Intermediate Resistance States Research Articles

Thermal dependence of the current in TiN/Ti/HfO2/W memristors at different intermediate conduction states

The dependence of the current in TiN/Ti/HfO2/W devices on the temperature is investigated in the range from 78 K to 340 K. Resistive switching cycles at 78 K are conducted to explore the thermal dependence in filament configurations with different intermediate resistance states. The less conductive states show an increase of the current as the temperature rises, while the fully formed filament displays a metallic-like behavior. A comprehensive model, based on the Stanford Model including a series resistance, is proposed and successfully validated by experimental data. The interplay between the ohmic and non-linear components in the model for different filament states is analyzed, emphasizing the dominance of the non-linear component (and its corresponding thermal dependence) in partially formed filaments and the prevalence of the ohmic component in the fully formed filament, which shows a decreasing current as the temperature rises. A complete compact model for simulation of circuits including the thermal dependence of these devices is developed.

Neuromorphic one-shot learning utilizing a phase-transition material

Design of hardware based on biological principles of neuronal computation and plasticity in the brain is a leading approach to realizing energy- and sample-efficient AI and learning machines. An important factor in selection of the hardware building blocks is the identification of candidate materials with physical properties suitable to emulate the large dynamic ranges and varied timescales of neuronal signaling. Previous work has shown that the all-or-none spiking behavior of neurons can be mimicked by threshold switches utilizing material phase transitions. Here, we demonstrate that devices based on a prototypical metal-insulator-transition material, vanadium dioxide (VO2), can be dynamically controlled to access a continuum of intermediate resistance states. Furthermore, the timescale of their intrinsic relaxation can be configured to match a range of biologically relevant timescales from milliseconds to seconds. We exploit these device properties to emulate three aspects of neuronal analog computation: fast (~1 ms) spiking in a neuronal soma compartment, slow (~100 ms) spiking in a dendritic compartment, and ultraslow (~1 s) biochemical signaling involved in temporal credit assignment for a recently discovered biological mechanism of one-shot learning. Simulations show that an artificial neural network using properties of VO2 devices to control an agent navigating a spatial environment can learn an efficient path to a reward in up to fourfold fewer trials than standard methods. The phase relaxations described in our study may be engineered in a variety of materials and can be controlled by thermal, electrical, or optical stimuli, suggesting further opportunities to emulate biological learning in neuromorphic hardware.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy.

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

Exploring Multi‐Bit Logic In‐Memory with Memristive HfO2‐Based Ferroelectric Tunnel Junctions

AbstractThe increasing demand for data movement and energy consumption in physically separate von Neumann architectures, where the processor and memory are distinct entities, highlights the severity of the memory‐wall problem. Thus, memristor‐based logic‐in‐memory (LiM) has garnered significant interest as it is a paradigm that enables both logic and memory functionalities to be performed within a single device. Ferroelectric tunnel junctions (FTJs) have the advantages of low energy consumption and high scalability; thus, they are particularly appropriate for use as analog memristors in LiMs. Precise and accurate programming of the analog resistance states in FTJs is essential to achieve an efficient memristor‐based LiM design. However, existing switching models for FTJs cannot completely consider the ferroelectric domain switching, which challenges multi‐bit LiM operations. This particularly applies in cases involving updates from one intermediate resistance state (IRS) to another IRS, such as transitioning from State 01 to 10 in 2‐bit LiM. In this study, a novel phenomenon associated with domain switching is observed in the IRS of an HZO FTJ device and applied it to an LiM implementation. These findings can contribute to the operational accuracy and update rules of not only FTJ‐based LiMs but also other FTJ applications, such as crossbar arrays.

Encoding multistate charge order and chirality in endotaxial heterostructures

High-density phase change memory (PCM) storage is proposed for materials with multiple intermediate resistance states, which have been observed in 1T-TaS2 due to charge density wave (CDW) phase transitions. However, the metastability responsible for this behavior makes the presence of multistate switching unpredictable in TaS2 devices. Here, we demonstrate the fabrication of nanothick verti-lateral H-TaS2/1T-TaS2 heterostructures in which the number of endotaxial metallic H-TaS2 monolayers dictates the number of resistance transitions in 1T-TaS2 lamellae near room temperature. Further, we also observe optically active heterochirality in the CDW superlattice structure, which is modulated in concert with the resistivity steps, and we show how strain engineering can be used to nucleate these polytype conversions. This work positions the principle of endotaxial heterostructures as a promising conceptual framework for reliable, non-volatile, and multi-level switching of structure, chirality, and resistance.

Modeling the conduction mechanisms of intrinsic multi-level states in HfOx-based resistive random access memory

Multi-level cell storage technology based on resistive random access memory (RRAM) with multi-level state characteristics is more attractive in achieving low-cost ultra-high-density nonvolatile memory. Although a large number of literatures have reported the multi-level state characteristics of RRAM, so far there is no unified model that can well explain the intrinsic reasons for the existence of intermediate resistance state (IRS) and the switching mechanism between different resistance states. Multi-level state characteristics are observed by I–V characteristic measurements on RRAM with TiN/HfOx/barrier layer/TiN stacks fabricated using a commercialized 28 nm CMOS process. Compared to other published resistive switching models, the proposed model based on trap-assisted tunneling is more consistent with the measured. The model can reproduce the multi-level state characteristics based on the mechanism that interaction between defects and oxygen vacancies at the interface of HfOx and TiN electrode, resulting in the formation of multiple weak conductive filaments. Furthermore, the wide spread of high resistance state (HRS) and the switching between HRS and IRS are determined by the distance of tunneling gap. As HfOx-based RRAM will soon be commercialized, it is becoming very urgent to clarify the switching mechanisms of multi-level state characteristics and propose a universal model. Consequently, this work satisfied the current demand and significantly advanced the understanding and development of commercialized, cost-effective, high-density multi-bit HfOx-based RRAM technology.

Thin-film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity.

A design concept of phase-separated amorphous nanocomposite thin films is presented that realizes interfacial resistive switching (RS) in hafnium oxide-based devices. The films are formed by incorporating an average of 7% Ba into hafnium oxide during pulsed laser deposition at temperatures ≤400°C. The added Ba prevents the films from crystallizing and leads to ∼20-nm-thin films consisting of an amorphous HfOx host matrix interspersed with ∼2-nm-wide, ∼5-to-10-nm-pitch Ba-rich amorphous nanocolumns penetrating approximately two-thirds through the films. This restricts the RS to an interfacial Schottky-like energy barrier whose magnitude is tuned by ionic migration under an applied electric field. Resulting devices achieve stable cycle-to-cycle, device-to-device, and sample-to-sample reproducibility with a measured switching endurance of ≥104 cycles for a memory window ≥10 at switching voltages of ±2 V.Each device can be set to multiple intermediate resistance states, which enables synaptic spike-timing-dependent plasticity. The presented concept unlocks additional design variables for RS devices.

Successive crystallization in indium selenide thin films for multi-level phase-change memory

Increasing the storage density of phase-change memory is a challenging subject towards the universal memory commercialization. Multi-level data storage can be implemented in phase-change memory cells by designing multiple stages of crystallization of phase-change materials, which is favorable to push forward the application of phase-change memory in storage-class memory and the emerging in-memory computing. Here, the structure and properties of In2Se3 phase-change thin films with multi-level phase transitions are investigated. The In2Se3 thin films changes resistance states twice upon heating with high crystallization temperature and good thermal stability. The amorphous In2Se3 thin films undergo partial crystallization of the Se phase followed by crystallization of the γ-In2Se3 phase, with successive stages of crystallization accounting for the three distinct levels of resistivity. The phase-change memory cell based on In2Se3 thin films shows a continuous switching process of high resistance state, intermediate resistance state and low resistance state with low drift coefficient in SET-RESET operations at a speed as fast as 60 ns, and the RESET power is as low as 311 pJ, indicating that the In2Se3 thin films have good performance of three-state storage property and can effectively improve the memory density.

Bipolar resistive switching with multiple intermediate resistance states in Mn3O4 thin film

Bipolar resistive switching with multiple intermediate resistance states in Mn3O4 thin film

Pixel Antenna Optimization Using the Adjoint Method and the Method of Moving Asymptote

The pixel concept offers a convenient and efficient method for antenna design. To meet the given specified antenna requirements or constraints, heuristic algorithms are often used to optimize the connection states for the hardwires between pixels, which are usually assumed to have the binary states, “short” or “open”. An alternative optimization strategy is proposed in this work, where the Boolean constraints on the hardwires are first relaxed to continuously tunable resistances and the globally convergent method of moving asymptotes (GCMMA) is applied to optimize the resistances. To speed up the computation, the adjoint method is used to simplify the calculation of the gradient information required in GCMMA. Resistances that are found as being above or below a certain threshold are then judged as “open” or “short”, respectively. Finally, the remaining intermediate resistance states are binarized using the successive exhaustive Boolean optimization (SEBO) method. Four different optimization strategies are proposed in terms of the objective function implemented in GCMMA and whether the second-stage SEBO is iterative or not. Two broadband endfire pixel antennas having horizontal linear polarization and high radiation efficiency are designed and fabricated correspondingly to demonstrate the feasibility of our proposed methods.

Multilevel resistive switching with negative differential resistance in Al/NiO/ZnFe2O4/ITO ReRAM device

The Resistive Random Access Memory devices have emerged as an energy-efficient alternative to Von Neumann computers by enabling in-memory computing. Here we demonstrate bipolar resistive switching in thin films of Nickel oxide (NiO) and Zinc Ferrite (ZFO) using a simple Al/NiO/ZFO/ITO configuration, making them a possible candidate for the next generation memory devices. The fabricated device demonstrated excellent resistive switching behavior with high endurance for up to 1000 cycles, good retention for more than 104 s, and a good resistance ratio of HRS to LRS ∼102. Ohmic conduction was observed in the LRS, while in the HRS, along with ohmic conduction, space charge limited current (SCLC) and Schottky mechanisms were observed. Besides the LRS and HRS, a number of stable intermediate resistance states can also be obtained during the RESET process using different stop voltages, which makes the current device a multilevel resistive switching device.

Memristors Based on Many-Layer Non-Stoichiometric Germanosilicate Glass Films

Nonstoichiometric GeSixOy glass films and many-layer structures based on them were obtained by high-vacuum electron beam vapor deposition (EBVD). Using EBVD, the GeO2, SiO, SiO2, or Ge powders were co-evaporated and deposited onto a cold (100 °C) p+-Si(001) substrate with resistivity ρ = 0.0016 ± 0.0001 Ohm·cm. The as-deposited samples were studied by Fourier-transformed infrared spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. A transparent indium–tin–oxide (ITO) contact was deposited as the top electrode, and memristor metal–insulator–semiconductor (MIS) structures were fabricated. The current–voltage characteristics (I–V), as well as the resistive switching cycles of the MIS, have been studied. Reversible resistive switching (memristor effect) was observed for one-layer GeSi0.9O2.8, two-layer GeSi0.9O1.8/GeSi0.9O2.8 and GeSi0.9O1.8/SiO, and three-layer SiO2/a–Ge/GeSi0.9O2.8 MIS structures. For a one-layer MIS structure, the number of rewriting cycles reached several thousand, while the memory window (the ratio of currents in the ON and OFF states) remained at 1–2 orders of magnitude. Intermediate resistance states were observed in many-layer structures. These states may be promising for use in multi-bit memristors and for simulating neural networks. In the three-layer MIS structure, resistive switching took place quite smoothly, and hysteresis was observed in the I–V characteristics; such a structure can be used as an “analog” memristor.

Multilevel resistance state tuned by change in crystallization mode of superlattice-like Sb/Si3N4 films

Phase change materials (PCMs) have been attracting increasing interest in computer bionics and artificial intelligence neuron systems. However, the influence of interface between Si3N4 and PCMs on phase chang behavior is still unclear. In this paper, phase change performance and crystallization behavior of homogeneous Sb film are adjusted by changing the thickness of the Si3N4 film and the iteration period. Sb film covered by Si3N4 has better thermal stability, better data retention, and higher crystallization rate due to that the stable Si3N4 can reduce the long-range order and inhibit the nano-size growth of Sb crystal grains. The low element diffusion between Sb and Si3N4 is favorable to stabilize the nano-interface and significantly improve the resistance drift. The resistance polymorphism including high, intermediate, and low resistance states can be obtained by annealing at different temperatures. The ability to achieve such a multilevel resistance state is ascribed to the nucleation-dominated crystallization mode.

Tri-level resistive switching characteristics and conductive mechanism of HfO<sub>2</sub>/NiO<sub>x</sub>/HfO<sub>2</sub>

With the extensive integration of portable computers and smartphones with "Internet of Things" technology, further miniaturization, high reading/writing speed and big storage capacity are required for the new generation of non-volatile memory devices. Compared with traditional charge memory and magnetoresistive memory, resistive random access memory(RRAM) based on transition metal oxides is one of the promising candidates due to its low power consumption, small footprint, high stack ability, fast switching speed and multi-level storage capacity.<br>Inspired by the excellent resistive switching characteristics of NiO and HfO<sub>2</sub>, NiO<sub>x</sub> films were deposited by magnetron sputtering on the Pt<111> layer and the polycrystalline HfO<sub>2</sub> film, respectively. Their microstructures, resistive switching characteristics and conductive mechanisms were studied. X-ray diffractometer data shows the <111> preferred orientation for the NiO<sub>x</sub> film deposited on the Pt<111> layer but the <100> preferred one for the film deposited on the polycrystalline HfO<sub>2</sub> layer. X-ray photoelectron depth profile of Ni2p core level reveals that the NiO<sub>x</sub> film is the mixture of oxygen-deficient NiO and Ni<sub>2</sub>O<sub>3</sub>. NiO<sub>x</sub>(111) films show bipolar resistive switching(RS) characteristics with a clockwise current-voltage(<i>I-V</i>) loop, but its ratio of the high resistance to the low resistance(<i>R</i><sub>H</sub>/<i>R</i><sub>L</sub>) is only~10, and its endurance is also poor. The NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack exhibits bipolar RS characteristics with a counterclockwise <i>I-V</i> loop. The <i>R</i><sub>H</sub>/<i>R</i><sub>L</sub> is greater than 10<sup>4</sup>, the endurance is about 10<sup>4</sup> cycles, and the retention time exceeds 10<sup>4</sup> s. In the initial stage, the HfO<sub>2</sub>/NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack shows similar bi-level RS characteristics with that of the NiO<sub>x</sub>(200)/HfO<sub>2</sub> stack. However, in the middle and the last stages, it's <i>I-V</i> curves gradually evolve into tri-level RS characteristics with a "two-step Set process" in the positive voltage region, showing potential applications in multilevel nonvolatile memory devices and brain-like neural synapses. Its <i>I-V</i> curves in the high and the low resistance states follow the relationship of Ohmic conduction (<i>I</i>∝<i>V</i>), while the <i>I-V</i> curves in the intermediate resistance state are dominated by the space-charge-limited-current mechanism (<i>I</i>∝<i>V</i><sup>2</sup>). The tri-level RS phenomena are attributed to the coexistence of the oxygen-vacancy conductive filaments in the NiO<sub>x</sub>(200) film and the space charge limited current in the upper HfO<sub>2</sub> film.

Coupling a phase field model with an electro-thermal solver to simulate PCM intermediate resistance states for neuromorphic computing

A key attribute for Phase-Change Memories to be used for neuromorphic computing is the possibility to create intermediate resistance states by the progressive crystallization during the successive application of set pulses. To complete an experimental approach, we studied progressive crystallization by simulation. Insight on the real mechanisms of progressive crystallization is obtained by coupling Joule heating, diffusion of heat and a phase change model relying on the Phase Field Method.

Remotely tuned multistate resistive switching in MoS2/NiMnIn thin film heterostructure for highly flexible ReRAM application

In this article, the Cu/MoS2/NiMnIn memory structure was fabricated over stainless steel substrate for flexible electronics applications. The bipolar resistive switching characteristics were observed in two different fabricated devices (i) without sulfur vacancies (D1) and (ii) with sulfur vacancies (D2) in MoS2 thin film. Three different resistance states such as the high resistance state (HRS), intermediate resistance state (IRS) and low resistance state (LRS) have been detected for the D2 memory device. It could be ascribed to the two filamentary models based on the movement of sulfur vacancies and the formation of Cu metallic filament with applied bias voltage. Moreover, the Ohmic and modified space charge limited conduction mechanisms clearly explain the current conduction in different resistance states of device D2. The fabricated MoS2 thin film-based memory structure exhibits stable resistive switching behavior with a high OFF/ON ratio of ∼3.6 × 103, good consistency of ∼3600 endurance cycles and excellent data retention capability up to 3000 s. Moreover, the remote tuning of device D2 was thoroughly investigated and an appreciable change in SET voltage was detected with external temperature and magnetic field. Additionally, the external magnetic field altered the switching states and enhanced the multi-bit data storage capability of the memory device. This can be ascribed to the effect of the Lorentz force on ionic movement in the presence of the external magnetic field. The mechanical flexibility of the memory structure was tested for 1000 bending cycles at various bending angles in both the tensile and compressive bending modes. Hence, the present study opens up new ways for the futuristic flexible device for high data storage and neuromorphic computing applications.

Reproducible Non-Volatile Multi-State Storage and Emulation of Synaptic Plasticity Based on a Copper-Nanoparticle-Embedded HfOx/ZnO Bilayer with Ultralow-Switching Current and Ideal Data Retention.

The multilevel properties of a memristor are significant for applications in non-volatile multi-state storage and electronic synapses. However, the reproducibility and stability of the intermediate resistance states are still challenging. A stacked HfOx/ZnO bilayer embedded with copper nanoparticles was thus proposed to investigate its multilevel properties and to emulate synaptic plasticity. The proposed memristor operated at the microampere level, which was ascribed to the barrier at the HfOx/ZnO interface suppressing the operational current. Compared with the stacked HfOx/ZnO bilayer without nanoparticles, the proposed memristor had a larger ON/OFF resistance ratio (~330), smaller operational voltages (absolute value < 3.5 V) and improved cycle-to-cycle reproducibility. The proposed memristor also exhibited four reproducible non-volatile resistance states, which were stable and well retained for at least ~1 year at 85 °C (or ~10 years at 70 °C), while for the HfOx/ZnO bilayer without copper nanoparticles, the minimum retention time of its multiple resistance states was ~9 days at 85 °C (or ~67 days at 70 °C). Additionally, the proposed memristor was capable of implementing short-term and long-term synaptic plasticities.

The Structural Modulation of Amorphous 2D Tungsten Oxide Materials in Magnetron Sputtering

Abstract2D oxide materials have gained tremendous attention in the applications. Herein, a synthesis route of 2D WO3 materials via magnetron sputtering is reported. A deposition between 2D monolayers and thin film structure are accomplished according to the temperature of substrate. And 2D monolayers are only formed on a cooled substrate. Ag doping helps to exfoliate 2D WO3 into freestanding monolayers, in which the thickness of 2D monolayer is only ≈3 nm. The ultralarge size of 2D WO3 shows unique features from the traditional 2D material. Resistive switching device and photocatalysis are discussed as examples of application. There is a clear intermediate resistance state in the device with 2D structure. And the failure of resistive switching device is closely related with the 2D structure of WO3. In the application of photocatalysis, an improved two‐step exfoliation, achieving a stack of WO3 monolayers with a large internal volume, is utilized. The enhancements of photocatalysis are obtained with 2D WO3 after exfoliation.

Investigation of the Temperature Effect on Electrical Characteristics of Al/SiO2/n++-Si RRAM Devices.

In this work, we investigate the effect of temperature on the electrical characteristics of Al/SiO2/n++-Si RRAM devices. We study the electroforming process and show that forming voltage and time-to-breakdown are well described by Weibull distribution. Experimental current–voltage characteristics of Al-SiO2-(n++Si) structures are presented and discussed at different temperatures. We show that some intermediate resistance states can be observed at higher temperatures. In our analysis, we identify Space Charge Limited Conduction (SCLC) as the dominating transport mechanism regardless of the operating temperature.

Electrical and structural properties of binary Ga–Sb phase change memory alloys

Material properties of Ga–Sb binary alloy thin films deposited under ultra-high vacuum conditions were studied for analog phase change memory (PCM) applications. Crystallization of this alloy was shown to occur in the temperature range of 180–264 °C, with activation energy >2.5 eV depending on the composition. X-ray diffraction (XRD) studies showed phase separation upon crystallization into two phases, Ga-doped A7 antimony and cubic zinc-blende GaSb. Synchrotron in situ XRD analysis revealed that crystallization into the A7 phase is accompanied by Ga out-diffusion from the grains. X-ray absorption fine structure studies of the local structure of these alloys demonstrated a bond length decrease with a stable coordination number of 4 upon amorphous-to-crystalline phase transformation. Mushroom cell structures built with Ga–Sb alloys on ø110 nm TiN heater show a phase change material resistance switching behavior with resistance ratio >100 under electrical pulse measurements. TEM and Energy Dispersive Spectroscopy (EDS) studies of the Ga–Sb cells after ∼100 switching cycles revealed that partial SET or intermediate resistance states are attained by the variation of the grain size of the material as well as the Ga content in the A7 phase. A mechanism for a reversible composition control is proposed for analog cell performance. These results indicate that Te-free Ga–Sb binary alloys are potential candidates for analog PCM applications.