Chapter Four - Nature of the Resistive Switching Phenomena in TiO2 and SrTiO3: Origin of the Reversible Insulator–Metal Transition

In Mott‐type resistive switching phenomena, which are based on the metal–insulator transition in strongly correlated materials, the presence of an abrupt temperature‐driven transition in the material is considered essential for achieving high‐speed and large‐resistance‐ratio switching. However, this means that the freedom of material/device design in applications is significantly reduced for this type of switching by the strict requirement of transition abruptness. Here, high‐speed, abrupt resistive switching with a switching time of 140 ns is demonstrated in epitaxial films of Ca2RuO4/LaAlO3 (001), which is a material with a nonthermal metal–insulator transition driven by current, despite the complete absence of an abrupt thermal transition in the resistivity–temperature characteristics. Highly smooth negative‐differential‐resistance behavior, very high cycling stability, and an endurance over 106 cycles are also demonstrated in the current–voltage and current–time characteristics, which confirm the nonstochastic nature of the abrupt switching. These results suggest that strict control of the resistivity–temperature characteristics is not necessarily required in a material with a nonthermal‐type metal–insulator transition to obtain high‐speed resistive switching because of the independence of the dynamics from those of the thermal transition, and this phenomenon potentially has important advantages in resistive switching applications.

Transition metal compounds showing a metal-insulator transition (MIT) show complex behavior due to strongly correlated electron effects and offer attractive properties for nano-electronics applications, which cannot be obtained with regular semiconductors. MIT based nano-electronics, however, remains unproven, and MIT devices are poorly understood. We point out and single out one of the major hurdles preventing MIT-electronics: obtaining a high Off resistance and high On-Off resistance ratio in an MIT switch. We show a path toward an MIT switch fulfilling strict Off and On resistance criteria by: (1) Obtaining understanding of the VO2-interface, a protoypical MIT material interface. (2) Introducing a MIT tunnel junction concept to tune switch resistances. In this junction, the metal or insulating phase of the MIT material controls how much current flows through. Adapting the junction's parameters allows tuning the MIT switch's Off and On resistance. (3) Providing proof of principle of the junction and its switch resistance tuning capability, experimentally in two forms. (4) Showing theoretically how stringent Off and On resistance specifications can be fulfilled. The prototypical VO2 MIT results in an abrupt change in bulk electrical resistivity at ∼68 °C. We show that the VO2 MIT manifests itself in an abrupt interfacial transition of current across a VO2-barrier interface forming a tunnel junction. In a first tunnel junction form, a two orders of magnitude abrupt change in contact resistivity induced by the bulk MIT is shown in VO2-metal contact structures. VO2-metal contact properties are discussed in detail, and the work function of VO2 is found to be 5.2eV(25 °C)−5.3eV(90 °C). In a second junction form, an abrupt change in tunneling current of up to an order of magnitude caused by the bulk MIT is shown to be present in VO2-insulator-metal capacitor structures with atomic layers deposition (ALD) Al2O3 and HfO2 barrier layers. The capacitors show the feasibility of using the MIT to switch a component to a high Off resistance state. Current and capacitance-voltage characteristics of the capacitors are analyzed as well as voltage or field dependent MITs at VO2 interfaces. The abrupt change in current across the VO2 interface is shown to be driven by the change in free carriers in bulk VO2 across the MIT.

Studies on dislocations in prototypic binary and ternary oxides (here TiO2 and SrTiO3) using modern TEM and scanning probe microscopy (SPM) techniques, combined with classical etch pits methods, are reviewed. Our review focuses on the important role of dislocations in the insulator-to-metal transition and for redox processes, which can be preferentially induced along dislocations using chemical and electrical gradients. It is surprising that, independently of the growth techniques, the density of dislocations in the surface layers of both prototypical oxides is high (109/cm2 for epipolished surfaces and up to 1012/cm2 for the rough surface). The TEM and locally-conducting atomic force microscopy (LCAFM) measurements show that the dislocations create a network with the character of a hierarchical tree. The distribution of the dislocations in the plane of the surface is, in principle, inhomogeneous, namely a strong tendency for the bundling and creation of arrays or bands in the crystallographic <100> and <110> directions can be observed. The analysis of the core of dislocations using scanning transmission electron microscopy (STEM) techniques (such as EDX with atomic resolution, electron-energy loss spectroscopy (EELS)) shows unequivocally that the core of dislocations possesses a different crystallographic structure, electronic structure and chemical composition relative to the matrix. Because the Burgers vector of dislocations is per se invariant, the network of dislocations (with additional d1 electrons) causes an electrical short-circuit of the matrix. This behavior is confirmed by LCAFM measurements for the stoichiometric crystals, moreover a similar dominant role of dislocations in channeling of the current after thermal reduction of the crystals or during resistive switching can be observed. In our opinion, the easy transformation of the chemical composition of the surface layers of both model oxides should be associated with the high concentration of extended defects in this region. Another important insight for the analysis of the physical properties in real oxide crystals (matrix + dislocations) comes from the studies of the nucleation of dislocations via in situ STEM indentation, namely that the dislocations can be simply nucleated under mechanical stimulus and can be easily moved at room temperature.

Electric-field-induced resistance switching (RS) phenomena have been studied for over 60 years in metal/dielectrics/metal structures. In these experiments a wide range of dielectrics have been studied including binary transition metal oxides, perovskite oxides, chalcogenides, carbon- and silicon-based materials, as well as organic materials. RS phenomena can be used to store information and offer an attractive performance, which encompasses fast switching speeds, high scalability, and the desirable compatibility with Si-based complementary metal—oxide—semiconductor fabrication. This is promising for nonvolatile memory technology, i.e., resistance random access memory (RRAM). However, a comprehensive understanding of the underlying mechanism is still lacking. This impedes faster product development as well as accurate assessment of the device performance potential. Generally speaking, RS occurs not in the entire dielectric but only in a small, confined region, which results from the local variation of conductivity in dielectrics. In this review, we focus on the RS in oxides with such an inhomogeneous conductivity. According to the origin of the conductivity inhomogeneity, the RS phenomena and their working mechanism are reviewed by dividing them into two aspects: interface RS, based on the change of contact resistance at metal/oxide interface due to the change of Schottky barrier and interface chemical layer, and bulk RS, realized by the formation, connection, and disconnection of conductive channels in the oxides. Finally the current challenges of RS investigation and the potential improvement of the RS performance for the nonvolatile memories are discussed.

Resistive switching (RS) phenomena are reversible changes in the metastable resistance state induced by external electric fields. After discovery ∼50 years ago, RS phenomena have attracted great attention due to their potential application in next-generation electrical devices. Considerable research has been performed to understand the physical mechanisms of RS and explore the feasibility and limits of such devices. There have also been several reviews on RS that attempt to explain the microscopic origins of how regions that were originally insulators can change into conductors. However, little attention has been paid to the most important factor in determining resistance: how conducting local regions are interconnected. Here, we provide an overview of the underlying physics behind connectivity changes in highly conductive regions under an electric field. We first classify RS phenomena according to their characteristic current–voltage curves: unipolar, bipolar, and threshold switchings. Second, we outline the microscopic origins of RS in oxides, focusing on the roles of oxygen vacancies: the effect of concentration, the mechanisms of channel formation and rupture, and the driving forces of oxygen vacancies. Third, we review RS studies from the perspective of statistical physics to understand connectivity change in RS phenomena. We discuss percolation model approaches and the theory for the scaling behaviors of numerous transport properties observed in RS. Fourth, we review various switching-type conversion phenomena in RS: bipolar-unipolar, memory-threshold, figure-of-eight, and counter-figure-of-eight conversions. Finally, we review several related technological issues, such as improvement in high resistance fluctuations, sneak-path problems, and multilevel switching problems.

Resistive random access memory based on the resistive switching phenomenon is emerging as a strong candidate for next generation non-volatile memory. So far, the resistive switching effect has been observed in many transition metal oxides, including strongly correlated ones, such as, cuprate superconductors, colossal magnetoresistant manganites and Mott insulators. However, up to now, no clear evidence of the possible relevance of strong correlation effects in the mechanism of resistive switching has been reported. Here, we study Pr0.7Ca0.3MnO3, which shows bipolar resistive switching. Performing micro-spectroscopic studies on its bare surface we are able to track the systematic electronic structure changes in both, the low and high resistance state. We find that a large change in the electronic conductance is due to field-induced oxygen vacancies, which drives a Mott metal-insulator transition at the surface. Our study demonstrates that strong correlation effects may be incorporated to the realm of the emerging oxide electronics.

Resistive switching (RS) phenomena in oxides have received a large interest for ultra-scaled and high-density non-volatile memories, and many prototypes have been proposed at industrial level. Recently, RS have been also exploited for new type of applications, such as reconfigurable logic and synaptic electronics. In the latter field, the interest is towards RS devices which can be used to fabricate artificial synapses, able to emulate the synaptic functions of biological synapses, and to be integrated with standard CMOS circuits to build neuromorphic systems. These RS devices, also named memristive systems, are of particular interest due to their simple two terminal structure, low power operation, high-scalability, low thermal budget fabrication and, depending on material system, easy integration into CMOS based platform. This talk will first introduce the current state of the art and materials systems investigated for non-volatile memories as well as synaptic devices for neuromorphic circuits, highlighting the materials/device differences versus target application. Then, the talk will present our recent advancements on HfO2 and Al-doped-HfO2 based RS devices. The oxide layers (binary and doped oxides) are deposited by atomic layer deposition and the resistive switching properties are analysed from micro- to nanoscale. The fabrication of high-density and nanoscale HfOx-based memristive devices is achieved by block-copolymer lithography. Furthermore, in view of application of these devices as synaptic elements, the long term plasticity, as potentiation and depression typical of biological synapses, are characterized by various pulsed operation schemes. Special emphasis is given to programming algorithms based on a train of identical pulses. It will be shown that a careful choice of the pulse amplitude/pulse width combination is fundamental to achieve an analogue modulation of the device.

Resistive switching (RS) phenomena are nowadays one of the most studied topics in the area of microelectronics. It can be observed in Metal–Insulator–Metal (MIM) structures that are the basis of resistive switching random-access memories (RRAMs). In the case of commercial use of RRAMs, it is beneficial that the applied materials would have to be compatible with Complementary Metal-Oxide-Semiconductor (CMOS) technology. Fabricating methods of these materials can determine their stoichiometry and structural composition, which can have a detrimental impact on the electrical performance of manufactured devices. In this study, we present the influence of the Ar/N2 ratio during reactive magnetron sputtering of titanium nitride (TiN) electrodes on the resistive switching behavior of MIM devices. We used silicon oxide (SiOx) as a dielectric layer, which was characterized by the same properties in all fabricated MIM structures. The composition of TiN thin layers was controlled by tuning the Ar/N2 ratio during the deposition process. The fabricated conductive materials were characterized in terms of chemical and structural properties employing X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis. Structural characterization revealed that increasing the Ar content during the reactive sputtering process affects the crystallite size of the deposited TiN layer. The resulting crystallite sizes ranged from 8 Å to 757.4 Å. The I-V measurements of fabricated devices revealed that tuning the Ar/N2 ratio during the deposition of TiN electrodes affects the RS behavior. Our work shows the importance of controlling the stoichiometry and structural parameters of electrodes on resistive switching phenomena.

Non-volatile memory technology is significant in the market of electronics products. Until now, Flash memory has dominated the market of non-volatile memories, but due to its scalability problem, scientists have also been calling for new devices. Resistive random access memory ReRAM has attracted great attention due to its potential for flash memory replacement in next generation nonvolatile memory applications. This device is based on resistive switching (RS) phenomenon for operation, which is reversible and can be reversed repeatedly. In this work, device fabrication used doped graphene oxide with copper (GO + 1% Cu) and copper oxide (CuO) thin films. ITO (Indium Tin Oxide) and aluminum were used as contacts. Thin films are obtained by the dip coating technique. The mechanism of the resistive switching effect in doped graphene oxide thin film based devices has been investigated by macroscopic current-voltage (IxV) measurements. The RS originates from the formation and rupture of conducting filaments. In addition, switching effect in these study devices has a clear dependence on the electrode material. ReRAM device structures show unipolar resistive switching behavior.

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials and advanced to the commercial manufacturing stage. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. These are many resistivity changing mechanisms for the oxide based ReRAM structures such as the insulator–metal transition in perovskite oxides and conductive-bridging (CBRAM). In our research, we focus on the CBRAM with nano-electrolyte reaction, whose advantage is the low forming voltage. The CBRAM with nano-electrolyte reaction comprises the generation and rupture of a metal filament using a metal such as Ag and Cu acting as a fast mobile ion in oxides. Hafnium oxide (HfO2), which is used as a high-k gate insulator for advanced complementary metal-oxide-semiconductor (CMOS) technologies, has shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials.1) To put the oxide based ReRAM on practical applications, understanding on controls of metal/oxide interface is essentially important. Here, we employed hard x-ray photoelectron spectroscopy (HX-PES) under bias operation. HX-PES is a powerful tool for investigating the electronic structure and chemical state of the surface/interface of stacking structures for nanoelectronics devices without any degradation because it has a longer photoelectron mean free path than conventional x-ray photoelectron spectroscopy using Al Kα radiation (hν= 1486.6 eV). The detection depth of HX-PES with an energy of 6 keV is approximately three times deeper than that of conventional XPS, so the photoelectron from a metal/oxide interface, which works as an electrical device, can be detected by HX-PES. With this method, bias-induced compositional changes around the metal/oxide interface during device operation have been directly observed. HX-PES was performed at the SPring-8 BL15XU undulator beamline. The incident X-ray energy and the total energy resolution were 5.95 keV and 240 meV, respectively. We have demonstrated resistance switching using HfO2 film with a Cu top electrode for nonvolatile memory applications, and revealed the Cu diffusion into the HfO2 layer during the conductive filament formation process. Resistive switching was clearly observed in the Cu/HfO2/Pt structure by performing current-voltage measurements. The current step from a high resistive state to a low resistive state was of the order of 103-104, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing HX-PES under bias operation. The application of a bias to the structure reduced the Cu2O state at the interface and the intensity ratio of Cu 2p3/2/Hf 3d5/2, providing evidence of Cu2O reduction and Cu diffusion into the HfO2 layer. These results also provide evidence that the resistance switching of the Cu/HfO2/Pt structure originates in a solid electrolyte reaction containing Cu ions. HX-PES also revealed the top or bottom electrode dependences of interface reduction or oxidization, and ion migration behaviors.2-5) In the presentation, we will show the details of the correlation between the switching mechanism and the interface reaction in the electrode/high-k dielectrics based ReRAM structure.6) We are grateful to HiSOR, Hiroshima Univ. and JAEA/SPring-8 for the development of HX-PES at BL15XU of SPring-8. The HX-PES measurements were performed under the approval of the NIMS Beamline Station (Proposal Nos. 2009A 4600, 2010B 4600, 2011A 4611, 2011B 4613, and 2012A 4613).

We have studied the resistive switching (RS) phenomenon in series of BiFeO3 thin films of thickness of 40 – 154 nm deposited by PLD technique on conducting Nb-doped substrate of SrTiO3 and with Pt top electrodes. It was found that 154 nm film demonstrates the interface-provided I–V characteristic of Schottky diode when the applied voltage does not exceed the threshold value Vd = 1.3 V. The RS phenomenon appears as the current hysteresis loop during the 0 → Vm → 0 → −Vm → 0 voltage sweep cycle, provided that the maximal stop-voltage Vm is larger than Vd. For thinner films neither diode-like I–V behavior nor substantial RS effect were observed. The results are interpreted in terms of the filamentary model of the mobile oxygen vacancies.

Non-volatile resistive switching in the Mott insulator (V1−xCrx)2O3

The conventional memory technologies are facing the scaling issues as the semiconductor devices are rapidly approaching the miniaturization limits. Recently the electric-field controlled multilevel resistive memory switching phenomenon in metal oxides has attracted considerable attention to develop next generation low power, high speed, rugged and high density nonvolatile resistive random access memory (RRAM) devices. Multilevel switching of the resistance in RRAM devices promises high capacity memory with capability of storing multiple bits in one device. Generally the materials employed for this application are in polycrystalline form which limits the continuous downscaling when memory cell size becomes comparable to the grain diameter. Amorphous materials free from grain boundaries are capable of offering homogeneous structure to avoid such issue. It is hence well reasoned that amorphous high-k gate dielectrics, which have already been demonstrated to be compatible with semiconductor transistor technologies, may be good choices for RRAM applications. Amongst other high-k gate dielectrics materials currently being explored for the development of RRAM, lanthanum based amorphous high-k oxides have emerged as potential candidates. In this paper we report, for the first time (to the best of our knowledge), multilevel resistive switching (RS) in RRAM devices based on amorphous thin films of LaGdO3(LGO), their resistive switching characteristics, and associated conduction mechanisms.About 50 nm thick amorphous LGO films were grown on commercial Pt/TiO2/SiO2/Si substrates using pulsed laser deposition at a substrate temperature of ~ 300 ºC and oxygen partial pressure of ~ 2×10-3 Torr. Top electrodes of ~ 70 nm thick Pt film with a typical diameter of ~ 80 µm were used to construct MIM capacitors. The RS characteristics and conduction mechanisms of these devices were studied through current-voltage (I-V) measurements in the top-bottom configuration. The as grown devices were found to be initially in high resistance state (HRS) ~ 40 MΩ and did not show any resistance switching behavior until the applied bias voltage was increased to ~7 V (initial forming voltage) with a current compliance (ICC) of 10 mA at which device switched to low resistance stae (LRS). After forming process the device showed reliable and repeatable switching between HRS to LRS with nearly constant resistance ratio ~ 106 and well defined and non-overlapping switching voltages. To achieve multilevel resistance switching, the compliance currents was varied in the range of 1-10 mA during the set process i.e. during switching from HRS to LRS. It was observed that the higher the ICC imposed on the device, the lower resistance value was obtained in LRS. Under different ICC values of 1, 5 and 10mA, three different resistances of LRS having values ~ 2400, 170 and 10Ω were obtained with a nearly constant value of resistance of HRS. All the four states showed good endurance up to 50 cycles provided the desired ICC is maintained along with retention characteristics over 103 seconds. The temperature dependent measurement of resistances of the device in different resistance states revealed that all the three low resistance states were metallic with increasing resistance temperature coefficient (α) on increasing resistance of LRS while HRS showed semiconducting behavior. Based on this and XPS studies we propose that the observed resistive switching can be explained by the formation/rupture of conductive filaments formed out of oxygen vacancies and metallic Gd atom while multilevel conduction in the device can be explained by the variation in the diameter of conducting filament which was further supported by variation in α. The observed reproducible and nonvolatile multilevel resistive switching, good contrast in HRS to LRS resistance ratio, high endurance, and amorphous structure make LGO as a promising material for the future nonvolatile multi-bit RRAM devices.

In the exploration of materials for neuromorphic computing, Graphene Oxide (GO) stands out as a promising organic candidate due to its low-cost fabrication, flexibility, and tunable chemical properties. However, the underlying mechanisms of resistive switching in GO remain unclear. We believe that previous studies may have undervalued the role of GO as the resistive switching medium, focusing instead on various metallic electrodes. In this study, we designed experiments to pinpoint the origin of resistive switching in Graphene Oxide-based Resistive Switching Random Access Memories (RRAM). To investigate the resistive switching mechanisms, we fabricated GO-based RRAMs with three different levels of oxidation: high (3), medium (1.5), and low (0.5). Comparisons using X-ray diffraction analysis (XRD), Raman spectroscopy, and Fourier Transform Infrared (FTIR) spectroscopy confirmed successful adjustments in functional groups, which are expected to play a role in the resistive switching phenomena within GO. The devices created demonstrated stable SET/RESET cycles, with an endurance of 10² cycles and a retention time of 10⁴ seconds. Modifying the oxidation levels unveiled forming-free and analog resistive switching capabilities for the GO(1.5) devices, accompanied by minimal variability—an essential feature for neuromorphic computing. Furthermore, the overshoot transition during the SET process exhibited a slope that was 2.5 times lower for the GO(1.5) RRAM, which is crucial for linear synaptic evolution. We also observed a direct correlation between the Ion/Ioff ratio and oxidation concentration, indicating that oxygen-based functional groups within the GO layers are key components in the resistive switching behavior of these devices. In conclusion, we propose a multifilamentary mechanism for resistive switching that accounts for the differing electrical characteristics of medium and highly oxidized GO-RRAMs.

Charge transport and low-frequency conductance noise in metal-nanoparticle embedded one-dimensional conducting polymer nanotubes: multiple resistive switching phenomena