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As a high-k material, hafnium oxide (HfO2) has been used in gate dielectrics for decades. Since the discovery of polar phase in Si-doped HfO2 films, chemical doping has been widely demonstrated as an effective approach to stabilize the ferroelectric phase in HfO2 based thin films. However, the extra capping layer deposition, post-growth annealing and wake-up effect are usually required to arouse the ferroelectricity in HfO2 based thin films, resulting in the increase of complexity for sample synthesis and the impediment of device application. In this study, the ferroelectricity is observed in non-capped dopant-free HfO2 thin films prepared by pulsed laser deposition (PLD) without post-growth annealing. By adjusting the deposited temperature, oxygen pressure and thickness, the maximum polarization up to 14.7 μC/cm2 was obtained in 7.4 nm-thick film. The fraction of orthorhombic phase, concentrations of defects and size effects are considered as possible mechanisms for the influences of ferroelectric properties. This study indicates that PLD is an effective technique to fabricate high-quality ferroelectric HfO2 thin films in the absence of chemical doping, capping layer deposition and post-growth annealing, which may boost the process of nonvolatile memory device application.

This paper reports the improvement of electrical, ferroelectric and endurance of Hf0.5Zr0.5O2 (HZO) thin-film capacitors by implementing W electrode. The W/HZO/W capacitor shows excellent pristine 2Pr values of 45.1 μC/cm2 at ±6 V, which are much higher than those of TiN/HZO/W (34.4 μC/cm2) and W/HZO/TiN (26.9 μC/cm2) capacitors. Notably, the maximum initial 2Pr value of W/HZO/W capacitor can reach as high as 57.9 μC/cm2 at ±7.5 V. These strong ferroelectric polarization effects are ascribed to the W electrode with a fairly low thermal expansion coefficient which provides a larger in-plane tensile strain compared with TiN electrode, allowing for enhancement of o-phase formation. Moreover, the W/HZO/W capacitor also exhibits higher endurance, smaller wake-up effect (10.1%) and superior fatigue properties up to 1.5 × 1010cycles compared to the TiN/HZO/W and W/HZO/TiN capacitors. Such improvements of W/HZO/W capacitor are mainly due to the decreased leakage current by more than an order of magnitude compared to the W/HZO/TiN capacitor. These results demonstrate that capping electrode material plays an important role in the enhancement of o-phase formation, reduces oxygen vacancies, mitigates wake-up effect and improves reliability.

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

When scaling the lateral size of a ferroelectric random access memory (FeRAM) device down to the nanometer range, the polarization switching-induced displacement current becomes small and challenging to detect, which greatly limits the storage density of FeRAM. Here, we report the observation of significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching in BiFeO3 thin films via conductive atomic force microscopy. Interestingly, this injected current can be effectively modulated by applying mechanical force. As the loading force increases from ∼50 to ∼750 nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by 2-3 orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of the injected current may be attributed to the mechanical force-induced changes in the barrier height and interfacial layer width.

Precise control of ferroelectric and multiferroic domain states at the nanoscale is of considerable interest due to the potential to boost the development of next-generation low-energy-consumption ...

Ferroelectric topological objects (e.g. vortices, skyrmions) provide a fertile ground for exploring emerging physical properties that could potentially be utilized in future configurable nanoelectronic devices. Here, we demonstrate quasi-one-dimensional metallic high conduction channels along two types of exotic topological defects, i.e. the topological cores of (i) a quadrant vortex domain structure and (ii) a center domain (monopole-like) structure confined in high quality BiFeO3 nanoisland array, abbreviated as the vortex core and the center core. We unveil via phase-field simulations that the superfine (< 3 nm) metallic conduction channels along center cores arise from the screening charge carriers confined at the core whereas the high conductance of vortex cores results from a field-induced twisted state. These conducting channels can be repeatedly and reversibly created and deleted by manipulating the two topological states via an electric field, leading to an apparent electroresistance effect with an on/off ratio higher than 103. These results open up the possibility of utilizing these functional one-dimensional topological objects in high-density nanoelectronic devices such as ultrahigh density nonvolatile memory., manuscript (21 pages, 4 figures) plus supplementary information

A capping layer is known to be critical for stabilizing the ferroelectric (FE) orthorhombic phase (o-phase) in a HfO2-based thin film. Here, vanadium oxide (VOx), a functional oxide exhibiting the insulator-metal transition, is used as a novel type of a capping layer for the Hf0.5Zr0.5O2 (HZO) thin film. It is demonstrated that the VOx capping layer (VCL) can enhance the FE properties of the HZO thin film comprehensively. Specifically, the HZO thin film with a VCL shows large remanent polarization (2Pr ≈36.9 μC/cm2), relatively small coercive field (Ec ≈1.09 MV/cm), high endurance (up to 109 cycles), and long retention (>105 seconds). The enhanced FE properties may be attributed to the VCL-induced stabilization of the FE o-phase and suppression of oxygen vacancies at the interface. Furthermore, the HZO thin film with a VCL exhibits a successive rightward shift of polarization-voltage (P-V) hysteresis loop as the temperature increases. This is well correlated with the insulator-metal transition in a VCL, which can modulate the interfacial built-in field and thus cause the P-V loop shift. It is therefore demonstrated that a VCL not only enhances the FE properties of HZO thin films but also provides a temperature degree of freedom to modulate the FE properties, which may open up a new pathway to develop HfO2-based FE memories with high performance and novel functionalities.

Nowadays the development of machine vision is oriented toward real-time applications such as autonomous driving. This demands a hardware solution with low latency, high energy efficiency, and good reliability. Here, we demonstrate a robust and self-powered in-sensor computing paradigm with a ferroelectric photosensor network (FE-PS-NET). The FE-PS-NET, constituted by ferroelectric photosensors (FE-PSs) with tunable photoresponsivities, is capable of simultaneously capturing and processing images. In each FE-PS, self-powered photovoltaic responses, modulated by remanent polarization of an epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 layer, show not only multiple nonvolatile levels but also sign reversibility, enabling the representation of a signed weight in a single device and hence reducing the hardware overhead for network construction. With multiple FE-PSs wired together, the FE-PS-NET acts on its own as an artificial neural network. In situ multiply-accumulate operation between an input image and a stored photoresponsivity matrix is demonstrated in the FE-PS-NET. Moreover, the FE-PS-NET is faultlessly competent for real-time image processing functionalities, including binary classification between ‘X’ and ‘T’ patterns with 100% accuracy and edge detection for an arrow sign with an F-Measure of 1 (under 365 nm ultraviolet light). This study highlights the great potential of ferroelectric photovoltaics as the hardware basis of real-time machine vision.

Perceiving nanoscale ferroelectric phenomena from real space is of great importance for elucidating underlying ferroelectric physics. During the past decades, nanoscale ferroelectric characterization has mainly relied on the Piezoresponse Force Microscopy (PFM) invented in 1992, however, the fundamental limitations of PFM have made the nanoscale ferroelectric studies encounter significant bottlenecks. In this study, a high-resolution non-contact ferroelectric measurement, named Non-Contact Heterodyne Electrostrain Force Microscopy (NC-HEsFM), is introduced. It is demonstrated that NC-HEsFM can operate on multiple eigenmodes to perform ideal high-resolution ferroelectric domain mapping, standard ferroelectric hysteresis loop measurement, and controllable domain manipulation. By using a quartz tuning fork (QTF) sensor, multi-frequency operation, and heterodyne detection schemes, NC-HEsFM achieves a real non-contact yet non-destructive ferroelectric characterization with negligible electrostatic force effect and hence breaks the fundamental limitations of the conventional PFM. It is believed that NC-HEsFM can be extensively used in various ferroelectric or piezoelectric studies with providing substantially improved characterization performance. Meanwhile, the QTF-based force detection makes NC-HEsFM highly compatible for high-vacuum and low-temperature environments, providing ideal conditions for investigating the intrinsic ferroelectric phenomena with the possibility of achieving an atomically resolved ferroelectric characterization.

Progress in energy storage ceramics for applications has been of interest owing to their clean green nature and excellent ferroelectric/dielectric performances. In this study, lead-free Sm-doped 0.95BaTiO3–0.05Bi(Mg0.5Ti0.5)O3 (BT–BMT–xSm) relaxor ferroelectric ceramics were synthesized and confirmed to be an appropriate material for storage devices. After the introduction of Sm2O3 into BT–BMT ceramics, their hysteresis loops gradually shrank and became slender, which is beneficial to achieving high efficiency energy storage in ceramic capacitors. The maximum recoverable energy density for the BT–BMT–xSm (x = 0.03) sample reached 3.86 J cm−3 at normal temperature, and its corresponding energy efficiency reached 81.6%. Moreover, it was found from impedance spectroscopy that the breakdown field strength is related to the oxygen defect content in the samples. Ceramic samples with fewer defects have better insulation performance, which means that a high breakdown field strength will be obtained. With proper doping, enhanced relaxor behaviour and breakdown field strength were obtained to achieve excellent energy storage properties.

The ferroelectricity of Hf0.5Zr0.5O2 (HZO) thin films has been usually reported to be induced by metallic capping layer. In this work, we successfully obtained ferroelectricity of HZO thin films induced by ultrathin insulating Al2O3 capping layers. The ferroelectric properties of HZO thin films induced by Al2O3 capping layers with different thickness have been investigated. It was found that the Al2O3(2 nm)/HZO (20 nm) stack shows excellent ferroelectric properties. The Au/Al2O3(2 nm)/HZO (20 nm)/TiN capacitor exhibits a very low leakage current of $\sim {4.2}\times {10}^{\text {-9}}$ A at 4 V and a maximum $2{P}_{r}\approx 32.3~\mu \text{C}$ /cm2 at sweeping voltages between ±8 V. In addition, the capacitor also shows excellent endurance properties up to 108 cycles. Our work demonstrated that dielectric films like Al2O3 can be adopted to be used as capping layer to generate excellent ferroelectric properties of HfO2-based thin films, which will contribute to their future applications in ferroelectric memory and negative capacitance transistors.

Complex oxides with tunable structures have many fascinating properties, though high-quality complex oxide epitaxy with precisely controlled composition is still out of reach. Here we have successfully developed solution-based single-crystalline epitaxy for multiferroic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3–(x)CaTiO3 (BTFM–CTO) solid solution in large area, confirming its ferroelectricity at the atomic scale with strong spontaneous polarization. Careful compositional tuning leads to a bulk magnetization of 0.07 ± 0.035 μB/Fe at room temperature, enabling magnetically induced polarization switching exhibiting a large magnetoelectric coefficient of 2.7–3.0 × 10−7 s/m. This work demonstrates the great potential of solution processing in large-scale complex oxide epitaxy and establishes novel room-temperature magnetoelectric coupling in epitaxial BTFM–CTO film, making it possible to explore a much wider space of composition, phase, and structure that can be easily scaled up for industrial applications.

Achieving high power conversion efficiencies (PCEs) in ferroelectric photovoltaics (PVs) is a longstanding challenge. Although recently ferroelectric thick films, composite films, and bulk crystals have all been demonstrated to exhibit PCEs >1%, these systems still suffer from severe recombination because of the fundamentally low conductivities of ferroelectrics. Further improvement of PCEs may therefore rely on thickness reduction if the reduced recombination could overcompensate for the loss in light absorption. Here, a PCE of up to 2.49% (under 365-nm ultraviolet illumination) was demonstrated in a 12-nm Pb(Zr0.2Ti0.8)O3 (PZT) ultrathin film. The strategy to realize such a high PCE consists of reducing the film thickness to be comparable with the depletion width, which can simultaneously suppress recombination and lower the series resistance. The basis of our strategy lies in the fact that the PV effect originates from the interfacial Schottky barriers, which is revealed by measuring and modeling the thickness-dependent PV characteristics. In addition, the Schottky barrier parameters (particularly the depletion width) are evaluated by investigating the thickness-dependent ferroelectric, dielectric and conduction properties. Our study therefore provides an effective strategy to obtain high-efficiency ferroelectric PVs and demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. An approach to boost the power conversion efficiencies (PCEs) of ferroelectric photovoltaics (PVs) is proposed based on the Schottky barrier effect. This approach leverages the thinning of a ferroelectric film to somewhere close to the depletion width, which can simultaneously suppress the recombination and lower the series resistance. Using this approach, we achieve a PCE up to 2.49% (under 365-nm ultraviolet illumination) in the 12-nm Pb(Zr0.2Ti0.8)O3 ultrathin films. Our study provides insightful guidance on how to design and tailor the ferroelectric films to achieve high PCEs, and also demonstrates the great potential of ferroelectrics for use in ultrathin-film PV devices. Eliminating stray electrical effects in ultra-thin films can help optimize an unconventional solar energy technology. Ferroelectric materials have internal dipoles that spontaneously move photogenerated charges toward external circuits, producing higher power outputs than predicted by theory. Zhen Fan from South China Normal University in Guangzhou and colleagues now report that the dimensions of ferroelectric thin films distinctly affect how efficiently they convert sunlight into electricity. Measurements of solar cells containing lead-zirconium-titanate ferroelectrics with different thicknesses revealed a jump in conversion efficiencies when the film reached a thickness of 12 nanometers. Further analysis showed that this thickness correlates with the solar cell’s ‘depletion width’, a zone formed when metal electrodes contact the film. The electric field in the depletion zone complements the pushing actions of the ferroelectric dipoles, lowering electrical losses compared to thicker ferroelectric films.

A physical understanding of the nature of spatially modulated phases (SMPs) in rare-earth-doped antiferrodistortive (AFD) multiferroics and how they behave close to surfaces and interfaces is lacking. Here the emergence of the antiferroelectric (AFE), ferroelectric (FE), or ferrielectric (AFE-FE) spatial modulation in the vicinity of the morphotropic phase transition in ${\mathrm{La}}_{x}{\mathrm{Bi}}_{1--x}\mathrm{Fe}{\mathrm{O}}_{3}$ ($x\ensuremath{\sim}0.2$) is explored on the atomic level using high-resolution scanning transmission electron microscopy (HRSTEM). The suppression, or ``melting,'' of the AFE-type SMP in the vicinity of the AFD twin wall/surface junction is revealed by HRSTEM in ${\mathrm{La}}_{0.22}{\mathrm{Bi}}_{0.78}\mathrm{Fe}{\mathrm{O}}_{3}$ films and explained by the hybrid approach combining Landau-Ginzburg-Devonshire (LGD) phenomenology and the semimicroscopic four-sublattice model (FSM). The LGD-FSM approach reduces the problem of AFE (or AFE-FE) SMP emergence and stability to the thermodynamic analysis of the free-energy functional with AFE, FE, and AFD long-range order parameters and two master parameters: the FE-AFE coupling strength between four neighboring A sites and the nonstoichiometry factor, which are proportional to the variations of La concentration in ${\mathrm{La}}_{x}{\mathrm{Bi}}_{1--x}\mathrm{Fe}{\mathrm{O}}_{3}$ films. We establish that the surface-induced melting of SMPs and the associated broadening of AFE AFD domain walls minimize the film free energy under certain conditions imposed on the master parameters and gradient energy below the critical value. The observed behavior provides insight into the origin of SMPs in AFD multiferroics.

Coupling effects among mechanical, electrical and magnetic parameters in a material provide exciting opportunity for creating novel device functionalities. Here, bilayer composite films combining magnetic shape memory alloy Ni-Mn-Ga (NMG) and antiferroelectric (AFE) Pb0.97La0.02Zr0.95Ti0.05O3 (PLZT) have been fabricated. X-ray diffraction, magnetization, and magnetic domain measurements demonstrated an austenite-martensite transformation of NMG top-layer during the annealing. Meanwhile, an AFE to ferroelectric transition of the PLZT sub-layer is observed, which can be attributed to the interfacial stress coupling. The quantitative residual stress analysis revealed that the change of residual stress in the PLZT layer is up to 300 MPa during the annealing, confirming the ferroelastic stress transfer between the NMG and PLZT layers. These results open up a new pathway for interfacial elastic coupling driven multi-functionalities in composite films.

A novel two-step nano-patterning method is proposed to fabricate epitaxial ferroelectric BiFeO 3 (BFO) nanoring array, which maintains well-epitaxial structure and possesses strong ferroelectricity demonstrated by X-ray diffraction (XRD) and piezoresponse force microscopy (PFM). The ferroelectric polarizations were examined by PFM, revealing the reversible switching behavior under an electric field. This novel method could also be extended to other oxide material systems. The fabrication of high quality ferroelectric nanoring structure provides the possibility to explore novel functionalities (e.g., ferroelectric vortices) and offers application potentials for the high-density non-volatile memory devices.

A strain-driven orthorhombic (O) to rhombohedral (R) phase transition is reported in La-doped BiFeO3 thin films on silicon substrates. Biaxial compressive epitaxial strain is found to stabilize the rhombohedral phase at La concentrations beyond the morphotropic phase boundary (MPB). By tailoring the residual strain with film thickness, we demonstrate a mixed O/R phase structure consisting of O phase domains measuring tens of nanometers wide within a predominant R phase matrix. A combination of piezoresponse force microscopy (PFM), transmission electron microscopy (TEM), polarization–electric field hysteresis loop (P–E loop), and polarization maps reveal that the O-R structural change is an antiferroelectric to ferroelectric (AFE-FE) phase transition. Using scanning transmission electron microscopy (STEM), an atomically sharp O/R MPB is observed. Moreover, X-ray absorption spectra (XAS) and X-ray linear dichroism (XLD) measurements reveal a change in the antiferromagnetic axis orientation from out of plane...

Negative differential resistance (NDR) has been extensively investigated for its wide device applications. However, a major barrier ahead is the low reliability. To address the reliability issues, we consider ferroelectrics and propose an alternative mechanism for realizing the NDR with deterministic current peak positions, in which the NDR results from the polarization switching-induced charge injection and subsequent charge trapping at the metal/ferroelectric interface. In this work, ferroelectric Au/BiFe0.6Ga0.4O3 (BFGO)/Ca0.96Ce0.04MnO3 (CCMO) nanocapacitors are prepared, and their ferroelectricity and NDR behaviors are studied concurrently. It is observed that the NDR current peaks are located at the vicinity of coercive voltages (Vc) of the ferroelectric nanocapacitors, thus evidencing the proposed mechanism. In addition, the NDR effect is reproducible and robust with good endurance and long retention time. This study therefore demonstrates a ferroelectric-based NDR device, which may facilitate the ...

Recent research progress in (K,Na)NbO3 (KNN)-based lead-free piezoelectric ceramics has attracted increasing attention for their applications to microsystems or microelectromechanical systems (MEMS) in the form of thin films. This work demonstrates that high-quality KNN-based epitaxial films can be synthesized by a conventional sol–gel method, whose phase structure and domain characteristics have been investigated with emphasis on the temperature effect. A monoclinic MC structure is observed at room temperature in KNN-based epitaxial films, which is close to but different from the orthorhombic phase in bulk counterparts. Piezoresponse force microscopy (PFM) at elevated temperatures reveals continuous changes of ferroelectric domains in KNN films during heating and cooling cycles between room temperature and 190 °C. A distinct change in domain morphology is observed upon heating to 110 °C, accompanied by a clear variation of dielectric permittivity suggesting a thermotropic phase transition, which is revea...

$R\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ is of critical concern when a memory device stores data. While ferroelectric diodes continue to emerge as a promising memory technology, their retention behavior and underlying physics have not been well investigated. This paper proposes a theoretical model for retention loss in ferroelectric diodes, combining polarization relaxation kinetics and polarization-controlled Schottky emission. The model describes well the behavior of metal/BiFeO${}_{3}$/La${}_{0.7}$Sr${}_{0.3}$MnO${}_{3}$ diodes, and explains the dependences of retention-loss rate on BiFeO${}_{3}$ thickness and electrode screening. These findings should help to guide the development of reliable ferroelectric memory devices.

Binary ferroelectric materials such as hafnium oxide have been intensively studied, which are expected to exhibit robust ferroelectricity comparable to the perovskite-based ferroelectrics at nanoscale. Here, using the combination of X-ray diffraction (XRD) associate with Transmission Electron Microscope (TEM), we reveal the epitaxial growth of Al-doped HfO2 possessing various phase structures on different oriented SrTiO3 substrates. The well-oriented films show different ferroelectricity indicated by piezoresponse force microscopy (PFM). The film grown on the (111) SrTiO3 substrate shows the largest electromechanical effect. These results, with the analyses of structural characterizations, demonstrate the orientation control of phase transitions and ferroelectricity in Al-doped HfO2 thin films.

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 Å ZOCL, the 2P r 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.

We report a systematic study on the magnetic, dielectric, and magnetoelectric (ME) properties of the Y-type hexaferrite (Ba0.4Sr1.6)Zn2(Fe11.3Al0.7)O22 single crystal. The phase diagrams were established by the observed magnetic anomalies under magnetic field and temperature scans for the in-plane and out-of-plane cases. Intrinsic large anisotropy in the magnetodielectric and converse ME effects was revealed. The spin-driven electric polarizations induced in the ab-plane and out-of-plane were found to be closely related to the transverse conical and alternating longitudinal conical spin structures, respectively. The amplitudes of varied magnetization ΔM are about ∼0.128 μB/f.u and ∼0.0178 μB/f.u. in the E oscillating between ±1 MV/m at 100 K, corresponding to the converse ME effect coefficient of ∼3500 ps/m and ∼480 ps/m for the in-plane and out-of-plane cases, respectively. All these results demonstrate the essential and unique spin-order-induced anisotropic ferroelectricity and ME properties in these Y-type hexaferrites.

SrTiO3 remains at the core of research on oxide electronics, owing to its fascinating properties and wide applications as a commercial substrate. Heterojunctions based on Nb-doped SrTiO3 (NSTO), including both metal/NSTO Schottky junctions (MSJs) and NSTO-based ferroelectric tunnel junctions (FTJs) have received considerable attention due to the colossal electroresistance (CER) effect. However, the mechanism underpinning the CER effect is still poorly understood. Here, we conduct a comparative study on the CER effects in Au/NSTO MSJs and Au/BaTiO3/NSTO FTJs. The two types of heterojunctions show many similarities in resistive switching characteristics, including hysteretic current–voltage curves with asymmetric shapes, absence of critical switching fields, switching times on the scale of ∼1.0 μs, and resistance relaxations of the Curie–von Schweidler type. These results suggest that the CER effects in the MSJs and FTJs may have a common origin, i.e., charge trapping/detrapping, as further revealed by scanning Kelvin probe microscopy. Using temperature-dependent current–voltage, capacitance–voltage, and photo-response measurements, we demonstrate that charge trapping/detrapping could modify both the Schottky barrier profile and the tunneling process, and in turn lead to different transport mechanisms in different voltage regimes. The charge trapping/detrapping-induced CER effect can be well described by a metal–insulator–semiconductor (MIS) model, which reproduces the hysteretic current–voltage curves fairly well over a large range of voltage sweeping and thus provides a unified framework for the CER effects in certain NSTO-based heterojunctions.

We propose a novel and facile method to fabricate epitaxial ferroelectric BiFeO3 (BFO) nanorings using anodized alumina (AAO) template-assisted PLD and ion-beam etching. The morphology and dimensions of these nanorings are revealed by atomic force microscopy (AFM) and confirmed by transmission electron microscopy (TEM). The typical dimensions of the BFO nanorings are 12 nm inner diameter with 30 nm wall thickness and heights of around 10 nm. The X-ray diffraction (XRD), reciprocal space mapping (RSM) and TEM data demonstrate the epitaxial structure of these nanorings. Moreover, the ferroelectric properties are investigated by piezoresponse force microscopy (PFM), showing the polarization reversal behaviors of the isolated BFO nanorings, which have great potential applications for high-density non-volatile memory devices. In addition, this novel method could also be extended to other material systems (such as BaTiO3, ZnO and Au). Furthermore, the fabrication of high quality nanoring structures may open a pathway to explore related emerging physical phenomena (e.g., ferroelectric vortices).

Ferroelectric nanocapacitors with simultaneously tunable resistance and photovoltaic effect have great potential for realizing high-density non-volatile memories and multifunctional opto-electronic nanodevices. Here, using a polystyrene sphere template method, we developed well-ordered Au nanoelectrode arrays on super-tetragonal BiFeO3 (T-BFO)/La0.7Sr0.3MnO3 (LSMO) epitaxial thin films, forming Au/T-BFO/LSMO nanocapacitors. The nanocapacitors exhibited switchable resistance states and photovoltaic responses, controllable by the ferroelectric polarization of T-BFO. Owing to the giant polarization of T-BFO, both giant electroresistance (ON/OFF current ratio >20 000) and noticeable photovoltage (∼0.4 V) were achieved in the Au/T-BFO/LSMO nanocapacitors. These results demonstrate that the T-BFO-based nanocapacitors are promising for applications in high-density memories with multiple routes for non-destructive readout, as well as other multifunctional nanodevices.

In recent years, there is a surge of research interest in exotic ferroelectric topological states, motivated by their rich emerging physical properties and potential applications in nanoelectronic devices. Here, we demonstrate the observation of a sort of complex center-type topological domain structures, which exhibit a quadrant center-type (with polarization pointing to the center) topological texture for an in-plane polarization component and a cylinder domain pattern along the vertical direction, in rhombohedral structured Pb(Zr0.7Ti0.3)O3 (R-PZT) nanoislands. Such a center domain state exhibits a rather high stability, which can well maintain its topological texture after heating to above curie temperature and subsequently cooling down to room temperature. Moreover, it allows erasure by a scanning electric field, yet it can also be recovered by a similar heating and cooling process. The observation of these unique topological textures in R-PZT nanoislands might provide a good playground for further exploring their topological phase transition properties, emerging novel functionalities, and application potential.

Ferroelectric-based electronic devices have excellent low-energy characteristics due to the highly insulating property, in which the Joule heating can be neglected. The recent discovery of electrically switchable cracks in ferroelectric/alloy film heterostructure gave rise to a new way to construct transistor, where the opening and closing of crack switched the channel current off and on. Here, we observed the complementary switching of cracks for the first time, and demonstrated a complementary inverter without any additional process to set different types of transistors, which was implemented by forming the n- and p-type transistor in conventional complementary metal oxide semiconductor (CMOS) technology. The complementary states were generated spontaneously once the cracks were induced and varied with the change of applied input voltage polarity. The low ON resistance and near-zero OFF state leakage current result in the high current on/off ratio (~107) and allow for the low dynamic and static power dissipation. Further, the switching of cracks is coupled to the surrounding ferroelectric polarization states, offering the non-destructive readout operation. We believe that our work provides a very simple route to build complementary logic gates and paves a way for the energy-efficient electronic devices, while promoting the “crack nanoelectronics”.

Defect engineering has been a powerful tool to enable the creation of exotic phases and the discovery of intriguing phenomena in ferroelectric oxides. However, accurate control the concentration of defects remains a big challenge. In this work, ion implantation, that can provide controllable point defects, allows us the ability to produce a controlled defect-driven true super-tetragonal (T) phase with enhanced tetragonality in ferroelectric BiFeO3 thin films. This point defect engineering is found to drive the phase transition from the as-grown mixed rhombohedral-like (R) and tetragonal-like (MC) phase to true tetragonal (T) symmetry. By further increasing the injected dose of He ion, we demonstrate an enhanced tetragonality super-tetragonal (super-T) phase with the largest c/a ratio (~ 1.3) that has ever been experimentally achieved in BiFeO3. A combination of morphology change and domain evolution further confirm that the mixed R/MC phase structure transforms to the single-domain-state true tetragonal phase. Moreover, the re-emergence of R phase and in-plane stripe nanodomains after heat treatment reveal the memory effect and reversible phase transition. Our findings demonstrate the control of R-Mc-T-super T symmetry changes and the creation of true T phase BiFeO3 with enhanced tetragonality through controllable defect engineering. This work also provides a pathway to generate large tetragonality (or c/a ratio) that could be extended to other ferroelectric material systems (such as PbTiO3, BaTiO3 and HfO2) which may lead to strong polarization enhancement.

Epitaxial strain, imparted by an underlying substrate, is a powerful pathway to drive phase transitions and dramatically alter properties in oxide heterostructures, enabling the emergence of new ground states and the enhancement of ferroelectricity, piezoelectricity, superconductivity and ferromagnetism. However, the limitation of commercially available single-crystal substrates and the lack of continuous strain tunability preclude the ability to take full advantage of strain engineering for further exploring novel properties and exhaustively studying fundamental physics in complex oxides. Here we report an approach for imposing continuously tunable, large epitaxial strain in oxide heterostructures beyond substrate limitations by inserting an interface layer through tailoring its gradual strain relaxation. Taking BiFeO3 as a model system, we demonstrate that the introduction of an ultrathin interface layer allows the creation of a desired strain that can induce phase transition and stabilize a new metastable super-tetragonal phase as well as morphotropic phase boundaries overcoming substrate limitations. Furthermore, continuously tunable strain from tension to compression can be generated by precisely adjusting the thickness of the interface layer, leading to the first achievement of continuous O-R-T phase transition in BiFeO3 on a single substrate. This proposed route could be extended to other oxide heterostructures, providing a platform for creating exotic phases and emergent phenomena.

Coupling effects among spin, charge, and lattice in a strongly correlated system are critical for next generation spintronic and data storage devices. However, the complex effects are elusive and difficult to distinguish their contributions to polarization modulation. Here we tailored the polarization by co-doping of non-magnetic Y and Eu at A-sites in DyMn2O5. The structure, specific heat, magnetism, and ferroelectricity of the polycrystalline Dy1-x(Eu0.24Y0.76)xMn2O5 ceramics were comprehensively explored. Interestingly, the co-doping does not cause lattice distortion of DyMn2O5, and all the ceramics are orthorhombic structures, while the independent Dy3+ spin order and the Dy3+-Mn3+ coupling can be suppressed. With increasing the co-doping content x, the spins related properties associated with the Dy3+-Mn4+-Dy3+ sub-lattice are progressively inhibited, while they keep less disturbance in the Mn3+-Mn4+-Mn3+ block. Moreover, the spin coupling of Dy3+-Mn3+ ions is stronger again the magnetic field than that of Dy3+-Mn3+. Our results enhance the understanding of ferrielectricity in DyMn2O5, and provide a method for controlling the polarization in the multiferroic manganite coexisting 3d and 4f elements.

Conductive ferroelectric domain walls--ultra-narrow and configurable conduction paths, have been considered as essential building blocks for future programmable domain wall electronics. For applications in high density devices, it is imperative to explore the conductive domain walls in small confined systems while earlier investigations have hitherto focused on thin films or bulk single crystals, noting that the size-confined effects will certainly modulate seriously the domain structure and wall transport. Here, we demonstrate an observation and manipulation of conductive domain walls confined within small BiFeO3 nano-islands aligned in high density arrays. Using conductive atomic force microscopy (CAFM), we are able to distinctly visualize various types of conductive domain walls, including the head-to-head charged walls (CDWs), zigzag walls (zigzag-DWs), and typical 71{\deg} head-to-tail neutral walls (NDWs). The CDWs exhibit remarkably enhanced metallic conductivity with current of ~ nA order in magnitude and 104 times larger than that inside domains (0.01 ~ 0.1 pA), while the semiconducting NDWs allow also much smaller current ~ 10 pA than the CDWs. The substantially difference in conductivity for dissimilar walls enables additional manipulations of various wall conduction states for individual addressable nano-islands via electrically tuning of their domain structures. A controllable writing of four distinctive states by applying various scanning bias voltages is achieved, offering opportunities for developing multilevel high density memories.

There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in the bulk. For example, 180° ferroelectric domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors, including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180° stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180° stripe domains with {112} domain walls were observed in films

We report a facile and cost-competitive nanopatterning route, using Ar ion beam etching through a monolayer polystyrene sphere (PS) array placed on a ferroelectric epitaxial thin film, to fabricate ordered ferroelectric nanodot arrays. Using this method, well-ordered BiFeO3 epitaxial nanodots, with tunable sizes from ∼100 to ∼900 nm in diameter, have been successfully synthesized. Interestingly, a plethora of exotic nanodomain structures, e.g., stripe domains, vortex and antivortex domains, and single domains, are observed in these nanodots. Moreover, this novel technique has been extended to produce Pb(Zr,Ti)O3 nanodots and multiferroic composite Co/BiFeO3 nanodots. These observations enable the creation of exotic domain structures and provide a wide range of application potentials for future nanoelectronic devices.

Robust and reversible polar topological center domains were found in BiFeO3 nanodots, which are individually controllable., The exotic topological domains in ferroelectrics and multiferroics have attracted extensive interest in recent years due to their novel functionalities and potential applications in nanoelectronic devices. One of the key challenges for these applications is a realization of robust yet reversibly switchable nanoscale topological domain states with high density, wherein spontaneous topological structures can be individually addressed and controlled. This has been accomplished in our work using high-density arrays of epitaxial BiFeO3 (BFO) ferroelectric nanodots with a lateral size as small as ~60 nm. We demonstrate various types of spontaneous topological domain structures, including center-convergent domains, center-divergent domains, and double-center domains, which are stable over sufficiently long time but can be manipulated and reversibly switched by electric field. The formation mechanisms of these topological domain states, assisted by the accumulation of compensating charges on the surface, have also been revealed. These results demonstrated that these reversibly switchable topological domain arrays are promising for applications in high-density nanoferroelectric devices such as nonvolatile memories.

Ferroelectric photovoltaic materials have attracted intensive interest due to their intriguing above-bandgap photovoltage, while the low photocurrent limits their further device applications. In this work, both enhanced and tunable photovoltaic effect (PVE) are demonstrated in sandwiched structure of Graphene/tetragonal-like BiFeO3/Ca0.96Ce0.04MnO3 (Graphene/T(-like) BFO/CCMO). The epitaxial BFO film is grown on the CCMO buffered LaAlO3 substrate by pulsed laser deposition and the mechanically exfoliated graphene is transferred directly onto the BFO film as top electrode. The optimized open circuit voltage (Voc) and short circuit current density (Jsc) are measured to be −0.88 V and 2.56 mA/cm2, respectively. Moreover, the photovoltaic response can be modulated by controllable ferroelectric polarization, whereby both the Voc and Jsc show piezoresponse-like hysteresis behaviors against voltage. The notably enhanced PVE is likely a result of the combination effects of large polarization in the T(-like) BFO film, partially unscreened depolarization field, high transmittance and conductivity of the graphene top electrode. This work clearly indicates the potential of Graphene/ferroelectric photovoltaic devices for memory and energy conversion applications.

Recent advances in scanning transmission electron microscopy (STEM) have enabled direct visualization of the atomic structure of ferroic materials, enabling the determination of atomic column positions with ~pm precision. This, in turn, enabled direct mapping of ferroelectric and ferroelastic order parameter fields via the top-down approach, where the atomic coordinates are directly mapped on the mesoscopic order parameters. Here, we explore the alternative bottom-up approach, where the atomic coordinates derived from the STEM image are used to explore the extant atomic displacement patterns in the material and build the collection of the building blocks for the distorted lattice. This approach is illustrated for the La-doped BiFeO3 system.The full analysis procedure is available as an interactive paper in a form of a Google Colab (Jupyter) notebook where a classical paper organization is augmented with code cells that appear hidden by default (when viewed in Google Colab). This should allow a reader to retrace the analysis and, more importantly, it enables the readers to use the same codes for their data. The same paper is also available in a standard pdf format (without code).

Ferroelectric diodes are promising for fast, low-power nonvolatile memory, but a major roadblock to this technology is a lack of reliability and reproducibility, the origins of which remain poorly understood. Through experiment and modeling, the authors show that the electroresistance ratio, resistive-switching polarity, and high- and low-resistance states all can be significantly affected by interfacial charge injection and trapping in such a device. This work carries important implications for the development of reliable resistive-switching memory.

The polarization imprint along with the photovoltaic (PV) effect has been studied in Pt/Pb(Zr0.3Ti0.7)O3/SrRuO3 ferroelectric capacitors. It is shown that the positive DC poling induces the imprint with a downward direction whereas the negative DC poling suppresses the imprint (i.e., rejuvenation). In the polarization up state, the imprinted capacitor exhibits degraded PV properties compared with the rejuvenated one. This may be because the imprint reduces the number of upward domains, thus lowering the driving force for the PV effect. In the polarization down state, however, the rejuvenated capacitor enters the imprinted state spontaneously. This rejuvenation-to-imprint transition can be further aggravated by applying positive voltages and ultraviolet illumination. It is proposed that the domain pinning/depinning, which are associated with the oxygen vacancies and trapped electrons modulated by polarization, voltage, and illumination, may be responsible for the polarization imprint and rejuvenation. Our ...

We have comparatively studied the dielectric, ferroelectric, conduction, and photovoltaic properties of Ti/BaTiO3 (BTO)/SrRuO3 (SRO) and Pt/BTO/SRO capacitors. The resistive switching (RS) is observed in the Pt/BTO/SRO capacitor while it is absent in the Ti/BTO/SRO capacitor, which may be attributed to the interfacial layer existing between Pt and BTO and the Ti/BTO Ohmic interface, respectively. Further analyses on the conduction mechanisms suggest that the RS may be caused by the opening/closing of conduction paths in the Pt/BTO interfacial layer, whereas the polarization is ruled out as the origin of RS because of the inconsistency between the RS switching voltages and coercive voltages. On the other hand, it is observed that the photovoltaic effects (PVEs) in both Ti/BTO/SRO and Pt/BTO/SRO capacitors are electrically unswitchable and the open-circuit voltages of the two capacitors are similar in magnitude, implying that the PVE is driven by an internal bias field rather than the polarization-induced f...

Efficient and cost-competitive fabrication of high-quality ferroelectric and multiferroic nanostructures is of general interest. In this work, a top-down nano-patterning technique is developed by the Ar+ ion beam etching in combination with the sacrificed ultrathin anodic alumina (AAO) mask. This technique is demonstrated by preparation of the epitaxial BiFeO3 (BFO) nanostructures of various geometries, including nanodot and anti-nanodot arrays. The lateral dot size is as small as ∼60 nm and an ultrahigh dot density of ∼60 Gbit/inch2 is achieved. It is revealed that the etching process involves sequential shape evolution of both the AAO mask and the underlying BFO film, resulting in the nanodots and anti-nanodots arrays of various geometries. The as-etched BFO nanodots array exhibits well-established ferroelectric domain structures and reversible polarization switching, as examined by piezoresponse force microscopy (PFM). It is suggested that this technique is extendable to fabrication of a wide range of functional oxide nanostructures for potential nanoelectronic applications.