Valence Change Mechanism Research Articles

α-CsPbI3 Quantum Dots ReRAM with High Air Stability Working by Valance Change Filamentary Mechanism.

The current memory system is facing obstacles to improvement, and ReRAM is considered a powerful alternative. All-inorganic α-CsPbI3 perovskite-based ReRAM working by electrochemical mechanism is reported, but the electrochemically active electrode raised difficulty in long-term stable operation, and bulk α-CsPbI3 device can not show resistive switching behavior with an inert metal top electrode. Herein, by making the α-CsPbI3 into QDs and applying it to the device with inert Au as the top electrode, the devices working by valence change mechanism are successfully fabricated. The large surface-to-volume ratio made an abundant amount of iodine vacancies and facile migration of vacancies allowed the device to work by valence change mechanism. The devices show reliable electrical characteristics, 800 cycles endurance and retention for over 4 × 104 s, and air stability for 1 month. This work demonstrates that applying the QDs can improve the stability and enable a new type of working mechanism in ReRAM.

Understanding the Reversible Transition of Unipolar and Bipolar Resistive Switching Characteristics in Solution-Derived Nanocrystalline Au-Co3O4 Thin-Film Memristors.

Nanocrystalline Au-Co3O4 thin films were fabricated through a facile solution-processing method, and voltage-polarity-dependent resistive switching (RS) characteristics including variability of Set/Reset voltages, stability of cycling endurance, and carriers transport mechanism were studied in the Pt/Au-Co3O4/Pt memory devices. The switching voltages of the Set and Reset processes in the bipolar RS memristors exhibited lower variability as compared to the unipolar resistance switching devices. Moreover, the switching performance of cycle-to-cycle endurance in bipolar mode had a smaller fluctuation than those of unipolar switching behavior. Based on the current-voltage curve fitting analysis, it was found that Ohmic-conduction behaviors dominated the carriers transport of low-resistance state regardless of unipolar or bipolar switching behaviors. The carrier transport of high resistance state was governed following Poole-Frenkel emission and Schottky emission mechanisms in the unipolar and bipolar switching modes, respectively. The physical switching mechanism of Pt/Au-Co3O4/Pt memory devices was proposed using the model by means of growth and disruption of conducting filaments, involved in memory effects of thermochemical mechanism and valence change mechanism in the unipolar and bipolar RS, respectively. The proposed model following the finite element method revealed the roles of electrical field distribution and temperature gradient to further clarify the RS mechanism. Our results open a door for understanding and optimizing oxide-based thin-film resistance switching memory devices.

Resolving the Relaxation of Volatile Valence Change Memory

AbstractMemristive devices based on the valence change mechanism are highly interesting candidates for data storage and hardware implementation of synapses in neuromorphic circuits. Although long‐term retention is often required for data storage applications, a slight resistance drift of the low resistive state (LRS) is observed even for stable devices. For other devices, the LRS has been observed to decay rapidly to the high resistive state (HRS). These types of devices are of interest for neuromorphic circuits where short‐term plasticity is required. In this work, the LRS relaxation of volatile, crystalline Pt/SrTiO3/Nb:SrTiO3: devices is investigated in detail, yielding time constants ranging from milliseconds to seconds. The decay is analyzed in terms of the Gibbs free energy gradient for the contribution of oxygen ion migration. A relaxation model based on drift‐diffusion dynamics is presented. The model may serve as a tool for developing guidelines and design rules for future volatile memristive technology based on Schottky barrier mediated electron transport.

Atomic Layer Deposition Films for Resistive Random‐Access Memories

AbstractResistive random‐access memory (RRAM) stands out as a promising memory technology due to its ease of operation, high speed, affordability, exceptional stability, and potential to enable smaller memory devices with sizes under 10 nm. This has drawn significant attention, with atomic layer deposition (ALD) emerging as an ideal technology to tackle the challenges of nanoscale fabrication in the micro‐ and nanomanufacturing industry. ALD offers technological advantages such as functional multiple‐layer stacking, doping capabilities, and incorporating oxygen reservoirs or reactive layers. These factors contribute to achieving more intriguing, stable, and reliable nonvolatile resistance switching behaviors in RRAM. Specifically, ALD greatly benefits RRAM, that relies on the valence change mechanism, where high‐k transition metal oxides are commonly used as switching materials, and precise control over oxygen vacancies is achievable. This review provides a comprehensive overview of ALD films used in RRAM, delves into resistive switching properties and microscopic mechanisms in binary and ternary oxides and nitrides, and explores the impact of ALD‐prepared electrodes. Furthermore, the current status and future prospects of ALD‐based RRAM are highlighted, which is poised to catalyze further advancements in the fields of information storage and neural networks.

A Flexible Nickel-Oxide-Based RRAM Device Prepared Using the Solution Combustion Method

Binary metal oxide materials, such as nickel oxide, are widely used in flexible resistive variable memory devices due to advantages such as their easily controllable material composition, simple structural composition, and good compatibility between manufacturing processes and complementary metal oxide processes. In this work, a solution combustion method was employed to prepare NiOx thin films for use as a resistive switching layer of resistance random-access memory. The formation temperature of the NiOx layer in the RRAM device was kept as low as 175 °C, making the device compatible with flexible substrates. With polyethylene naphthalenediate as the substrate, the NiOx-based RRAM exhibited obvious bipolar resistance-switching properties, superb bending resistance, and good stability. Through theoretical fitting and structural characterization, the conduction mechanisms were attributed to the combination of the space-charge-limited current and Ohmic mechanisms, while the valence change mechanism was determined to be the resistive switching mechanism. This study demonstrates a low-temperature and scalable approach to constructing NiOx-based RRAM devices on flexible substrates.

Accurate evaluation method for HRS retention of VCM ReRAM

Long-term retention is one of the major challenges concerning the reliability of redox-based resistive switching random access memories based on the valence change mechanism (VCM). The stability of the programmed state has to be ensured over several years, leaving a sufficient read window between the states, which is even more challenging at large statistics. Thus, the underlying physical mechanisms have to be understood and experimental data have to be evaluated accurately. Here, it shows that the retention behavior of the high resistive state (HRS) is more complex than that of the low resistive state and requires a different evaluation method. In this work, we experimentally investigate the retention behavior of 5M VCM devices via accelerated life testing and show the difficulties of commonly used evaluation methods in view of the HRS. Subsequently, we present a new evaluation method focusing on the standard deviation of the HRS current distribution. Hereby, an activation energy for the degradation process can be extracted, which is essential for the prediction of the devices’ behavior under operating conditions. Furthermore, we reproduce the experimentally observed behavior with our 3D Kinetic Monte Carlo simulation model. We confirm the plausibility of our evaluation method and are able to connect the calculated activation energy to the migration barriers of oxygen vacancies that we implemented in the model and that we believe play a key role in the experimentally observed degradation process.

Low-Temperature Solution-Based Molybdenum Oxide Memristors.

Solution-based memristors have gained significant attention in recent years due to their potential for the low-cost, scalable, and environmentally friendly fabrication of resistive switching devices. This study is focused on the fabrication and characterization of solution-based molybdenum trioxide (MoO3) memristors under different annealing temperatures (200 to 400 °C). A MoO3 ink recipe is developed using water as the main solvent, enabling a simplified and cost-effective fabrication process. Material analysis reveals the presence of a Mo6+ oxidation state and an amorphous structure in the films annealed up to 250 °C. Electrical tests confirm a bipolar resistive switching behavior in the memristors according to the valence change mechanism (VCM). Endurance tests demonstrate stable memristors, indicating their robust nature after multiple cycles. Memristors annealed at 250 °C exhibit a nonvolatile behavior with a retention time up to 105 s under ambient air conditions. The high reproducibility observed in these memristors highlights their potential for practical applications and scalability.

In Operando Near‐Field Optical Investigation of Memristive Ta2O5 Thin Film Devices with a Graphene Top Electrode

AbstractResistive switching devices based on metal oxides are candidates for nonvolatile memory storage. They often rely on the valence change mechanism, the field‐induced movement of donor ions leading to nanoscale conductive paths in filamentary‐type devices. Devices usually consist of a transition metal oxide like Ta2O5 sandwiched between two metal electrodes. Critical parameters of the devices, such as cycle‐to‐cycle variability, Roff/Ron ratio, and endurance depend on the morphology and composition of the filaments. However, investigating filaments on the nanoscale is cumbersome, and commonly applied techniques such as conductive atomic force or transmission electron microscopy require delaminating the metal top electrode, inhibiting in operando investigations over many switching cycles. Here, the authors use infrared scattering‐type scanning near‐field optical microscopy (s‐SNOM) to investigate resistive switching in Ta2O5 films with a graphene top electrode in operando and reveal individual filaments on the device level. By selecting an appropriate illumination frequency, the authors can trace the evolution of filaments and the joule heating‐induced retraction of the top electrode until device failure. s‐SNOM promises a deeper understanding of resistive switching devices’ microscopic switching behavior and applies to a wide range of resistive switching oxides, such as HfO2, SrTiO3, and SiO2.

Origin of discrete resistive switching in chemically heterogeneous vanadium oxide crystals.

Phase changes in oxide materials such as VO2 offer a foundational platform for designing novel solid-state devices. Tuning the V : O stoichiometry offers a vast electronic phase space with non-trivial collective properties. Here, we report the observation of discrete threshold switching voltages (Vth) with constant ΔVth between cycles in vanadium oxide crystals. The observed threshold fields over 10 000 cycles are ∼100× lower than that noted for stoichiometric VO2 and show unique discrete behaviour with constant ΔVth. We correlate the observed discrete memristor behaviour with the valence change mechanism and fluctuations in the chemical composition of spatially distributed VO2-VnO2n-1 complex oxide phases that can synergistically co-operate with the insulator-metal transition resulting in sharp current jumps. The design of chemical heterogeneity in oxide crystals, therefore, offers an intriguing path to realizing low-energy neuromorphic devices.

Eightwise Switching Mechanism in Memristive SrTiO3Devices and Its Implications on the Device Performance

Filamentary‐type resistive switching devices based on the valence change mechanism (VCM) have been studied extensively during the past decades, due to their great performance as nonvolatile memories and their promising nature for novel computing architectures. For many materials exhibiting the VCM, two different polarities can be observed, namely, counter‐eightwise and eightwise switching. The more technologically relevant materials, such as TaOxand HfOx, usually exhibit the counter‐eightwise resistive switching mode, however, the nature of its abrupt switching limits the potential for their implementation in more analog systems. On the contrary, the eightwise resistive switching mode exhibits a more gradual switching profile, which makes it a more suitable candidate for neuromorphic and in‐memory computing applications. Contrary to its counterpart, the eightwise switching mode has not been fully understood for several years, hampering the improvement of memristive devices based on this mode. This review presents the major advancements that have been made to understand the mechanism behind the eightwise switching mode and its implications on the properties of the memristive devices. The works presented here are based on the model system of SrTiO3, however, the obtained knowledge and the strategies to tailor their properties can be transferred to other technologically relevant systems.

Ternary Łukasiewicz logic using memristive devices

Memristive devices based on the Valence Change Mechanism (VCM) are promising devices for storage class memory, neuromorphic computing and logic-in-memory (LIM) applications. They are suited for such a wide range of applications, due to their possibility for extreme dense integration, low power consumption and multilevel capabilities. Through LIM concepts, Boolean logic operations can be performed directly in memory. In many of these concepts, the resistance state of the device is interpreted as the logical input and output of the logic function, which is why these concepts are called ‘stateful’ logic. Most of the proposed ideas, however, are defined based on only binary switching VCM devices and neglect their multi-level capabilities. Extending LIM concepts towards multinary logic, e.g. a ternary logic, would increase the data density inside the memory array and reduce the number of devices required to perform a certain operation. In this work, we discuss two possibilities of realizing a ternary logic based on the analog switching in the RESET or in the SET direction. For both directions we verify the logic functionality by showing the basic operations of implication, negation and false operation, which together form a functionally complete logic. Additionally, for both switching directions, we discuss a 41-Trit ( 64-Bit) addition. For all investigations the physics-based compact model JART VCM v1b is used, which has been verified on the RESET direction multilevel properties of the TaO devices.

Correlation between Electronic Structure, Microstructure, and Switching Mode in Valence Change Mechanism Al2O3/TiOx‐Based Memristive Devices

AbstractMemristive devices with valence change mechanism (VCM) show promise for neuromorphic data processing, although emulation of synaptic behavior with analog weight updates remains a challenge. Standard filamentary and area‐dependent resistive switching exhibit characteristic differences in the transition from the high to low resistance state, which is either abrupt with inherently high variability or gradual and allows quasi‐analog operation. In this study, the two switching modes are clearly correlated to differences in the microstructure and electronic structure for Pt/Al2O3/TiOx/Cr/Pt devices made from amorphous layers of 1.2 nm Al2O3 and 7 nm TiOx by atomic layer deposition. For the filamentary mode, operando spectromicroscopy experiments identify a localized region of ≈50 nm in diameter of reduced titania surrounded by crystalline rutile‐like TiO2, highlighting the importance of Joule heating for this mode. In contrast, both oxide layers remain in their amorphous state for the interfacial mode, which proves that device temperature during switching stays below 670 K, which is the TiO2 crystallization temperature. The analysis of the electronic conduction behavior confirms that the interfacial switching occurs by modulating the effective tunnel barrier width due to accumulation and depletion of oxygen vacancies at the Al2O3/TiOx interface. The results are transferable to other bilayer stacks.

Probabilistic model of resistance jumps in memristive devices.

Resistance switching memory cells such as electrochemical metallization cells and valence change mechanism cells have the potential to revolutionize information processing and storage. However, the creation of deterministic resistance switching devices is a challenging problem that is still open. At present, the modeling of resistance switching cells is dominantly based on deterministic models that fail to capture the cycle-to-cycle variability intrinsic to these devices. Herewith we introduce a state probability distribution function and associated integrodifferential equationto describe the switching process consisting of a set of stochastic jumps. Numerical and analytical solutions of the equationhave been found in two model cases. This work expands the toolbox of models available for resistance switching cells and related devices and enables a rigorous description of intrinsic physical behavior not available in other models.

An Atomistic Model of Field-Induced Resistive Switching in Valence Change Memory

In valence change memory (VCM) cells, the conductance of an insulating switching layer is reversibly modulated by creating and redistributing point defects under an external field. Accurately simulating the switching dynamics of these devices can be difficult due to their typically disordered atomic structures and inhomogeneous arrangements of defects. To address this, we introduce an atomistic framework for modeling VCM cells. It combines a stochastic kinetic Monte Carlo approach for atomic rearrangement with a quantum transport scheme, both parametrized at the ab initio level by using inputs from density functional theory. Each of these steps operates directly on the underlying atomic structure. The model thus directly relates the energy landscape and electronic structure of the device to its switching characteristics. We apply this model to simulate field-induced nonvolatile switching between high- and low-resistance states in a TiN/HfO2/Ti/TiN stack and analyze both the kinetics and stochasticity of the conductance transitions. We also resolve the atomic nature of current flow resulting from the valence change mechanism, finding that conductive paths are formed between the undercoordinated Hf atoms neighboring oxygen vacancies. The model developed here can be applied to different material systems to evaluate their resistive switching potential, both for use as conventional memory cells and as neuromorphic computing primitives.

A Study of the Electroforming Process in 1T1R Memory Arrays

For reproducible resistive switching in memristive devices, electroforming is a crucial process. However, a deeper understanding of the electroforming process is still lacking due to unavailability of a proper simulation tool. Here, we propose a physics-based compact model for the electroforming of valence change mechanism (VCM) memristive devices. The developed JART VCM Forming model is experimentally validated with the ZrOx-based memristive device. Furthermore, the electroforming process in different 1T1R memristive arrays is simulated with this model. The study shows that the electrical characteristics of each device in the array after the forming process are influenced by word/bit line series resistance. In addition, control effects depending on the channel width and applied gate voltage of transistor in the 1T1R cell are also investigated with the compact model simulation.

Tailor-made synaptic dynamics based on memristive devices

The proliferation of machine learning algorithms in everyday applications such as image recognition or language translation has increased the pressure to adapt underlying computing architectures towards these algorithms. Application specific integrated circuits (ASICs) such as the Tensor Processing Units by Google, Hanguang by Alibaba or Inferentia by Amazon Web Services were designed specifically for machine learning algorithms and have been able to outperform CPU based solutions by great margins during training and inference. As newer generations of chips allow handling of and computation on more and more data, the size of neural networks has dramatically increased, while the challenges they are trying to solve have become more complex. Neuromorphic computing tries to take inspiration from biological information processing systems, aiming to further improve the efficiency with which these networks can be trained or the inference can be performed. Enhancing neuromorphic computing architectures with memristive devices as non-volatile storage elements could potentially allow for even higher energy efficiencies. Their ability to mimic synaptic plasticity dynamics brings neuromorphic architectures closer to the biological role models. So far, memristive devices are mainly investigated for the emulation of the weights of neural networks during training and inference as their non-volatility would enable both processes in the same location without data transfer. In this paper, we explore realisations of different synapses build from memristive ReRAM devices, based on the Valence Change Mechanism. These synapses are the 1R synapse, the NR synapse and the 1T1R synapse. For the 1R synapse, we propose three dynamical regimes and explore their performance through different synapse criteria. For the NR synapse, we discuss how the same dynamical regimes can be addressed in a more reliable way. We also show experimental results measured on ZrOx devices to support our simulation based claims. For the 1T1R synapse, we explore the trade offs between the connection direction of the ReRAM device and the transistor. For all three synapse concepts we discuss the impact of device-to-device and cycle-to-cycle variability. Additionally, the impact of the stimulation mode on the observed behavior is discussed.

Charge Transport inside TiO2 Memristors Prepared via FEBID.

We fabricated memristive devices using focused electron beam-induced deposition (FEBID) as a direct-writing technique employing a Pt/TiO2/Pt sandwich layer device configuration. Pinching in the measured current-voltage characteristics (i-v), the characteristic fingerprint of memristive behavior was clearly observed. The temperature dependence was measured for both high and low resistive states in the range from 290 K down to about 2 K, showing a stretched exponential behavior characteristic of Mott-type variable-range hopping. From this observation, a valence change mechanism of the charge transport inside the TiO2 layer can be deduced.

In-Gap States of HfO2 Nanoislands Driven by Crystal Nucleation: Implications for Resistive Random-Access Memory Devices

Envisioned extremely scaled, high-performance memory devices request to conduct the step from thin semiconductor films to nanoscale structures and the use of promising high-k materials such as hafnium oxide (HfO2). HfO2 is well suited for use in resistive random-access memory (ReRAM) devices based on the valence change mechanism. Here, we provide a decidedly scaled system, namely, HfO2 nanoislands that are grown by van der Waals epitaxy on highly oriented pyrolytic graphite (HOPG). The electronic and structural properties of these well-separated, crystalline HfO2 nanoislands are investigated by scanning probe methods as well as ab initio methods. The topography reveals homogeneously formed HfO2 nanoislands with areas down to 7 nm2 and a thickness of one unit cell. They exhibit several acceptor- and donor-like in-gap states in addition to the bulk band gap, implying bulk properties. X-ray photoelectron spectroscopy indicates hafnium carbide formation as one possible origin for defect states. Going further to the crystal nucleation of HfO2, nanocrystals with a diameter of 2.7–4.5 Å are identified next to carbon vacancies in the topmost HOPG layer, indicating that carbon is incorporated into the islands at early nucleation stages. A precise description of these nuclei is accomplished by the simulation of small HfmOn(:C) clusters (m = 3 to 10; n = 3 to 22) with and without carbon incorporation using ab initio methods. The comparison of the theoretically determined lowest-energy clusters and electronic states with the experimental results allows us to identify the structure of the most relevant HfO2 sub-nanometer crystals formed during the first nucleation steps and the nature of the in-gap states found at the surfaces of HfO2 nanoislands. That way, a model system is derived that consists of distinct structural units, related to surface states or defect states, respectively, that will promote the tailoring of in-gap states of smallest HfO2 structures and thus the scalability of memory devices.

(Digital Presentation) Resistive Switching and Brain-Inspired Computing in Perovskite Nanowires and Quantum Wires

In the past decade, halide perovskites (HPs) have shot to fame in the genre of optoelectronics and photovoltaics owing to their large absorption co-effcient, high color purity, tunable bandgap and long charge diffusion lengths. Besides these traits, HPs also possess innumerable charge transport pathways, inherent hysteresis, high charge-carrier and ionic mobilities which render them as ideal candidates for resistive random access switching memories (RRAMs). However owing to material and electrical instability associated with HP thin-film devices, the figures-of-merits (FOMs) namely retention, endurance and switching speed were not up to the state of-the-art standard until recently. In order to revolutionize HP Re-RAMs we devised a unique device structure where we replaced the thin-film architecture with vertically aligned high density HP nanowires and quantum wires embedded in a porous alumina membrane (PAM) sandwiched between metallic silver and aluminum contacts. The excellent passivation provided by the PAM imparted the requisite electrical and material stability to the environmentally delicate HPs by drastically reducing the surface diffusion pathways and thereby thwarting the moisture induced attacks. Extrapolated retention time as high as 28.3 years and measured device endurance of a million cycles were obtained. Utilizing the single crystalline HP nanowires and quantum wires and their associated high ionic and electronic mobilities, switching speed as fast as 100 ps was also obtained. These FOMs represent record values for HP RRAMs ever reported. Furthermore a 14 nm lateral size HP quantum wire RRAM cell was fabricated and a cross-bar device architecture with a unique sneaky path mitigation scheme were developed, which successfully exhibited the scalability potential of our devices. We further coupled the optoelectronic and switching behaviors and were able to obtain optical programmability among the low resistance states. Besides data storage, the HP nanowires and quantum wires were employed in developing neuromorphic devices enabled with low power and high precision computing capabilities. Specifically, we obtained robust multi-level states in two types of brain-inspired devices capable of performing analog processing tasks by using silver as the top electrode and precisely controlling the current injection in the monocrytalline switching medium and by using indium doped tin oxide as the top electrode and inducing a novel valence change mechanism in the HP nanowires triggering the gradual conductance change. All in all, our nanowire and quantum wire devices propel HP RRAMs to the state-of-the-art standard in multifarious applications concerning future data storage and neuromorphic computing.

Modified Dynamic Physical Model of Valence Change Mechanism Memristors.

Valence change-type resistance switching behaviors in oxides can be understood by well-established physical models describing the field-driven oxygen vacancy distribution change. In those models, electroformed residual oxygen vacancy filaments are crucial as they work as an electric field concentrator and limit the oxygen vacancy movement along the vertical direction. Therefore, their movement outward by diffusion is negligible. However, this situation may not be applicable in the electroforming-free system, where the field-driven movement is less prominent, and the isotropic oxygen vacancy diffusion by concentration gradient is more significant, which has not been given much consideration in the conventional model. Here, we propose a modified physical model that considers the change in the oxygen vacancies' charged state depending on their concentrations and the resulting change in diffusivity during switching to interpret the electroforming-free device behaviors. The model suggests formation of an hourglass-shaped filament constituting a lower concentration of oxygen vacancies due to the fluid oxygen diffusion in the thin oxide. Consequently, the proposed model can explain the electroforming-free device behaviors, including the retention failure mechanism, and suggest an optimized filament configuration for improved retention characteristics. The proposed model can plausibly explain both the electroformed and the electroforming-free devices. Therefore, it can be a standard model for valence change memristors.