Resistive Switching Properties of Sol–Gel-Derived V-Doped SrTiO3 Thin Films

V-doped and undoped SrTiO₃ (V:STO and STO) thin films on Pt/Ti/SiO₂/Si substrates were synthesized using a sol–gel method to form metal–insulator–metal (MIM) structures. Coexistence of the bipolar and unipolar resistive switching (BRS and URS) modes in Pt/STO/Pt and Pt/V:STO/Pt structures was observed as a irreversible transition from BRS to URS on adjustment of the compliance current (I _comp). Both states were stable and reproducible over 60 cycles, and the maximum operating voltage of the Pt/STO/Pt was reduced from 10 V to 2 V by doping with V. Linear fitting of current–voltage curves suggests that space-charge-limited leakage was the limiting leakage mechanism for these two devices. Based on these results, a switching mechanism based on filament theory is proposed to explain both resistive switching modes.     

Effect of oxygen concentration on resistive switching behavior in silicon oxynitride film

SiO_xN_y films with different oxygen concentrations were fabricated by reactive magnetron sputtering, and the resistive switching characteristics and conduction mechanism of Cu/SiO_xN_y/ITO devices were investigated. The Cu/SiO_xN_y/ITO device with SiO_xN_y deposited in 0.8-sccm O₂ flow shows a reliable resistive switching behavior, including good endurance and retention properties. As the conductivity of SiO_xN_y increases with the increase of the oxygen content dynamical electron trapping and detrapping is suggested to be the conduction mechanism. The temperature dependent I-V measurement indicates that the carrier transport can be ascribed to the hopping conduction rather than the metallic conductive filament.     

Impact of Bi Deficiencies on Ferroelectric Resistive Switching Characteristics Observed at p‐Type Schottky‐Like Pt/Bi1–δFeO3 Interfaces

This work reports a resistive switching effect observed at rectifying Pt/Bi_1–δFeO₃ interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO₃, Bi‐deficient Bi_1–δFeO₃ films act as a p‐type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi_1–δFeO₃ interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage‐sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse‐voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >10⁵ cycles and data retention of >10⁵ s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi_1–δFeO₃ interfaces to nonvolatile memory.     

Resistive Random Access Memory Cells with a Bilayer TiO2/SiOX Insulating Stack for Simultaneous Filamentary and Distributed Resistive Switching

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO₂, SiO_X, and n⁺⁺Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.     

Mechanism of resistive switching in films based on partially fluorinated graphene

The mechanism of resistive switching in films of partially fluorinated graphene is investigated. The films are obtained on the basis of a number of graphene suspensions with different composition and various degree of fluorination. The dependence of the value of resistive switching on the presence of organic additives (N-methylpyrrolidone and dimethylformamide) in the suspension composition and the degree of fluorination are found experimentally. It is shown that, for films obtained from a suspension without organic components, no resistive switching is observed independently of the degree of fluorination. The most profound effect of ~10–20 times is found for films containing dimethylformamide. A physical model for the films is proposed; according to this model, the dependence under observation is related to the presence of encapsulated functional groups formed from dimethylformamide molecules manifesting electrical activity in the investigated films. The measured current–voltage characteristics of the films are described in the best way by the Frenkel–Poole model with an activation energy of 0.08 eV of centers responsible for conduction. The value of switching depends on the degree of fluorination of the initial suspensions. The greatest effect is observed at a degree of fluorination of the suspensions of ~CF _0.25. Suspensions of partially fluorinated graphene investigated in this study are suitable for fabricating device structures by the 2D-printing technology.     

How Does Moisture Affect the Physical Property of Memristance for Anionic–Electronic Resistive Switching Memories?

Memristors based on anionic–electronic resistive switches represent a promising alternative to transistor‐based memories because of their scalability and low power consumption. To date, studies on resistive switching have focused on oxygen anionic or electronic defects leaving protonic charge‐carrier contributions out of the picture despite the fact that many resistive switching oxides are well‐established materials in resistive humidity sensors. Here, the way memristance is affected by moisture for the model material strontium titanate is studied. First, characterize own‐processed Pt|SrTiO_3‐δ|Pt bits via cyclic voltammetry under ambient conditions are thoroughly characterized. Based on the high stability of a non‐volatile device structures the impact of relative humidity to the current–voltage profiles is then investigated. It is found that Pt|SrTiO_3‐δ|Pt strongly modifies the resistance states by up to 4 orders of magnitude as well as the device's current–voltage profile shape, number of crossings, and switching capability with the level of moisture exposure. Furthermore, a reversible transition from classic memristive behavior at ambient humidity to a capacitively dominated one in dry atmosphere for which the resistive switching completely vanishes is demonstrated for the first time. The results are discussed in relation to the changed Schottky barrier by adsorbed surface water molecules and its interplay with the charge transfer in the oxide.     

Effect of an external bias voltage on the photoelectric properties of silicon MIS/IL structures

The mechanism by which an external bias voltage influences the photoelectric properties of Al/tunneling-thin SiO₂/p-Si structures with an induced inversion layer is investigated theoretically. A characteristic feature of the structure studied is the presence of a special inversion grid. Between this grid and the substrate a positive voltage is applied. Relations expressing the functional dependence on the bias voltage of the structure parameters and the output electrical characteristics of photocells, which are based on it, are obtained. The results of numerical calculations illustrating the effectiveness of using a bias voltage to increase the efficiency of photocells based on Al/SiO₂/p-Si structures with an induced inversion layer are presented. Fiz. Tekh. Poluprovodn. 31, 1273–1277 (October 1997)     

Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

Direct Observation of Conversion Between Threshold Switching and Memory Switching Induced by Conductive Filament Morphology

Volatile threshold switching (TS) and non‐volatile memory switching (MS) are two typical resistive switching (RS) phenomena in oxides, which could form the basis for memory, analog circuits, and neuromorphic applications. Interestingly, TS and MS can be coexistent and converted in a single device under the suitable external excitation. However, the origin of the transition from TS to MS is still unclear due to the lack of direct experimental evidence. Here, conversion between TS and MS induced by conductive filament (CF) morphology in Ag/SiO₂/Pt device is directly observed using scanning electron microscopy and high‐resolution transmission electron microscopy. The MS mechanism is related to the formation and dissolution of CF consisting of continuous Ag nanocrystals. The TS originates from discontinuous CF with isolated Ag nanocrystals. The results of current–voltage fitting and Kelvin probe force microscopy further indicate that the TS mechanism is related to the modulation of the tunneling barrier between Ag nanocrystals in CF. This work provides clearly experimental evidence to deepen understanding of the mechanism for RS in oxide‐electrolyte‐based resistive switching memory, contributing to better control of the two RS behaviors to establish high‐performance emerging devices.     

Exploiting Memristive BiFeO3 Bilayer Structures for Compact Sequential Logics

Resistive switching devices are considered as one of the most promising candidates for the next generation memories and nonvolatile logic applications. In this paper, BiFeO₃:Ti/BiFeO₃ (BFTO/BFO) bilayer structures with optimized BFTO/BFO thickness ratio which show symmetric, bipolar, and nonvolatile resistive switching with good retention and endurance performance, are presented. The resistive switching mechanism is understood by a model of flexible top and bottom Schottky‐like barrier heights in the BFTO/BFO bilayer structures. The resistive switching at both positive and negative bias make it possible to use both polarities of reading bias to simultaneously program and store all 16 Boolean logic functions into a single cell of a BFTO/BFO bilayer structure in three logic cycles.     

Resistive switching characteristics of MIM structures based on oxygen-variable ultra-thin HfO2 and fabricated at low temperature

We report the resistive switching characteristics of Metal-Insulator-Metal (MIM) structures fabricated at low temperature and having different concentrations of oxygen vacancies in the insulator. The oxygen modulation in HfO₂ is promoted by a very simple variation of standard thermal Atomic-Layer Deposition (ALD), so that different exposure times to H₂O during each half-cycle of the hafnium oxide deposition are used (being Tetrakis Dimethylamino Hafnium–TDMAH the other precursor). We show the correlation of the stoichiometry with the forming voltage, conduction mechanisms and resistance windows of memory devices. All structures present a bipolar operation mode in which the resistive switching mechanism is related to the migration of oxygen vacancies inside the dielectric. These MIM devices have a simple structure, low power consumption and they are fabricated using a very low thermal budget of only 250 °C, thus enabling their integration at the Back-End of Line (BEOL) stage of an integrated circuit in order to increase the density of memory arrays in at least one order of magnitude.     

Low‐Dimensional Lead‐Free Inorganic Perovskites for Resistive Switching with Ultralow Bias

3D organic–inorganic and all‐inorganic lead halide perovskites have been intensively pursued for resistive switching memories in recent years. Unfortunately, instability and lead toxicity are two foremost challenges for their large‐scale commercial applications. Dimensional reduction and composition engineering are effective means to overcome these challenges. Herein, low‐dimensional inorganic lead‐free Cs₃Bi₂I₉ and CsBi₃I₁₀ perovskite‐like films are exploited for resistive switching memory applications. Both devices demonstrate stable switching with ultrahigh on/off ratios (≈10⁶), ultralow operation voltages (as low as 0.12 V), and self‐compliance characteristics. 0D Cs₃Bi₂I₉‐based device shows better retention time and larger reset voltage than the 2D CsBi₃I₁₀‐based device. Multilevel resistive switching behavior is also observed by modulating the current compliance, contributing to the device tunability. The resistive switching mechanism is hinged on the formation and rupture of conductive filaments of halide vacancies in the perovskite films, which is correlated with the formation of AgI_x layers at the electrode/perovskite interface. This study enriches the library of switching materials with all‐inorganic lead‐free halide perovskites and offers new insights on tuning the operation of solution‐processed memory devices.     

Organic nonpolar nonvolatile resistive switching in poly(3,4-ethylene-dioxythiophene): Polystyrenesulfonate thin film

In this paper, the reproducible nonpolar resistive switching is demonstrated in devices with the sandwiched structure of Au/poly(3,4-ethylene-dioxythiophene): polystyrenesulfonate/Au for nonvolatile memory application. The switching between high resistance state (OFF-state) and low resistance state (ON-state) does not depend on the polarity of the applied voltage bias, which is different from both the WORM characteristics and the bipolar switching characteristics reported before. The resistive ratio between the ON- and OFF-state is on the order of 10³ and increases with the device area decreasing. Both the ON- and OFF-state of the memory devices are stable, showing no significant degradation over 10⁴ s under continuous readout testing. It is proposed that the reduction and oxidation of PEDOT: PSS film might be the switching mechanism.     

Size effect on the electrical conduction and noise of RuO2-based thick film resistors

Temperature dependence of sheet resistance for a generic RuO₂-based resistor with a composition of 20wt.% RuO₂-80wt.% glass (63wt.% PbO-25wt.% B₂O₃-12wt.% SiO₂) is evaluated. A combined tunnel/parallel conduction model is employed to describe the resistance behavior with respect to the temperature variation. The geometry of the resistive film, such as the aspect ratio and thickness, cast a significant effect on the electrical characteristic of the thick film assembly. It is observed that shorter resistive films exhibit smaller resistivity as compared to that of the longer film. Thinner resistive films have smaller resistivity as compared to the thicker ones. In addition,1/f noise is the dominating contribution in the thick film resistor. The presence of1/f noise can be qualitatively explained with the aid of the tunneling mechanism.     

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices

Hafnium oxide (HfO_x)‐based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two‐terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfO_x/TiN‐based metal–insulator–metal structures as model systems, it is shown that a well‐controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half‐integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfO_x will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.     

Investigation on switching behavior of ZrO2 thin film for memory device applications

In this paper, we have investigated the resistive switching behavior of nanostructured zirconium oxide (ZrO₂) thin film deposition by spin coating. The metal (silver) electrodes were patterned by electrohydrodynamic inkjet printing technique. The X-ray diffraction and Fourier transform infra-red spectra confirmed that the presence of monoclinic phase in the as deposited ZrO₂ thin film. The field emission scanning electron microscopic image revealed the uniform deposition of ZrO₂ thin film with spherical morphology. The as-fabricated Ag/ZrO₂/Ag memory device exhibited the characteristic bipolar resistive switching behavior under consecutive dc sweep. The possible mechanism of the bipolar resistive switching has been discussed in detail. The endurance and retention analysis of the fabricated device revealed the stability of the device. Our results ensure the promising applications of ZrO₂ thin film in the memory device applications.     

The investigation into charge degradation of MIS structures under strong electric field by a method of controlled current load

A new method for studying charge degradation of MIS structures by applying a controlled current load to the structure and taking the time dependence of the voltage is suggested. It allows designers (without switching the structure) to monitor changes in the charge state of MIS structures under the conditions when the capacitance is charged and the charge is injected into the insulator. Charge degradation of metal-PSG-passivated thermal silicon dioxide-insulator structures was studied. It was found that both the SiO₂ space charge and the density of fast surface states generated by tunnel electron injection from the SiO₂ electrode decrease once current load has disappeared.     

C-AFM study on multi - resistive switching modes observed in metal–organic frameworks thin films

Metal-organic framework (MOF) materials have recently attracted much attention for use in resistive random-access memory due to the advantages of having high insulative properties, well-defined structures, a large specific surface area, and an adjustable pore size. In this study, the memory device based on zirconium (IV)-carboxylate MOF (UiO-66) nanoparticles exhibits the low operation voltage (V < 0.5 V), high ON/OFF ratio (~10⁴), excellent endurance (5 × 10² cycles), and longtime retention (10⁴ s). To clarify the resistive switching mechanism of the Ag/PVA-MOF/FTO device, conductive atomic force microscopy (C-AFM) was used. The results indicate that all of the electrode, Zr₆ clusters of UiO-66, PVA, and UiO-66 conjugation have simultaneous contributions to the resistance switching behavior. The resistive switching can be controlled either by the electron hopping process between Ag⁺ ions and Zr₆ nodes in threshold mode or the formation/rupture of the metal filaments in the bipolar switching mode. Interestingly, inherent characteristics of MOF materials, such as high porosity and large size cages (octahedral, tetrahedral), strongly influence the transport properties and switching mechanism of the device which is also discussed in detail. These resistive switching characteristics and mechanisms of UiO-66 could provide a thorough understanding for future research and application not just for UiO-66 but also for the general MOFs materials.     

The resistive switching characteristics of a Ti/Gd2O3/Pt RRAM device

We successfully fabricated the Gd₂O₃ film for the application of resistive random access memory (RRAM). The resistive switching behavior of the Ti/Gd₂O₃/Pt capacitor structure could be both operated under positive or negative bias. However, there was a significant difference on the switching properties. The switching behavior under positive bias operation was more stable, had less voltage and resistance fluctuation, and had longer endurance than that of the negative one. We propose that the anode electrode plays an important role in the switching characteristics and may be the cause of the asymmetry of the I-V curves between positive and negative operation.     

Hot carrier effect—model, mechanism and effects on C-V and I-V characteristics in MOS structures

The effects of hot carrier injection on C-V and I-V characteristics in MOS structures are discussed. The charge trapping and generation of interface states caused by the hot carrier effect lead to C-V characteristic curve distortion, flatband voltage shift and, under a constant voltage, SiO₂ leakage current shift with time. The mechanisms of these shifts are dealt with. The tⁿ physical models of shifts are put forward. The phenomenon observed in experiments is well explained.