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Ferroelectric HfO2 has attracted much attention owing to its superior ferroelectricity at an ultra-thin thickness and good compatibility with Si-based complementary metal–oxide–semiconductor (CMOS) technology. However, the crystallization of polar orthorhombic phase (o-phase) HfO2 is less competitive, which greatly limits the ferroelectricity of the as-obtained ferroelectric HfO2 thin films. Fortunately, the crystallization of o-phase HfO2 can be thermodynamically modulated via interfacial stress engineering. In this paper, the growth of improved ferroelectric Al doped HfO2 (HfO2:Al) thin films on (111)-oriented Si substrate has been reported. Structural analysis has suggested that nonpolar monoclinic HfO2:Al grown on (111)-oriented Si substrate suffered from a strong compressive strain, which promoted the crystallization of (111)-oriented o-phase HfO2 in the as-grown HfO2:Al thin films. In addition, the in-plane lattice of (111)-oriented Si substrate matches well with that of (111)-oriented o-phase HfO2, which further thermally stabilizes the o-phase HfO2. Accordingly, an improved ferroelectricity with a remnant polarization (2 P r) of 26.7 μC/cm2 has been obtained. The results shown in this work provide a simple way toward the preparation of improved ferroelectric HfO2 thin films. [ABSTRACT FROM AUTHOR]

Detecting ferroelectricity at micro- and nanoscales is crucial for advanced nanomaterials and materials with complicated topography. Switching spectroscopy piezoresponse force microscopy (SSPFM), which involves measuring piezoelectric hysteresis loops via a scanning probe microscopy tip, is a widely accepted approach to characterize polarization reversal at the local scale and confirm ferroelectricity. However, the local hysteresis loops acquired through this method often exhibit unpredictable shapes, a phenomenon often attributed to the influence of parasitic factors such as electrostatic forces and current flow. Our research has uncovered that the deviation in hysteresis loop shapes can be caused by spontaneous backswitching occurring after polarization reversal. Moreover, we've determined that the extent of this effect can be exacerbated when employing inappropriate SSPFM waveform parameters, including duration, frequency, and AC voltage amplitude. Notably, the conventional 'pulse-mode' SSPFM method has been found to intensify spontaneous backswitching. In response to these challenges, we have redesigned SSPFM approach by introducing the positive up-negative down (PUND) method within the 'step-mode' SSPFM. This modification allows for effective probing of local piezoelectric hysteresis loops in ferroelectrics with reversible piezoresponse while removing undesirable electrostatic contribution. This advancement extends the applicability of the technique to a diverse range of ferroelectrics, including semiconductor ferroelectrics and relaxors, promising a more reliable and accurate characterization of their properties. [ABSTRACT FROM AUTHOR]

The overall performance of the multilayer resulting in a sol-gel bismuth ferrite (BiFeO3) film will be primarily determined by the properties of the first layer, but this has yet to receive much attention, even though chemical and morphological defects of this layer can accumulate as the number of layers increases. Here, we perform an optical, conductive, and ferroelectric study of first layer (L 1) dip-coating sol-gel BiFeO3 films using two routes that vary only in the dissolvent; the first one is based on 2-methoxyethanol (MOE), and the second one on acetic acid (AA) with some MOE (AA-MOE). Tauc plots reveal a band gap of 2.43 eV and 2.75 eV for MOE (30 ± 5 nm thick) and AA-MOE (35 ± 5 nm thick) films, respectively. MOE films showed a dielectric function with features at ∼2.5 eV, ∼3.1 eV, and ∼3.9 eV, which were associated with charge-transfer transitions, but such features are absent in AA-MOE films. Advanced atomic force microscopy techniques were used to identify the fine features or defects of the BiFeO3 films: The conductive maps show that the charge transport pathways in both film routes are controlled by nanometer defects rather than grain or grain boundary defects. Current-voltage curves reveal high conductive pathway at a lower voltage for the MOE films than for AA-MOE films. The piezoelectric coefficient for MOE films was ∼20% higher than AA-MOE films. Both deposition methods yield ferroelectric films with an electromechanical strain controlled by the piezoelectric effect and minimal contribution from electrostriction. An optimization for the AA-MOE-based route in the withdrawal speed results in a significant reduction of morphological defects and a more than twofold increase in the piezoelectric coefficient. Our results broaden the understanding of optical and ferroelectric BiFeO3 films based on a chemical solution by dip-coating. [ABSTRACT FROM AUTHOR]

In this study, platinum (Pt) and tungsten (W), two materials with dissimilar coefficients of thermal expansion (CTE) and work functions (WF), are used as the top electrode (TE) and the bottom electrode (BE) in metal/ferroelectric/metal (MFM) structures to explore the ferroelectricity of hafnium zirconium oxide (HZO) with a thickness less than 10 nm. The electrical measurements indicate that a higher CTE mismatch between HZO and TE/BE is beneficial for enhancing the ferroelectric properties of nanoscale HZO thin films. The different WFs of TE and BE generate a built-in electric field in the HZO layer, leading to shifts in the hysteresis loops and the capacitance–voltage characteristics. The structural characterizations reveal that the preferred formation of the orthorhombic phase in HZO is dominated by the W BE. The device in which W is used as the TE and BE (the W/HZO/W MFM structure) presents the optimal ferroelectric performance of a high remanent polarization (2 P r = 55.2 μ C cm−2). The presence of tungsten oxide (WO x ) at the W/HZO interfaces, as revealed by high-resolution transmission microscopy, is also responsible for the enhancement of ferroelectric properties. This study demonstrates the significant effects of different CTEs and WFs of TE and BE on the properties of ferroelectric HZO thin films. [ABSTRACT FROM AUTHOR]

WTe2 recently attracted considerable attention as a layered material exhibiting ferroelectricity, giant magnetoresistance, and pressure-induced superconductivity. In this study, we performed Raman spectroscopy on bulk WTe2, including the low-frequency region. An unfamiliar Raman peak (P0) was found at approximately 9 cm−1 in addition to the seven already known peaks (P1–P7). Furthermore, the angular and polarization dependence of the spectra revealed the peak P0 had A 1 symmetry. The symmetry is consistent with the experimental results and first-principles calculations by other groups. Our work paves the way for studying the symmetry of the low-frequency phonons in atomic-layer ferrolectric films using Raman spectroscopy. [ABSTRACT FROM AUTHOR]

In this work, the effects of top electrode (TE) and bottom electrode (BE) on the ferroelectric properties of zirconia-based Zr0.75Hf0.25O2 (ZHO) thin films annealed by post-deposition annealing (PDA) are investigated in detail. Among W/ZHO/BE capacitors (BE = W, Cr or TiN), W/ZHO/W delivered the highest ferroelectric remanent polarization and the best endurance performance, revealing that the BE with a smaller coefficient of thermal expansion (CTE) plays a vital role in enhancing the ferroelectricity of fluorite-structure ZHO. For TE/ZHO/W structures (TE = W, Pt, Ni, TaN or TiN), the stability of TE metals seems to have a larger impact on the performance over their CTE values. This work provides a guideline to modulate and optimize the ferroelectric performance of PDA-treated ZHO-based thin films. [ABSTRACT FROM AUTHOR]

HfO2-based ferroelectric thin films have attracted significant interest for semiconductor device applications due to their compatibility with complementary metal oxide semiconductor (CMOS) technology. One of the benefits of HfO2-based ferroelectric thin films is their ability to be scaled to thicknesses as low as 10 nm while retaining their ferroelectric properties; a feat that has been difficult to accomplish with conventional perovskite-based ferroelectrics using CMOS-compatible processes. However, reducing the thickness limit of HfO2-based ferroelectric thin films below the sub 5 nm thickness regime while preserving their ferroelectric property remains a formidable challenge. This is because both the structural factors of HfO2, including polymorphism and orientation, and the electrical factors of HfO2-based devices, such as the depolarization field, are known to be highly dependent on the HfO2 thickness. Accordingly, when the thickness of HfO2 drops below 5 nm, these factors will become even more crucial. In this regard, the size effect of HfO2-based ferroelectric thin films is thoroughly discussed in the present review. The impact of thickness on the ferroelectric property of HfO2-based thin films and the electrical performance of HfO2-based ferroelectric semiconductor devices, such as ferroelectric random-access-memory, ferroelectric field-effect-transistor, and ferroelectric tunnel junction, is extensively discussed from the perspective of fundamental theory and experimental results. Finally, recent developments and reports on achieving ferroelectric HfO2 at sub-5 nm thickness regime and their applications are discussed. [ABSTRACT FROM AUTHOR]

Ferroelectric field effect transistor (FeFET) memories with hafnium zirconium oxide (HZO) ferroelectric gate dielectric and ultrathin InO x channel exhibit promising applicability in monolithic three-dimensional (M3D) integrated chips. However, the inferior stability of the devices severely limits their applications. In this work, we studied the effect of single cycle of atomic-layer-deposited Al–O bonds repeatedly embedded into an ultrathin InO x channel (∼2.8 nm) on the Hf0.45Zr0.55O x FeFET memory performance. Compared to the pure InO x channel, three cycles of Al–O bonds modified InO x channel (IAO-3) generates a much larger memory window (i.e. drain current ratio between the programmed and erased devices) under the same program conditions (+5.5 V/500 ns), especially after post-annealing at 325 °C for 180 s in O2 (1238 versus 317). Meanwhile, the annealed IAO-3 FeFET memory also shows quite stable data retention up to 104 s, and much more robust program/erase stabilities till 105 cycles. This is because the modification of strong Al–O bonds stabilizes the oxygen vacancies and reduces the bulk trap density in the channel. Furthermore, it is indicated that the program and erase efficiencies increase gradually with reducing the channel length of the memory device. By demonstrating markedly improved performance of the HZO FeFET memory with the ultrathin IAO-3 channel, this work provides a promising device for M3D integratable logic and memory convergent systems. [ABSTRACT FROM AUTHOR]

Moiré superlattices have emerged as a highly controllable quantum platform for exploration of various fascinating phenomena, such as Mott insulator states, ferroelectric order, unconventional superconductivity and orbital ferromagnetism. Although remarkable progress has been achieved, current research in moiré physics has mainly focused on the single species properties, while the coupling between distinct moiré quantum phenomena remains elusive. Here we demonstrate, for the first time, the strong coupling between ferroelectricity and correlated states in a twisted quadrilayer MoS2 moiré superlattice, where the twist angles are controlled in sequence to be ∼ 57°, ∼ 0°, and ∼ –57°. Correlated insulator states are unambiguously established at moiré band filling factors v = 1, 2, 3 of twisted quadrilayer MoS2. Remarkably, ferroelectric order can occur at correlated insulator states and disappears quickly as the moiré band filling deviates from the integer fillings, providing smoking gun evidences of the coupling between ferroelectricity and correlated states. Our results demonstrate the coupling between different moiré quantum properties and will hold great promise for new moiré physics and applications. [ABSTRACT FROM AUTHOR]

The artificial synapses are basic units in the hardware implementation of neuromorphic computing, whose performances should be gradually modulated under external stimuli. The underlying mechanism of the increasing and decreasing device conductance is still unclear in the Hf0.5Zr0.5O2 based synapses. In this study, the Hf0.5Zr0.5O2 capacitors with different stack orders are fabricated in atomic layer deposition, whose ferroelectric properties are investigated by analyzing the capacitanceâ€"voltage and polarization-voltage curves. The enhanced ferroelectricity is found after the rapid thermal annealing treatment for all the TiN/Hf0.5Zr0.5O2/TiN, TiN/HfO2-ZrO2/TiN and TiN/ZrO2-HfO2/TiN devices. In the device with poor ferroelectricity, the conductance gradually decreases under both positive and negative identical pulse schemes, which corresponds to the gradual dissolution process of the conductive filaments established in the initial pulse. For the capacitors with strong ferroelectricity, dual-direction conductance modulation can be observed due to the partial domain switching process, which can emulate the potentiation and depression process of biological synapses. [ABSTRACT FROM AUTHOR]

A concise overview of ferroelectric ceramics and composites, which constitute the backbone of transducer applications such as actuators, sensors, energy harvesting, and storage are provided. For piezoelectric composites based on ferroelectric ceramics, underlying principles are developed using linear equations of state for piezoelectricity and the concepts of crystalline anisotropy, symmetry, composite connectivity, and form factor engineering as unifying themes. Emphasis is given to diphasic piezoelectric composites which have revolutionized transducer engineering in fields such as sonar and medical ultrasound imaging. It is shown that through judicious selection of a ferroelectric ceramic in conjunction with an inactive material such as a polymer or metal, effective material properties exceeding the performance of single-phase ceramics can be achieved. The application of traditional composite strategies and advances in fabrication technologies to rapidly growing fields such as dielectric energy storage and piezoelectric energy harvesting are also discussed. [ABSTRACT FROM AUTHOR]

Combining the experimental characterization with the large-scale density functional theory calculations based on finite-element discretization (DFT-FE), we address the stabilization of polar orthorhombic phases (o-HfO2) in Al:HfO2 nanofilms by means of the atomic registry distortions and lattice deformation caused by Al substitutional defects (AlHf) and Schottky defects (2AlHf + VO) in tetragonal phases (t-HfO2) or monoclinic phases (m-HfO2). The phase transformation directly from the t-HfO2 into polar o-HfO2 are also elucidated within a heterogeneous distribution of Al dopants in both t-HfO2 bulk crystal structure and Al:HfO2 nanofilm. It is revealed using large-scale DFT calculations that the Al substitutional defects (AlHf) or the Schottky defect (2AlHf + VO) could induce the highly extended atomic registry distortions or lattice deformation in the t- and m-HfO2 phases, but such effects are greatly diminished in ferroelectric orthorhombic phase. By purposely engineering the multiple AlHf defects to form dopant-rich layers in paraelectric t-HfO2 nanofilm or bulk crystal, the induced extended lattice distortions surrounding the defect sites exhibit the shearing-like atomic displacement vector field. The large-scale DFT calculations further predicted that the shearing-like microscopic lattice distortions could directly induce the phase transformation from the t-HfO2 into polar orthorhombic phase in both Al:HfO2 bulk crystal and nanofilms, leading to the large remanent polarization observed in Al:HfO2 nanofilms with the presence of Al-rich layers. The current study demonstrates that the ferroelectricity of HfO2 bulk crystal or thin film can be optimized and tuned by delicately engineering both the distribution and concentration of Al dopants in atomic layer deposition without applying the top capping electrode, providing the extra flexibility for designing the HfO2 based electronic devices in the future. [ABSTRACT FROM AUTHOR]

An investigation was conducted with regard to the effect of etching process on the ferroelectric (FE) characteristics of different device structures with Al-doped HfO2 thin films; further, the effect of the rapid thermal annealing temperature on the FE properties was elucidated using metal-ferroelectric-metal (MFM) capacitors using TiN electrodes with varying thickness and 4 at.% Al-doped HfO2 FE layer. The capacitors were annealed at different temperatures after lithography and etching process; this was aimed at incorporating the FE-orthorhombic phase. The samples annealed after patterning were able to obtain improved FE characteristics due to the amount of tensile stress. The MFM devices that were initially patterned were also studied as a reference. We found that even though it required higher temperature and shorter time to introduce the FE phase, it exhibited more stable as well as promising FE properties and electrical performances with a relatively large remnant polarization (2 P r ∼ 60 ÎĽ C cmâˆ'2), a coercive electric field of approximately 2 MV cmâˆ'1 and high switching current density with less leakage. Our results indicate how the FE properties of the HfO2-based thin films can be engineered through suitable process sequence and post-annealing conditions, thereby verifying the applicable flexibility of FE-HfO2 for semiconductor device integration. [ABSTRACT FROM AUTHOR]

We investigated the ferroelectric properties of barium titanate (BTO) nanocube self-assembled monolayers with and without heat treatment using piezoresponse force microscopy (PFM). Observations of polarization switching behavior confirmed that BTO nanocube monolayers about 15 nm thick are ferroelectric, even without heat treatment. Vertical PFM phase imaging of the monolayers revealed that heat treatment changed the ferroelectric polarization distribution in the monolayers at 800 °C. Atomic force microscopy and transmission electron microscopy suggested that this change originated from the residual stress caused by mechanical interactions between neighboring BTO nanocubes and between the monolayers and the substrate. [ABSTRACT FROM AUTHOR]

The effects of field cycling of Al0.78Sc0.22N capacitors on ferroelectric properties are investigated. In the first hundreds of switching cycles, the reduction in the switching voltage was observed, possibly due to the formation of nitrogen-vacancy to facilitate the atom displacements. With further switching cycles, fatigue effect was observed, especially for domains with low switching voltage. The leakage current analysis indicates continuous downward band bending with the number of switching cycles, effectively reducing the Schottky barrier height for electrons. The breakdown of Al0.78Sc0.22N films is triggered by the Joule heat due to excessive leakage current. The mechanism is in contrast to conventional ferroelectric materials, where the breakdown is triggered by Joule heat at the local conductive filaments. [ABSTRACT FROM AUTHOR]

We examined the wake-up effect in a TiN/Hf0.4Zr0.6O2/TiN structure. The increased polarization was affected by the cumulative duration of a switched electric field and the single application time of the field during each switching cycle. The space-charge-limited current was stable, indicating that the trap density did not change during the wake-up. The effective charge density in the space-charge region was extracted from capacitanceâ€"voltage curves, which demonstrated an increase in free charges at the interface. Based on changing characteristics in these properties, the wake-up effect can be attributed to the redistribution of oxygen vacancies under the electric field. [ABSTRACT FROM AUTHOR]

The ferroelectricity of wurtzite, Sc x Ga1âˆ' x N (x  = 0.35â€"0.44), was demonstrated in a metalâ€"ferroelectricâ€"metal capacitor. The remanent polarization (P r) obtained from positive-up negative-down measurements was high, exceeding 120 ÎĽ C cmâˆ'2. The coercive field (E c) of Sc0.44Ga0.56N was approximately 3.6 MV cmâˆ'1 at 300 K, which decreased to 3 MV cmâˆ'1 at 473 K. We observed that regardless of the host material, the Sc concentration governed the E c value in Sc x Ga1âˆ' x N and Sc x Al1âˆ' x N, suggesting that the polarization switching started in the wurtzite unit cells containing Sc. Additionally, the E c of Sc x Ga1âˆ' x N was lower than that of Sc x Al1âˆ' x N when P r was equivalent. [ABSTRACT FROM AUTHOR]

The sputtering power dependence of 40 nm thick Al0.7Sc0.3N ferroelectric properties was characterized from 200 to 300 W. X-ray rocking curve revealed higher orientated growth into the c -axis with higher sputtering power. Films formed by high power showed reduced leakage current with a higher breakdown field, enabling one to apply the high field for ferroelectric switching. A high remnant polarization (P r) of 130 μ C cm−2 was obtained with a coercive field (E c) of 6 MV cm−1. The switching cycle test revealed a wake-up effect for all the films; increasing the leakage current and modifying the E c. We anticipate the change is attributed to the existence and the generation of nitrogen vacancies (V N) in the films. [ABSTRACT FROM AUTHOR]

Ferroelectric and magnetic properties are investigated for Bi2Fe4O9 nanoparticles with different shapes (cuboid and sphere-like) synthesized by hydrothermal and sol-gel method. The magnetic study reveals that coercivity, Neel temperature and remanent magnetization strongly depend on shape of the particle. The nanoparticle with sphere-like shape exhibits magnetization curve of antiferromagnetic (AFM) ordering with ferromagnetic (FM) component. As the particle shape changes from sphere-like to cuboid, the AFM component is dominating over the ferromagnetic component. A small exchange bias is also observed at low temperature in both the sphere-like and cuboid nanoparticle. The coercivity, remanent magnetization and Neel temperature of sphere-like nanoparticle is greater than cuboid nanoparticle. Ferroelectric measurement shows the remanent polarization of cuboid is greater than sphere-like nanoparticle but the coercivity is almost same. This Bi2Fe4O9 nanoparticle shows a small change in polarization under magnetic field. The polarization value decreases with magnetic field increases. The magnetoelectric coupling-measured by change of remanent polarization under magnetic field are found to be greater in Bi2Fe4O9 sphere-like nanoparticles. These shape dependent magnetic and ferroelectric properties are coming because of shape anisotropy. [ABSTRACT FROM AUTHOR]

Its an absolute pleasure to be able to write this article to honor Professor John Goodenough on his 100th birthday. John, here is wishing you many more years of wonderful science with mirth and laughter! I have had the pleasure of knowing John for more than two decades and also following in his footsteps (albeit at a great distance from him), working on complex correlated oxides for my entire professional career, starting from the Hi-TC cuprates, manganites that display colossal magnetoresistance, ferroelectricity and most recently looking at the coupling between electricity and magnetism in multiferroics as well as the intricacies of spin transport in correlated oxides. This article is written on behalf of many colleagues, collaborators, and researchers in the field of complex oxides as well as current and former students and postdocs who continue to enable and undertake cutting-edge research in the field of multiferroics, magnetoelectrics, and broadly correlated electron materials physics as well as the pursuit of electric-field control of magnetism. What I present is something that is extremely exciting from both a fundamental science and applications perspective and has the potential to revolutionize the field of microelectronics. To realize this potential will require numerous new innovations, both in the fundamental science arena as well as translating these scientific discoveries into real applications. Thus, this article attempts to bridge the gap between fundamental materials physics and the actual manifestations of the physical concepts into real-life applications, a spirit that John’s entire career has embraced. [ABSTRACT FROM AUTHOR]

A large coercive field E C of HfO2 based ferroelectric devices poses critical performance issues in their applications as ferroelectric memories and ferroelectric field effect transistors. A new design to reduce E C by fabricating nanolaminate Hf0.5Zr0.5O2/ZrO2 (HZZ) thin films is used, followed by an ensuing annealing process at a comparatively high temperature 700 °C. High-resolution electron microscopy imaging detects tetragonal-like domain walls between orthorhombic polar regions. These walls decrease the potential barrier of polarization reversal in HfO2 based films compared to the conventional domain walls with a single non-polar spacer, causing about a 40% decrease in E C. Capacitance versus electric field measurements on HZZ thin film uncovered a substantial increase of dielectric permittivity near the E C compared to the conventional Hf0.5Zr0.5O2 thin film, justifying the higher mobility of domain walls in the developed HZZ film. The tetragonal-like regions served as grease easing the movement of the domain wall and reducing E C. [ABSTRACT FROM AUTHOR]

Ever since the ferroelectricity of complementary metal-oxide semiconductor (CMOS) compatible HfO2-based materials was discovered, numerous studies have been conducted on their ferroelectric (FE) properties and device applications. In particular, pure-HfO2 FE materials without external doping have attracted considerable attention owing to their excellent robustness against variation because variations that appear in conventional doped-HfO2 FEs are not observed in electrical characteristics induced by dopant fluctuations in pure-HfO2 FEs. Studies on metal/FE/insulator/semiconductor (MFIS) stack are required to apply the ferroelectricity of pure-HfO2 to memory devices that are completely compatible with Si-based CMOS processes. In pure-HfO2 based MFIS stacks, the polarization tends to reduce with increasing thickness of the HfO2, although the leakage current diminishes. To overcome the tradeoff between the polarization and leakage current with respect to the thickness of the HfO2, an Al2O3 layer was inserted between the HfO2 layers to form a laminated FE structure. By employing the laminated FE, leakage current was effectively suppressed by the Al2O3 and lower HfO2 layers, and polarization was enhanced by the FE sum of the upper and lower HfO2 layers. Therefore, an MFIS structure with maximized polarization and minimized leakage current was successfully demonstrated using laminated FE. In addition, the feasibility of the proposed MFIS with laminated FE for nonvolatile memory device applications was confirmed by verifying the multistate operations of a FE tunnel junction. [ABSTRACT FROM AUTHOR]

The primary order parameter of hybrid improper ferroelectricity (HIF) is not spontaneous polarisation but nonpolar modes such as oxygen octahedral rotations (OORs), antipolar, or even Jahnâ€"Teller distortions; therefore, the HIF mechanism may be applied as an effective pathway to tune electronic bandgaps, control orbitals, and create multiferroicity. Most of the current experimental research on HIF is focused on layered perovskite oxides; therefore, this review focuses on the recent progress of experimental studies on HIF materials with the Ruddlesdenâ€"Popper (Râ€"P) and Dionâ€"Jacobson (Dâ€"J) structures. Experimental research on double-layered Râ€"P oxides is included, and the linear relationship between the Curie temperature and tolerance factor has been established. Moreover, the coexistence of polar and weak ferromagnetic phases has been observed in iron-based double-layered Râ€"P oxides at room temperature. The recent discovery of ferroelectricity in A-site cation-ordered triple-layered Râ€"P oxides has significantly expanded the HIF field. HIF has also been confirmed in caesium- and rubidium-based double-layered Dâ€"J oxides, and complex OOR modes have been observed in Dâ€"J oxides. Although significant progress has been achieved for HIF materials in layered perovskite oxides, extensive research is required to reveal the mysteries of HIF and to create single-phase multiferroics in HIF materials. [ABSTRACT FROM AUTHOR]

Ferroelectric tunnel junction (FTJ) has been considered as a promising candidate for next-generation memory devices due to its non-destructive and low power operations. In this article, we demonstrate the interlayer (IL) engineering in the FTJs to boost device performances. Through the analysis on the material and electrical characteristics of the fabricated FTJs with engineered IL stacks, it is clearly found that the insertion of an Al2O3 layer between the SiO2 insulator and the pure-HfOx FE improves the read disturbance (2Vc = 2.2 V increased), the endurance characteristics (tenfold improvement), and the cell-to-cell TER variation simultaneously without the degradation of the ferroelectricity (less than 5%) and the polarization switching speeds through grain size modulation. Based on these investigations, the guidelines of IL engineering for low power ferroelectric devices were provided to obtain stable and fast memory operations. [ABSTRACT FROM AUTHOR]

The distinct spin amplitude wave was reported in a highly frustrated magnetic compound Fe _2 GeO _4 , which is very different from observations on other members of the M _2 GeO _4 (M = Fe, Co, and Ni) family, raising interest in this compound for some additional emergent phenomena. In particular, this non-uniform spin order allows the intrinsic connection between ferroelectric polarization and magnetically gradient structure to probe the potential linear magnetoelectric (ME) effect. In this work, we address this issue and investigate the magnetism of Fe _2 GeO _4 single crystal that hosts two successive anomalies at antiferromagnetic (AFM) Néel temperatures T _N1 ∼ 7.5 K and T _N2 ∼ 6.7 K, respectively. Our results reveal a remarkable metamagnetic transition in the magnetization as a function of the magnetic field, occurring at a critical magnetic field H _c ∼ 4.1 T when applied along the [110] and [1–10] directions, while such transition along the [001] direction is pointedly absent. Further exploration uncovers two predominant off-diagonal ME coefficients α _yz and α _zy in the incommensurate AFM phase between T _N1 and T _N2 . Additionally, all components of the linear ME tensor remain non-vanishing in the canting AFM phase below T _N2 . This indicates the ME mechanisms for the two phases that may be driven by different magnetic structures. All these presented results are sufficient for us to draw a non-trivial ME phase diagram, which is beneficial to understanding the ME behavior of Fe _2 GeO _4 . Therefore, our study implies that Fe _2 GeO _4 , an unusual frustrated magnet, provides a platform for manipulating the fascinating ME effect in the spinel structure.

Aurivillius-type bismuth layer-structured ferroelectric (BLSF) Bi _5 Ti _3 FeO _15 (BTF) has recently attracted considerable attention as a typical multiferroic material because ferroelectric and magnetic orders coexist, but bulk BTF exhibits antiferromagnetic (AFM) orders and negligible intrinsic magnetoelectric (ME) coupling effects. In this study, nickel-substituted Bi _5 Ti _3 FeO _15 (Bi _5 Ti _3 Fe _0.5 Ni _0.5 O _15 , abbreviated as BTF-Ni) was synthesized using a solid-state reaction method to explore and enhance both the magnetic and ferroelectric properties of BTF. Polarization-electric field P - E loops indicate that the BTF-Ni exhibits considerable maximum polarization P _m of 11.9 μ C/cm ^2 and remnant polarization P _r of 5.8 μ C/cm ^2 , but still keeps a very high ferroelectric Curie temperature (FE T _c ) of 1029 K, which are much superior to those of pure BTF. Moreover, magnetization-magnetic field M-H loops indicate that BTF-Ni exhibits significant ferromagnetic properties with a large saturation magnetization M _s of 60 memu/g, low coercive field H _c of 31 Oe at room temperature, and a high ferromagnetic Curie temperature (FM T _c ) of 698 K, whereas pure BTF has an antiferromagnetic Néel temperature ( T _N ) of 80 K. Our work suggests that nickel-substituted BTF is a potential room-temperature magnetoelectric multiferroic material.

Ferroelectric random-access memory (FRAM) based on conventional ferroelectric materials is a non-volatile memory with fast read/write operations, high endurance, and 10 years of data retention time. However, it suffers from destructive read-out operation and lack of CMOS compatibility. HfO2-based ferroelectric tunnel junctions (FTJ) may compensate for the shortcomings of FRAM by its CMOS compatibility, fast operation speed, and non-destructive readout operation. In this study, we investigate the effect of ferroelectric and interface film thickness on the tunneling electroresistance or ON/OFF current ratio of the Hf0.5Zr0.5O2/Al2O3 based FTJ device. Integrating a thick ferroelectric layer (i.e. 12 nm Hf0.5Zr0.5O2) with a thin interface layer (i.e. 1 nm Al2O3) resulted in an ON/OFF current ratio of 78. Furthermore, to elucidate the relationship between ON/OFF current ratio and interfacial properties, the Hf0.5Zr0.5O2-Al2O3 films and Ge-Al2O3 interfaces are examined via time-of-flight secondary ion mass spectrometry depth profiling mode. A bilayer oxide heterostructure (Hf0.5Zr0.5O2/Al2O3) is deposited by atomic layer deposition (ALD) on the Ge substrate. The ON/OFF current ratio is enhanced by an order of magnitude when the Hf0.5Zr0.5O2 film deposition mode is changed from exposure (H2O) ALD to sequential plasma (sequential O2–H2) ALD. Moreover, the interfacial engineering approach based on the in situ ALD H2-plasma surface pre-treatment of Ge increases the ON/OFF current ratio from 9 to 38 by reducing the interfacial trap density state at the Ge-Al2O3 interface and producing Al2O3 with fewer oxygen vacancies as compared to the wet etch (HF + H2O rinse) treatment of the Ge substrate. This study provides evidence of strong coupling between Hf0.5Zr0.5O2 and Al2O3 films in controlling the ON/OFF current ratio of the FTJ. [ABSTRACT FROM AUTHOR]

Two-dimensional multiferroics, which simultaneously possess ferroelectricity and magnetism in a single phase, are well-known to possess great potential applications in nanoscale memories and spintronics. On the basis of first-principles calculations, a CrNCl2 monolayer is reported as an intrinsic multiferroic. The CrNCl2 has an antiferromagnetic ground state, with a NĂ©el temperature of about 88 K, and it exhibits an in-plane spontaneous polarization of 200 pC/m. The magnetic moments of CrNCl2 mainly come from the d5 xy orbital of the Cr cation, but the plane of the d xy orbital is perpendicular to the direction of the ferroelectric polarization, which hardly suppresses the occurrence of ferroelectricity. Therefore, the multiferroic exits in the CrNCl2. In addition, like CrNCl2, the CrNBr2 is an intrinsic multiferroic with antiferromagnetic-ferroelectric ground state while CrNI2 is an intrinsic multiferroic with ferromagnetic-ferroelectric ground state. These findings enrich the multiferroics in the two-dimensional system and enable a wide range of applications in nanoscale devices. [ABSTRACT FROM AUTHOR]

Morphotropic phase boundaries (MPBs) show substantial piezoelectric and dielectric responses, which have practical applications. The predicted existence of MPB in HfO2–ZrO2 solid solution thin film has provided a new way to increase the dielectric properties of a silicon-compatible device. Here, we present a new fabrication design by which the density of MPB and consequently the dielectric constant ϵr of HfO2–ZrO2 thin film was considerably increased. The was controlled by fabrication of a 10 nm [1 nm Hf0.5Zr0.5O2 (ferroelectric)/1 nm ZrO2 (antiferroelectric)] nanolaminate followed by an appropriate annealing process. The coexistence of orthorhombic and tetragonal structures, which are the origins of ferroelectric (FE) and antiferroelectric (AFE) behaviors, respectively, was structurally confirmed, and a double hysteresis loop that originates from AFE ordering, with some remnant polarization that originates from FE ordering, was observed in P–E curve. A remarkable increase in ϵr compared to the conventional HfO2–ZrO2 thin film was achieved by controlling the FE–AFE ratio. The fabrication process was performed at low temperature (250 °C) and the device is compatible with silicon technology, so the new design yields a device that has possible applications in near-future electronics. [ABSTRACT FROM AUTHOR]

The single-phase multiferroic Bi5Fe1 âˆ' x Ni x Ti3O15 (BFNT- x, x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized by a sol-gel auto-combustion method, and their microstructures, ferroelectric, magnetic, and dielectric properties were investigated in detail. All samples belong to layer-perovskited Aurivillius phase containing four perovskite units sandwiched between two Biâ€"O layers. Ni substitution can not only improve ferroelectricity but also enhance the magnetic properties. The BFNT-0.2 sample shows the largest remnant polarization (2 P r ∼ 11.6 ÎĽC/cm2) and the highest remnant magnetization (2 M r ∼ 0.244 emu/g). The enhancement of the magnetic properties may mainly originate from the spin canting of Fe/Ni-O octahedra via Dzyaloshinskiiâ€"Moriya (DM) interaction. In order to explore the influence of valance state of magnetic ions on the properties, the x-ray photoelectron spectroscopy was carried out. Furthermore, structural, ferroelectric, and magnetic transitions were also investigated. [ABSTRACT FROM AUTHOR]

Neuro-inspired deep learning algorithms have shown promising futures in artificial intelligence. Despite the remarkable progress in software-based neural networks, the traditional von-Neumann hardware architecture has suffered from limited energy efficiency while facing unprecedented large amounts of data. To meet the performance requirements of neuro-inspired computing, large-scale vector-matrix multiplication is preferred to be performed in situ, namely compute-in-memory. Non-volatile memory devices with different materials have been proposed for weight storage as synaptic devices. Among them, HfO2-based ferroelectric devices have attracted great attention because of their low energy consumption, good complementary-metal-oxide-semiconductor (CMOS) compatibility and multi-bit per cell potential. In this review, recent trends and prospects of the ferroelectric synaptic devices are surveyed. First, we present the three-terminal synaptic devices based on the ferroelectric field effect transistor (FeFET), and discuss the switching physics of the intermediate states, the back-end-of-line integration and the 3D NAND architecture design. Then, we introduce a hybrid precision synapse concept that leverages the volatile charges on the gate capacitor of the FeFET and the non-volatile polarization on the gate dielectric of the FeFET. Lastly, we review two-terminal synaptic devices using the ferroelectric tunnel junction (FTJ) and ferroelectric capacitor (FeCAP). The design margins of the crossbar array with FTJ and FeCAP analyzed. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) ferroelectric materials are promising substitutes of three-dimensional perovskite based ferroelectric ceramic materials. Yet most studies have been focused on the construction of non-centrosymmetric 2D van der Waals materials and only a few are constructed experimentally. Herein, we experimentally demonstrate the co-existence of voltage-tunable out-of-plane (OOP) and in-plane (IP) ferroelectricity in few-layer InSe prepared by a solution-processable method and fabricate ferroelectric semiconductor channel transistors. The reversible polarization can initiate instant switch of resistance with high ON/OFF ratios and a comparable subthreshold swing of 160 mV/dec under gate modulation. The origins of such unique OOP and IP ferroelectricity of the centrosymmetric structure are theoretically analyzed. [ABSTRACT FROM AUTHOR]

For a given three different Si doping concentrations at room and high temperatures, the threshold voltage shift (ΔVth) on silicon-doped hafnium-oxide-based ferroelectric field effect transistor (FeFET) is experimentally investigated. It turned out that charge trapping in the gate stack of FeFET (versus polarization switching in the gate stack of FeFET) adversely affects ΔVth. Charge trapping causes the positive ΔVth, while polarization switching causes the negative ΔVth. The dominance of polarization switching is predominantly determined by the total remnant polarization (2Pr), which can be controlled by adjusting Si doping concentration in the hafnium-oxide layer. As the Si doping concentration increases from 2.5% to 3.6%, and 5.0%, 2Pr decreases 19.8 μC cm−2 to 15.25 μC cm−2, and 12.5 μC cm−2, which leads to ΔVth of −0.8 V, −0.09 V, and +0.1 V, respectively, at room temperature. At high temperature, the effect of polarization switching is degraded due to the decreased Pr, while the effect of charge trapping is very independent of temperature. For those three different Si doping concentrations (i.e. 2.5%, 3.6%, and 5.0%), at the high temperature, ΔVth of FeFET is −0.675 V, −0.075 V, and +0.15 V, respectively. This experimental work should provide an insight for designing FeFET for memory and logic applications. [ABSTRACT FROM AUTHOR]

The present report leads to the development of a lead-free perovskite system (Ba Ca)(Sn0.11Zr0.05Ti0.84)O3 (BCSZTx); 0 0.20, exhibiting relaxor ferroelectric behaviour in an average cubic structure. The x-ray diffraction measurements have shown a simple cubic phase with Pm m space group for all the compositions. Despite having a centrosymmetric cubic phase, a slim hysteresis loop has been observed via PE loop measurements. Raman spectroscopic measurements have revealed the presence of local ordering in the macroscopic cubic matrix, corresponding to 'A' and 'B' sites. The cooperative behaviour of 'A' and 'B' site off-centered (local) atoms leading to microscopic polar symmetry in the macroscopically cubic matrix is held responsible for the observed relaxor ferroelectric nature. Owing to the aforementioned contrapositive behaviour, these ceramics have exhibited a high value of dielectric constant. Eventually, we have clearly observed a decisive role of Ca2+ dopant at 'A' site in BCSZTx ceramic system leading to the enhancement in the relaxor ferroelectric behaviour and dielectric properties. The presence of a slim hysteresis loop along with broad and diffuse dielectric nature makes these ceramics, a potential candidate for energy storage applications. [ABSTRACT FROM AUTHOR]

By adoption of a high permittivity ZrO2 capping layer (ZOCL), enhanced ferroelectric properties were achieved in the Hf0.5Zr0.5O2 (HZO) thin films. For HZO thin film with 10 Ĺ ZOCL, the 2Pr value can reach as high as ∼43.1 μC cm−2 under a sweep electric field of 3 MV cm−1. In addition, a reduced coercive field of 1.5 MV cm−1 was observed, which is comparable to that of HZO with metallic CL. Furthermore, the homogeneity of ferroelectric orthorhombic phase in HZO films was observed to be clearly increased, as evidenced by nanoscale piezoelectric force microscopy measurements. These results demonstrate that ZOCL is very favorable for high performance ferroelectric HZO films and their future device applications. [ABSTRACT FROM AUTHOR]

From the viewpoint of interface quality within the gate stack, it is crucial to accurately evaluate the device performance of ferroelectric field-effect transistors (FeFETs). In this article, a measurement scheme composed of pulse configurations is designed to avoid dynamic changes in the threshold voltage of the FeFET during charge pumping (CP) measurements. Three types of control devices experiencing different levels of bias stress are prepared for investigations of the origins of device performance degradation. The frequency dependence of applied voltage levels to induce variations in CP current (ICP) are suggested to result from charge-injection events into deteriorated interface trap sites. These behaviors were also verified by the pulsed current–voltage (I–V) method using high-frequency gate voltage (VGS). The frequency dispersion of memory window (MW) variations and their VGS-sweep polarity dependences well reflect the interfacial behavior related to the charge injection of the stressed devices. The trap/detrap times and locations of trapped charges are examined by the energy distribution of trap sites calculated from the obtained ICP values. Thus, the MW obtained from the FeFET should be accurately analyzed to separate the charge-injection components from ferroelectric field effects by means of the proposed useful guidelines using the CP and pulsed I–V methods. [ABSTRACT FROM AUTHOR]

The moiré-pattern with slightly interlayer-twisted bilayer two-dimensional (2D) materials has recently been receiving substantial attention. One of the major characters for these intriguing structures is the appearance of low-energy ultra-flat bands and thus a package of new physics associated with strong electron correlation emerges. However, such new physics may become vague unless the twist-angle θ is sufficiently small such as θ ∼ 1°, making practical applications and control-flexibility hard to handle. In this work, we explore the possible flat-band moiré physics in recently concerned 2D bilayer α-In2Se3 which is antiferroelectric with sublayer out-of-plane (OP) polarizations, addressing the potential role of polarization-bound charges in modulation of electron–electron correlation and interlayer hybridization, based on the extensive first-principles calculations. On one hand, it is found that the low energy band becomes extremely flat in the bilayer α-In2Se3 moiré-pattern with relatively large twist-angle, e.g. θ = 13.17°, which is more easily accessible experimentally. On the other hand, the impact of the sublayer OP polarizations on the band structure is asymmetric, and the flattening effect is much more remarkable for the end-to-end polarization alignment but weaker for the head-to-head alignment. This work thus opens a broad roadmap for technological access to artificial fabrication of novel moiré-patterned 2D materials by means of low-dimensional ferroelectricity. [ABSTRACT FROM AUTHOR]

We use density functional theory to investigate the possibility of polar and multiferroic states in free-standing, perovskite-based nanodots at the atomic limit of miniaturization: single unit cells with terminations which allow centro-symmetry. We consider both A-O and B-O2 terminations for three families of nanodots: (i) A = Ba with B = Ti, Zr, and Hf; (ii) A = Ca and Sr with B = Ti; and (iii) A = Na and K with B = Nb. We find all A–O terminated dots to be non-polar and to exhibit cubic symmetry (except for K8NbO6), regardless of the presence of ferroelectricity in the bulk. In contrast, all the B–O2 terminated nanodots considered relax to a non-cubic ground state. Rather surprisingly, all of these structures exhibit polar ground states (except NaNb8O12). We propose a new structural parameter, the cluster tolerance factor (CTF), to determine whether a particular chemistry will result in a polar ground state nanodot, analogous to the Goldschmidt factor for bulk ferroelectrics. In addition, we find that all A–O terminated (except Ca8TiO6) and all polar B–O2 terminated nanodots are magnetic, where none show magnetism in the bulk. As with bulk systems, multiferroicity in the B–O2 terminated dots originates from separation between spin density in peripheral B atoms and polarity primarily caused by the off-center central A atom. Our findings stress that surface termination plays a crucial role in determining whether ferroelectricity is completely suppressed in perovskite-based materials at their limit of miniaturization. [ABSTRACT FROM AUTHOR]

Obtaining large electrocaloric (EC) effects in ferroelectric materials is a prerequisite for incorporating them into advanced solid-state cooling devices. Based on phase-field simulations, we propose an effective approach for improving the EC effect near room temperature in compositionally graded ferroelectric (CGFE) thin films (e.g., Pb1 − xSrxTiO3) by controlling the compositional gradient along the direction of the thickness of the films. The simulation results reveal the formation of a stable needle-like domain structure in CGFE films with large composition gradients, while a periodic striped domain structure is found in small-composition-gradient CGFE films. The temperature dependence of polarization can be tailored by controlling the composition gradient in the films. With rising temperatures, a notable transition from a needle-like to a vortex domain structure is observed in CGFE films, which is distinguishable from that observed in homogeneous ferroelectric films. Our work demonstrates that the EC effect (the adiabatic temperature change, ΔT) can be greatly improved by the needle-to-vortex domain transition that occurs when the composition gradient is increased. When the composition gradient increases, a larger EC effect is achieved at lower temperatures. Large-composition-gradient films exhibit multiple peaks of ΔT. The local concentration of the total energy near the root of the needle domains is also found to drive the needle-to-vortex domain transition, giving rise to an enhancement of the EC effect in CGFE films. Our study provides a potential pathway for designing ferroelectric thin films with enhanced EC properties for energy-efficient solid-state refrigeration. [ABSTRACT FROM AUTHOR]

The Al:HfO2 ferroelectric nanofilms with different total thicknesses and distributions of Al-rich strips are prepared using atomic layer deposition (ALD) in an uncapped configuration. The synergistic interplay between the number of Al-rich layers and the thickness of total film offers the additional flexibility to boost the ferroelectricity of the resulting Al:HfO2 nanofilms. By carefully optimizing both the ALD cycles for dopant layer and the total film thickness in the preparation, the HfO2 nanofilms in post-deposition annealing can exhibit excellent ferroelectricity. The highest remanent polarization (2Pr) of 51.8 μC cm−2 is obtained in a 19.4 nm thick Al:HfO2 nanofilm at the dopant concentration of 11.1 mol% with a three ALD cycles for Al-rich strips. Remarkable remanent polarization value observed in the uncapped electrode clamping film paves a new way to explore the origin of ferroelectricity in hafnium oxide nanofilms. The observed ferroelectricity of the nanofilm is affected neither by the presence of an interface between the upper electrode and the film nor the choices of the materials of upper electrode in the measurement, ensuring a high flexibility in the designing and fabrication of the relevant devices in the future. [ABSTRACT FROM AUTHOR]

PbZr0.2Ti0.8O3 (PZT) gate insulator with the thickness of 30 nm is grown by pulsed laser deposition (PLD) in AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs). The ferroelectric effect of PZT AlGaN/GaN MIS-HEMT is demonstrated. The polarization charge in PZT varies with different gate voltages. The equivalent polarization charge model (EPCM) is proposed for calculating the polarization charge and the concentration of two-dimensional electron gas (2DEG). The threshold voltage (Vth) and output current density (IDS) can also be obtained by the EPCM. The theoretical values are in good agreement with the experimental results and the model can provide a guide for the design of the PZT MIS-HEMT. The polarization charges of PZT can be modulated by different gate-voltage stresses and the Vth has a regulation range of 4.0 V. The polarization charge changes after the stress of gate voltage for several seconds. When the gate voltage is stable or changes at high frequency, the output characteristics and the current collapse of the device remain stable. [ABSTRACT FROM AUTHOR]

Numerical simulations have been performed to study hydrogen interstitial diffusion and ferroelectric degradation in lead titanate films. The computational method consists of two parts: hydrogen diffusion property calculations using density functional theory (DFT) and ferroelectric property calculations using molecular dynamics (MD) simulations. The hydrogen interstitial diffusion path and the activation energy have been obtained using DFT calculations. With the obtained diffusion properties, the distributions of hydrogen impurities have been calculated using Fick's second law and used as input to MD simulations, with various diffusion times and hydrogen surface concentrations. It has been found that magnitudes of remnant polarization and coercive electric fields decrease with increasing the diffusion time and the hydrogen surface concentration. The distribution of hydrogen impurities is another critical factor to ferroelectric responses. Compared to a uniform distribution of hydrogen impurities, the nonuniform distribution diminishes the ferroelectric properties more severely. A heavily defective region due to the nonuniform hydrogen distribution reduces the effective film thickness and induces the in-plane ferroelectric domain formations. [ABSTRACT FROM AUTHOR]

Ferroelectric field-effect transistors (FETs) with a metal-ferroelectric-metal-insulator-semiconductor (MFMIS) gate stack were fabricated and characterized to elucidate the key process parameters and to optimize the process conditions for guaranteeing nonvolatile memory operations of the device when the undoped HfO2 was employed as ferroelectric gate insulator. The impacts of top gate (TG) for the MFM part on the memory operations of the MFMIS-FETs were intensively investigated when the TG was chosen as metal Pt or oxide ITO electrode. The ferroelectric memory window of the MFMIS-FETs with ITO/HfO2/TiN/SiO2/Si gate stack increased to 3.8 V by properly modulating the areal ratio between two MFM and MIS capacitors. The memory margin as high as 104 was obtained during on- and off-program operations with a program pulse duration as short as 1 μs. There was not any marked degradation in the obtained memory margin even after a lapse of retention time of 104 s at 85 °C and repeated program cycles of 10,000. These obtained improvements in memory operations resulted from the fact that the choice of ITO TG could provide effective capping effects and passivate the interfaces. [ABSTRACT FROM AUTHOR]

Sr-doped Ba0.7La0.3TiO3 (BSLTO) thin films are deposited by pulsed laser deposition, and their microstructure, conductivity, carrier transport mechanism, and ferroelectricity are systematically investigated. The x-ray diffraction measurements demonstrate that Sr-doping reduces the lattice constant of BSLTO thin films, resulting in the enhanced phonon energy in the films as evidenced by the Raman measurements. Resistivity-temperature and Hall effect measurements demonstrate that Sr can gradually reduce electrical resistivity while the electron concentration remains almost unchanged at high temperatures. For the films with semiconducting behavior, the charge transport model transforms from variable range hopping to small polaron hopping as the measurement temperature increases. The metalic conductive behaviors in the films with Sr = 0.30, 0.40 conform to thermal phonon scattering mode. The difference in charge transport behavior dependent on the A-site cation doping, is clarified. It is revealed that the increasing of phonon energy by Sr doping is responsible for lower activation energy of small polaron hopping, higher carrier mobility, and lower electrical resistivity. Interestingly, the piezoelectric force microscopy (PFM) results demonstrate that all the BSLTO films can exhibit ferroelectricity, especially for the room temperature metallic conduction film with Sr = 0.40. These results imply that Sr-doping could be a potential way to explore ferroelectric metal materials for other perovskite oxides. [ABSTRACT FROM AUTHOR]

Due to their structural flexibility, tunability and easy processing, molecular ferroelectric materials have emerged to complement the well-known ferroelectric perovskite oxides. A number of compounds have been investigated but, with the exception of polymeric polyvinylidene fluoride, most have not succeeded in achieving their potential, mainly because of their poor ambient stability. It is only recently, with the surge of electronic type ferroelectricity, that molecular ferroelectrics with superior properties have been synthesized, making them competitive with respect to oxides. Nevertheless, the uniaxial nature of most of the compounds still represents the main bottleneck because it limits the transposition of the bulk properties into films. A possible solution is represented by the use of multiaxial molecular compounds or of low-dimensional systems. In this review, we highlight the main achievements in this field and summarize the open questions to be addressed if molecular ferroelectrics are to be exploited in devices. [ABSTRACT FROM AUTHOR]

The single event effects (SEEs) on ferroelectric Hf0.5Zr0.5O2 capacitor-based non-volatile static random access memory (nvSRAM) were investigated by simulation. A nvSRAM cell integrated with two ferroelectric Hf0.5Zr0.5O2 capacitors is proposed in this study. A macro-model of the ferroelectric Hf0.5Zr0.5O2 capacitor, extracted from the real fabricated devices, is utilized for simulation analysis. Fundamental store and recall operations of the proposed nvSRAM design have been demonstrated. An independent double exponential current source was utilized and injected into specific circuit nodes to simulate the heavy ion induced single event transient current. The simulation results show that the transient pulse current is possible to upset the logic state of the memory cell from 1 to 0, but whether it can recover in a short time period after the upset errors depends on the exact value of linear energy transfer for the injected particles. In addition, increasing the remnant polarization (Pr) and decreasing the coercive voltage (Vc) and film thickness of ferroelectric capacitors can mitigate the influence of SEEs, which provides guidance for process hardening techniques aiming at space applications. [ABSTRACT FROM AUTHOR]

Since the discovery of ferroelectricity in doped/alloyed HfO2 and ZrO2 thin film, many device engineers have been attracted to its sustainable ferroelectricity at the thickness of a few nanometer. While most of the previous studies have mainly focused on the ferroelectric properties of the thermally atomic layer deposited (THALD) Hf0.5Zr0.5O2 (HZO), the plasma-enhanced ALD (PEALD) HZO has not received much attention. In this work, a direct comparison between the two types of HZO thin films is carried out, where we found that a tradeoff exists between these two fabrication methods. While the THALD HZO was able to maintain a higher cycling endurance, the PEALD HZO showed more stable characteristics over the cycling with reduced wake-up and fatigue effects, in addition to better tolerance against breakdown under high electric field. Furthermore, the PEALD HZO could be crystallized with post deposition annealing at 350 °C, which is of great interest for the back-end-of-line compatibility with silicon fabrication processes. [ABSTRACT FROM AUTHOR]

New photovoltaic materials have been extensively searched for in the last decade for clean and renewable solar energy conversion. Among them, the organic–inorganic hybrid lead halide perovskite solar cell is of current interest to the solar cell community because of its low processing cost, ease of fabrication and high power conversion efficiency. These perovskites show great potential to be photovoltaic absorbers with excellent optoelectronic properties, for example, high absorption coefficient, long charge carrier diffusion length, moderate ambipolar mobilities, tunable bandgap, etc. The unique physical and chemical properties of these materials are based on the crystal structure of the perovskites, named after the mineral form of CaTiO3. Most inorganic materials with a perovskite crystal structure typically show ferroelectric properties. Ferroelectricity in methylammonium lead tri-iodide (MAPbI3), an absorbing material in perovskite solar cells with a perovskite crystal structure, is now a topic of debate. Some theoretical and experimental investigations have shown the presence of a ferroelectric domain in these perovskites, although a lot of controversial results and conclusions also exist in the literature. In this review, notable signs of progress in the theoretical and experimental studies regarding ferroelectricity in organic-inorganic hybrid perovskites are summarized. In addition, future improvements of ferroelectric perovskites are proposed, paving the way towards prospective high-performance flexible and wearable ferroelectric solar cells and optoelectronic devices. [ABSTRACT FROM AUTHOR]

In the present work, detailed analyses are carried out to study the impact of temperature on the device performance of a single-gated polarity-controllable–ferroelectric–field-effect transistor (PC–FE–FET). Further, the impact of unintentional variations (≤±5%) in ferroelectric (FE) material parameters are extensively investigated. Various device characteristics are studied by implementing the baseline approach. Due to the integration of an FE layer, the proposed device shows remarkable improvements in current drivability, transconductance and transconductance generation factor, and even at elevated temperatures, sub-60 subthreshold swing values are achieved in comparison to the conventional polarity-controllable–FET (PC-FET) for both n- and p-modes of operation. It is demonstrated that the PC–FE–FET shows remarkable stability towards variations in FE parameters. [ABSTRACT FROM AUTHOR]

Tunnel field-effect transistors (TFETs) are strong candidates for 'Internet of Things' electronic devices, due to their ultra-low power consumption. In this work, we propose a novel nanoscale silicon-on-insulator double-gate ferroelectric tunneling field-effect transistor (SOI DG-FeTFET), and single-event-transient (SET) effects are investigated by means of two-dimensional technology computer-aided design simulations. In addition, we perform systematic analysis and comparison with a silicon-on-insulator double-gate ferroelectric field-effect transistor (SOI DG-FeET). The simulation results show that the peak value of the drain transient current achieved by our DG-FeTFET is up to 2.72 × 10–4 A at 10 MeV·cm2 mg−1, which is much higher than the on-state current (Ion) ∼ 7.63 × 10−5 A at Vd = 0.5 V. Moreover, our results show that the DG-FeTFET is more susceptible to SET effects than FeFET. The results also show that the bipolar amplification effect can be neglected, and that the drift-diffusion mechanism is dominant in the carrier collection process in the DG-FeTFET. The transient responses of the single event effect are also strongly correlated with the strike location of heavy ions, with the most sensitive part being close to the area where interband tunneling occurs and the electric field is the strongest. These analyses may prove relevant for applications SOI DG-FeTFETs/FeFETs in environments where radiation is present. [ABSTRACT FROM AUTHOR]