Electric Field-induced Resistance Switching Research Articles

Resistive switching characteristics of W/TiO2/ITO devices

Materials with resistance-switching properties are proving to be promising for the novel non-volatile memories-resistive random-access memory (ReRAM). Despite extensive research on electric field-induced resistance switching, there are diverse opinions about its reliability and working mechanism. In this work, we analysed the impact of SET and RESET voltages on the switching characteristics and the possible current conduction mechanism of W/TiO2/ITO devices. By simple solution processing and spin coating, titanium dioxide thin films were grown on ITO, glass, silicon, and quartz substrates. The precursor and solvent used for the synthesis of the active layer were titanium isopropoxide and ethanol, respectively. Polycrystalline films with an anatase structure were formed after annealing at 400 °C with a 20 nm average crystallite size, as evidenced by X-ray diffraction patterns. The crystalline nature of the fabricated film has been found to improve on annealing. Using Raman spectroscopy, the chemical bonding states of the film were studied. A UV–visible spectrometer study confirms that anatase TiO2 possesses an indirect band gap transition with a 3.42 eV band gap. The device shows reversible, reproducible, stable, and bipolar resistance-switching behaviour as indicated by current–voltage (I-V) measurements, and it was found that there exists a minimum voltage (∼-4 V) to set the device into an ON state. Compared to crystalline TiO2, theamorphous active layer offers much better switching characteristics, such as good cyclic endurance with a Roff-Ronratio ∼ 10.The current–voltage characteristics were fitted to various carrier transport mechanismssuch as Ohmic conduction, Schottky emission (SE), Space-charge-limited conduction (SCLC), and Poole-Frenkel emission (PF). Among these, the ohmic and SE mechanisms were detected at low and high fields, respectively. Schottky barrier modulation at the metal-oxide interface has been identified as the motive for switching.

Observation of CCNR-type electrical switching in Zn0.3Mn0.7+xSixFe2−2xO4 spinel ferrite series

A nonvolatile memory effect exhibited by electric field-induced resistance switching has been the topic of intense research not only due to its applications as resistive random access memory but also from the basic physics point of view. Among several binary and ternary mixed oxide compounds, the ones which possess magnetic ions have shown a great promise. Spinel ferrite system Zn0.3Mn0.7+xSixFe2−2xO4 with varying x is investigated for its novel electrical switching properties. Both temperature and applied voltage dependence of current-controlled negative resistance-type electrical switching showed better than 200% of resistive switching ratios. Bulk polycrystalline samples showed composition x dependence of resistive switching. The current–voltage characteristics are modeled for low and high applied field regime, and the presence of space-charge-limiting current is confirmed. Thin films of the ferrite system grown by pulsed laser deposition showed almost nonexistence of resistive switching, suggesting that the bulk composition of the compound has a major role to play against the film–electrode interface.

Resistive Switching Behavior of Au/TiO2/NiCr Structure

Thin film memristor behavior depends on the choice of top and bottom electrode material and metal oxide layer. In the present work, the influence of the top and bottom electrode layers on the electric field-induced resistance switching phenomena of TiO2-based MIM was studied. Au/TiO2/NiCr and NiCr/TiO2/Au structures were fabricated by depositing TiO2 active layer by reactive dc magnetron sputtering technique between thermally evaporated NiCr and Au electrodes. TiO2 layer was sputtered with a cathode power of 150[Formula: see text]W keeping the other deposition parameters constant. TiO2 films were analyzed by X-ray diffractometer, field emission scanning electron microscopy and UV–Visible spectrophotometer to investigate their structural, morphological and optical behavior. Structural studies of the films reveal that the samples were amorphous. And the [Formula: see text]–[Formula: see text] analysis for the fabricated MIM structure was analyzed.

Electric field-induced resistive switching, magnetism, and photoresponse modulation in a Pt/Co0.03Zn0.97O/Nb:SrTiO3 multi-function heterostructure

A Co0.03Zn0.97O (CZO) thin film was epitaxially grown on a Nb doped (001) SrTiO3 (NSTO) single-crystal substrate by pulsed laser deposition to form a Pt/CZO/NSTO heterostructure. This device exhibits stable bipolar resistive switching, well retention and endurance, multilevel memories, and a resistance ratio of high resistance state (HRS)/low resistance state (LRS) up to 7 × 105. Under the illumination of a 405 nm laser, the HRS of the device showed distinct photoelectricity with an open-circuit voltage of 0.5 V. A stronger ferromagnetism was observed at the HRS than at the LRS. The above phenomenon is attributable to the accumulation and migration of oxygen vacancies at the interface of CZO/NSTO. Our results demonstrated a pathway towards making multifunctional devices that simultaneously exhibit resistive switching, photoelectricity, and ferromagnetism.

Biocompatible Er:(Na,K)NbO3 nanofibers

Dense homogeneous fabric composed from continuous bead-free erbium-doped sodium potassium niobate (Er:NKN) 100 μm long and 100-200 nm in diameter nanofibers was sintered by sol-gel calcination assisted electrospinning technique. Er doping with the concentration of 2 at.% provides readily detectable room-temperature broad-band photoluminescence (PL) centered at λPL = 0.55 and 0.98 μm being pumped, respectively, with 532 and 785 nm lasers. Electric field induced resistance switching and strong electric rectification effect were found in nanoporous sandwich Au/Er:NKN/Pt capacitive cell. Memristor-type current-voltage I-V characteristics originate from the electrochemical migration of oxygen vacancies at the n-type Er:NKN oxide/high work function Pt cathode junction interface.

(Invited) Hard X-Ray Photoelectron Spectroscopic Study on High-k Dielectrics Based Resistive Random Access Memor

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).

Magnetic characterization and electrical field-induced switching of magnetite thin films synthesized by atomic layer deposition and subsequent thermal reduction

Magnetite (Fe3O4) of high quality was prepared by combining atomic layer deposition (ALD) with a subsequent thermal reduction process. The reduction process in hydrogen atmosphere was investigated by in situ x-ray diffraction studies as a function of temperature. A complete reduction to Fe3O4 was confirmed within a narrow temperature window during the thermal treatment. Magnetic characterization of magnetite thin films as a function of temperature, applied magnetic field and magnetic field orientation were performed. The highly stoichiometry- and impurity-sensitive Verwey transition was observed in magnetic and electrical measurements. Moreover, the isotropic point at which the magnetocrystalline anisotropy of magnetite vanishes was unveiled. Both findings prove, first, the formation of the magnetite phase against the undesired maghemite and, second, the quality of the ALD thin films to be comparable with samples grown by molecular beam epitaxy. The magnetic easy and hard axis could be found to be in-plane and out-of-plane, respectively. Consistent with angular-dependent studies of the coercive field, additionally performed first-order reversal curve measurements revealed a complex micromagnetic structure with different magnetization reversal paths for both configurations. Finally, electric field-induced resistive switching was studied in detail being in perfect agreement with results of single-crystalline samples. The presented data and its analysis support the assumption of previous works of the magnetization reversal in magnetite nanotubes, suggest improvement for future magnetization studies of nanostructures by exploiting the isotropic point and might open new paths for low-cost resistive switching devices.

(Invited) Photoelectron Spectroscopic Study on High-k Dielectrics Based Nanoionics-Type ReRAM Structure under Bias Operation

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. An oxide sandwiched between two metal electrodes shows reversible electric field–induced resistance switching behaviors. Hafnium oxide (HfO2), which is used as a high-k gate insulator, has also shown resistance switching phenomena and been increased interest in the use of HfO2 and related oxides as potential ReRAM materials [1]. For the oxide based ReRAM with the conductive filament formation model, two mechanisms of resistance switching have been proposed. One is the nanoionics model, which comprises the generation and rupture of a metal filament using a metal such as Cu acting as a fast mobile ion in oxides. The other model is that of oxygen vacancy nucleation at the metal/oxide interface. 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 to examine the electronic structure of Pt or Cu/HfO2 ReRAM structures in an operating device. 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. The interface electronic states of 10-nm-thick Pt or Cu top electrodes/HfO2/Pt structures were measured with HX-PES in the SPring-8 BL15XU undulator beamline. The incident X-ray energy was 5.95 keV. In the case of the Pt/HfO2 interface, applying a forward bias increased the Pt–O bonding peak as shown in Fig 1, indicating evidence of Pt electrode oxidization and oxygen vacancy formation around the interface [2]. In contrast, the application of a bias to the Cu/HfO2 interface reduced the copper oxide bonding state, providing evidence of oxygen reduction and Cu diffusion into the HfO2 layer [3,4]. We achieved direct observation of oxygen migration at the metal/HfO2 interface under device operation, which is the key to controlling the electrical properties of oxide based ReRAM. Based on these results, we demonstrated the control of the switching voltage and the initial conductive filament formation process of the Ta−Nb binary oxide ((TaxNb1−x)2O5) ReRAM structure using a combinatorial method [5]. The relationship between the interface structure and the electrical properties also will be discussed in detail at the presentation.

Detailed analyses of electric field-induced resistance switching behavior of SrFeO3−x film

We investigated the electric field-induced resistance change of SrFeO3−x film as a candidate for memory material. SrFeO3−x film showed hysteresis in its current–voltage curve and distinct pulse-switching properties. The resistance of the sample can be switched by the pulse voltage with the length of 50 ns. The relaxation time of the voltage analyses revealed that the resistance-switching properties originate from both the oxygen migration and the charge injection process. Based on these properties, we found that the magnetic state can also be changed by the application of the electric field.

Pulsed laser-deposited VO2 thin films on Pt layers

VO2 films were deposited on Pt (111)/TiO2/SiO2/Si (001) substrates by means of a pulsed laser deposition technique. An x-ray diffraction peak at 2θ = 39.9° was deconvoluted into two pseudo-Voigt profiles of Pt (111) and VOx-originated components. The VOx diffraction peak was more obvious in a VOx/Pt (111)/Al2O3 (0001) sample, having a narrower width compared with a VO2/Al2O3 (0001) sample. Temperature-controlled Raman spectroscopy for the VOx/Pt/TiO2/SiO2/Si sample has revealed the monoclinic VO2 phase at low temperature and the structural phase transition at about 72 °C in a heating process. The electronic conductive nature at the high temperature phase was confirmed by near normal incidence infrared reflectivity measurements. Out-of-plane current-voltage characteristics showed an electric field-induced resistance switching at a voltage as low as 0.2 V for a 50 nm-thick film. A survey of present and previous results suggests an experimental law that the transition voltage of VO2 is proportional to the square root of the electrodes distance.

Pulse-induced alternation from bipolar resistive switching to unipolar resistive switching in the Ag/AgOx/Mg0.2Zn0.8O/Pt device

Electric field-induced resistive switching (RS) and related effects are studied for the ZnO-based device Ag/AgOx/Mg0.2Zn0.8O/Pt. The system exhibits a bipolar resistive switching (BRS) for the current (I)–voltage (V) cycles, with the set/reset voltage distributing in a narrow region around 0.15 V/0.16 V. The high to low resistance ratio is ∼10, and the resistive state is well retainable. However, the RS becomes unipolar (unipolar resistive switching—URS) when electric pulses are applied, with a fairly wide distribution of the set/reset voltages, though the resistive state is still well retainable. It was further found that a backward transition from the URS to the BRS state can be occasionally triggered by simply performing I–V cycling in the negative branch, which shows the strong competition of the BRS and URS states. Both the BRS and URS states were stable and reproducible over 90 cycles. Possible mechanisms for the BRS and URS state and their mutual transition were discussed.

CCNR type high field instability in Ti4+-substituted Mn–Zn ferrites

Polycrystalline samples of the spinel ferrite system Zn0.3Mn0.7+xTixAl0.1Fe1.9−2xO4 (x = 0.0–0.3) have been studied for electrical switching in the temperature range from 300 to 700 K and applied voltage range from 0 to 400 V. High electric field induced resistance switching of nearly 200% has been observed for all the compositions. The dependence of temperature and applied electric field strength on switching has been examined. The electric field and temperature required for switching increase with increase in Mn–Ti concentration. The results have been interpreted considering previous models and have been ascribed to the existence of space charge limiting current in the system. The switching observed, is of current-controlled negative resistance (CCNR)-type.

Electric field-induced resistance switching in (Bi2O3)0.7(Y2O3)0.3 films

Single-phase (Bi2O3)0.7(Y2O3)0.3 (BYO) films are obtained by pulse laser deposition under the temperatures from 300 to 500 °C and the oxygen pressure of 90 Pa. Electric field-induced resistance switching is observed in the Ag/BYO/Pt structure. Postannealing in 750 °C improves the repeatability of the resistance switching and narrows the distribution of the set and reset voltages triggering the resistance switching. A linear variation of reset current with resistance is obtained for the Ag/BYO/Pt systems, either as-prepared or postannealed, despite the great dispersion of these two quantities. A reduction in the activation energy, from ∼0.63 to ∼0.33 eV, for the migration of oxygen vacancies is observed after the forming process. Possible mechanisms are discussed.

EFFECT OF LIGHT IRRADIATION ON ELECTRIC-FIELD-INDUCED RESISTANCE SWITCHING PHENOMENON IN PLANAR VO2/c-Al2O3 STRUCTURE

A simple device of VO 2 film with two terminal electrodes revealed current jump phenomena at certain applied voltage. In this study, we investigated the effect of light irradiation on the electric field-induced resistance switching phenomenon. Based on changes of current–voltage characteristics under light irradiation, we discussed the effect of light irradiation on this device.

Electrically-controlled Resistance Switching Accompanied by Ultra-high-J Domains of VO2 Films

AbstractThe electric-field-induced resistance switching (EIRS) phenomenon on a VO2 planar-type junction fabricated on a Al2O3 (0001) substrate was studied by performing current-voltage (I-V) measurement and optical microscope observation simultaneously. It was confirmed that current density J of the low-resistance-state (LRS) region is maintained constant at approximately 1.6 × 106 A/cm2, while the volume of the LRS region was changed according to the current. A survey of the previous I-V traces on EIRS of VO2 revealed that almost all the junctions so far had shown non-zero V-intercepts, which are attributed to the volume change of the LRS regions. The maintenance of high-J in the LRS region is considered to be related to the electrically-induced metallic phase mechanism reported in perovskite-type manganites.

SrFeO3-x (x < 1) 薄膜における酸素欠損数と電界誘起抵抗スイッチング特性の相関

SrFeO3-x (x < 1) 薄膜における酸素欠損数と電界誘起抵抗スイッチング特性の相関

Effect of pressure on the electric-field-induced resistance switching ofVO2planar-type junctions

${\text{VO}}_{2}$ thin-film planar-type junctions with the size of $1500\text{ }\ensuremath{\mu}\text{m}$ in width and $10\text{ }\ensuremath{\mu}\text{m}$ in length were fabricated on ${\text{Al}}_{2}{\text{O}}_{3}$ (0001) substrates, and the electric-field-induced resistance switching (EIRS) characteristics were investigated at hydrostatic pressures up to 2 GPa and room temperature. The resistance in the electric-field-induced low resistance state (E-LRS) was revealed to be insensitive to the hydrostatic pressure, in contrast to the apparent suppression of resistance in the temperature-induced low resistance state. The critical current to induce E-LRS is increased as the pressure is increased, while the voltage at the transition is independent of the pressure. These results indicate a picture in ${\text{VO}}_{2}$ that not a Joule heating but a reversible breakdown triggers EIRS, and that the electric current of huge density maintains filamentary E-LRS regions.

Resistance Switching Effect at Perovskite-Oxide Interfaces

Electric field induced resistance switching and memory effect has been investigated for perovskite-oxide heterojunctions consisting of various metal electrodes and p-type or n-type semiconducting perovskite oxides such as Pr0.7Ca0.3MnO3 and Nb-doped SrTiO3(Nb : STO). The metal/perovskite-oxide heterojunction devices show either ohmic or rectifying current-voltage (I-V ) characteristics similar to those of a Schottky junction, depending on the work function of the metals. In addition, the rectifying I-V characteristics have large hysteresis that can host the resistance switching effect. The dependence of the I-V characteristics on the Nb-doping level in the heteroepitaxial SrRuO3/Nb : STO junctions suggests that the threshold of resistance switching is determined by the potential profile of the Schottky barrier under bias voltage. From the experimental results, we propose the possible origin of the resistance switching effect, that is a charging effect at the Schottky-like metal/perovskite-oxide interfaces.