility of charged domain walls in wurtzite ferroelectrics.In situ biasing experiments in ferroelectrics are commonly utilized to study dynamic processes using in situ bias holders in the TEM; 69 however, in wurtzite ferroelectrics, these studies have been hindered by the high electric fields required to switch the polarization.Electrode migration and/or electrode grain growth for such thin specimens breakdown the samples before the polarization reversal is attained.To avoid this limitation, electron beam-induced charging has been utilized as an alternative method to observe in situ switching pathways in Al 0.94 B 0.06 N, 59 which enabled the identification of a transient antipolar phase.This antipolar phase has a b-BeO-like structure, as predicted by density functional theory (DFT) calculations, 59,70 indicating a sequential switching mechanism as shown in Fig. 2(e).
Although a significant body of literature exists for Al 1Àx Sc x N, other wurtzite ferroelectrics, such as Al 1Àx B x N and Zn 1Àx Mg x O, need to be systematically explored.In addition, significant efforts need to be placed in identifying switching mechanisms for a variety of compositions, as well as characterizing the electrode interface region to better understand the local structure evolution during polarization inversion.Additional strategies need to be designed to characterize the effect of local distortion, defects, and bonding states at the electrode interfaces on the switching mechanisms and wake-up processes in wurtzite ferroelectrics, taking advantage of the picometer resolution that can be achieved in modern STEM instrumentation.

IV. SCANNING PROBE MICROSCOPY, LOCAL PROBES
A characteristic feature of ferroelectric materials is the formation of ferroelectric domain structures, which form to minimize depolarization fields and accommodate strain fields and disorder 71 (e.g., ions, adsorbates, planar defects, and grain boundaries).Ferroelectric domain dynamics are directly connected to the fundamental physics of ferroelectric materials, namely, the nature of the order parameter that governs ferroelectric behavior.A breakthrough in understanding the static and dynamic properties of domain structures in ferroelectric materials arrived with the invention of piezoresponse force microscopy (PFM), a subset of atomic force microscopy (AFM). 72,73In PFM, the application of electrical bias to a conductive scanning probe microscopy tip results in an electromechanical strain, and consequently a surface deformation.Here, the surface deformation is directly detected via a laser reflected from an AFM tip.Similarly, the application of constant bias to the probe can be used to modify domain structures and explore phenomena such as domain nucleation and wall motion with nanometer scale resolution.Furthermore, DC bias sweeps with concurrent PFM detection yield the local electromechanical hysteresis loop that, on a qualitative level, can be interpreted similarly to macroscopic P-E loops. 74or the last two decades, PFM has been utilized to study piezoelectric nitride semiconductors such as AlN.Previously, nitride semiconductors were believed to be non-ferroelectric, and the PFM contrast observed in these materials was attributed to regions with metal and nitrogen polar surface terminations, respectively. 75Since the discovery of ferroelectricity in nitrides, several authors have reported PFM measurements of local piezoelectric properties and grain-related polarization patterns.Copyright 2023 The American Association for the Advancement of Science (AAAS). 59he recent emergence of wurtzite ferroelectrics and the early demonstration of phenomena, such as wake up, 76  It is important to note that an important aspect of polarization switching from air-exposed surfaces in ferroelectric wurtzites is the possible coupling to surface electrochemical phenomena.Previously, such dynamics were discovered in the well-studied LaAlO 3 -SrTiO 3 (LAO-STO) system, in which ionic screening of the polarization charge by hydroxides and protons (water reduction cycle) is a necessary component of the polarization switching in the LAO layer. 77,78It is argued that similar behavior can be present on wurtzite surfaces, in which the surface termination changes upon polarization switching.Large voltages needed to switch the polarization can enhance surface electrochemistry effects when measured in ambient environments, which necessitates additional techniques to further evaluate polarization reversal pathways, domain wall motion, and the defects associated with the process.

V. OPTICAL PROPERTIES A. Photoluminescence
As previously stated, some wurtzite domain walls have a structure analogous to the inversion domain boundaries that are also observed in the absence of ferroelectric behavior.Inversion domain boundaries in AlN have been linked to structural changes associated with the formation of Al-O bonds and intentional oxygen exposure during growth 79,80 (e.g., when the polarity inverts at an interface due to oxygen accumulation).Strong photoluminescence (PL) occurs at inversion boundaries in III-V semiconductors due to both the localized electric fields limiting non-radiative recombination and the defects that tend to cluster there. 66,67By extension, PL offers a path to monitor domain wall motion while also providing a probe of defect evolution during switching.However, domains are typically smaller than the diffraction limit of standard optical characterization techniques.For this reason, PL will likely find its most widespread usage as a tool to understand defect evolution throughout device lifetime.Photoluminescence probes electronic states in a material by quantifying the emission of light as charge relaxes back to equilibrium after photoexcitation, and is well established in its ability to identify and differentiate defects in wurtzite materials. 81,82Defects, meanwhile, have been suggested as a factor limiting the endurance of wurtzite ferroelectrics.For example, increased leakage currents in Al 1Àx Sc x N devices have been attributed to positively charged defect accumulation near the contacts. 45,83,84While positively charged defects reducing the Schottky barrier height are consistent with the measured increase in leakage current, direct experimental examination of the defect states evolving with switching is lacking.
With this motivation, the evolution of photoluminescence was examined as Al 0.93 B 0.07 N metal-ferroelectric metal capacitors went Applied Physics Letters PERSPECTIVE pubs.aip.org/aip/aplthrough wake-up.Wake-up was performed utilizing a procedure similar to that of Ref. 76 after which the top metal contacts were removed allowing for photoluminescence of the underlying Al 0.93 B 0.07 N. Systematic changes are observed in the photoluminescence shape and intensity in the 1.7-2.7 eV energy range (see Fig. 4).Specifically, PL-intensity decreases with wake-up even as the spectral signal near 1.8 eV increases relative to that near 2.2 eV.These sub-bandgap modes are located at energies typically associated with oxygen defect complexes, suggesting that these complexes are being modified by the wake-up process. 82,85Future work will center on definitively assigning these features and assessing the implications on ferroelectric behavior and leakage currents.PL, as a nondestructive measurement technique, provides a means of efficiently monitoring defect contributions to the switching of wurtzite ferroelectrics through the depth of the heterostructure.

B. Second harmonic generation
Historically, the interest in III-V semiconductors originated from robust linear optical properties.1][92] This precluded their extensive use in additional applications in the fields of nanophotonics, biological imaging, telecommunications, and quantum computing.Alloying with Sc increases the second order non-linear optical coefficients of AlN by an order of magnitude, opening up opportunities in this design space.The combination of appreciable non-linear optical coefficients and optical transparency into the deep UV range due to the ultrawide bandgap gives Al 1Àx Sc x N a distinct advantage vs other materials.Now combined with the ability to achieve quasi-phase matching through periodic poling from polarization reversal, ferroelectric wurtzites are uniquely positioned for new photonic device architectures such as mirrorless optical parametric oscillators.The wide transparency range of ferroelectric wurtzites also promotes broadband photonic functionalities, such as Kerr comb generation, which utilizes third order (v (3) zzz ) harmonic effects. 93In Al 1Àx Sc x N, a maximum value of d 33 ¼ v (2) zzz ¼ 62.3 6 5.6 pm/V at 1550 nm for 36% Sc content was reported, 21 roughly twice that of LiNbO 3 , the most common material studied for CMOS compatible nonlinear optical systems.Recently, quasi-phase matching (QPM) has been explored in 200 nm thick Al 1Àx Sc x N with periodic poling to generate domain widths of approximately 220 nm for a periodicity of 440 nm. 94This gives significant promise to generating even smaller feature sizes with extreme ultraviolet (EUV) lithographic processes utilized in advanced technology nodes.More importantly, wurtzite nitrides like Al 1Àx Sc x N can be grown directly on Si without heterogeneous integration, unlike LiNbO 3 .Currently, commercially produced LiNbO 3 with low optical and acoustic losses is ion sliced and wafer bonded to integrate on Si. 95,96 The enhancement of nonlinear optical coefficients in Al 1Àx B x N with x is less pronounced compared to Al 1Àx Sc x N, as shown in Fig. 5(b) with the largest coefficient, d 33 , remaining effectively unchanged, while an enhancement of d 31 and d 15 was observed.Maximum values of d 33 ¼ 10.7 6 1.4 pm/V, d 31 ¼ 0.9 6 0.1 pm/V, and d 15 ¼ 1.2 6 0.07 pm/V at 800 nm were measured via second harmonic generation (SHG) at 11% B content. Figure 5(c) demonstrates a quasiphase matched structure with domains 400 nm in length prepared through poling with a biased AFM tip in PFM measurements. 97In Zn 1Àx Mg x O, a complex relationship between Mg content and nonlinear susceptibilities was observed.A maximum enhanced value of d 33 ¼ 10.1 6 2.7 was reported with 23% Mg content, almost 50% greater than single crystal ZnO. 98In addition, the electro-optic properties have been reported, with an effective Pockels coefficient of 7.6 6 0.2 pm/V in films with 28% Mg content, a threefold enhancement over previously reported ZnO films. 99The difference in non-linear optical properties between Zn 1Àx Mg x O, Al 1Àx Sc x N, and Al 1Àx B x N points toward lattice and electron anharmonicity being key factors for tailoring non-linear optical responses in wurtzite ferroelectrics.

VI. ELECTRICAL PROPERTIES
The ultrawide bandgaps of wurtzite ferroelectrics arise, in part, from strong interatomic bonds in the crystals.This has important implications on the electrical characteristics of these materials.There are now several homologues of ferroelectric wurtzite compounds, including Al 1Àx Sc x N, Al 1Àx B x N, Al 1Àx Y x N, 28 Ga 1Àx Sc x N, 100 and Zn 1Àx Mg x O, and it is likely that the richness of the composition space will continue to grow.These compounds show large, electrically reversible polarizations (typically with remanent polarizations from 80 to 130 lC/cm 2 ), and bandgaps from 3 to 6.2 eV. Figure 6 compares the measured polarization-electric field hysteresis loops of important ferroelectric materials, including several of the wurtzite-structured ferroelectrics with PbZr 1Àx Ti x O 3 and Hf 1Àx Zr x O 2 .It is apparent that the high-field polarization properties of the wurtzite ferroelectrics are characterized by high remanent polarizations coupled with high coercive fields.For example, most of the AlN-based compositions have remanent polarizations in excess of 100 lC/cm 2 , with coercive fields that exceed 4 MV/cm at room temperature for measurements at 100 Hz. 13,14 It is apparent that Sc modifications lower the coercive field The coercive fields of Zn 1Àx Mg x O wurtzites, like those of the AlN-based compositions, are strongly temperature dependent.As is typical of ferroelectric materials, in the wurtzites, there is not a single activation energy for polarization reversal.Instead, the temperaturedependent pseudo-activation energies are $20 to 40 meV. 101These are considerably below the activation energies that would be expected for an intrinsic coercive field (e.g., uniform switching of the polarization throughout the film volume at once) and strongly suggest the existence of mobile interfaces such as domain walls as the means of polarization reversal.In Al 1Àx B x N, the switching occurs through a non-polar intermediate structure, which is structurally reminiscent of an inversion domain wall in a nitride, or indeed a small slice of a feldspar-like structure.This allows the polarization reversal process, and the commensurate bond-breaking to be conducted a few atoms at a time, rather than simultaneously through the entire structure.
In their as-deposited state, wurtzite ferroelectrics behave like linear dielectrics for the first several cycles and must be "woken-up" to display polarization switching.This has been shown in Al 1Àx B x N and Zn 1Àx Mg x O, where the wake up process is highly rate dependent.If field cycles are a few Hz, full wake up will occur in one cycle, whereas 1 kHz cycling may require several hundred.In at least some cases, the wake-up phenomenon is related to the fact that films are unipolar (or nearly so) as grown and the process involves generating enough nuclei of the opposite orientation to enable full switching of the polarization.It is also noted that some ferroelectrics wurtzites have imprint and hence some voltage asymmetry in the measured P-E loops, which decreases after field cycling, which is linked to the wakeup process.Once woken up, the hysteresis loops of the wurtzite ferroelectrics are typically quite square, suggesting comparatively abrupt switching relative to many other ferroelectric materials.Recent work suggests that the nucleation rate goes through a maximum during the growth and impingement phases. 102This differs from reports on switching in perovskite ferroelectrics, including the Kolmogorov-Avrami-Ishibashi (KAI) model, 103 or the nucleation limited switching (NLS) model. 104he ultimate switching speeds of the wurtzite ferroelectrics are as-yet unknown.
It should be noted that there are numerous reports of hysteresis loops in the wurtzites that are rounded at the top and/or the bottom of the loop, indicating a substantial contribution of leakage currents.Artifacts of this type inflate the apparent remanent polarization.In  cases where sufficient power is available in the measurement electronics, this can be mitigated by measurements at higher frequencies.Where that is not possible, PUND measurements can be useful, though as usual, care should be taken when a small number is derived as the difference between two large numbers (that is, PUND measurements are useful but not fail-safe in very leaky samples).
In part because of the very high coercive fields characteristic of the wurtzite ferroelectrics to date, their data retention properties are excellent, considerably out-performing the polarization retention times of ferroelectrics such as PZT or Hf 1Àx Zr x O 2 (Fig. 7).For example, Al 0.93 B 0.07 N films with W top and bottom electrodes retained >200 lC/cm 2 of opposite state signal margin after baking for 1000 h at 200 C.In Zn 0.64 Mg 0.36 O, no polarization loss was observed over the same time frame.In Al 0.7 Sc 0.3 N, data can be retained for at least 1000 s exposures to 400 C. 105 Even further, Al 1Àx Sc x N retains its polarization state after heating to 1100 C, demonstrating wurtzite ferroelectrics are the only ferroelectrics currently capable of operating in this extreme temperature regime. 50This is reasonable, since it is likely that none of wurtzite ferroelectrics demonstrated to date undergo a Curie temperature to a non-ferroelectric prototype phase prior to melting.Thus, the change of spontaneous polarization with temperature is not accelerated as the vicinity of a phase transition is not reached.
Among the remaining challenges in the wurtzite ferroelectrics is demonstration of high cycle lifetime on bipolar switching.Many of the available films undergo dielectric breakdown following cycling of thousands to millions of cycles.These aspects will have to be improved to enable integrated memory applications where cycling requirements necessitate at least 10 15 cycles.It is anticipated that this cycling would be improved as deleterious film defect densities decrease (especially at electrode interfaces as film thickness decreases) to allow for continued voltage scaling.Overall, cycling performance is expected to increase the most if the coercive fields of the wurtzites could be reduced without significantly degrading the bandgap.

VII. FIRST PRINCIPLES CALCULATIONS
To further aid in the discovery of novel highly piezoelectric and ferroelectric wurtzite materials and improve their suitability for memory applications, computational methods such as the density functional theory (DFT), molecular dynamics, and machine learning (ML) have been employed to uncover the structure-property relationships, especially those that govern the switching mechanisms in alloyed wurtzite semiconductors.Although significant theoretical work has gone into understanding the physical properties of wurtzite semiconductors such as AlN and GaN, ferroelectric behavior opens up new opportunities to understand the physics of wurtzite materials.Previously, the absolute magnitudes of spontaneous polarization in wurtzite semiconductors could not be experimentally measured as the spontaneous polarizations could not be reversed.Heterojunctions, such as A 1Àx Ga x N-GaN, offered the possibility to measure polarization discontinuities and relative polarization values, but ferroelectric behavior changed this notion.Thus, a fundamental understanding of polarization in wurtzites pays dividends to heterostructures that do not even utilize the polarization reversal (e.g., commercial Al 1Àx Ga x N-GaN transistors as power amplifiers).
To this end, several additional important ongoing areas for wurtzite ferroelectrics involve the understanding and engineering of coercive field values, ferroelectric domain nucleation and growth, and interfacial effects with substrates.For example, one of the drawbacks to technological adoption of wurtzite materials for integrated memory applications is the ultra-large coercive field.The large coercive fields (>2 MV/cm) exceed what is needed to ensure a sufficient memory window and threshold voltage stability in scaled devices.Strategies to lower the coercive field involve insight from first principles calculations and work toward understanding how the structure transitions from metal polarity to nitrogen polarity or vice versa. 12,106A primary way to elucidate the polarization reversal mechanism from DFT is to calculate the minimum energy pathway between the different bulk polarization states using the nudged elastic band (NEB) method. 107ecently, NEB simulations predicted a lower reversal barrier in Al 1Àx B x N compared to AlN. 59 Moreover, the intermediate state in Al 1Àx B x N is an anti-polar state rather than the hexagonal-BN phase in AlN.This type of analysis provides structural insight into the effects of alloying elements on the bulk wurtzite that enable ferroelectricity, enabling control over the coercive field.As an extension of DFT, highthroughput methods have been used to discover and characterize new wurtzite solid solutions.Using these larger datasets of possible materials, machine learning (ML) featurization has been employed to understand the structure-property relationships that govern these ferroelectric responses and discover dopants to improve the material response.An example of this is shown in Fig. 8. Another promising avenue to enhance device functionality for wurtzite ferroelectrics is lowering the coercive field and thus leakage current densities using strain, both local and interfacial.9][110][111] Moreover, first-principles studies have demonstrated that applying an in-plane a-b expansion (e.g., tensile strain) during thin film growth is expected to lower the barrier of polarization reversal. 112Using DFT and ML in the future is expected to pay dividends toward determining the impacts of defects, doping, and interfaces on the material strain, which vary based on deposition technique.Combined, these are crucial action levers to control the ferroelectric properties of wurtzite-based materials.

VIII. DEVICES AND EMERGING APPLICATIONS
The emerging ecosystem of ferroelectric wurtzite materials requires careful heterostructure design to efficiently emphasize the new features these materials offer and minimize deleterious effects that accompany these new properties.For example, the large polarizations in ferroelectric wurtzites necessitate electrodes with large carrier concentrations to compensate the polarization charge.This is prevalent when utilizing degenerately doped semiconductors as electrodes or incorporation in heterostructures with 2DEGs and 2DHGs in HEMT heterostructures.Incorporating ferroelectric behavior into nitride HEMTs holds the promise of integrating memory functionality with digital electronics.Recent reports of ferroelectric nitride HEMT heterostructures have demonstrated sub-60 mV/decade subthreshold slopes at room temperature 113,114 and gain at frequencies in excess of 100 GHz. 115Future work in this area will involve exploring the aspects of ultra-thin ferroelectric nitride layers and heterostructure design and depolarization effects.In addition, the prospect of negative drain induced barrier lowering (nDIBL) associated with ferroelectric behavior would be beneficial to improving the limitations set by short channel effects in scaled devices. 116,117The large electric fields (e.g., >3 MV/cm) associated with polarization reversal in ferroelectric wurtzites imply challenges when integrating with GaN devices (the GaN breakdown field is $3 to 3.4 MV/cm, depending on doping levels) 118,119 and necessitate use of electrode geometries and heterostructures that minimize electric field crowding to prevent premature dielectric breakdown and device failure.Field plate devices and associated heterostructures that reduce peak electric fields are expected to improve high-field performance and stability.In addition, polarization-graded heterostructures, [120][121][122] made possible by the polarization-induced doping, provide opportunities to reduce peak electric fields and assess the physics of ferroelectric three-dimensional electron gases (3DEGs) and generate built-in p-n junctions from polarization fields.Also, the strong non-linear optical responses shown by wurtzite ferroelectrics come at the expense of increased optical losses at visible wavelengths and beyond.These losses need to be reduced to allow for light propagation on centimeter length scales and to take advantage of the quasi-phase matching offered from periodic poling of domain arrays.The ability to precisely etch and pattern heterostructures, generate materials with low defect densities, as well as smooth domain wall interfaces, will be beneficial for reducing optical losses in waveguides and in devices incorporating nonlinear phenomena.For acoustic devices, periodically poled resonators utilizing Al 1Àx Sc x N alloys have recently shown promise in generating higher frequency harmonic modes. 123,124Improving the quality factor at higher frequencies through heterostructure design and/or materials development will continue to be a significant challenge as scaling to high frequencies comes with associated tradeoffs.
The aspects in the previous paragraph also apply to existing device designs studied for CMOS integrated memory.For example, three common ferroelectric memory devices being studied with Hf 1Àx Zr x O 2 alloys are ferroelectric FETs (FeFETs), ferroelectric tunnel junctions (FTJs), and ferroelectric random access memories (FeRAM) in the form of 1 transistor-1 capacitor (1T-1C) architectures. 125For wurtzite-based FeFETs, the large remanent polarization values cannot be completely compensated by conventional semiconductor channels.For reference, 100 lC/cm 2 would equate to a 6.25 Â 10 14 /cm 2 mobile carrier sheet charge density.An additional approach could be to integrate wurtzites with amorphous In 2 O 3 , which has a large carrier n type concentration of $1 Â 10 20 /cm 3 while still maintaining a large electron saturation velocity of $1 Â 10 7 cm/s. 126,127Tuning the In 2 O 3 thickness will tailor the total charge that can be accumulated at the interface and allow for threshold voltage modulation.For FTJs and FeRAM, the large remanent polarizations of wurtzites are expected to be advantageous to providing more tunneling current modulation and more charge from the capacitor, respectively.Recent work demonstrating ternary content-addressable memory (TCAM) based on back-endof line (BEOL) compatible Al 1-x Sc x N ferroelectric diodes is compelling. 128FeRAM with wurtzites is feasible with large polarization charges available, allowing capacitor area scaling reduction for the same charge sense amplifier typically used in FeRAM.Both sets of devices would require extensive work in reduction in the films' coercive fields to operate at CMOS technology voltages ($3.3 to 0.7 V) and demonstrate low leakage currents for the FeRAM.Opportunities exist for ferroelectrics in NAND type architectures, which operate at larger voltages ($15 to 20 V) and at slower speeds and fewer endurance cycles than the main memory cache.CMOS compatible ferroelectrics offer the ability to lower the NAND programming voltage 129 and reduce the number of layers in the stack, thus improving threshold voltage shift variation concerns and improving memory density.However, the ability to aggressively stack ferroelectric capacitors in 3D geometries to compete with NAND density remains an open area of investigation and innovation.

IX. CONCLUSIONS
Ferroelectric wurtzite materials are being extensively studied for potential use in integrated memory.In addition, ferroelectric wurtzites offer the ability to merge logic and memory functionalities in digital electronics and demonstrate enhanced non-linear optical responses for on-chip systems and integration in existing III-nitride photonic platforms.Significant progress has been made in recent years in demonstrating ferroelectricity in ultrathin layers toward CMOS voltage ranges, which also opens up applications in quantum heterostructures and allows for increased flexibility in heterostructure processing and design, such as strain engineering.Future research directions can build on the knowledge gained from Hf 1Àx Zr x O 2 and related systems to further drive voltage reductions and improve fatigue and endurance performance.In addition to competing with Hf 1Àx Zr x O 2 for use in integrated memory, ferroelectric wurtzites hold distinct advantages for use in high voltage, high temperature, and harsh environment memory applications due to the strong chemical bonds in the wurtzite crystal structure.A more thorough understanding of defect densities that contribute to leakage currents, polarization reversal mechanisms toward lowering the coercive field, and synthesis and design of new materials with reduced defects and different defect tolerances will help guide further heterostructure design and the ultimate utilization of this emerging class of materials.

FIG. 2 .
FIG. 2. (a) ADF-STEM image of an inversion domain boundary (IDB) between an N-polar domain and Al-polar in AlN.Adapted from Stolyarchuk et al., Sci.Rep. 8, 14111 (2018).Copyright 2018 Author(s), licensed under a Creative Commons Attribution (CC BY) license. 39(b) STEM-EDS maps on 10% Sc film showing Sc enrichment at the grain boundary in the intensity profiles (right).Reproduced with permission from Sandu et al., Phys.Status Solidi A 216, 1800569 (2019).Copyright 2023 John Wiley and Sons. 55(C) Differentiated differential phase contrast (dDPC)-STEM image of Al 0.94 B 0.06 N overlapped with a polarization vector map at each unit cell.Reproduced with permission from Calderon et al., Science 380, 1034 (2023).Copyright 2023 The American Association for the Advancement of Science (AAAS). 59(D) Annular dark field (ADF)-STEM images for N-polar atomic structure in the as-deposited Al 1Àx Sc x N and Al-polar atomic structure in the switched Al 1Àx Sc x N, the polarization inversion is attained ex situ.Reproduced with permission from Wolff et al., J. Appl.Phys.129, 034103 (2021).Copyright 2021 AIP Publishing LLC. 62(e) Atomic models, STEM image simulations, and experimental images for the N-polar, nonpolar, and Al-polar in Al 0.94 B 0.06 N occurring during a switching event.Reproduced with permission from Calderon et al.Science 380, 1034 (2023).Copyright 2023 The American Association for the Advancement of Science (AAAS).59 have opened questions regarding the spatially heterogeneous nanoscale mechanisms and domain dynamics associated with local polarization switching.Due to the relatively short history of ferroelectric wurtzites, few studies characterizing the local ferroelectric structure have been reported.As an example, ferroelectric switching is clearly shown in Al 0.94 B 0.06 N and Zn 0.64 Mg 0.36 O thin films.Figures 3(a) and 3(b) show post þ60 V/À60 V DC poling band excitation PFM amplitude and phase images, respectively, for 150 nm thick Zn 0.64 Mg 0.36 O. Briefly, band excitation PFM uses a non-sinusoidal signal with a defined band in frequency space to independently detect the resonance frequency and response amplitude to mitigate topographic crosstalk.Here, clear remanent polarization switching is observed as indicated by written domain patterns and 180 phase switching.Similarly, Figs.3(e) and 3(f) show post þ15 V/À15 V poling with band excitation for 20 nm thick Al 0.94 B 0.06 N with signatures of polarization reversal.

FIG. 3 .
FIG. 3. Band excitation PFM of Zn 1Àx Mg x O [(a)-(d)] and Al 1Àx B x N films [(e)-(h)] showing piezoresponse amplitude and phase changes as a result of polarization reversal.Amplitude changes are linked to electromechanical changes that occur in the vicinity of adjacent domains, in this case a poled region.A uniform amplitude across a poled region with a difference in magnitude in an oppositely poled region is attributed to a finite electrostatic contribution.Polarization reversal from the nitrogen polar to metal polar state causes a phase shift of 180 (p radians).Here, square domain patterns are written by applying opposite polarity DC biases.The phase-voltage loops resemble polarization-voltage loops in electrical measurements.The measurements were performed in ambient for Zn 1Àx Mg x O and in a glovebox environment for Al 1Àx B x N.

FIG. 4 .
FIG.4.Photoluminescence of Al 0.93 B 0.07 N using a 405 nm wavelength excitation acquired at varying degrees of wake-up, with a decreasing intensity for increasing number of electrical measurement cycles.This change in the PL response is indicative of defect evolution and a change of defect concentrations during multiple measurements of polarization reversal cycles.

FIG. 5 .
FIG. 5. (a) Experimental values for bandgap vs in-plane lattice constant of existing III-nitride semiconductors and new ternary wurtzite ferroelectrics of Al 1Àx B x N, Al 1Àx Sc x N, and Zn 1Àx Mg x O alloys.The ellipses show trends for the alloy systems.(b) Measured second order nonlinear susceptibilities from SHG measurements for ternary wurtzite ferroelectrics.The purple bar highlights that all of these compounds show appreciable SHG behavior at ultraviolet wavelengths and enhanced SHG coefficients relative to binary constituents ZnO and AlN for most cation alloying contents.Periodic poling from DC bias in PFM measurements for 20 nm thick Al 1Àx B x N (x ¼ 0.07) films, generating uniform domains down to 400 nm in width, indicating suitably for quasi-phase matched heterostructures at ultraviolet wavelengths.Reproduced with permission from Suceava et al., Opt.Mater.Express 13, 1522 (2023).Copyright 2023 Optica Publishing Group under the terms of the Open Access Publishing Agreement.97

FIG. 6 .
FIG.6.Measured polarization-electric field hysteresis loops for ternary wurtzite ferroelectric materials and technologically relevant ferroelectrics HZO and PZT.It is noted that the coercive field is frequency dependent and the shape of the loops can be influenced by conduction mechanisms at high field, which are dependent on film thickness and measurement frequency.

FIG. 7 .
FIG. 7. Polarization retention as a function of bake temperature and time for wurtzite ferroelectrics compared to commercial PZT and Hf 0.5 Zr 0.5 O 2 films.Wurtzite Al 0.93 B 0.07 N and Zn 0.64 Mg 0.36 O show excellent same state polarization margin.

FIG. 8 .
FIG. 8. (a) Schematic of wurtzite supercell in VESTA software utilized in simulations, utilizing different cations and analyzing the changes in internal bond angles (a-a, c-a) and internal parameter (u).c and a correspond to the crystallographic axes.The atoms shown depict atomic radii (e.g., Al and N), not ionic radii (Al þ3 and N À3 ).Magnitudes of spontaneous and piezoelectric polarization in the wurtzites are known to be sensitive to c/a ratios, u values, and bond angles.(b) Process flow utilizing machine learning and first principles calculations to accelerate materials discovery and design.