Reproducible Switching Research Articles

Effects of Electrodes on the Filament Formation in HfO2- Based Resistive Random Access Memory

The resistance switching characteristics of HfO2-based resistive random access memory (RRAM) with Cu/HfO2/ITO and TiN/HfO2/ITO structures were investigated. Results show that both devices exhibit a stable and reproducible bipolar switching behavior during successive cycles. The formation of Cu conducting filaments is believed to be the reason for the resistive switching of Cu/HfO2/ITO device. For TiN top electrode, the interfacial layer between TiN and HfO2 film is formed and acts as an oxygen reservoir therefore, oxygen vacancy filaments are responsible for the resistive switching of TiN/HfO2/ITO device.

A sub-micrometer resolution hard X-ray microprobe system of BL8C at Pohang Light Source.

A microprobe system has been installed on the nanoprobe/XAFS beamline (BL8C) at PLS-II, South Korea. Owing to the reproducible switch of the gap of the in-vacuum undulator (IVU), the intense and brilliant hard X-ray beam of an IVU can be used in X-ray fluorescence (XRF) and X-ray absorption fine-structure (XAFS) experiments. For high-spatial-resolution microprobe experiments a Kirkpatrick-Baez mirror system has been used to focus the millimeter-sized X-ray beam to a micrometer-sized beam. The performance of this system was examined by a combination of micro-XRF imaging and micro-XAFS of a beetle wing. These results indicate that the microprobe system of the BL8C can be used to obtain the distributions of trace elements and chemical and structural information of complex materials.

Reversible switching of ferromagnetism in ZnCuO nanorods by electric field

The reproducible switching of ferromagnetism in ZnCuO nanorods by applying a reversible electric field has been realized. High-resolution transmission electron microscopy images showed a hexagonal wurtzite structure with no detectable trace of secondary phase or precipitation of Cu impurity in the ZnCuO nanorods. The Cu concentrations in the ZnCuO nanorods were tested by energy dispersive spectroscopy and x-ray photoelectron spectroscopy and found to be about 2.7 at. %. The switching mechanism is confirmed in terms of the formation and rupture of conductive filaments, with oxygen vacancies (VO) localized mainly on surface of the ZnCuO nanorods. Subsequently, the variation of VO concentration during the resistive switching process modulates the ferromagnetism of the ZnCuO nanorods. The saturation magnetization at low resistance state is apparently 6.4 times larger than that at high resistance state for an Au/ZnCuO/ITO structure. An indirect double-exchange model has been used to explain the ferromagnetism in ZnCuO nanorods.

Ferroelectric behavior and reproducible Bi-stable resistance switching property in K-doped ZnO thin films as candidate for application in non-volatile memories

Potassium-doped ZnO thin films electrodeposited on indium tin oxide (ITO) coated glass substrates exhibited ferroelectric behavior with a remnant polarization of 0.2µC/cm2. Especially, wave forms showing the applied input voltage Vi and output voltage Vo were obtained for Al/ZnO:K/ITO structure. It exhibits a superposition of Vi (input) and Vo (output) signal from Al/ZnO:K/ITO structure with a clear phase shift between the two wave forms which again confirms that the observed ferroelectric hysteresis curve is not related to leaky dielectric materials. The current–voltage characteristics of Al/ZnO:K/ITO structures measured for several cycles revealed bi-stable switching characteristics. The reproducible bi-stable switching characteristics for the mentioned structures had good retention in one particular resistance state. Around one order of switching was realized between low and high resistance states. The switching property thought to be polarization induced originating out from the ferroelectric properties of the potassium doped ZnO thin film. The switching between ZnO:K/ITO interface is assumed to be critical for stability in switching for several cycles. Possible application of this structure in non-volatile memories is explored.

Automation Highlights from the Literature

Automation Highlights from the Literature

Bi-stable resistive switching in an array of $$\hbox {Cu/Cu}_x\hbox {O/Au}$$ Cu/Cu x O/Au nanowires

A resistive switching system comprising of metal–insulator–metal \((\hbox {Cu/Cu}_x\hbox {O/Au})\) sandwich-structured nanowires embedded within polycarbonate membrane has been investigated. The system switches from non-Ohmic high resistive state (HRS) to Ohmic low resistive state on application of a critical bias of \(\sim \)2.5 V. The bipolar switching can be performed by applying current bias as well. Driving two suitable currents, \(I_{\mathrm{set}}= 50\,\upmu \hbox {A}\) and \(I_{\mathrm{reset}}= -0.5\,\hbox {mA},\) we observe highly reproducible switching between two stable resistive states. The switching is initiated by establishment of filamentary conduction path commonly formed in oxide materials. However, the main charge transport in the HRS is governed with modified activated behavior, which is obvious from the antisymmetric, reversible I–V characteristic following \(I=aV\exp [(-E_A+b|V|)/k_BT]\) where a, b and \(E_A\) are constants. The exponential term corresponds to charge generation by field-enhanced thermal activation process, whereas the linear term is related to mobility.

All-Printed Paper Memory

We report the memory device on paper by means of an all-printing approach. Using a sequence of inkjet and screen-printing techniques, a simple metal–insulator–metal device structure is fabricated on paper as a resistive random access memory with a potential to reach gigabyte capacities on an A4 paper. The printed-paper-based memory devices (PPMDs) exhibit reproducible switching endurance, reliable retention, tunable memory window, and the capability to operate under extreme bending conditions. In addition, the PBMD can be labeled on electronics or living objects for multifunctional, wearable, on-skin, and biocompatible applications. The disposability and the high-security data storage of the paper-based memory are also demonstrated to show the ease of data handling, which are not achievable for regular silicon-based electronic devices. We envision that the PPMDs manufactured by this cost-effective and time-efficient all-printing approach would be a key electronic component to fully activate a paper-based circuit and can be directly implemented in medical biosensors, multifunctional devices, and self-powered systems.

Submicronic VO2–PVP composites coatings for smart windows applications and solar heat management

Large surface and chemically stable thermochromic composite coatings consisting of IR active sub-micron VO2 particles embedded in a passive polymeric host matrix of Polyvinylphenol were synthesized by dip and spin coating process. The hybrid composite coatings are IR transparent and IR reflective below and above the switching temperature of ∼68°C. Although these cost effective hybrid composite coatings are two-dimensional morphologically (1–3 nonpercolated submicron VO2 crystallites within the coating thickness), they exhibit a significant reversible IR transmission modulation with temperature of ∼45% between 25°C and 100°C at 1mm. This satisfactory reproducible thermochromic optical switching characteristic could address the impediment to greater market penetration of thermochromic VO2 based technology relatively to the high cost coatings engineered by vacuum technologies. From technological applications perspective, the global production of glass which could regulate solar heat by using such hybrid thermochromic coatings, could be part of 1 billion m2/year with ∼25% for building sector and about ∼11% for the automotive industry. Likewise, such cost effective hybrid thermochromic coatings could play a significant role in the minimization of air conditioning load and thus energy consumption in both building and automotive sectors. As hinted to in its 2012 annual report, the International Energy Council’s predictions have estimated that with ∼2billionsm2 of coated windows worldwide with smart coatings, energy saving in the building and automotive economic fields is equivalent of CO2 reduction by about ∼100 millions of tons.

Multilevel resistive switching effect in sillenite structure Bi12TiO20 thin films

Multilevel resistive switching effect has been demonstrated in Pt/Bi12TiO20/Pt structures, in which the sillenite structure Bi12TiO20 thin films were prepared by a chemical solution deposition method. The non-volatile multilevel resistive switching can be easily realized by changing either the compliance current during set process or reset voltage during reset process. The Pt/Bi12TiO20/Pt devices show excellent switching parameters such as reproducible switching effect, low set/reset voltage, high cell yield, good endurance, and long retention time (up to 104 s). The dominated conduction mechanisms were Ohmic conduction in low resistance state (LRS) and lower voltage region of high resistance state (HRS), and Schottky emission in higher voltage region of HRS. In the reset process, the switching from LRS to HRS consists of two parts: an abrupt transition due to thermal effect and a gradual transition owing to electric field-induced migration of oxygen vacancies. On the basis of the measurements of the resistance-temperature dependence in different resistance states and analysis of current-voltage characteristics, we explained the resistive switching behavior using a conductive filament-related model.

COMPARATIVE STUDY OF SPIN INJECTION AND TRANSPORT IN Alq3 AND Co–PHTHALOCYANINE-BASED ORGANIC SPIN VALVES

Recent experimental reports suggest the formation of a highly spin-polarized interface ("spinterface") between a ferromagnetic (FM) Cobalt ( Co ) electrode and a metal-phthalocyanine (Pc) molecule. Another report shows an almost 60% giant magnetoresistance (GMR) response measured on Co / H 2 Pc -based single molecule spin valves. In this paper, we compare the spin injection and transport properties of organic spin valves with two different organic spacers, namely Tris(8-hydroxyquinolinato) aluminum ( Alq 3) and CoPc sandwiched between half-metallic La 0.7 Sr 0.3 MnO 3 (LSMO) and Co electrodes. Alq 3-based spin valves exhibit clear and reproducible spin valve switching with almost 35% negative GMR at 10 K, in accordance with previous reports. In contrast, cobalt-pthalocyanine ( CoPc )-based spin valves fail to show clear GMR response above noise level despite high expectations based on recent reports. Investigations of electronic, magnetic and magnetotransport properties of electrode/spacer interfaces of LSMO/ CoPc / Co devices offer three plausible explanations for the absence of GMR: (1) CoPc films are strongly chemisorbed on the LSMO surface. This improves the LSMO magnetic properties but also induces local traps at the LSMO interface for spin-polarized charge carriers. (2) At the Co / CoPc interface, diffusion of Co atoms into the organic semiconductor (OS) layer and chemical reactivity between Co and the OS deteriorates the FM properties of Co . This renders the Co / CoPc interface as unsuitable for efficient spin injection. (3) The presence of heavy Co atoms in CoPc leads to large spin–orbit coupling in the spacer. The spin relaxation time in the CoPc layer is therefore considerably smaller compared to Alq 3. Based on these findings, we suggest that the absence of GMR in CoPc -based spin valves is caused by a combined effect of inefficient spin injection from FM contacts and poor spin transport in the CoPc spacer layer.

Engineering of the Chemical Reactivity of the Ti/HfO2 Interface for RRAM: Experiment and Theory.

The Ti/HfO2 interface plays a major role for resistance switching performances. However, clear interface engineering strategies to achieve reliable and reproducible switching have been poorly investigated. For this purpose, we present a comprehensive study of the Ti/HfO2 interface by a combined experimental-theoretical approach. Based on the use of oxygen-isotope marked Hf*O2, the oxygen scavenging capability of the Ti layer is clearly proven. More importantly, in line with ab initio theory, the combined HAXPES-Tof-SIMS study of the thin films deposited by MBE clearly establishes a strong impact of the HfO2 thin film morphology on the Ti/HfO2 interface reactivity. Low-temperature deposition is thus seen as a RRAM processing compatible way to establish the critical amount of oxygen vacancies to achieve reproducible and reliable resistance switching performances.

Memory resistive switching in CeO2-based film microstructures patterned by a focused ion beam

Heteroepitaxial CeO2 (80nm)/La0.5Sr0.5CoO3 (500nm) film structure has been pulsed laser deposited on a sapphire substrate. The Ag/CeO2 microjunctions patterned by a focused ion beam on a La0.5Sr0.5CoO3 film exhibit reproducible reversible switching between a high resistance state (OFF) with insulating properties and a semiconducting or metallic low resistance state (ON) with resistance ratios up to 104. The influence of micro-scaling and defects formed at the cell boundaries during etching on its electrical characteristics has been analyzed. The appearance of a switching channel at the moment of the electrical forming, responsible for the memory effect, has been proved, along with a mechanism of a self-healing electrical breakdown.

Investigation of bipolar resistive switching characteristics in Si<SUB align="right">3N<SUB align="right">4-based RRAM with metal-insulator-silicon structure

Fab-friendly Ti/Si 3 N 4 /p + -Si stacked RRAM device was fabricated. Reproducible bipolar switching occurs under a reasonable operation voltage (<3 V) owing to forming-less process. In addition, self-compliance less than 1 mA is helpful for a circuit design. For high density, the feasibility of multi-level cell (MLC) operation is demonstrated using gradual set/reset during resistance transition.

Lateral spin transfer torque induced magnetic switching at room temperature demonstrated by x-ray microscopy

Changing and detecting the orientation of nanomagnetic structures, which can be used for durable information storage, needs to be developed towards true nanoscale dimensions for keeping up the miniaturization speed of modern nanoelectronic components. Therefore, new concepts for controlling the state of nanomagnets are currently in the focus of research in the field of nanoelectronics. Here, we demonstrate reproducible switching of a purely metallic nanopillar placed on a lead that conducts a spin-polarized current at room temperature. Spin diffusion across the metal-metal (Cu to CoFe) interface between the pillar and the lead causes spin accumulation in the pillar, which may then be used to set the magnetic orientation of the pillar. In our experiments, the detection of the magnetic state of the nanopillar is performed by direct imaging via scanning transmission x-ray microscopy (STXM).

Resistance Switching in Metal Oxide Thin Films and its Memory Application

We explore the unipolar resistance switching effect in sandwich structures based on Nb, Ta and Zr oxide thin films. The structures were fabricated by pulsed laser deposition and low temperature anodic oxidation methods. After electroforming process memory cells demonstrate reproducible switching between low and high resistance states with a resistance ratio around 102. Nonvolatile resistance storage was traced within 40 days. The low-temperature anodic oxidation of Nb was found to be suitable to fabricate flexible nonvolatile memory elements. The parameters of resistive switching are not degraded after 100000 flexing.

Threshold Switching and Conductance Quantization in Al/HfO2/Si(p) Structures

Volatile threshold switching and non-volatile memory switching modes of resistive switching are reported in Al/HfO2/Si(p) metal–oxide–semiconductor structures with different values of current compliance limit during electroforming. When the current is limited to below 100 µA, a reproducible threshold switching loop is reported under injection from the p-type silicon substrate. The conduction in the low resistance state is linear above a voltage threshold called holding voltage and the conductance is a non-integer multiple of the quantum of conductance. Depending on the size of the conducting filament created during the electroforming process, one or several quasi-one dimensional quantum subbands are found to contribute to the current. Abrupt transitions between different discrete conductance values are reported during increasing and decreasing voltage sweeps. These results provide strong experimental evidence suggesting that the conduction filament behaves as a quantum wire (QW). No structural instability of the filament has to be invoked to explain either the highly structured conduction properties or the set and reset switching transitions. It is claimed that the whole phenomenology can be understood by electron injection from the valence band into a narrow conducting path which behaves as a QW.

Voltage-Triggered Ultrafast Phase Transition in Vanadium Dioxide Switches

Electrically driven metal-insulator transition in vanadium dioxide (VO2) is of interest in emerging memory devices, neural computation, and high speed electronics. We report on the fabrication of out-of-plane VO2 metal-insulator-metal (MIM) structures and reproducible high-speed switching measurements in these two-terminal devices. We have observed a clear correlation between electrically-driven ON/OFF current ratio and thermally-induced resistance change during metal-insulator transition. It is also found that sharp metal-insulator transition could be triggered by external voltage pulses within 2 ns at room temperature and the achieved ON/OFF ratio is greater than two orders of magnitude with good endurance.

Forming-free bipolar memristive switching of ZnO films deposited by cyclic-voltammetry

Cyclic voltammetry deposition (CV-D) was applied to deposit ZnO films on indium-tin-oxide (ITO) glass. Delineated in details are the optimizations of CV-D parameters which bestow enhanced nucleation of ZnO grains. The result is much superior coverage of the CV-D thin films as compared to those obtained by conventional electrochemical deposition. Memristive devices in the stacks of Pt/ZnO/ITO were prepared and tested. The devices in which ZnO prepared by CV-sweeping within ±0.9V for 6 cycles then fix-potential-deposited at −0.75V for 300s show reproducible forming-free bipolar switching operation at voltages ≤±1V. The cycle-life in DC sweeping mode is at least 200 cycles. The electrical conduction belongs to space-charge-limited-current conduction mechanism, which is fitted to extract carrier mobility 0.97cm2/Vs and carrier concentration 8.6×1018cm−3. Chemical bonding status and concentration profiles of oxygen and zinc were examined by electron spectroscopy and an Auger electron nanoscope. Gradient oxygen bonding status in ZnO film, typical of cyclic voltammetry deposition, facilitates the electrical switching at low voltages.

Uniform Complementary Resistive Switching in Tantalum Oxide Using Current Sweeps

Tantalum oxide (TaOx) is an advantageous material system to realize complementary resistive switches, but the ON-window starts to shrink after a few cycles in the voltage sweep mode. A power analysis reveals the presence of power boosts during conventional voltage sweeps in the nonswitching part cell. This power stress is significantly reduced by the use of current sweeps with voltage compliance, leading to very reproducible switching. The endurance is raised to >; 30000 cycles.

Detailed mechanism for the orthogonal polarization switching of gold nanorod plasmons

In this work, we describe an electro-optic material capable of orthogonally switching the polarization of the localized surface plasmon resonance scattering of single gold nanorods, independent of their orientation. Liquid crystal samples are prepared in a sandwich configuration with electrodes arranged so that an applied voltage induces alignment-switching of the liquid crystal molecules covering individual gold nanorods. Due to the birefringence of the nematic liquid crystal, the reorientation in the nematic director alignment causes a change in the output polarization of the scattered light. We propose the underlying mechanism to be based on a homogeneous nematic to twisted nematic phase transition and provide support for it via Jones calculus by modelling the effect of ideally aligned homogeneous nematic and twisted nematic phases on polarized light transmitted through the sample. In the model, we include the effects of sample thickness and surface plasmon resonance wavelength, expressed in terms of the phase retardation, χ, on the observed output polarization. We find four distinctively different trends for the output polarization as a function of the incident polarization as χ is varied. Two of these cases provide reproducible orthogonal polarization switching of the surface plasmon resonance while maintaining a high degree of polarization. These results are verified experimentally with liquid crystal cells of different thicknesses. The deviation of the experimental samples from ideal behaviour can be explained by the inherent variations in the surface plasmon resonance maximum and local cell thickness.