Conventional Ferroelectric Materials Research Articles

Tunnel junctions based on interfacial two dimensional ferroelectrics

Van der Waals heterostructures have opened new opportunities to develop atomically thin (opto)electronic devices with a wide range of functionalities. The recent focus on manipulating the interlayer twist angle has led to the observation of out-of-plane room temperature ferroelectricity in twisted rhombohedral bilayers of transition metal dichalcogenides. Here we explore the switching behaviour of sliding ferroelectricity using scanning probe microscopy domain mapping and tunnelling transport measurements. We observe well-pronounced ambipolar switching behaviour in ferroelectric tunnelling junctions with composite ferroelectric/non-polar insulator barriers and support our experimental results with complementary theoretical modelling. Furthermore, we show that the switching behaviour is strongly influenced by the underlying domain structure, allowing the fabrication of diverse ferroelectric tunnelling junction devices with various functionalities. We show that to observe the polarisation reversal, at least one partial dislocation must be present in the device area. This behaviour is drastically different from that of conventional ferroelectric materials, and its understanding is an important milestone for the future development of optoelectronic devices based on sliding ferroelectricity.

Engineering interfacial polarization switching in van der Waals multilayers.

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here we show that deliberate interlayer rotations in multilayer van der Waals heterostructures modulate both the spatial ordering and switching dynamics of polar domains. The engendered tunability is unparalleled in conventional bulk ferroelectrics or polar bilayers. By means of operando transmission electron microscopy we show how alterations of the relative rotations of three WSe2 layers produce structural polytypes with distinct arrangements of polar domains with either a global or localized switching response. Furthermore, the presence of uniaxial strain generates structural anisotropy that yields a range of switching behaviours, coercivities and even tunable biased responses. We also provide evidence of mechanical coupling between the two interfaces of the trilayer, a key consideration for the control of switching dynamics in polar multilayer structures more broadly.

Mild chemistry synthesis of ultrathin Bi2O2S nanosheets exhibiting 2D-ferroelectricity at room temperature.

Modern technology demands miniaturization of electronic components to build small, light, and portable devices. Hence, discovery and synthesis of new non-toxic, low cost, ultra-thin ferroelectric materials having potential applications in various electronic and optoelectronic devices are of paramount importance. However, achieving room-temperature ferroelectricity in two dimensional (2D) ultra-thin systems remains a major challenge as conventional three-dimensional ferroelectric materials lose their ferroelectricity when the thickness is brought down below a critical value owing to the depolarization field. Herein, we report room-temperature ferroelectricity in ultra-thin single-crystalline 2D nanosheets of Bi2O2S synthesized by a simple, rapid, and scalable solution-based soft chemistry method. The ferroelectric ground state of Bi2O2S nanosheets is confirmed by temperature-dependent dielectric measurements as well as piezoelectric force microscopy and spectroscopy. High resolution transmission electron microscopy analysis and density functional theory-based calculations suggest that the ferroelectricity in Bi2O2S nanosheets arises due to the local distortion of Bi2O2 layers, which destroys the local inversion symmetry of Bi2O2S.

High, Multiple, and Nonvolatile Polarizations in Organic-Inorganic Hybrid [(CH3)3(CH2CH2Cl)N]2InCl5·H2O for Memcapacitor.

Dielectrics with high, nonvolatile, and multiple polarizations are required for fabricating memcapacitors that enable high parallelism and low energy consumption in artificial neuromorphic computing systems as artificial synapses. Conventional ferroelectric materials based on displacive and order-disorder types generally have difficulty meeting these requirements due to their low polarization values (∼150 μC/cm2) and persistent electrical hysteresis loops. In this study, we report a novel organic-inorganic hybrid (CETM)2InCl5·H2O (CETM = (CH3)3(CH2CH2Cl)N) exhibiting an intriguing polarization vs electric field (charge vs voltage) "hysteresis loop" and a record-high nonvolatile polarization over 30 000 μC/cm2 at room temperature. The polarization is highly dependent on the period and amplitude of the ac voltage, showing multiple nonvolatile states. Electrochemical impedance spectroscopy, time-dependent current behavior, disparate resistor response in the dehydrated derivative (CETM)2InCl5, and the negative temperature dependence of ionic conductance support that the memcapacitor behavior of (CETM)2InCl5·H2O stems from irreversible long-range migration of protons. First-principles calculations further confirm this and clarify the microscale mechanism of anisotropic polarization response. Our findings may open up a new avenue for developing memcapacitors by harnessing the benefits of ion migration in organic-inorganic hybrids.

Strain as a Global Factor in Stabilizing the Ferroelectric Properties of ZrO2

AbstractSince the discovery of ferroelectricity in doped HfO2 and ZrO2 thin films over a decade ago, fluorite‐structured ferroelectric thin films have attracted much research attention due to their excellent scalability and complementary metal‐oxide semiconductor compatibility compared to conventional perovskite ferroelectric materials. Although various factors influencing the formation of the ferroelectric properties are identified, a clear understanding of the causes of the phase formation have been difficult to determine. In this work, ZrO2 films deposited by atomic layer deposition and chemical solution deposition have resulted in films with completely different structural properties. Regardless of these differences, a general relationship between strain and phase formation is established, leading to a more unified understanding of ferroelectric phase formation in undoped ZrO2 films, which can be applied to other fluorite‐structured films.

Reversible fatigue-rejuvenation procedure and its mechanism in Hf0.5Zr0.5O2 epitaxial films

HfO2-based ferroelectrics, such as Hf0.5Zr0.5O2, arouse great attention in recent years because of their CMOS compatibility and robust nano-scale ferroelectricity. However, fatigue is one of the toughest problems for ferroelectric applications. The fatigue mechanism of HfO2-based ferroelectrics is different from conventional ferroelectric materials, and research on the fatigue mechanism in HfO2-based epitaxial films have been rarely reported. In this work, we fabricate 10 nm Hf0.5Zr0.5O2 epitaxial films and investigate the fatigue mechanism. The experimental data show that the remanent ferroelectric polarization value decreased by 50% after 108 cycles. It is worth noting that the fatigued Hf0.5Zr0.5O2 epitaxial films can be recovered through applying electric stimulus. Combined with the temperature-dependent endurance analysis, we propose that fatigue of our Hf0.5Zr0.5O2 films comes from both phase transition between ferroelectric Pca21 and antiferroelectric Pbca as well as defects generation and dipole pinned. This result offers a fundamental understanding of HfO2-based film system, and could provide an important guideline for subsequent studies and future applications.

Ca2FeOsO6/Sr2FeOsO6 superlattice: Multiferroicity above room temperature with giant hybrid-improper ferroelectric polarization and high photovoltaic efficiency

Using first-principles calculations, the structure and electronic properties of the room-temperature ferrimagnetic ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice are investigated. We show that the superlattice hosts hybrid-improper ferroelectricity despite the fact that bulk ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ realizes an ${a}^{0}{a}^{0}{c}^{\ensuremath{-}}$ tilting pattern of the O octahedra. The magnitude is comparable to that of conventional ferroelectric materials and is found to increase under both compressive and tensile strain. In contrast to competing superlattices, a ferrimagnetic critical temperature above room temperature is realized. An indirect-to-direct band-gap transition is observed between $+$1% and $+$2% strain, coming along with localization of the valence and conduction states on different transition-metal sublattices, which enables efficient electron-hole separation upon photoexcitation. The potential gradient due to the ferroelectric polarization supports the electron-hole separation and a spectroscopic limited maximum efficiency of 27% confirms excellent potential in solar cell applications. The tunable room-temperature ferroelectricity, high critical temperature of the ferrimagnetic ordering with high magnetization, and favorable photoabsorption properties of the ${\mathrm{Ca}}_{2}{\mathrm{FeOsO}}_{6}/{\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ superlattice open up a broad range of technological applications.

Highly degenerate 2D ferroelectricity in pore decorated covalent/metal organic frameworks.

Two-dimensional (2D) ferroelectricity, a fundamental concept in low-dimensional physics, serves as the basis of non-volatile information storage and various electronic devices. Conventional 2D ferroelectric (FE) materials are usually two-fold degenerate, meaning that they can only store two logical states. In order to break such limitation, a new concept of highly degenerate ferroelectricity with multiple FE states (more than 2) coexisting in a single 2D material is proposed. This is obtained through the asymmetrical decoration of porous covalent/metal organic frameworks (COFs/MOFs). Using first-principles calculations and Monte Carlo (MC) simulations, Li-decorated 2D Cr(pyz)2 is systematically explored as a prototype of highly degenerate 2D FE materials. We show that 2D FE Li0.5Cr(pyz)2 and LiCr(pyz)2 are four-fold and eight-fold degenerate, respectively, with sizable spontaneous electric polarization that can be switched across low transition barriers. In particular, the coupling between neighbouring electric dipoles in LiCr(pyz)2 induces novel ferroelectricity-controlled ferroelastic transition and direction-controllable hole transport channels. Moreover, three-fold and six-fold degenerate ferroelectricity is also demonstrated in P-decorated g-C3N4 and Ru-decorated C2N, respectively. Our work presents a general route to obtain highly degenerate 2D ferroelectricity, which goes beyond the two-state paradigm of traditional 2D FE materials and substantially broadens the applications of 2D FE compounds.

Narrow Bandgap Inorganic Ferroelectric Thin Film Materials

AbstractHarvesting solar energy to solve the energy crisis and environmental pollution is a research hotspot in the photovoltaic industry today. Conventional ferroelectric materials have a bandgap between 3.0 and 3.8 eV, and their low absorption coefficients and photocurrent densities limit the photovoltaic applications of ferroelectric materials. Inorganic ferroelectric thin film materials with narrow bandgap (1.1–2.0 eV) have high optical absorption coefficient and broad spectral response, wide bandgap tuning range (1.2–3.0 eV), as well as high photocurrent density and switchable ferroelectric photoelectric effect, which have great potential for photovoltaic and optoelectronic applications in solar cells, photodetection, photocatalysis. This review summarizes the recent developments of narrow bandgap inorganic ferroelectric thin‐film materials and aims to stimulate research interest and efforts in ferroelectric photovoltaic materials.

Two‐dimensional In2Se3: A rising advanced material for ferroelectric data storage

AbstractFerroelectric memory is a promising candidate for next‐generation nonvolatile memory owing to its outstanding performance such as low power consumption, fast speed, and high endurance. However, the ferroelectricity of conventional ferroelectric materials will be eliminated by the depolarization field when the size drops to the nanometer scale. As a result, the miniaturization of ferroelectric devices was hindered, which makes ferroelectric memory unable to keep up with the development of integrated‐circuit (IC) miniaturization. Recently, a two‐dimensional (2D) In2Se3 was reported to maintain stable ferroelectricity at the ultrathin scale, which is expected to break through the bottleneck of miniaturization. Soon, devices based on 2D In2Se3, including the ferroelectric field‐effect transistor, ferroelectric channel transistor, synaptic ferroelectric semiconductor junction, and ferroelectric memristor were demonstrated. However, a comprehensive understanding of the structures and the ferroelectric‐switching mechanism of 2D In2Se3 is still lacking. Here, the atomic structures of different phases, the dynamic mechanism of ferroelectric switching, and the performance/functions of the latest devices of 2D In2Se3 are reviewed. Furthermore, the correlations among the structures, the properties, and the device performance are analyzed. Finally, several crucial problems or challenges and possible research directions are put forward. We hope that this review paper can provide timely knowledge and help for the research community to develop 2D In2Se3 based ferroelectric memory and computing technology for practical industrial applications.image

Field cycling behavior and breakdown mechanism of ferroelectric Al0.78Sc0.22N films

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.

Van der Waals force-induced intralayer ferroelectric-to-antiferroelectric transition via interlayer sliding in bilayer group-IV monochalcogenides

Two-dimensional materials with ferroelectric properties break the size effect of conventional ferroelectric materials and unlock unprecedented potentials of ferroelectric-related application at small length scales. Using first-principles calculations, a sliding-induced ferroelectric-to-antiferroelectric behavior in bilayer group-IV monochalcogenides (MX, with M = Ge, Sn and X = S, Se) is discovered. Upon this mechanism, the top layer exhibits a reversible intralayer ferroelectric switching, leading to a reversible transition between the ferroelectric and antiferroelectric states in the bilayer MXs. Further results show that the interlayer van der Waals interaction, which is usually considered to be weak, can actually generate an in-plane lattice distortion and thus cause the breaking/forming of intralayer covalent bonds in the top layer, leading to the observed anomalous phenomenon. This unique property has advantages for energy harvesting over existing piezoelectric and triboelectric nanogenerators. The interlayer sliding-induced big polarization change (40 μC cm−2) and ultrahigh polarization changing rate generate an open-circuit voltage two orders of magnitude higher than that of MoS2-based nanogenerators. The theoretical prediction of power output for this bilayer MXs at a moderate sliding speed 1 m s−1 is four orders of magnitude higher than the MoS2 nanogenerator, indicating great potentials in energy harvesting applications.

Screening transition metal-based polar pentagonal monolayers with large piezoelectricity and shift current

Two-dimensional (2D) materials entirely composed of pentagon motifs are of interest for their wide applications. Here, we demonstrate that in-plane polar symmetry can exist in ternary pentagonal monolayers, where the induced electric polarization is not associated with specific conditions, such as ferroelectric phase transition, strain gradient, and layer stacking, but is an intrinsic structural property coming from the orderly arranged polar bonds. Based on the high-throughput screening method and first-principles calculations, we find eight stable 2D polar transition metal compounds with a number of intriguing properties. In particular, their piezoelectric coefficients are three orders of magnitude larger than those of 2D elemental and binary pentagonal structures, and their bulk photovaltaic shift current can reach up to 300 μA V−2, superior to that of 2D conventional ferroelectric materials such as GeS. Our identified pentagonal monolayers not only expand the family of 2D pyroelectric materials, but also hold potential for energy conversions.

Identifying the Transition Order in an Artificial Ferroelectric van der Waals Heterostructure.

Two-dimensional semiconducting ferroelectrics can enable new technology for low-energy electronic switching. However, conventional ferroelectric materials are usually electrically insulating and suffer from severe depolarization effects when downscaled to atomic thickness. Following recent work, we show that robust ferroelectricity can be obtained from nonferroelectric semiconducting 2H-WSe2 by creating R-stacked bilayers with broken inversion symmetry. Here, we identify that the phase transition order of this artificial ferroelectric heterostructure is first-order, with a discontinuous jump in the order parameter across the phase transition temperature. The Curie temperature has been experimentally determined as 353 K. Using the Landau-Devonshire theory, we further determine the Curie-Weiss temperatures to be 351.2 K. We additionally demonstrate the robustness of this artificial ferroelectric material using consecutive polarization measurements, where no appreciable deterioration was detected.

Effect of ferroelectric and interface films on the tunneling electroresistance of the Al2O3/Hf0.5Zr0.5O2 based ferroelectric tunnel junctions

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.

Investigation of the wake-up process and time-dependent imprint of Hf0.5Zr0.5O2 film through the direct piezoelectric response

Ferroelectric HfO2-based thin films, which have been attracting a great deal attention because of their potential use in various applications, are known for their unique properties, such as a large time-dependent imprint and wake-up effect, which differentiate them from conventional ferroelectric materials. In this study, direct piezoelectric measurement was employed to investigate the state of polarization during the retention and wake-up process without applying an electric field. The polarization-electric field hysteresis loop of a sputtered Hf0.5Zr0.5O2 (HZO) film with a thickness of 10 nm showed a time-dependent imprint at room temperature during polarization retention, and the internal electric field that generated the imprint gradually increased from 0.05 to 0.6 MV/cm. While a space charge density of more than 1 μC/cm2 is required to form such an internal electric field, it was found that the magnitude of the direct piezoelectric response did not change at all during polarization retention. On the other hand, both the remanent polarization and direct piezoelectric response increased during the wake-up process. Based on the difference in the variation over time of these two characteristics, we concluded that the non-ferroelectric layer exists at the interface between the HZO film and TaN electrode and gradually transitions to ferroelectric phases through the electric field cycle.

Progress of two-dimensional ferroelectrics in physical and chemical applications

Ferroelectric materials are intriguing and technologically useful because the spontaneous electric polarization is electrically switchable, leading to fascinating properties and wide range of applications such as field effect transistor, non-volatile memories and sensors. In general, the emergence of ferroelectricity requires an asymmetric structure. Previous studies of ferroelectrics have mainly focused on ABO3 perovskite compounds such as BaTiO3, PbTiO3. For technological demands, the size of the devices is decreasing dramatically that conventional ferroelectric materials will suffer from many constraints at nanoscale. For example, when the thickness of ABO3 film falls below a critical value, the ferroelectricity would probably disappear. Therefore, it is essential to search for the low-dimensional materials with stable ferroelectricity. Atomically thin two-dimensional (2D) materials which exhibit abundant electronic, optical and mechanical properties would be ideal candidates and a variety of 2D ferroelectric materials have been proposed accordingly. The first part of this article reviews the recent theoretical and experimental progress of 2D ferroelectric materials. In contrast to conventional 2D materials, 2D ferroelectrics have reversible spontaneous electric polarization which would give rise to an electrostatic potential from the bottom to the top surface. The electrostatic potential difference on two surfaces would lead to distinct band edge alignments and different electron transfer when they are respectively integrated with other materials, thus changing the properties of adjacent layers. On the other hand, switching the polarization direction could change the surface’s charge density, making it possible to modulate the binding strength of adsorbed species. According to the Sabatier principle, the adsorbate-surface binding strength must be optimal to achieve the maximum efficiency. A ferroelectric surface with tunable activity can provide a platform to enhance specific catalytic performance. What’s more, the intrinsic electric field in 2D ferroelectric materials is also beneficial to separate the photo-generated carriers and inhibit their recombination, which are of great importance in photocatalytic reactions. It is worth noting that, the opposite electric polarizations in ferroelectric materials are bistable, which means the electrical switching of these two states is nonvolatile. The states will remain even the external electric field is retracted. Therefore, it is expected that this intrinsic electric field would be an effective way to achieve nonvolatile control of the properties of adjacent materials and reaction activity. To this end, in the middle part, we focus on four respects, namely the effects of ferroelectric polarization on magnetism, electronic properties, gas adsorption and photocatalytic water splitting to show the applications of 2D ferroelectric materials. Up to now, great efforts have been devoted in ferroelectric materials with out-of-plane spontaneous polarization. Compared to out-of-plane ferroelectricity, in-plane ferroelectrics have unique advantage in terms of preventing vertical perturbations. With the rapid progress of the experimental techniques, 2D ferroelectric materials would hold great potential to be applied in a broader range of applications. At last, a brief overview for 2D ferroelectric materials is presented.

Large electrostrain induced by reversible domain switching in ordered ferroelectric nanostructures with optimized geometric configurations

Large electromechanical response of ferroelectric materials is particularly appealing for applications in functional devices, such as sensors and actuators. For conventional ferroelectric materials, however, the mechanical strain under an external electric field, i.e. the electrostrain, is often limited by the intrinsic electromechanical property of the materials. Domain engineering has been suggested as a practical way to overcome this limitation and to enhance the electrostrain. Here, we show from phase-field simulations that reversible domain switching in ordered ferroelectric nanostructures with optimized geometric configurations can enhance the electrostrain significantly. In the presence of an external electric field, the domains in such nanostructures can switch from a multi-domain state confined by the geometric configurations to a mono-domain state. It is interesting that the domains can switch back to the multi-domain state due to strong internal depolarization fields once the electric field is removed. As a result, accompanying the reversible domain switching behavior, a large and reversible electrostrain can be obtained. Going further, it is found that the temperature dependence of the large electrostrain is similar to that of polarization in such nanostructures. The present work opens a perspective to obtaining large electrostrain in nanoscale ferroelectrics, which holds great promise for designing electromechanical functional devices with high performance.

Double Swing Quiescent-Current: An Experimental Detection Method of Ferroelectricity in Very Leaky Dielectric Films

Ferroelectric materials are very attractive for many technological applications. In the scenario of conventional CMOS technology, they are very suitable to fabricate nonvolatile memories (FeRAM) or as the gate stack of a transistor to realize a FeFET. Conventional ferroelectric materials, like PZT or SBT, exhibit very high permittivity and low coercive field and very thick films must be grown to obtain wide enough memory windows. Because of that, new ferroelectric materials must be investigated.Applications compatible with CMOS technology require very thin layers of dielectrics. Moreover, it is difficult to obtain new insulating materials with low conductivity in the early stages of their technological development. Therefore, ferroelectric properties can be masked by high leakage currents. This can result in the abandonment of these materials before the growing conditions are optimized. In addition, the conventional measurements of ferroelectric properties use techniques in which the electric field is varied with relatively high frequency signals that result in displacement currents that represent an additional term which is added to leakage currents and further conceals ferroelectric behavior. Traditional Sawyer-Tower techniques do not allow the detection of ferroelectricity when leakage and/or displacement currents are excessively important and even experimental values of polarization may be overestimated when affected by charge terms corresponding to the parasitic phenomena mentioned above.Basically, the double swing quiescent-current (DSQC) consists on measuring the current of the sample while varying the applied voltage as follows: a) To obtain the positive remanent polarization, we apply two identical voltage sweeps from zero to values high enough to polarize the sample. The swing rate is chosen slow enough to consider the measure process as a succession of quasi-stationary states. On the first sweep the current has three contributions: leakage, displacement and polarization itself. Since the material has already been polarized in the first sweep, the component due to ferroelectric polarization is no present in the second one. The difference between the curves of the first and second sweeps provide the curve due to the polarization current. b) Once the material is positively polarized, we repeat the same process but applying negative voltage sweeps to negatively polarize the material.To check the validity of the technique, we have performed previous tests with conventional ferroelectric materials. In Figures 1 and 2 we represent the current-voltage and charge-voltage characteristics obtained. We verify that the obtained results match the specifications for these materials.Afterwards, we have applied the DSQC method to characterize the ferroelectric behavior of amorphous SiO2-Nb2O5 nanolaminates and mixture films. The films were grown by atomic layer deposition. The films were grown at 300 oC from Nb(OC2H5)5, Si2(NHC2H5)6, and O3 to thicknesses ranging from 13 to 130 nm. The niobium to silicon atomic ratio was varied in the range of 0.11-7.20.In Figure 3 we observe a complete voltage current cycle. Starting from 0 V, the voltage increases linearly to a value of 12 volts and then decreases to -12 volts. Finally, the voltage returns to zero. A swing rate of 5 mV/s has been used to obtain this curve. In this figure you can observe the two peaks corresponding to the positive and negative polarization. It is also noted that the current is not zero when the sample is already polarized due to the existence of leakage and displacement currents.To remove these components two sweeps must be performed in each direction. In Figures 4 and 5 we present the result of applying double sweep for positive and negative voltages, respectively. The difference between the two sweeps provides the ferroelectric polarization current. We note that this current returns to zero once the sample has been polarized.In summary, we demonstrate that double swing quiescent-current technique allows to detect ferroelectric behavior even in highly conductive materials and obtain reliable values of the remaining polarization that are not affected by the parasitic effects of leakage and displacement currents. Figure 1

Stable Subloop Behavior in Ferroelectric Si-Doped HfO2.

The recent demand for analogue devices for neuromorphic applications requires modulation of multiple nonvolatile states. Ferroelectricity with multiple polarization states enables neuromorphic applications with various architectures. However, deterministic control of ferroelectric polarization states with conventional ferroelectric materials has been met with accessibility issues. Here, we report unprecedented stable accessibility with robust stability of multiple polarization states in ferroelectric HfO2. Through the combination of conventional voltage measurements, hysteresis temperature dependence analysis, piezoelectric force microscopy, first-principles calculations, and Monte Carlo simulations, we suggest that the unprecedented stability of intermediate states in ferroelectric HfO2 is due to the small critical volume size for nucleation and the large activation energy for ferroelectric dipole flipping. This work demonstrates the potential of ferroelectric HfO2 for analogue device applications enabling neuromorphic computing.