Resistive switching effects in oxide sandwiched structures

Resistive switching (RS) behaviors have attracted great interest due to their promising potential for the data storage. Among various materials, oxide-based devices appear to be more advantageous considering their handy fabrication and compatibility with CMOS technology, though the underlying mechanism is still controversial due to the diversity of RS behaviors. In this review, we focus on the oxide-based RS memories, in which the working mechanism can be understood basically according to a so-called filament model. The filaments formation/rupture processes, approaches developed to detect and characterize filaments, several effective attempts to improve the performances of RS and the quantum conductance behaviors in oxide-based resistive random access memory (RRAM) devices are addressed, respectively.     

Resistive switching phenomena of tungsten nitride thin films with excellent CMOS compatibility

We report the resistive switching (RS) characteristics of tungsten nitride (WN_x) thin films with excellent complementary metal-oxide-semiconductor (CMOS) compatibility. A Ti/WN_x/Pt memory cell clearly shows bipolar RS behaviors at a low voltage of approximately ±2.2 V. The dominant conduction mechanisms at low and high resistance states were verified by Ohmic behavior and trap-controlled space-charge-limited conduction, respectively. A conducting filament model by a redox reaction explains the RS behavior in WN_x films. We also demonstrate the memory characteristics during pulse operation, including a high endurance over >10⁵ cycles and a long retention time of >10⁵ s.     

Resistive switching in metal-insulator-metal structures based on germanium oxide and stabilized zirconia

Bipolar resistive switching in metal-insulator-metal structures based on a double-layer insulator composed of a layer of yttria-stabilized zirconia (YSZ) containing 12 mol % Y₂O₃ and a layer of GeO _x is studied. It is shown that the incorporation of an additional GeO _x layer into the structure leads to a significant decrease in the variation of resistive switching parameters at both negative and positive voltages. Au/Zr/GeO _x /YSZ/TiN structures exhibit a high stability of the resistance ratio in high-resistance and low-resistance states during cyclic switching. The studied structures can be used for designing next-generation nonvolatile memory elements.     

Resistive switching characteristics of ytterbium oxide thin film for nonvolatile memory application

This paper studies the effects of both the positive and negative forming processes on the resistive switching characteristics of a Pt/Yb₂O₃/TiN RRAM device. The polarity of the forming process can determine the transition mechanism, either bipolar or unipolar. Bipolar behavior exists after the positive forming process, while unipolar behavior exists after the negative forming process. Furthermore, the bipolar switching characteristics of the Pt/Yb₂O₃/TiN device can be affected by using a reverse polarity forming treatment, which not only reduces the set and reset voltage, but also improves the on/off ratio.     

Resistive switching in copper oxide nanorods: a bottom up approach applicable for enhanced scalability

Reversible, stable and reproducible resistive switching in a parallel network of Cu2O nanorods, observed in the present study, highlights the advantages of using nanorods in comparison to normally used thin films. Unipolar and symmetric current-voltage characteristics of the metal/insulator/metal structure consisting of Hg top contact/Copper oxide (Cu2O) nanorods/Ag bottom contact in a sandwich configuration shows electroforming at about 11 V, reproducible reset and set points at 0.53 +/- 0.03 and 4.2 +/- 0.02 V and a high OFF/ON resistance ratio > 10(3). Slope of current-voltage characteristics and current contrast in CAFM mapping indicate that filamentary conduction mechanism is responsible for resistive switching. This study sets the foundation for fabricating a nanorods based resistive random access memory device and thus a manifold increase in the device scalability.     

Effects of oxygen stoichiometry on resistive switching properties in amorphous tungsten oxide films

The bipolar resistance switching in WO_3 + δ films sandwiched by Al and Pt electrodes was investigated by changing additional oxygen content (δ). Reliable switching voltages and retention were observed for all samples. As δ increases the bi-stable current-voltage characteristics fluctuate leading to unstable switching power consumption. An analysis of the temperature dependence of the bi-stable resistance states revealed additional features that thermionic emission and metallic conduction co-contribute to the electrical transport of the resistance states. The authors propose that the observed resistance switching is due to the combined effects of potential modification near the interface and the formation of a metallic channel.     

Local resistance switching at grain and grain boundary surfaces of polycrystalline tungsten oxide films

Resistance switching behavior has been investigated in as-prepared and oxygen-annealed polycrystalline tungsten oxide films using conductive atomic force microscopy. The oxygen-annealed film appeared more insulative than the as-prepared films. The local current distributions demonstrated the lower conductivity at the grain boundaries than at the grains in the oxygen-annealed films. Reversible resistance switching behavior only occurred at the grain surface region of the oxygen-annealed films and the resistance switching process was described by the local valence change of tungsten ions induced by electrochemical migration of protons or oxygen vacancies. This different resistance switching behavior between the grain and grain boundary surface was attributed to the different oxygen vacancy density caused by the post-annealing process. The present results would be especially meaningful for the fabrication of nanoscale resistive nonvolatile memory devices.     

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spin valves mechanically coupled to a ferroelectric substrate. The switching between low- and high-resistance states results from the strain-driven magnetization reorientations by about 90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weak cubic magnetocrystalline anisotropy. Such reorientations, not requiring external magnetic fields, can be realized experimentally by applying moderate electric field to a thick substrate (bulk or membrane type) made of a relaxor ferroelectric having ultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giant magnetoresistance may be employed as electric-write nonvolatile magnetic memory cells with nondestructive readout.     

Structures and chemistry of tungsten powder from doped and undoped tungsten blue oxide

Blue tungsten oxide, both without dopant and doped with aluminium, silicon and potassium was reduced to tungsten by heating at 750 and 900° C in flowing hydrogen. The morphology, chemistry, and crystal structure were determined by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and Auger electron spectroscopy. Undoped oxide reduced at a lower rate than did the doped oxide. The potassium, silicon and aluminium appeared to be distributed non-uniformly on the surface of the doped oxide particles. Nucleation of tungsten crystals began earlier in the doped oxides leading to a bimodal distribution of crystal sizes in the doped material. There were pockets of potassium inside the tungsten crystals from the doped blue oxide. This suggests that the tungsten crystals nucleated earlier and at regions of high potassium concentration on the surface of the doped blue oxide. Tungsten was then transported from the surrounding oxide to the nucleus to grow the crystals trapping the potassium inside the crystal. Nucleation of the tungsten crystals occurred more homogeneously on the surface of the oxide that was low in potassium and, of course, on the undoped oxide particles, leading to the smaller size of the tungsten crystals from the undoped oxide.     

Resistive switching behavior of (Zn1−xMgx)O films prepared by sol-gel processes

Highly c-axis oriented sol-gel (Zn_1 − xMg_x)O films were deposited on Pt/Ti/SiO2/Si substrates. Resistive switching behaviors with stable switching and high resistance ratio were demonstrated for the Pt/(Zn_0.9Mg_0.1)O/Pt stacks. The effect of the film thickness and the annealing temperature on resistive switching was discussed. Higher substitution of Mg for Zn results in higher resistance of (Zn_1 − xMg_x)O films, which is beneficial for resistive switching to occur at thinner film thickness. The mechanisms dominating the low and the high resistance states are Ohmic conduction and Poole-Frenkel emission, respectively. The resistance ratio varies from 140 to 1000, which is much higher than the value 25 reported recently for sol-gel (Zn_0.8Mg_0.2)O films. Films annealed at higher annealing temperatures possess higher resistance ratio.     

Crystallization and thermochromism in tungsten oxide films annealed in vacuum

The crystalline phases formed in tungsten oxide (WO_{3 ? x} ) films upon isothermal step annealing in vacuum and air at various temperatures within 500?C750°C have been studied. The films were deposited onto silica glass substrates by reactive dc magnetron sputtering. It is established that the observed thermochromism is related to the presence of an oxygen-deficient WO_{3 ? x} phase belonging to the hexagonal system, which is intensely formed as the annealing temperature increases from 650 to 750°C.     

Preparation and characterizations of tungsten oxide electrochromic nanomaterials

Nanoscaled tungsten oxide thin films were fabricated by galvanostatic electrodeposition. The effect of preparation parameters such as tungsten ions concentration, pH, current density and annealing on the properties and performance of WO₃ thin films electrochromic materials was investigated. XRD, SEM–EDS, TEM, FTIR, UV–VIS spectrophotometry, and electrochemical measurements were utilized to characterize the structural and compositional properties as well as the electrochromic behaviour of the prepared thin films. Triclinic WO₃ structure was prepared at 0.1 M W⁺ and current density of 0.5 mA cm⁻², while at 0.2 M W⁺ and 1 mA cm⁻², orthorhombic structure was revealed. High energy gap of 3.5 eV with diffusion coefficient of 6.81 × 10⁻¹¹ cm² S⁻¹ and coloration efficiency of 62.68 cm² C⁻¹ were obtained for the films prepared at pH 2, 1 mA cm⁻², and 0.1 M W⁺.     

Structural, electrochemical and optical comparisons of tungsten oxide coatings derived from tungsten powder-based sols

Tungsten trioxide (WO₃) electrochromic coatings have been formed on indium tin oxide-coated glass substrates by aqueous routes. Coating sols are obtained by dissolving tungsten powder in acetylated (APTA) or plain peroxotungstic acid (PTA) solutions. The structural evolution and electrochromic performance of the coatings as a function of calcination temperature (250 °C and 400 °C) have been reported. Differential scanning calorimetry and X-ray diffraction have shown that amorphous WO₃ films are formed after calcination at 250 °C for both processing routes; however, the coatings that calcined at 400 °C were crystalline in both cases. The calcination temperature-dependent crystallinity of the coatings results in differences in optical properties of the coatings. Higher coloration efficiencies can be achieved with amorphous coatings than could be seen in the crystalline coatings. The transmittance values (at 800 nm) in the colored state are 35% and 56% for 250 °C and 400 °C-calcined coatings, respectively. The electrochemical properties are more significantly influenced by the method of sol preparation. The ion storage capacities designating the electrochemical properties are found in the range of 1.62–2.74 × 10^− 3 (mC cm^− 2) for APTA coatings; and 0.35–1.62 × 10^− 3 (mC cm^− 2) for PTA coatings. As a result, a correlation between the microstructure and the electrochromic performance has been established.     

Solution precursor plasma sprayed tungsten oxide particles and coatings

Solution precursor plasma spray process was used to deposit single particles and coatings of tungsten oxide (WO₃), and the microstructures of single particles and coatings were characterized by field emission scanning electron microscopy. The effects of substrate temperature and spraying distance on the microstructure of single particles and coatings were studied. In the case of WO₃ particles, the particle spheroidization degree became better as the increase in substrate temperature. When the substrate temperature increased up to 200°C, bubble-like morphologies appeared. For the deposited WO₃ coatings, a highly porous structure was obtained when a 100?mm spraying distance was used. Besides, the grain size of coatings decreased through increasing the spraying distance from 60 to 100?mm.     

Gas-chromism in ultrasonic spray pyrolyzed tungsten oxide thin films

A simple and inexpensive ultrasonic spray pyrolysis (USP) technique has been employed to deposit tungsten oxide (WO₃) thin films by spraying 2.0 mM aqueous ammonium metatungstate solution onto the amorphous glass substrates kept at 250°C. The films were further annealed at 400°C for 4 h in air. X-ray diffraction (XRD) technique was used to determine the crystallinity and to identify the WO₃ phases. It was found that the films were sub-stoichiometric, WO_3-z. To study gas-induced properties, a catalyzing layer of platinum (Pt) was sputtered onto it. The gas-induced electrical and optical properties of Pt/WO₃/glass samples were studied and results reported. It was found that electrical resistivity decreased by a factor of 10 within 2 min and stabilized after 15 min, after H₂ gas exposure. Similarly the optical transmittance of the samples attenuated from 55% to 10% within 15–20 min. The reversible changes in electrical resistivity and optical transmittance were observed when the samples were exposed to oxygen. The response times and sensitivity of the samples were estimated.     

Green synthesis and catalytic function of tungsten oxide nanoparticles

In this paper, a green chemical synthetic route was developed to synthesize WO3 nanoparticles with an average size of 70 nm. The products were characterized in detail by multiform techniques: X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photoluminescence of the obtained WO3 nanoparticles was also investigated. The effects of the hydrothermal temperature on the crystalline phase and morphology of the products have been studied systematically. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methylene blue (MB) aqueous solution. The electrocatalytic activity was characterized using voltammetric techniques. The results showed that the obtained WO3 nanoparticles have an excellent photocatalytic and electrocatalytic performance for the MB.