Thin Film Memristor Research Articles

Laser assembly of CeO2 nanobrushes and their resistive switching performance

Memristors in a metal/oxide/metal configuration emerge as an advanced non-volatile information storage technology due to their high operating speed and low power consumption. Understanding the role of the microstructure of the active oxide layer in its resistive switching (RS) performance is scientifically important for revealing the conduction mechanism of oxides, and technically critical for the development of high-performance memristors. Here, self-assembly of vertically aligned CeO2 nanobrushes, a typical RS material, is demonstrated in pulsed laser epitaxy. The growth map of CeO2 is fully explored, and optimized growth conditions for CeO2 nanobrushes are established. Microstructure and crystallinity characterizations reveal the single-crystalline epitaxy nature of CeO2 nanobrushes. A nanobrush memristor exhibits a threshold switching feature with an On/Off ratio of 50, as 16 times high as a thin-film memristor, and excellent endurance of up to 500 cycles and retention time of up to 104 seconds. Theoretical fitting of the I-V curves of the thin-film and nanobrush memristors show interfacial switching in the thin-film memristor and filamentary switching in the nanobrush memristor. The distinct RS mechanisms between thin films and nanobrushes suggest the fundamental role of the structural design of active oxide layers in the RS performance of oxide-based memristors.

Improvement of Memristive Characteristic of In-Ga-Zn-O Thin-Film Memristor by Conductive Buffer Layers

Improvement of Memristive Characteristic of In-Ga-Zn-O Thin-Film Memristor by Conductive Buffer Layers

A library of 2D electronic material inks synthesized by liquid-metal-assisted intercalation of crystal powders

Solution-processable 2D semiconductor inks based on electrochemical molecular intercalation and exfoliation of bulk layered crystals using organic cations has offered an alternative pathway to low-cost fabrication of large-area flexible and wearable electronic devices. However, the growth of large-piece bulk crystals as starting material relies on costly and prolonged high-temperature process, representing a critical roadblock towards practical and large-scale applications. Here we report a general liquid-metal-assisted approach that enables the electrochemical molecular intercalation of low-cost and readily available crystal powders. The resulted solution-processable MoS2 nanosheets are of comparable quality to those exfoliated from bulk crystals. Furthermore, this method can create a rich library of functional 2D electronic inks ( >50 types), including 2D wide-bandgap semiconductors of low electrical conductivity. Lastly, we demonstrated the all-solution-processable integration of 2D semiconductors with 2D conductors and 2D dielectrics for the fabrication of large-area thin-film transistors and memristors at a greatly reduced cost.

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.

Investigation of Grain Boundary Effects in Sm0.2Ce0.8O2-x Thin Film Memristors.

Cerium-based materials (CeO2-x) are of significant interest in the development of vacancy-modulated resistive switching (RS) memory devices. However, the influence of grain boundaries on the performance of memristors is very limited. To fill this gap, this study explores the influence of grain boundaries in cerium-based thin film resistive random-access memory (RRAM) devices. Sm0.2Ce0.8O2-x (SDC20) thin films were deposited on (100)-oriented Nb-doped SrTiO3 (NSTO) and (110)-oriented NSTO substrates using pulsed laser deposition (PLD). Devices constructed with a Pt/SDC20/NSTO structure exhibited reversible and stable bipolar resistive switching (RS) behavior. The differences in conduction mechanisms between single-crystal and polycrystalline devices were confirmed, with single-crystal devices displaying a larger resistance window and higher stability. Combining the results of XPS and I-V curve fitting, it was confirmed that defects near the grain boundaries in the SDC-based memristors capture electrons, thereby affecting the overall performance of the RRAM devices.

Thin-film memristor using an amorphous metal–oxide semiconductor with a gradient composition of conducting components

A thin-film memristor using an amorphous metal–oxide semiconductor (AOS) with a gradient composition of conducting components has been developed, which is another way to achieve memristive properties. The advantages are that conductivity distribution is already obtained as fabricated without forming operation, and an analog characteristic is obtained by optimizing the component composition in AOS. As the binary characteristic, the set operation induces the transition from a low conductance state to a high conductance state, whereas the reset operation does vice-versa. The switching ratio (SR) is high, 448. As the analog characteristic, the SR becomes greater rapidly as the V set increases, namely, the dynamic range is outstanding.

Modulating resistive switching in Ni0.5Zn0.5Fe2O4 memristors: The role of manganese doping concentrations

Memristors, recognized as key devices for mitigating the von Neumann bottleneck, show great potential in non-volatile memory technology. Their performance is fundamentally linked to the chemical composition and structural characteristics of their intermediate dielectric layer. Owing to the inherent advantages of metal oxides, such as their simplistic chemical makeup and superior thermal stability, memristors featuring a Metal-Insulator-Metal (MIM) sandwich structure with metal oxides as the intermediate layer have received extensive focus. In this study, a Pt/Ni0.5Zn0.5MnxFe2-xO4(NZMFO)/Pt composite film was prepared, and its resistance switching behavior was modulated by altering the doping concentration of Mnn+ in NZMFO. The experimental results indicate that altering the Mnn+ doping concentration does significantly impact the content of oxygen vacancies. As the concentration of Mnn+ increases from 0.10 to 0.20 at.%, the content of oxygen vacancies correspondingly rises from 46.58 % to 76.42 %. Concurrently, variations in Mnn+ doping concentration significantly alter the resistance switching properties of the Pt/NZMFO/Pt composite films. This includes elevating the set/reset voltage, diminishing the resistance ratio, and enhancing the durability with the augmentation of Mnn+ concentration. By meticulously adjusting the Mnn+ doping concentration, we can achieve a memristor characterized by superior performance across all metrics. Considering all performance aspects, this study determined that the Pt/NZMFO/Pt thin film memristor with x = 0.15 exhibits adequate endurance (100 cycles), high data retention, and stable set/reset voltage. This work underscores the critical role of Mnn+ doping concentration in optimizing the electrochemical properties of NZFO memristors, illustrating a strategic pathway to engineer memristive devices with enhanced performance for future non-volatile memory applications.

MOF thin film memristor prototype of 10×10 memory cells for automated electronic data recording

Metal–organic frameworks (MOFs) have recently emerged as a new class of functional materials for opto- and micro-electronic applications. Herein, the transition from laboratory samples of devices to their prototypes remains a challenge. Here we report a prototype of memristive device based on MOF (HKUST-1). The MOF thin film with a thickness of 130 nm and a size of 1 × 1 in. has been fabricated Layer-by-layer technique on a conductive substrate followed by the deposition of top Ag contacts. A fully automated process of applying voltage to write the binary data (at 0.8 ± 0.1 V), read it (by 0.4 V) and then erase (by inverted polarity with 0.4 ± 0.1 V) made it potentially possible to process arbitrary words for at least 10 cycles. The provided prototype, consisting of 10 × 10 memory cells, opens up prospects for real-life application of MOF-based logic elements, as well as reveals the challenges for their fabrication and exploitation.

Investigation of resistive switching properties in acceptor-induced Sr(Fe,Ti)O3 thin film memristor

In this work, the amorphous SrFe0.1Ti0.9O3-δ (a-SFT10) thin film was prepared on FTO/Glass substrate by chemical solution deposition process and then annealed at 500 ℃. Step Profiler measurements and SEM images revealed that the thickness of the a-SFT10 film was around 300 nm. The a-SFT10 thin film was found to have a smooth surface but was not crystalline. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of the film. Fe2+ and Fe3+ coexist in a-SFT10, according to XPS results, with Fe3+ being the dominating state. The mixed valence state of iron ions may be related to oxygen vacancies. Current-voltage characteristics were conducted to study the electrical properties. The uniform switching voltages, high switch ratio and good endurance were obtained. The ratio of high and low resistance (RH/RL) is up to 103 under positive bias voltage. The physical mechanisms underlying resistance switching behaviors were also investigated. The results show that the Ohmic, space charge limited current and Schottky conduction mechanisms dominated in different voltage ranges of A-SFT10 films, respectively. The oxygen vacancies associated with the acceptor doping play a significant role in the current characteristic in Au/a-SFT10/FTO/Glass device. Our findings suggest that the device has potential applications in future resistive random access memory.

The plasticity of synaptic memristor based on 2D-MoS2 thin film prepared in large-scale by a PLD-assisted CVD method

Two-dimensional MoS2 film was directly grown via a two-step chemical vapour deposition (CVD) method in a large-scale on a phosphorus doped silicon substrate, where a precursor of Mo was Pre-deposited by pulsed laser deposition (PLD) to ensure the homogeneity of Mo source. The film was confirmed to be dense, uniform and continuous nanocrystalline of MoS2 by the characterization of XRD, Raman, SEM, TEM and XPS. Then MoS2 film was employed as the channel materials to fabricate the memristor with lateral structure of Cu/MoS2/Cu. Consequently, the devices show stable nonvolatile resistance switching characteristics, such as low operating voltages, repeated cyclic sweeps (600 times) without decrease and long retention time (>104 s). Such mechanism of resistive switching can be attributable to the space-charge limited conduction dominated by defects. Importantly, a variety of biological synaptic functions, such as synaptic potentiation, depression and repetitive learning processes, are effectively simulated. Furthermore, the relationships between the resistance state and the parameters of pulse sequence were explored based on MoS2 thin film memristors. This research provides a feasible method for manufacturing MoS2 based on memristor, which is used for information and computing applications in neural morphological devices.

Ga-Sn-O Thin-Film Memristor and Analog Plasticity Characteristic

A Ga-Sn-O (GTO) thin-film memristor has been developed, and an analog plasticity characteristic has been observed. First, GTO thin-film memristors are fabricated by depositing three GTO layers in a stacked structure using sputtering. Next, the current-voltage characteristics are measured by varying the maximum applied voltage after a certain negative voltage, the hysteresis is observed, and the switching characteristics are evaluated, which is regarded as that an analog plasticity characteristic is observed. The parametric study is done for the stacked structure and deposition process, and the proposed operating mechanism is that oxygen vacancies reciprocate by applying negative and positive voltages. Finally, the pulse application characteristic shows the long-term potentiation and depression, which presents a practical possibility to utilize the GTO thin-film memristor for neuromorphic systems.

Solution-Processed Synaptic Memristors Based on Halide Perovskite Nanocrystals.

Exploring new materials and structures to construct synaptic devices represents a promising route to fundamentally approach novel forms of computing. Nanocrystals (NCs) of halide perovskites possess unique charge transport characteristics, i.e., ionic-electronic coupling, holding considerable promise for energy-efficient and reconfigurable artificial synapses. Herein, we report solution-processed thin-film memristors from all-inorganic CsPbBr3 perovskite NCs, functioning as an electrically programmable analog memory with good stability. The devices are demonstrated to successfully emulate a number of essential synaptic functions with low power consumption, including reversible potentiation and depression, short-term plasticity (STP), paired-pulse facilitation (PPF), and long-term plasticity (LTP), such as spike-number-dependent plasticity (SNDP), spike-rate-dependent plasticity (SRDP), spike-timing-dependent plasticity (STDP), and spike-voltage-dependent plasticity (SVDP). It is proposed that a coupled capacitive and inductive phenomenon originating from charge trapping and ion migration in CsPbBr3 NC films, controlled by the amplitude and timing of the programming pulses, defines the degree of synaptic plasticity. A transition emerges from the fast trap-related capacitive regime to a slow ionic inductive regime, which enables continuous change of the film resistance and the magnitude of the electronic current, analogous to the synaptic weight modulation in biological synapses.

Ferroelectric polymer thin-film memristors with asymmetric top electrodes

A type of ferroelectric polymer thin-film memristors is demonstrated to show their potential in the emulation of synaptic plasticity. The memristive characteristics of the device arise from the design of asymmetric top electrodes capacitively coupled with a floating bottom electrode, which enables the local modulation of ferroelectric polarization in the ferroelectric terpolymer film. Basic synaptic functions, such as continuous increase/decrease in synaptic weight and paired-pulse facilitation, are successfully emulated using the ferroelectric polymer memristors. This work provides a promising approach to implementing ferroelectric mechanisms in electronic synapses.

Back‐End‐of‐Line SiC‐Based Memristor for Resistive Memory and Artificial Synapse

AbstractTwo‐terminal memristor has emerged as one of the most promising neuromorphic artificial electronic devices for their structural resemblance to biological synapses and ability to emulate many synaptic functions. In this work, a memristor based on the back‐end‐of‐line (BEOL) material silicon carbide (SiC) is developed. The thin film memristors demonstrate excellent binary resistive switching with compliance‐free and self‐rectifying characteristics which are advantageous for the implementation of high‐density 3D crossbar memory architectures. The conductance of this SiC‐based memristor can be modulated gradually through the application of both DC and AC signals. This behavior is demonstrated to further emulate several vital synaptic functions including paired‐pulse facilitation (PPF), post‐tetanic potentiation (PTP), short‐term potentiation (STP), and spike‐rate‐dependent plasticity (SRDP). The synaptic function of learning‐forgetting‐relearning processes is successfully emulated and demonstrated using a 3 × 3 artificial synapse array. This work presents an important advance in SiC‐based memristor and its application in both memory and neuromorphic computing.

A TSTDP memristive synapse based on a comprehensive mathematical model of memory-TFT threshold voltage shift

New emerging nano-scale technologies like hydrogenated noncrystalline-silicon thin-film transistors (TFTs) and memristors, fabricated at low temperatures and over large areas, permit low-cost processing and 3D integration with CMOS cores. Here, we aim to propose a mathematical model which explains the memory-TFT threshold voltage shift due to the gate bias instability. Then, based on this mathematical approach, we propose a novel learning synapse composed of a voltage/flux driven memristor in parallel with a common-source memory-TFT with a memristive load. The proposed device realizes the triplet-based spike-timing-dependent plasticity rule (TSTDP) as a more realistic form of learning than the purely pair-based STDP (PSTDP). PSTDP is a synaptic learning rule which utilizes a constant-frequency pairing protocol to induce synaptic weight change and cannot explain the modification due to the frequency changes of spike pairs, and also the outcomes of triplet and quadruplet experiments. However, TSTDP improves the learning capabilities of the conventional PSTDP and reproduces the results of more electrophysiological experiments. In this paper, we apply various spike patterns like different-frequency and different-timing spike pairs, spike triplets, and quadruplets to the proposed device. Our simulations confirm a close match with the experimental data sets of real biological synapses.

Memristor–CMOS hybrid ultra-low-power high-speed multivibrators

Memristor–CMOS (MCM) technology enables fabrication of thin film memristors over the conventional CMOS devices and has the potential to significantly reduce the silicon-area and propagation delays in VLSI chips. The memristor not only has an extremely very useful characteristic of non-volatile memory, but also has the advantage of significantly lesser ON resistance $$R_{on}$$ and lesser undesired parasitic capacitance as compared to MOSFETs. Innovative MCM hybrid re-configurable circuits can outperform conventional CMOS-only design and hence are being considered as an important device for future digital VLSI applications. This paper presents applications of $${\mathrm{TiO}}_{2-x}$$ – $${\mathrm{TiO}}_{2}$$ memristor for digital multivibrator circuits at 45 nm CMOS technology node. The threshold adaptive memristor SPICE model has been used for design and performance-benchmarking of the proposed MCM multivibrators in circuit simulator (cadence®) at 45 nm technology node. The proposed new MCM mono-stable vibrator has a delay of merely 16 ps (98% lower delay than similar CMOS design) and requires a 45% lesser silicon area. Similarly, the proposed new MCM bi-stable vibrator has a delay of merely 5 ps (87% lower delay than similar CMOS design) and requires a 25% lesser silicon area. Moreover, the proposed MCM mono-stable and bi-stable multivibrators consume merely 0.1 $$\upmu$$ W and 0.5 $$\upmu$$ W of power, respectively, as compared to the 0.47 $$\upmu$$ W and 0.98 $$\upmu$$ W power required by corresponding CMOS-only multi-vibrators. The overall decrease in power delay product is 99% and 94%, respectively, in the proposed MCM mono-stable and bi-stable multivibrators as compared to the corresponding conventional CMOS-only multivibrators.

Electron Doping Effect in the Resistive Switching Properties of Al/Gd1-xCaxMnO3/Au Memristor Devices.

We report on the resistive switching (RS) properties of Al/Gd1–xCaxMnO3 (GCMO)/Au thin-film memristors. The devices were studied over the whole calcium substitution range x as a function of electrical field and temperature. The RS properties were found to be highly dependent on the Ca substitution. The optimal concentration was determined to be near x = 0.9, which is higher than the values reported for other similar manganite-based devices. We utilize an equivalent circuit model which accounts for the obtained results and allows us to determine that the electrical conduction properties of the devices are dominated by the Poole–Frenkel conduction mechanism for all compositions. The model also shows that lower trap energy values are associated with better RS properties. Our results indicate that the main RS properties of Al/GCMO/Au devices are comparable to those of other similar manganite-based materials, but there are marked differences in the switching behavior, which encourage further exploration of mixed-valence perovskite manganites for RS applications.

A Leaky-Integrate-and-Fire Neuron Model of Spontaneous Reset of Thin-Film Metal-Oxide Resistive Switches.

Resistive switches in crossbar arrays introduce one potential option to push past the limits of CMOS process scaling, with advantages including low switching thresholds (<3 V), high integrability with CMOS, and fast switching speeds (<10 ns). These typically employ a 1T1R scheme for each cell, where the transistor is deployed for selection and sneak path mitigation. However, when conductive filaments are formed in metal-oxide resistive switches, it is often the case that analog states are not thermodynamically favorable, and will spontaneously set or reset to a more stable state. This causes stochastic switching, variability, and non-reproducibility, in a manner which cannot be harnessed in stochastic gradient descent. Equally important is the memory leakage problem that is introduced. In this work, we present a generalized neuron model of resistive switching in the development of a phase plane characterization, and verify its operation by comparing it to our own in-house fabricated thin-film titanium-oxide memristor array. We show an alternative design methodology that draws inspiration from the leaky-integrate-and-fire neuron model. The advantages exhibited by such a methodology are to provide more biologically accurate neuronal model and to enable large scale simulations, demonstrated by the 30% improvement in speed over similar device models.

Egg-White-Based Polymer Memristors With Competing Electronic-Ionic Effect and Timescale-Dependent Current Modulation

Polymer thin film memristors based on the egg-white dielectric are achieved through a simple solution process, showing the coexistence of the clockwise and counterclockwise current hysteresis in a single device. Such unique memristive feature originates from two competing effects in the egg-white dielectric that possess different effective time ranges. The ion redistribution and charge trapping effects are dominant over relatively short and long timescales, respectively. The reversible and timescale dependent memristive characteristics in the polymer memristors may offer a new strategy towards organic electronic synapses.

SPICE Model of Current Polarity-Dependent Piecewise Linear Window Function for Memristors

Memristor and memristive systems are nonlinear systems. It is important to model them accurately. There are different memristor models and most of the models make use of window functions. In literature, there are various window functions. Recently, a piecewise linear (PWL) window function is used to model a memristor and memristive systems. Such a memristor with a PWL window function lacks a SPICE model. Also, in literature, there is current polarity dependent window functions proposed for memristors to model polarity dependent drift speed within the thin-film memristors. In this study, an alternative current-polarity dependent PWL window function is suggested to model a memristor, a different PWL function one for each current polarity is used, its SPICE model is made in LTSpice and also its simulation results are given. Such a model can be used to model the polarity dependent drift speed within the thin-film memristors.