Ferroelectric Resistive Switching Research Articles

Control of ferroelectricity in Ta-doped HfO2 and its non-zero-crossing current–voltage hysteresis behavior

Hafnium oxide (HfO2)-based ferroelectrics are being explored as potential candidates for ferroelectric memory devices due to their highly compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Enhancing the remanent polarization and investigating the underlying mechanism are crucial tasks. In the present study, tantalum (Ta) was introduced as a dopant to induce ferroelectric properties in HfO2, a large portion of orthorhombic phase was recognized in the as-grown Ta:HfO2 without further thermal treatment. The remanent polarization of Ta:HfO2 thin films can be optimized by adjusting the oxygen flow rates during the sputtering process. The influencing factors for enhanced ferroelectric performance include the control of Ta concentration, its valence state, and the presence of singly ionized oxygen vacancies, which are influenced by oxygen addition. Furthermore, the resistive switching behavior showing non-zero crossing current–voltage (I–V) hysteresis is associated with ferroelectricity and the presence of oxygen vacancies. A model has been proposed to explain the ferroelectric resistive switching with non-zero crossing I–V characteristics by considering the role of oxygen vacancies and polarization effects. This model suggests that the oxygen vacancies at the surface layer, along with ferroelectric polarization, play a crucial role in electron transport.

Nonvolatile ferroelectric resistive switching in α-In2Se3(2H) ferroelectric semiconductor junctions

Emerging two-dimensional van der Waals (vdW) based ferroelectric semiconductor junctions (FSJs) are promising for low-power and high-density applications. In this work, we report the vdW α-In2Se3(2H) based FSJs of different metal-In2Se3 interfaces. All vertical devices emerge with the nonvolatile ferroelectric resistive switching behavior. The Cr/α-In2Se3(2H)/Au FSJs show a typical ferroelectric resistive switching behavior under ∼1.0 V applied voltage. The switching voltage of the Ag/α-In2Se3(2H)/Au FSJs could be further decreased from 1.0 V to 0.8 V because of Ag migrations and ferroelectric reversal. Further, by optimizing the metal-In2Se3 interfaces, the Ag/α-In2Se3(2H)/Pt FSJs exhibit the ultralow switching voltage (∼0.3 V), excellent endurance (>225 cycles) and retention (>1 × 104 s). The results suggest that vdW α-In2Se3(2H) ferroelectric has great potential for low-power memristive devices.

Domain-modified engineering for low-power resistive switching in ferroelectric diodes

Neuromorphic devices based on ferroelectric resistive switching (RS) effects are promising to simulate the information recognition and memory of the human brain. However, the high power of RS elements in crossbar arrays is still an issue, limiting the neuromorphic applications. Here, we propose a domain-modified engineering for low-power RS in ferroelectric diodes by locally introducing relaxor ferroelectric units to lower domain switching barriers. A low-power RS of ∼ 70 μW, with large OFF/ON resistance ratio and high endurance, is achieved in Au/0.8BaTiO3-0.1Ba0.7Sr0.3TiO3-0.1BaTi0.7Zr0.3O3/Pt diodes, which is about 48.5% lower than that in Au/BaTiO3/Pt diodes. The interaction between macrodomains is depressed by domain modification engineering, lowering domain switching barriers, thereby operating voltage and power are significantly modulated. Meanwhile, good nonvolatility is obtained since the remanent polarization is partially maintained by the initial macrodomains and its decrease is slowed down by the relaxor units. This work provides a strategy to lower RS power by domain modification engineering for developing memristors and neuromorphic computing devices.

Dielectric properties and ferroelectric resistive switching mechanism in the epitaxial (111) BiFeO3 films

Resistive random access memory is promising for next-generation nonvolatile memory, which has high memory density, low power consumption, and fast read/write speed. However, the resistive switching (RS) mechanism in oxide ferroelectric thin film devices is still unclear. In this paper, Pt/BiFeO3/La0.5Sr0.5CoO3(Pt/BFO/LSCO) heterostructures were deposited on (111)SrTiO3(STO) substrates by magnetron sputtering. X-ray diffraction confirmed the epitaxial relationship between (111)BFO//(111)LSCO//(111)STO. Atomic force microscopy (AFM) showed that (111)BFO had an island structure with uniform grain distribution. The Pt/(111)BFO/(111)LSCO heterostructures exhibit obvious ferroelectricity and RS effects. The results of C-V analysis and I-V fitting indicate that the RS is caused by the space-charge-limited conduction mechanism.

Schottky Barrier Control of Self-Polarization for a Colossal Ferroelectric Resistive Switching.

Controlling the domain evolution is critical both for optimizing ferroelectric properties and for designing functional electronic devices. Here we report an approach of using the Schottky barrier formed at the metal/ferroelectric interface to tailor the self-polarization states of a model ferroelectric thin film heterostructure system SrRuO3/(Bi,Sm)FeO3. Upon complementary investigations of the piezoresponse force microscopy, electric transport measurements, X-ray photoelectron/absorption spectra, and theoretical studies, we demonstrate that Sm doping changes the concentration and spatial distribution of oxygen vacancies with the tunable host Fermi level which modulates the SrRuO3/(Bi,Sm)FeO3 Schottky barrier and the depolarization field, leading to the evolution of the system from a single domain of downward polarization to polydomain states. Accompanied by such modulation on self-polarization, we further tailor the symmetry of the resistive switching behaviors and achieve a colossal on/off ratio of ∼1.1 × 106 in the corresponding SrRuO3/BiFeO3/Pt ferroelectric diodes (FDs). In addition, the present FD also exhibits a fast operation speed of ∼30 ns with a potential for sub-nanosecond and an ultralow writing current density of ∼132 A/cm2. Our studies provide a way for engineering self-polarization and reveal its strong link to the device performance, facilitating FDs as a competitive memristor candidate used for neuromorphic computing.

Research progress of double perovskite ferroelectric thin films

Double perovskite ferroelectric thin films are completely new material systems derived from single perovskite. Their diversity of composition and structure and the tendency for spontaneous atomic ordering broaden the path for the development of ferroelectric thin films. The ordered double perovskite ferroelectric thin films lead to excellent ferroelectric, dielectric, magnetic, and optical properties, promising further applications in photovoltaic cells, information memory, and spintronic and photoelectric devices, where the intrinsic coupling and tuning of multiple properties could also push it into multifunctional intersecting devices. However, complex internal physical mechanisms and difficult preparation conditions have prevented its further development. Based on ordered/disordered ferroelectric thin films of double perovskites, this paper first discusses ordered characterization methods such as superstructure reflection/diffraction peaks, especially for epitaxial thin films, saturation magnetization (macroscopic), and scanning transmission electron microscopy (microscopic). In response to the generally poor ordering of present systems, the paper also reviews the internal structure of the material and the external synthesis conditions that affect the ordering, including the valence and radii of the cations, preparation methods, element substitution and strain engineering, in the hope of triggering further research into ordered double perovskite ferroelectrics. Combined with the current state of research on existing double perovskite ferroelectricity thin film systems, advances in the fields of ferroelectric photovoltaics, magnetoelectric coupling, dielectric tunability, resistive switching, and photoelectric coupling have been presented. Finally, the challenges facing the material system are discussed and an outlook is provided for the development of the field.

Effects of temperature and DC cycling stress on resistive switching mechanisms in hafnia-based ferroelectric tunnel junction

We propose an accurate and effective method, low-frequency noise (LFN) spectroscopy, to examine the resistive switching mechanism in ferroelectric tunnel junctions (FTJs) based on pure hafnium oxide (HfOx). Contrary to previous studies that primarily focused on the ferroelectric (FE) resistive switching (RS) in HfOx-based FTJs, the results of this study demonstrate that non-FE RS affected by the redistribution of oxygen vacancies also plays a significant role in determining the performance of FTJs. LFN spectroscopy is conducted in different conditions by changing the operating temperature and inducing DC cycling stress. The results reveal that the RS mechanism changes from FE to non-FE RS with increased program bias in all conditions. This change is facilitated by the rise in temperature and the number of DC cycling stress.

Ultrathin Nitride Ferroic Memory with Large ON/OFF Ratios for Analog In-memory Computing.

Computing in the analog regime using nonlinear ferroelectric resistive memory arrays can potentially alleviate the energy constraints and complexity/footprint challenges imposed by digital von Neumann systems. Yet the current ferroelectric resistive memories suffer from either low ON/OFF ratios/imprint or limited compatibility with mainstream semiconductors. Here, for the first time, ferroelectric and analog resistive switching in an epitaxial nitride heterojunction comprising ultrathin (≈5nm) nitride ferroelectrics, i.e., ScAlN, with potentiality to bridge the gap between performance and compatibility is demonstrated. High ON/OFF ratios (up to 105 ), high uniformity, good retention, (<20% variation after >105 s) and cycling endurance (>104 ) are simultaneously demonstrated in a metal/oxide/nitride ferroelectric junction. It is further demonstrated that the memristor can provide programmability to enable multistate operation and linear analogue computing as well as image processing with high accuracy. Neural network simulations based on the weight update characteristics of the nitride memory yielded an image recognition accuracy of 92.9% (baseline 96.2%) on the images from Modified National Institute of Standards and Technology. The non-volatile multi-level programmability and analog computing capability provide first-hand and landmark evidence for constructing advanced memory/computing architectures based on emerging nitride ferroelectrics, and promote homo and hybrid integrated functional edge devices beyond silicon.

Resistive switching polarity reversal due to ferroelectrically induced phase transition at BiFeO3/Ca0.96Ce0.04MnO3 heterostructures

Ferroelectric resistive switching (RS) devices with functional oxide electrodes allow controlled emergent phenomena at an interface. Here, we demonstrate RS polarity reversal due to ferroelectrically induced phase transition at a doped charge transfer insulator interface. For BiFeO3/Ca0.96Ce0.04MnO3 bilayers grown on a NdAlO3 substrate, by applying voltages to a Ca0.96Ce0.04MnO3 bottom electrode, the resistance changes from a high resistance state (HRS) to a low resistance state (LRS) during a positive voltage cycle (0 → 3 → 0 V), and from a LRS to a HRS during a negative voltage cycle (0 → −3 → 0 V). The RS polarity is completely opposite the expected RS behavior in ferroelectric heterostructures induced by polarization reversal. It is proposed that the unique resistance switching polarity is attributed to the band-filling controlled metal-insulator transition in a Ca0.96Ce0.04MnO3 film, triggered by ferroelectric based electrostatic doping. The results address the importance of ferroelectric field effect on the electronic properties of the interfacial system in ferroelectric/complex oxide-based resistive memory devices.

Understanding and modulation of resistive switching behaviors in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 multilayer junctions

The insertion layer in the ferroelectric multilayer junctions plays a key role in regulating the energy band structure of interface and their resistive switching behavior. Here, PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3/Nb:SrTiO3 (PZT/LSMO/NSTO) heterostructures with different LSMO thicknesses were prepared by pulsed laser deposition and their ferroelectric properties and resistive switching behaviors were investigated. It is found that the ferroelectric properties of the heterostructures almost do not change with the increasing in the thickness of LSMO, while the resistive switching behaviors are closely related to the LSMO thickness, and the maximum switching ratio of about 103 can be achieved in the PZT/LSMO/NSTO heterostructure with the LSMO thickness of 18 nm. By analyzing the I-V curves and energy band structures, it can be concluded that the resistive switching behaviors depend on the competition between the tunability of depletion layer width and the ability of ferroelectric filed effect. These results provide insights to understand ferroelectric resistive switching in ferroelectric heterostructures, and demonstrate an effective way to improve resistive switching performance by adjusting the band structure through the appropriate thickness of the insertion layer.

Existence of ferroelectric resistive switching memory in MoS2/PVDF heterojunction devices

The heterostructure device based on polyvinylidene fluoride (PVDF)/MoS2 films was succesfully prepared and showed a good hysteresis feature with a unique resistive switching perpromance, where the logarithmic I–V curve looks like a butterfly. The on-off ratio for the resistive switching in the device based on PVDF/MoS2 films reaches 2.5 × 102 and the resistive switching happens at −0.9 V and −4.9 V for a half loop. The influence of the introduction of MoS2 and ferroelectric PVDF film was also studied and compared to a device based on a single film. The underlying physical mechanism for the unique resistive transition was attributed to the polarization field from the ferroelectric polymer PVDF and the S vacancies in MoS2.

Bipolar Resistive Switching in Magnetostrictive Ni/PZT/Pt Structure for Non-Volatile Memory Applications

Resistive switching (RS) has significant potential for the forthcoming generation in logical and non-volatile memory devices. Owing to their switchable spontaneous polarization ferroelectric materials can be utilized for non-volatile memory applications, e.g., FeRAM, but their destructive readout scheme limits applications. The spontaneous electrical polarization of ferroelectric materials enhances the tuning charge properties at the ferroelectric/metal interface making them appropriate for RS applications. Here, the resistive switching performance of ferroelectric Pb(Zr0.6Ti0.4)O3 (PZT) thin film grown on flexible Ni substrate using pulsed laser deposition were investigated. The PZT film showed both entrenched P-E ferroelectric hysteresis loop and reversible resistive switching behaviors with DC voltage sweep. Bipolar switching between a low resistance state to a high resistance state with a large ON/OFF ratio of 102 and resistance retention potential up to 1010 cycles was observed. The current conduction mechanism can be understood by dual logarithm I-V curve fitting. The obtained results encourage the utilization of prepared Ni/PZT/Pt widget for the non-volatile memory applications.

Ferroelectric photovoltaic effect and resistive switching behavior modulated by ferroelectric/electrode interface coupling

The anomalous photovoltaic effect and resistive switching behaviors in ferroelectric materials attract much attention in recent years. Dozens of researches revealed that the two effects coexist and affect each other in electrode/ferroelectric/electrode structures. Therefore, the conductive mechanisms and research progresses of the two effects were discussed in this study, which suggested the interface coupling effect caused by polarization states led to switchable photovoltaic and different resistance states. On the other hand, electrode/ferroelectric/electrode structures have great potential in the application of high-density memories, and a novel non-volatile optoelectronic memory which can write multiple storage states and read information non-destructively can be realized by exploiting the two effects properly.

Couplings of Polarization with Interfacial Deep Trap and Schottky Interface Controlled Ferroelectric Memristive Switching

AbstractMemristors with excellent scalability have the potential to revolutionize not only the field of information storage but also neuromorphic computing. Conventional metal oxides are widely used as resistive switching materials in memristors. Interface‐type memristors based on ferroelectric materials are emerging as alternatives in the development of high‐performance memory devices. A clear understanding of the switching mechanisms in this type of memristors, however, is still in its early stages. By comparing the bipolar switching in different systems, it is found that the switchable diode effect in ferroelectric memristors is controlled by polarization modulated Schottky barrier height and polarization coupled interfacial deep states trapping/detrapping. Using semiconductor theories with consideration of polarization effects, a phenomenological theory is developed to explain the current–voltage behavior at the metal/ferroelectric interface. These findings reveal the critical role of the interaction among polarization charges, interfacial defects, and Schottky interface in controlling ferroelectric resistive switching and offer the guidance to design ferroelectric memristors with enhanced performance.

Effect of La0.67Sr0.33 MnO3 Insertion Layer on the Ferroelectric and Resistive Switching Behaviors of PbZr0.52Ti0.48O3/Nb:SrTiO3 Heterostructures

Recently, ferroelectric resistive switching (RS) effect in the ferroelectric/semiconductor heterostructures has been widely studied and the RS performance has been greatly improved. However, the relationships between ferroelectric and RS behaviors as well as interface structure of ferroelectric/semiconductor heterostructures need to be further studied. Herein, a [Formula: see text][Formula: see text]MnO3 (LSMO) layer with the thickness of 7 nm is inserted into [Formula: see text][Formula: see text]O3/Nb:SrTiO3 (PZT/NSTO) heterostructures, and its effects on the ferroelectric and RS behaviors are investigated. The PZT/NSTO heterostructures show significantly asymmetric ferroelectric loops, and the RS ratio in which can reach to three orders of magnitude. However, by inserting the LSMO layer, the ferroelectric loops became relatively symmetric, but the RS effect almost disappeared. It can be considered that the LSMO layer affects the interfacial energy band structure of the PZT/NSTO heterostructures, which makes ferroelectric polarization lose its effect on the modulation of the depletion layer width. Therefore, the existence of the adjustable depletion layer is very important for the RS effect of ferroelectric/semiconductor heterostructures.

Blocking of Conducting Channels Widens Window for Ferroelectric Resistive Switching in Interface‐Engineered Hf0.5Zr0.5O2 Tunnel Devices

AbstractFilms of Hf0.5Z0.5O2 (HZO) contain a network of grain boundaries. In (111) HZO epitaxial films on (001) SrTiO3, for instance, twinned orthorhombic (o‐HZO) ferroelectric crystallites coexist with grain boundaries between o‐HZO and a residual paraelectric monoclinic (m‐HZO) phase. These grain boundaries contribute to the resistive switching response in addition to the genuine ferroelectric polarization switching and have detrimental effects on device performance. Here, it is shown that, by using suitable nanometric capping layer deposited on HZO film, a radical improvement of the operation window of the tunnel device can be achieved. Crystalline SrTiO3 and amorphous AlOx are explored as capping layers. It is observed that these layers conformally coat the HZO surface and allow to increase the yield and homogeneity of ferroelectric junctions while strengthening endurance. Data show that the capping layers block ionic‐like transport channels across grain boundaries. It is suggested that they act as oxygen suppliers to the oxygen‐getters grain boundaries in HZO. In this scenario it could be envisaged that these and other oxides could also be explored and tested for fully compatible CMOS technologies.

Control of binary states of ferroic orders in bi-domain BiFeO3 nanoislands

Understanding switching mechanisms in multiferroics such as BiFeO3 (BFO) is an important challenge to control ferroic orders (ferroelectric or ferroelastic) as it could lead to the design of non-volatile memories based on magnetoelectric coupling. Here, we demonstrate an alternative way to control the binary states of ferroic orders by locally applying pressure and electric field in ferroelectric bi-domains confined in single BFO nanoislands. The study of the electronic transport properties and domain orientations using atomic force microscopy (AFM) based techniques enabled us to determine the electric and mechanical parameters at which ferroelectric and ferroelastic resistive switching can be observed. Nanoislands exhibited binary high and low resistance states without scaling effect, with high performance switching characteristics. Positive-forward rectifying behavior at high tip force was interpreted by the formation of a subsurface non-conductive interface due to the strain gradient. Ferroelastic switching at the surface was associated with a symmetry-breaking induced by electromechanical coupling between the AFM tip and the BFO thin film. It led to out-of-plane polarization pinning that allows performing only in-plane switching accompanied by nucleation and propagation of a conductive domain wall. The control of ferroic binary states by the electric field and pressure may pave the way for multilevel data storage devices.

Configurable Resistive Response in BaTiO3 Ferroelectric Memristors via Electron Beam Radiation.

Ferroelectric oxide memristors are currently in the highlights of a thriving area of research aiming at the development of nonvolatile, adaptive memories for applications in neuromorphic computing. However, to date a precise control of synapse-like functionalities by adjusting the interplay between ferroelectric polarization and resistive switching processes is still an ongoing challenge. Here, it is shown that by means of controlled electron beam radiation, a prototypical ferroelectric film of BaTiO3 can be turned into a memristor with multiple configurable resistance states. Ex situ and in situ analyses of current/voltage characteristics upon electron beam exposure confirm the quasi-continuous variation of BaTiO3 resistance up to two orders of magnitude under the typical experimental conditions employed in electron beam patterning and characterization techniques. These results demonstrate an unprecedented effective route to locally and scalably engineering multilevel ferroelectric memristors via application of moderate electron beam radiation.

Reversible transition of filamentary and ferroelectric resistive switching in BaTiO3/SmNiO3 heterostructures

We report a reversible transition between filamentary and ferroelectric resistive switching in BaTiO3/SmNiO3 heterostructures.

Electro and photon double-driven non-volatile and non-destructive readout memory in Pt/Bi0.9Eu0.1FeO3/Nb:SrTiO3 heterostructures

Ferroelectric resistive switching has recently attracted considerable attention as a promising candidate for next-generation non-volatile memory. In this work, we report an electro and photon double-driven bipolar resistive switching behavior in Pt /Bi0.9Eu0.1FeO3 (BEFO) /Nb-doped SrTiO3 (NSTO) heterostructures prepared via pulsed laser deposition. In addition to the polarization-based control of the resistive memory, a switchable photovoltaic effect is observed that can be used to detect the polarization direction non-destructively. Significantly, the electric field-modulated interfacial barrier can be further affected by photon-generated carriers. This phenomenon is attributed to the barrier modulation in the Pt /BEFO and BEFO /NSTO interfaces by electric field and photon excitation. These results indicate the feasibility of non-volatile and non-destructive readout from ferroelectric memory.