Growth of emergent simple pseudo-binary ferroelectrics and their potential in neuromorphic computing devices

Ferroelectric memory devices such as ferroelectric memristors, ferroelectric tunnel junctions, and field-effect transistors are considered among the most promising candidates for neuromorphic computing devices. The promise arises from their defect-independent switching mechanism, low energy consumption and high power efficiency, and important properties being aimed for are reliable switching at high speed, excellent endurance, retention, and compatibility with complementary metal–oxide–semiconductor (CMOS) technology. Binary or doped binary materials have emerged over conventional complex-composition ferroelectrics as an optimum solution, particularly in terms of CMOS compatibility. The current state-of-the-art route to achieving superlative ferroelectric performance of binary oxides is to induce ferroelectricity at the nanoscale, e.g., in ultra-thin films of doped HfO2, ZrO2, Zn1−xMgxO, Al−xScxN, and Bi1−xSmxO3. This short review article focuses on the materials science of emerging new ferroelectric materials, including their different properties such as remanent polarization, coercive field, endurance, etc. The potential of these materials is discussed for neuromorphic applications.


Neuromorphic computing and the promise of ferroelectric devices
Owing to the exponential increase in the quantity of information to be stored and processed, [1][2][3][4] there is a need for lower energy, faster and more cost-effective computing.To achieve this, new computing paradigms are highly sought after.The conventional von Neumann architecture, which is currently required to perform processing of large quantities of data, leads to low energy efficiency attributable to the separation of processing and memory units, which leads to a high energy cost when moving data back and forth between the two. 3,4The imminent end of Moore's Law will also limit computing capacity.Although in its early stage, neuromorphic, 'brain-like', computing has received significant attention as a possible alternative for undertaking data-intensive cognitive tasks quickly and efficiently.3][4] An analog synaptic device can be used to emulate the synaptic functionality in ANNs or SNNs for realizing cognitive tasks such as parallel computation and adaptive learning. 3,4The analog synaptic device is crucial in neuromorphic computing as a device with an effectively controllable conductance, which can be successfully arranged in a crossbar array to execute 'neural' tasks such as matrix vector multiplication.A multi-level non-volatile memory element can be used as an analog synaptic device and has the potential to implement potentiation (ability to gradually increase the conductance upon voltage pulses) and depression (ability to gradually decrease the conductance upon voltage pulses) for short-term and long-term plasticity.Non-volatile memory (NVM) devices can be based on resistive switching, phase change, and magnetic and ferroelectric materials, amongst others, and can imitate synapses by using just a single device or a few devices.5][6] Fig. 1 provides a schematic to illustrate the structures and functionalities of these devices.

Evolution of ferroelectric materials for non-volatile memory devices
In the electronics area, recent applications include ferroelectric non-volatile memory devices, which can be considered as the basic building blocks for neuromorphic computing devices. 4,7,8erroelectric materials possess the ability to switch, upon appropriate voltage application, the polarization direction of their remanent polarization (P r ), which is stable at zero electric field.This offers features like fast read/write speed, low power consumption, and high reliability compared with conventional floating-gate electrically erasable programmable read-only memory (EEPROM) and flash EEPROM devices, which require long read/write times and high voltages. 8Furthermore, ferroelectric materials can offer unique gradual domain switching characteristics, which provide the aforementioned multi-level conductance/resistance functionality.As a result, there is an increased interest in ferroelectric materials for non-volatile memory and neuromorphic applications.Fig. 2 shows the evolution of the number of publications per year since 1998, using the keywords 'ferroelectric materials and memory applications' in a Web of Science search.Up to 2013, the number of publications remained almost constant and then increased exponentially.This is linked to the discovery of new ferroelectric materials, as discussed below.
0][11][12][13][14][15][16][17] For instance, different companies (such as Cypress Semiconductor, Texas Instruments, and Fujitsu) installed a PZTbased FeRAM in applications, including wearable medical gadgets, smart cards, energy meters, airplane black boxes, radio frequency tags, and code storage in microcontrollers. 11SBT-based FeFETs with a huge memory window and almost infinite cycling endurance (410 12 ) were demonstrated by Sakai et al., 12 which later exhibited adequate operation in non-volatile logic (NVL) circuits 12 and 64 kbit NAND memory arrays. 13However, while recent investigations concentrated on the continued physical scalingdown of the metal-oxide-semiconductor field-effect transistor (MOSFET), 13,15 the integration of perovskite oxides with frontend semiconductor manufacturing processes faced difficulties,  particularly those related to perovskite etching, high crystallization temperature, hydrogen sensitivity, thickness, and cell size scaling beyond the 130 nm technology node. 9,11For instance, PZT experiences scalability problems, 9,11 because it often loses its polarization in thin films at the nanoscale due to large depolarization fields.Increasingly, 3D integration is required for high-density memory technology with constantly decreasing dimensions, which exacerbates the scaling difficulties for perovskite ferroelectric films at the industrial level. 11Furthermore, PZT contains lead, which makes it toxic with serious health implications.
Considering the aforementioned challenges of perovskites, the discovery of ferroelectricity in Si-doped hafnium oxide (Si:HfO 2 ) thin films has provided a promising alternative to resolve these problems. 15,16Following this exciting discovery, ferroelectricity in the nano-regime in Si:HfO 2 has attracted significant attention from the ferroelectrics community due to its compatibility with the complementary metal-oxide-semiconductor (CMOS) technology.Inducing ferroelectricity at the nanoscale is the current trend in non-volatile memories to realize applications such as high-density memory, storage class memory, neuromorphic computing, hardware security, etc. 17 Recent progress in this field has introduced new ferroelectric materials besides Si:HfO 2 with promising ferroelectric polarization and superior scalability to the nanoscale regime that are compatible with CMOS for the next-generation ferroelectric memories as shown in Fig. 3. [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Fig. 3 also provides some keywords for the enhancement of P r with moderate coercive field (E c ) values (1-2 MV cm À1 ), which can induce large memory windows in non-volatile memory devices such as FeFETs for non-destructive readout operations. 1 For FeFETs, however, a moderate P r is already ideal, as too high of a P r value can deteriorate the device reliability due to a high depolarization field and a high concentration of trapped charges. 1 Fig. 3 clearly provides a picture that orthorhombic (o-) Hf 0.5 Zr 0.5 O 2 is at an advanced stage compared with other ferroelectric materials in terms of achieving good P r , low E c , and excellent CMOS compatibility.Table 1 complements Fig. 3, showing further details such as preparation methods, electrode materials, and film thicknesses, which were used in the ferroelectric-based devices, together with other relevant key ferroelectric performance indicators such as required wake-up cycles (the number of switching cycles required to achieve the maximum P r ), endurance, Curie temperature (T c ), etc.With this as the background, this short review focuses on the breakthroughs in the newly introduced ferroelectric materials from the viewpoint of their ferroelectric properties and how they are beneficial for neuromorphic applications.Finally, we conclude the review article with a perspective on this burgeoning area.

HfO 2 and ZrO 2 -based films
Since the discovery of ferroelectricity in Si-doped hafnium oxide in 2011, doped hafnium oxide has been receiving significant attention. 35The discovery of CMOS-compatible and scalable ferroelectric materials has excited the ferroelectrics community. 35,36Following this, there have been extensive investigations into its fundamentals and applications.Hafnium oxide typically adopts a stable monoclinic phase (P2 1 /c) at room temperature, a tetragonal (P4 2 /mnc) structure at 42100 K, and a cubic (Fm% 3m) structure at 42800 K. 35,36 These phases are nonpolar, i.e., non-ferroelectric.So far, there have been a tremendous number of studies, experimental and computational, investigating the stabilization of meta-stable polar phases.Ferroelectricity in hafnium-and zirconium-based oxides arises from meta-stable, polar orthorhombic (o-) or rhombohedral (r-) phases with space groups Pca2 1 or R3m. 22,379][40][41][42][43][44][45][46][47] Out of all the dopants in hafnium oxide, zirconium shows the most promise owing to the robust ferroelectric properties upon introducing it.Wei et al. reported wake-up-free ferroelectric switching in ultra-thin Hf 0.5 Zr 0.5 O 2 films (thickness of 5 nm) by stabilizing the ferroelectric r-phase using a compressive epitaxial strain engineering strategy. 22Recently, the same group reported that the polarization switching in r-phase Hf 0.5 Zr 0.5 O 2 films (thickness of 6 nm) might be ascribed to reversible oxygen vacancy migration. 45In another work, Stylianidis et al. reported ferroelectricity in 5-nm-thick Hf 0.5 Zr 0.5 O 2 films also attributable to the polarization switching driven by oxygen vacancy migration. 46Yun et al., on the other hand, confirmed polarization switching in yttrium-doped HfO 2 with an o-phase with a small rhombohedral distortion attributable to ''true'' ferroelectricity, i.e., the reversible switching of the material's unit cell polarization. 47The electrical switchability of the yttrium-doped HfO 2 films studied using piezoresponse force microscopy revealed features of ''intrinsic'' ferroelectricity not obscured by extrinsic factors like charge injection.Overall, the phenomenon of ferroelectric switching in hafniabased films is still under debate regarding the origin of the observed polarization switching. 45,46The minimum film thickness that demonstrated ferroelectricity in Zr-doped HfO 2 (Hf 1Àx Zr x O 2 ) with x = 0.2 was 1 nm, which was grown by lowtemperature atomic layer deposition (ALD) on a silicon substrate, followed by rapid thermal annealing. 48Furthermore, the Zr-doped composition Hf 1Àx Zr x O 2 , with x = 0.5, is the most promising one for its excellent ferroelectric properties attributable to the stable ferroelectric orthorhombic phase (Pbc2 1 ) due to the ferroelectric phase transition in the HfO 2 -ZrO 2 solid solution. 380][41] Generally, an E c value of B1 MV cm À1 and a remanent polarization P r value of 415 mC cm À2 were achieved. 45,47ALD-grown HZO films tend to demonstrate wake-up behavior, which means that they require a number of applied voltage cycles prior to the actual demonstration of ferroelectric behavior or before reaching their maximum P r value.There have been multiple attempts to mitigate this wake-up effect and some reports demonstrated wake-up-free ALD-grown HZO through capping electrode engineering, mechanical stress, and plasma treatment. 10Furthermore, epitaxial HZO has been investigated, mainly using PLD.While PLD is currently not CMOS-compatible like ALD, it enables understanding of fundamental properties about the origin of ferroelectricity in HZO with highly crystallized single-phase films.HZO films have been grown on SrTiO 3 (STO) (001) with the La 0.67 Sr 0.33 MnO 3 (LSMO) buffer layer [55][56][57][58] and they typically demonstrate o-(111) oriented films.However, in 2018, Y. Wei et al. reported r-(111) ferroelectric films for the first time. 28The factors determining the formation of a specific ferroelectric phase remain unclear.Recent studies conducted by S. Estandia et al., 2021, 57 and S. Shi et al., 2023, 58 have highlighted the significant role of the bottom electrode in achieving the ferroelectric o-phase.Their findings indicate that MnO termination of LSMO is necessary for the formation of the o-phase.Epitaxial o-HZO films tend to have a high E c of 43 MV cm À1 (depending on their thickness) and P r 4 30 mC cm À2 .It is unclear at present which termination or other structural aspects of LSMO are needed to achieve the r-phase.In addition, recently, magnetron-sputtered r-Hf(Zr 1.129 )O 2 films have shown a very good P r of 22 mC cm À2 , an ultra-low E c of 0.65 MV cm À1 , and stable performance up to B10 12 cycles, which seem to solve the problem of high E c in o-phase HfO 2 -based films.The demonstrated stabilization of the r-phase and its associated ferroelectric properties are attributed to intercalated atoms that expand the lattice, thereby increasing both in-plane and outof-plane stresses, thus creating a more highly polarised structure, which is easier to switch due to its lower kinetic energy barrier between the two spontaneous polarization states. 180][51][52][53][54][55][56] The key parameters that determine the ideal requirement for a synaptic device are high P r , moderate E c , large on/off ratio, long cycling endurance, prolonged retention, and well-spaced conductance levels. 4For instance, ferroelectric materials can achieve multi-level P r states by varying the Review Materials Horizons applied electric switching field.Not only that, these materials maintain each state even after removal of the electric field.This multi-level P r can be used to modulate the conductance of the synaptic device. 59,60The low E c can extend the endurance lifetime and result in low-voltage switching for non-volatile memories. 61A large on/off ratio potentially increases the number of multi-level states by providing a large memory window. 4,62oreover, the endurance and retention of the ferroelectric materials allow the memory device to store the data over a long time. 62Therefore, we can say that ferroelectric-based nonvolatile memory devices are promising for synaptic elements in neuromorphic computing.
4][65][66] For instance, Halter et al. studied an Hf 0.57 Zr 0.43 O 2 thin-filmbased (thickness B9.6 nm) FeFET with WO x as the channel material for synaptic elements. 63  This journal is © The Royal Society of Chemistry 2024 energy (2.1 Â 10 À17 J mm À2 ).Furthermore, it offers multi-level programming spanning more than 4 bit depth and a stable retention over 1500 s.Moreover, the device is compatible with a back end of line (BEOL) integration into standard CMOS processes.Kim et al. reported Al-doped HfO 2 with improved switching speed (B107 mV per decade) and a good on/off ratio (1 Â 10 3 ), demonstrating its use as a potential synaptic transistor. 65By varying the amplitude and number of input pulses, this work was able to achieve potentiation/depression weight updates with minimal cycle-to-cycle variability and good linearity.With the performance data of this FeFET, a simulated pattern recognition accuracy of almost 90% was achieved for the Modified National Institute of Standard and Technology (MNIST) handwritten data set under optimal potentiation/ depression conditions.Furthermore, Al-doped HfO 2 suppresses electrically active charge traps typically arising from the direct deposition of HfO 2 on Si and favors analog conduction modulation at low operating voltages.
Be ´gon-Lours et al. developed a 3.5-nm-thick Hf 0.5 Zr 0.5 O 2 thin film transistor with an on/off ratio of 7 and a small device-todevice variability (o5%) suitable for neural network inference. 67urthermore, the device operates in the ohmic regime for the read-out, which is good for analog vector-matrix multiplication.The conduction modulation in the transistor may have originated from the motion of trapped hydrogen in the lattice and/or oxygen vacancies that form during ALD fabrication.These defects modify the conduction band energy profile and transport properties upon polarization switching.As a synaptic element, this transistor demonstrated a training accuracy of 92% after 36 epochs on the MNIST data set.Xi et al. experimentally demonstrated a CMOScompatible FeFET using an Hf 0.5 Zr 0.5 O 2 ferroelectric layer (thickness B10 nm, Fig. 4(a)) with a very high on/off ratio (3 Â 10 4 ) and low write energy consumption (B2 fJ) at low voltages (0.5 V) (Fig. 4(b)). 68This synaptic device was based on Hf 0.5 Zr 0.5 O 2 /NiSi 2 / Si and used partial polarization switching to modulate the Schottky barrier of the NiSi 2 /Si contacts for analog conduction modulation with symmetric potentiation/depression characteristics and high retention.
Very recently, Halter et al. reported an Hf 0.5 Zr 0.5 O 2 thin film FeFET (thickness B10 nm) (Fig. 4(c)) with good P r (B14 mC cm À2 ) and a very low write energy (1.2 fJ) at higher voltages (6 V). 69 In terms of neuromorphic behavior, using the NeuroSim framework, a good MNIST classification accuracy of 88% was achieved (Fig. 4(d)) due to fine-grained, quasi-continuous monotonic resistance changes with more than 200 steps between the low and high resistance states and a low cycle-to-cycle variability.Furthermore, a very minimal dynamic range loss with a very high endurance (410 10 cycles) and an outstanding retention (410 years) was achieved (Fig. 4(e) and (f)).As a result, the FeFET is promising for neural network inference and cognitive computing.
In a different study, Mikheev et al. fabricated a p-type Si/ Hf 0.5 Zr 0.5 O 2 /TiN-based second-order memristor using a 4-nmthin ferroelectric Hf 0.5 Zr 0.5 O 2 ultra-thin film to investigate the synaptic functions for neuromorphic computing. 70In firstorder memristors, the conductance is defined entirely by external stimuli and thus to emulate the temporal synapse response, an accurate overlapping of pre-and post-synaptic spikes is needed, which is not the case in biological synapses.In second-order memristors, the conductance is controlled by both the external stimuli and their instant internal state.These second-order memristors do not require particular spike shapes and their overlapping and thus allow a straightforward natural emulation of the frequency response.Mikheev et al. demonstrated conductivity (synaptic weight) modulation with R OFF /R ON B 8 via the gradual switching of polarization in ferroelectric domains of polycrystalline 4-nm-thin ferroelectric Hf 0.5 Zr 0.5 O 2 films.The built-in electric field and charge trapping/ de-trapping at the defect states at the Si interface contribute further to the temporal behavior, expressing modulation similar to synapses in biological systems.The work of Mikheev et al. also experimentally demonstrated synaptic functions such as shortterm plasticity, paired-pulse facilitation (PPF), paired-pulse depression (PPD), and spike-rate-dependent plasticity (SRDP), which are similar to biological synaptic systems.Mu ¨ller et al. developed another promising approach using an Fe memristor for the prospects of neuromorphic applications.A simple metalferroelectric-metal (MFM) structure utilizing an ultra-thin Hf 0.93 Y 0.07 O 2 film of thickness 4.5 nm displayed a very good on/off ratio (540) and a large range of accessible resistance states for future NVM. 66This work introduces polarization-modulated charge transport from Schottky-barrier-limited to Ohmic conduction, reporting a high on/off ratio for ultrathin epitaxial Hf 0.93 Y 0.07 O 2 films.The impact of the polarization reversal on the charge transport across a MFM device, utilizing a partially depleted insulating ferroelectric barrier, is demonstrated, which is conceptually different from those in FeTJs, where a fully depleted insulating ferroelectric barrier is used.
From these examples, it can be concluded that these HfO 2and ZrO 2 -based materials are all viable options for large-scale integration into neuromorphic hardware.However, one of the critical outstanding challenges with HfO 2 and ZrO 2 FeFETs are wake-up effects, about which no information has been provided in any of the reviewed examples.
Table 2 shows some key reports on HfO 2 -and ZrO 2 -based synaptic transistors, including features like P r , endurance pulse information, write pulse duration, on/off ratio, synaptic weight, and write energy.Fig. 5 complements Table 2 in terms of the key parameters (thickness, P r , endurance, the number of conductance levels o, and write energy) of HfO 2 -and ZrO 2 -based materials for application as synaptic devices.It can be concluded that reducing the thickness of the ferroelectric layer below 10 nm causes degradation of the potentiation/depression due to increased leakage currents during the program/erase process. 4Unfortunately, there is not much information available about the write energy and MNIST data accuracy except for Hf 0.5 Zr 0.5 O 2 thin film FeFETs, which are considered to be key neuromorphic parameters.FeS-FET device structures are not currently being investigated on a large scale, while FeTJs and Fe memristors have been scarcely investigated.
Just as with HfO 2 , pure or doped ZrO 2 -based thin films are promising for high-k gate dielectrics and are compatible with CMOS technology, and it is surprising that the material has

Review
2][73][74] In 2016, Fan et al. fabricated ZrO 2 films using magnetron sputtering and stabilized the ferroelectric o-phase by substrate-induced strain to form highly textured (111) ZrO 2 ferroelectric films. 72It was found that the o-phase is stabilized by applying compressive strain along the in-plane h11% 2i direction.Consequently, a phase transition from the tetragonal (t-) phase to the ferroelectric o-phase occurred in the ZrO 2 film.Lin et al. verified the existence of a ferroelectric o-phase in undoped ZrO 2 films (10 nm thick) made by remote plasma ALD. 73More recently, Wang et al. reported a P r of 8.5 mC cm À2 and a high relative permittivity of 39.6 in 30-nm-thick pure ZrO 2 films grown on Si (100) via chemical solution deposition. 23Ferroelectricity in pure ZrO 2 films is established after a wake-up operation of about 1000 cycles.According to Wang et al., the wake-up process induces ferroelectricity by changing the nonferroelectric t-phase to a ferroelectric o-phase via oxygen loss.
In addition, it was found that an increase in film thickness (40 nm and 50 nm) weakened the ferroelectric behavior and this was ascribed to the phase transformation from the o-phase to the cubic phase as strain relaxes with thickness.In the same year, Silva et al. showed ferroelectricity in r-phase ZrO 2 thin films without any wake-up process. 20The films were grown on (111)-oriented Nb:SrTiO 3 (Nb:STO) using ion beam sputtering.Ferroelectricity in these ZrO The films displayed a spontaneous polarization P s of 20 mC cm À2 and a P r of 10.8 mC cm À2 with an E c of 1.5 MV cm À1 .By converting an anti-ferroelectric t-phase to a ferroelectric o-phase, Cheema et al. introduced a strategy to achieve ferroelectricity in ZrO 2 films through reduced dimensionality. 71The  This journal is © The Royal Society of Chemistry 2024 films were deposited using ALD.This work reduced the ZrO 2 film thickness down to 5 Å and stabilized the ferroelectric ophase.The reduced dimensionality can be achieved through hydrostatic pressure, chemical pressure, or epitaxial strain. 71ery recently, Silva et al. stabilized the o-phase in 8-nm-thick ZrO 2 films deposited using ion beam sputtering on Nb:STO substrate. 20The films displayed a P r of 9.3 mC cm À2 and an E c of 1.2 MV cm À1 without any wake-up process.The lack of wake-up effect is attractive for next-generation memory devices.However, the potential application of ferroelectric ZrO 2 in neuromorphics is yet to be demonstrated.

Doped zinc oxide films
ZnO is regarded as a conventional non-ferroelectric II-VI wurtzite semiconductor material with a band gap B3.4 eV. 75heoretical investigations done by Moriwake et al., however, reported the possibility of a spontaneous polarization P s of B 90 mC cm À2 in wurtzite ZnO due to the structural distortion arising from the relative displacement of cations against anions along the c-axis. 75,76This is attributed to the tensile strain experienced along the ab-plane along the [0001] direction.Consequently, the potential barrier obstructing the polarization rotation is reduced and the non-polar P6 3 /mmc switches between two equivalent polar states (P6 3 mc), inducing ferroelectricity. 768][79] Onodera et al. reported ferroelectric behavior in wurtzite-structured Li-doped ZnO bulk ceramics with a modest P r of B 0.044 mC cm À2 at ambient temperature using P-E hysteresis loop measurements.Here, the Li ion occupies the off-centered positions, replacing Zn ions.This forms electric dipoles to induce ferroelectric polarization. 79Yang et al. reported ferroelectric polarization in V-doped ZnO thin films deposited on Si(111) substrates with a P r of B0.2 mC cm À2 using magnetron co-sputtering.The introduction of V 5+ at Zn 2+ sites affects the 3d-2p hybridization and induces a local dipole moment by changing the Zn(V)-O bond length. 77][83] Jacques et al. reported a high P r of B80 mC cm À2 and an E c of B3 MV cm À1 for epitaxial Zn 0.64 Mg 0.36 O (ZMO) films (thickness B300 nm) deposited using magnetron sputtering with pure Zn and Mg targets. 81To induce ferroelectric switching, an applied electric field greater than E c had to be applied for a sufficiently long duration to wake up the polarization states.The full realization of the ferroelectric switching in ZMO films was observed after 100 cycles with the application of an electric field of B5 MV cm À1 .The excess electric field of 5 MV cm À1 had to be applied to overcome the internal electric field in the ZMO films due to increased deep trap states attributed to the migration of defects at the interfaces. 81In Zn 1Àx Mg x O (ZnMgO) films of B500 nm thickness grown on single-crystal Al 2 O 3 , with x between 0.35 and 0.37, Ferri et al. reported ferroelectric switching with a P r exceeding 100 mC cm À2 and an E c less than 3 MV cm À1 . 82The pronounced wake-up process in the ferroelectric ZnMgO films concluded after 100 cycles with an applied electric field greater than E c .Ferri et al. also demonstrated the direct integration of ZnMgO films on flexible substrates such as kapton and polystyrene.This introduces the possibility of exploring, for the first time, ferroelectric switching in wurtzite solid solutions for neuromorphic computing.Despite the promising P r values, however, a considerable decrease of E c is needed to make ZnO realistic for neuromorphic applications.
To achieve this, we anticipate that different strategies such as epitaxial growth, doping, strain engineering, domain engineering, etc. can be used.

Scandium-doped aluminum nitride (Al 1Àx Sc x N) films
Pure aluminum nitride (AlN) with a wurtzite structure is generally a pyroelectric material as its polarization direction cannot be switched with an applied electric field. 69According to the literature, however, ferroelectricity in AlN can be induced by adding Sc to it.This is ascribed to the elongation of the Al-N bond parallel to the c-axis, which arises due to the formation of a metastable hexagonal phase introduced by Sc. [84][85][86][87][88] The layered hexagonal phase, particularly around the Sc site, is non-polar.5][86][87][88] As a result, Sc-doped AlN experiences ferroelectric switching.
Fichtner et al. reported a high P r of B100 mC cm À2 with an E c of B3 MV cm À1 in reactively sputtered Al 0.64 Sc 0.36 N films of thickness 400 nm deposited on oxidized Si(100) wafers. 84This excellent ferroelectricity in Al 0.64 Sc 0.36 N is associated with the existence of the hexagonal phase and compositional homogeneity, which allows only 1801 orientation of domains.Besides, the Al 0.64 Sc 0.36 N film exhibits a piezoelectric coefficient of 15.7 pm V À1 with an excellent symmetry around the field axis.It should be noted that the increased Sc content above a particular limit causes a linear decrease in E c .This was concluded from the composition analysis performed by Fichtner et al., where exceeding a certain threshold of Sc content impeded the ferroelectric properties again, favoring the formation of more of a non-polar layered hexagonal phase, which in turn distorts the wurtzite structure due to the expansion of the basal plane and the increasing length of the metalnitrogen bond. 84Wang et al. fabricated Al 0.71 Sc 0.29 N thin film capacitors to demonstrate ferroelectric switching. 89The 100 nm Al 0.71 Sc 0.29 N thin film was deposited on Pt-coated Ti using magnetron sputtering.A wake-up operation up to 50 cycles was required to grow and stabilize the ferroelectric domains in the Al 0.71 Sc 0.29 N thin film.The Al 0.71 Sc 0.29 N thin film displayed a P r of B100 mC cm À2 with an E c of B5.5 MV cm À1 near room temperature.Further work performed by Wang et al. reported a record high P r of B140 mC cm À2 and an E c of B6.5 MV cm À1 in 20-nm-thick Al 0.68 Sc 0.32 N thin films near room temperature. 90The film was deposited on a Pt (111)/Ti/SiO 2 / Si wafer using magnetron sputtering.The formation of a strongly c-axis-oriented Al 0.68 Sc 0.32 N film was elucidated using X-ray diffraction and rocking curve measurements, which confirmed its highly crystalline nature with a full width at half maximum of 2.71.The excellent ferroelectric switching in this Al 0.68 Sc 0.32 N film might be due to a combination of high crystalline nature, low film thickness, and a lower depolarization field at the surface oxide.Yasuoka et al. studied the effect of deposition conditions on Al 1Àx Sc x N thin films deposited on Pt(111)/Ti/SiO 2 /Si substrates using magnetron sputtering. 91The group investigated the effect of sputtering gas, film composition, and film thickness on the ferroelectric behavior of Al 1Àx Sc x N films.The Al 1Àx Sc x N films deposited under an N 2 atmosphere showed good ferroelectricity compared with an N 2 + Ar atmosphere.The smaller value of P r in the Al 1Àx Sc x N film deposited under an N 2 + Ar atmosphere might be attributed to the increased pinning of polarization states.The composition analysis of Al 1Àx Sc x N films in the range 0 o x o 0.34 displayed an increase in lattice parameter c up to 0.22 Å and showed a decreasing trend above this value.This indicates that the ferroelectricity is optimum for the composition x = 0.22 and is ascribed to the presence of more non-polar layered hexagonal phases for larger values of x, which contribute to the lengthening of the metal-nitrogen bond.Furthermore, the crystal structure analysis of the various thicknesses (48-140 nm) of Al 1Àx Sc x N films with x = 0.22 using rocking curve XRD analysis revealed that the preferential (002) orientation becomes predominant above 50 nm.Thus, the maximum values of P r and the dielectric constant were found to be 129 mC cm À2 and 15.9, respectively, for the Al 1Àx Sc x N films with x = 0.22 and 140 nm thickness. 91Ryoo et al. reported a high P r of B100 mC cm À2 and an E c of 45 MV cm À1 in 45-nmthick Al 0.7 Sc 0.3 N films deposited on a SiO 2 /Si substrate via sputtering. 27The enhanced ferroelectricity in the Al 0.7 Sc 0.3 N thin films arises from the imposed compressive strain, which eventually increases the c/a-lattice parameter ratio attributable to the enhanced (002)-preferred orientation.In another work, Kim et al. fabricated 30-nm-thick Al 0.70 Sc 0.30 N wurtzite thin films with a high P r of 100 mC cm À2 with an E c of 46 MV cm À1 via sputtering deposition on a SiO 2 /Si substrate.This is ascribed to the highly c-axis-oriented crystal structure of the film favouring very good switching speed, eliminating the slow motion of the domain wall.Furthermore, a fatigue-free behavior (o10 5 cycles) was observed in the film. 28Musavigharavi et al. grew 20-nm-thick ferroelectric Al 0.64 Sc 0.36 N on a SiO 2 /Si substrate by sputtering.The work reported a maximum P r of 30 mC cm À2 with an E c of 6.5 MV cm À1 .The large polarization is attributed to the formation of a highly crystalline wurtzite structure of Al 0.64 Sc 0.36 N at lower thickness with a preferential (002) orientation achieved normal to the SiO 2 /Si substrate. 29Wang et al., in 2023, reported ferroelectricity in ultra-thin Al 0.7 Sc 0.3 N deposited on a molybdenum (Mo) substrate using molecular beam epitaxy (MBE).The work displayed a P r of 23 mC cm À2 with an E c of 4 6 MV cm À1 . 30The group investigated the ferroelectric behavior of Al 0.7 Sc 0.3 N by varying the thickness from 100 to 5 nm.It is observed that the thickness decrease had a significant effect on P r (decreases) and E c (increases).The diminution in the ferroelectric polarization in the wurtzite Al 0.7 Sc 0.3 N ultra-thin films results from the high compressive strain induced by the scaling-down of the film thickness.Furthermore, in addition to the Janovec-Kay-Dunn (JKD) model (where E c is proportional to d À2/3 with the film thickness d), the formation of a non-switchable ''dead'' layer or dielectric interfacial layer, or a finite screening length of the electrodes, contributes to the increase in E c with the scaling down of the thickness.Zheng et al. developed ultra-thin Al 0.72 Sc 0.28 N films (thickness B5 nm) deposited on a sapphire substrate using physical vapor deposition and investigated their ferroelectric properties.The work showed P r 4 100 mC cm À2 with E c B9.2 MV cm À1 . 31This work suggests that the high E c value for lower thickness may be due to the decrease in the c/a ratio arising from the Sc alloying ratio or from the film stress.Liu et al. fabricated an FeFET device, as shown in Fig. 6(a), composed of 100-nm-thick sputtered Al 0.71 Sc 0.29 N integrated with a twodimensional (2D) molybdenum disulfide (MoS 2 ) channel. 32The device achieved a high P r of 80 mC cm À2 and an E c of 6 MV cm À1 .Furthermore, the device exhibited an excellent on/off ratio of B10 6 , an endurance of 10 4 cycles, and a good retention of 10 5 s, which was however measured at a gate voltage of AE40 V, which is a very high writing voltage (Fig. 6(b)).The 2D MoS 2 channel minimizes the depolarization field by compensating the incomplete charge induced by the ferroelectric Al 0.71 Sc 0.29 N layer, consequently providing prolonged retention and minimal readdisturb.Very recently, Wang et al. studied the ferroelectric and analog resistive switching of memristors in 5 nm wurtzite Al 0.7 Sc 0.3 N with gallium nitride (GaN) as the semiconductor channel grown on a sapphire substrate using MBE, as shown in Fig. 6(c). 92The device demonstrated its potential as a candidate for nonlinear ferroelectric resistive memory arrays with a high on/ off ratio of 10 5 , an endurance of 410 4 cycles, and a good retention of 10 5 s.Furthermore, the work displayed eight conductance states during potentiation and depression for the incremental pulse scheme with voltages ranging from À7.6 V to À8.24 V and 1.9 V to 3.564 V, respectively.However, some reports suggest that a minimum of ten conductance states during potentiation/depression are required for the effective training process by an ANN in the analog domain. 92Nevertheless, the device yielded a recognition accuracy of 92.9% from MNIST simulation (Fig. 6(d)).
Based on these promising demonstrations, we foresee that more studies on these high-P r Al 1Àx Sc x N films could produce further synaptic elements with multi-level polarization states essential for analog conduction modulation during potentiation/depression with CMOS compatibility.Strategies like domain and strain engineering, as well as thickness reduction, could potentially be explored for reducing E c to enhance the potential neuromorphic applicability.

Layered bismuth oxide films
Bismuth oxide is a classical ferroelectric material with a very high Curie temperature (B673 K) and excellent cycling endurance (B10 12 cycles). 93,94This discovery builds on the finding of in-plane ferroelectricity in BiO layers in complex layered bismuth oxide compositions, e.g., Bi 2 AlMnO 6 and Bi 2 NiMnO 6 .In these materials, the growth of the layered structure occurs more in the horizontal than in the vertical direction.6][97][98] Very recently, Yang et al. reported strong out-of-plane ferroelectricity in 1-nm-thin Sm-substituted bismuth oxide (Bi 1.8 Sm 0.2 O 3 ) grown on an Al 2 O 3 substrate using sol-gel spin-coating. 99The 1-nmthin Bi 1.8 Sm 0.2 O 3 (BSO) film exhibited a P r of 17 mC cm À2 , which is high compared with other ultra-thin films such as HZO, CuInP 2 S 6 , BaTiO 3 , BiFeO 3 , and PbZr 0.2 Ti 0.8 O 3 .Here, the Bi layers at the interface serve as strain bearers to relax the lattice mismatch from the substrate, thus maintaining an unstrained growth of the film.Even though the reduced thickness affects the ferroelectricity in the BSO film due to surface charges and leakage, the remarkable P r arises due to the incorporation of the Sm dopants, which preserves the ferroelectric phase in the BSO and maintains a high polarization by suppressing the depolarization field.Furthermore, Sm promotes the tolerance of the layered Bi structure to uphold the stability of the ferroelectric structure at low dimensions.A slightly thicker film of 4.56-nm-thin BSO displayed a high P r of 50 mC cm À2 with an E c   of about 0.75 MV cm À1 .In addition, the film exhibited an excellent cycling endurance (B10 8 cycles) and outstanding thermal stability (up to 493 K).The demonstrated E c and P r values at highly scaled film thicknesses are promising for neuromorphic applications such as non-destructive read/write operations and synaptic functionality.Hence, this new material warrants further deeper explorative studies of its neuromorphic behavior.
Very recently, Gia et al. demonstrated an FeTJ based on an ultra-thin BSO film of thickness 4.6 nm deposited on Nb:STO using a chemical solution method (Fig. 7(a)). 34The FeTJ exhibited long retention time (10 years, Fig. 7(b)) and exceptional cycling endurance (10 9 ).The stable tetragonal-like phase induced by Sm substitution provided prolonged retention in the BSO-based FeTJs by reducing the depolarization field.Furthermore, the BSO film showed an excellent P r of 50 mC cm À2 and a very low E c of 0.75 MV cm À1 .Moreover, the FeTJ displayed a large tunnelling electroresistance compared to other materials attributable to the large polarization maintained at the minute thickness (Fig. 7(c)).Consequently, both the ferroelectric barrier height and barrier width at the depletion region surface of Nb:STO were modulated.In this work, a single BSO-based FeTJ demonstrated 32 conductance states without any write-termination process.Hence, this new material warrants further deeper explorative studies of its neuromorphic behavior.
A summary comparing P r , E c , endurance, retention, and write energy of the different materials, together with neuromorphic performance parameters, is shown in Table 3.It is possible to conclude that Hf 0.5 Zr 0.5 O 2 partially meets the requirements of neuromorphic applications.In the case of Hf 0.5 Zr 0.5 O 2 , we took the best performance 56 from Fig. 5 to compare the neuromorphic parameters with other materials.However, these novel ferroelectrics are in the early stage of development and strong performance improvements are eagerly anticipated.

Concluding remarks
The recent discoveries of ferroelectricity in binary and pseudobinary nanoscale oxide thin films offer great possibilities for miniaturized non-volatile memory and neuromorphic computing devices.We have reviewed the most promising of these oxide thin film materials, and examples are summarized in Table 1.These new materials show high polarization in the nano-regime, which is particularly important for ultra-scaled devices.[67][68][69] However, HfO 2 -and ZrO 2 -based thin films typically struggle with parameters such as imprint, wakeup, fatigue, and consequently with uniform cycling endurance.Also, reducing the film thickness, as is required in some cases, may increase E c and therefore necessitate higher operating voltages, leading to higher leakage currents during program/ erase operations.
Based on the initial demonstrations of ferroelectricity in Sc-AlN, ZnMgO, and BiSmO 3 , there is potential for these materials for neuromorphic computing, too.However, while ZnMgO exhibits exceptional P r values comparable to HfO 2 and ZrO 2 thin films, as of yet the large coercive field and thickness of ZnMgO present challenges for its integration as a neuromorphic computing device.Also, Sc-doped AlN and ZnMgO suffer from problems of low endurance and the requirement for wake-up cycling.Furthermore, parameters like on/off ratio, potentiation/depression, write energy, synaptic weight, etc. remain unexplored in ferroelectric ZrO 2 and ZnMgO.Also, while very recently studied Bi 1Àx Sm x O 3 34 meets some performance parameter requirements for a synaptic element, all these performance parameters need to be explored in a substrate compatible with CMOS fabrication processes.For all the aforementioned oxide materials, strategies should be implemented to control the ferroelectric phase stability and reduce oxygen vacancy concentrations.More studies on epitaxial films are useful in this regard as they serve as model systems to tune the structure carefully without influences of grain boundaries and mixed phases.
With a good understanding of the fundamental properties of materials and their control, domains can then be engineered to achieve low-voltage multi-level synaptic plasticity with low variability for neuromorphic applications.For example, the controlled design of sufficiently many domains in a thin film would allow a more fine-grained tuning of multiple polarization levels for quasi-analog synaptic plasticity and at the same time would improve variability due to the suppression of stochastic switching of only a few domains.Finally, we note that there are also many opportunities to expand the aforementioned ferroelectric thin film systems for applications beyond memory and neuromorphic computing, namely in pyroelectric-and piezoelectric-based energy harvesters, transducers, energy storage, etc. [100][101][102][103][104][105] Emerging Technologies scheme (no.: CIET1819_24); and (vi) ERC advanced grant, EU-H2020-ERC-ADG no.882929, EROS.

Fig. 1
Fig. 1 Different types of ferroelectric non-volatile memories that are applicable to neuromorphic computing and their corresponding current responses.

Fig. 2
Fig. 2 Publications per year based on ferroelectric memories from the year 1998 up to 2023 [Analyze Results (webofscience.com)].

Fig. 3
Fig. 3 New ferroelectric materials and their possible applications.o-and r-indicate orthorhombic and rhombohedral phases, respectively.
This work showcased an Hf 0.57 Zr 0.43 O 2 /WO x stack as an FeFET with good voltage control and strong linearity and symmetry in potentiation and depression at different voltage amplitudes with identical pulse trains, which facilitate facile programming of learning algorithms.The FeFET has a short programming time of 40 ns and a low write

Fig. 4
Fig. 4 (a) Schematic diagram of an Hf 0.5 Zr 0.5 O 2 thin-film-based 3-terminal FeFET.(b) Calculated energy/spike consumption at different voltage pulse amplitudes. 68Copyright (2020) by American Chemical Society.(c) Illustration of an FeFET device based on an Hf 0.5 Zr 0.5 O 2 thin film with source (S), drain (D), gate (G), WO x channels, ferroelectric gate dielectric, access metal lines (M1, M2), plus S access metal line (M3) and D access metal line (M4).(d) MNIST classification performance of the FeFET with different degrees of non-idealities included: non-linearity factors and finite number of steps (purple), added cycle to cycle variation (grey), and added conductance range variation (blue).(e) Endurance measurement of the FeFET at 100 kHz.(f) Retention measurements at room temperature for the four programmed states 69 with extrapolation to 10 years.
2 thin films is attributed to the formation of a polar r-(R3m) phase under epitaxial compressive strain.Pole figure measurements of the 8-nm-thick ZrO 2 thin films confirmed the plane distances d 200 = d 020 = d 002 in the range between 2.55 Å and 2.50 Å, indicative of an r-space group.

Fig. 5
Fig. 5 Performance of HfO 2 -and ZrO 2 -based FeFETs for different key parameters that are required for neuromorphic computing devices.

Fig. 6
Fig. 6 (a) Schematic diagram of a 100-nm-thick ferroelectric AlScN-MoS 2 FeFET and (b) retention measurements made by monitoring the drain current at various time intervals of up to 100 000 s with a gate voltage of AE40 V.Copyright (2020) by American Chemical Society. 32(c) Illustration of an Fe memristor device using an AlScN ultra-thin film.(d) MNIST simulated classification performance of the AlScN Fe memristor showing the best recognition accuracy of 92.9% with non-linearity and cycle-to-cycle/device-to-device variations.92

92
Fig. 6 (a) Schematic diagram of a 100-nm-thick ferroelectric AlScN-MoS 2 FeFET and (b) retention measurements made by monitoring the drain current at various time intervals of up to 100 000 s with a gate voltage of AE40 V.Copyright (2020) by American Chemical Society. 32(c) Illustration of an Fe memristor device using an AlScN ultra-thin film.(d) MNIST simulated classification performance of the AlScN Fe memristor showing the best recognition accuracy of 92.9% with non-linearity and cycle-to-cycle/device-to-device variations.92

Fig. 7
Fig. 7 (a) Schematic diagram of a 4.6-nm-thin ferroelectric Bi 1.8 Sm 0.2 O 3 (BSO) FeTJ device structure and (b) retention measurement indicating prolonged retention for more than 10 years by extrapolation.(c) Comparison of tunnelling electroresistance (TER) values of the BSO FTJ device with other ferroelectric materials and thicknesses.34

Table 1
Key reports on (pseudo-)binary ferroelectric materials and their properties Material and thickness Method P r (mC cm À2 ) E c (MV cm À1 ) T c (K) *PLD -pulsed laser deposition, ALD -atomic layer deposition, IBSD -ion bean sputtering deposition, MBE -molecular beam epitaxy, PVDphysical vapor deposition, and CSM -chemical solution method

Table 2
Key reports on HfO 2 -and ZrO 2 -based synaptic memory devices and their properties