Analysis of quantum conductance,read disturb and switching statistics in HfO2 RRAM using conductive AFM

Most studies on resistance switching have been carried out at the device level with the standard electrical characterization setup, which allows for effective automated reliability test and extensive characterization of the lifetime of an RRAM device. However, it is equally important to be able to probe the switching phenomenon at the nanoscale so as to improve insight on the bias-dependent kinetic behavior of the filament during multiple reversible breakdown and recovery cycles. This study aims to do just that by probing HfO₂ blanket films (~ 4 nm) with a W bottom electrode using an ultra-sharp Pt-wire conductive AFM (CAFM) tip with an areal resolution of ~10–20 nm at ambient conditions. The use of the CAFM allows for a more reliable assessment of single filament evolution behavior as possible multiple filamentation events (common at the device level) are rare for such small probing areas. The role of oxygen vacancy induced filaments is studied here by using low compliance setting and moderate voltage levels, ensuring operation in the sub-quantum conductance regime. Our results show good repeatable switching trends and also provide insight on the quantum conductance phenomenon in oxygen vacancy based filaments. The read disturb trends in switching are investigated for the high resistance state (HRS) and the impact of tip-induced mechanical stresses on forming lifetime is also presented, which could serve as a motivator for further studies on non-volatile memory (NVM) reliability for flexible electronics devices and system on chip (SoC) applications.     

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM?) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO₂ and Ta/HfO₂ systems, thereby providing a complete explanation of the RRAM? switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.     

Oxygen Vacancy Creation,Drift, and Aggregation in TiO2‐Based Resistive Switches at Low Temperature and Voltage

Transmission electron microscopy with in situ biasing has been performed on TiN/single‐crystal rutile TiO₂/Pt resistive switching structures. Three elementary processes essential for switching: i) creation of oxygen vacancies by electrochemical reactions at low temperatures (<150 °C), ii) their drift in the electric field, and iii) their coalescence into planar faults (and dissociation from them) have been documented. The faults have a form of vacancy discs in {110} and {121} planes, are bound by partial dislocation loops, and are identical to Wadsley defects observed in nonstoichiometric TiO₂ annealed at high temperatures. The faults can be regarded as a precursor to the formation of oxygen‐deficient Magnéli phases, but 3D secondary phase inclusions have not been detected. Together, the observations shed light on the behavior of oxygen vacancies in relatively low electric fields and temperatures, suggesting that, in addition to the rather accepted notion of oxygen vacancy motion during the writing processes in resistive switching devices, such motion may occur even during reading, and may be accompanied by significant oxygen vacancy creation at modest device excitation levels.     

Physical Mechanisms behind the Field‐Cycling Behavior of HfO2‐Based Ferroelectric Capacitors

Novel hafnium oxide (HfO₂)‐based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite‐based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake‐up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake‐up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake‐up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.     

Atomic Cobalt Vacancy-Cluster Enabling Optimized Electronic Structure for Efficient Water Splitting

Vacancies created on a surface can alter the local electronic structure, thus enabling a higher intrinsic activity for the evolution of hydrogen and oxygen. Conventional strategies for vacancy engineering, however, have a strong focus on non-metal sulfur/oxygen defects, which have often overlooked metallic vacancies. Herein, evidence is provided that cobalt vacancies can be atomically tuned to have different sizes to achieve cobalt vacancy clusters through controlling the migration of iridium single atoms. The coalescence of Co vacancy clusters at the surface of an IrCo alloy results in an increased d-band level and eventually compromises H adsorption, leading to enhanced electrocatalytic activity toward the hydrogen evolution reaction. In addition, the Co vacancy clusters can improve the electronic conductivity with respect to the oxidized Co surface, which substantially aids in strengthening the adsorption of oxygen intermediates toward an effective oxygen evolution reaction at a low overpotential. These collective effects originate from the Co vacancy cluster and specifically enable highly efficient and stable water splitting with a low total overpotential of 384 mV in alkaline media and 365 mV in an acidic environment, achieving a current density of 10 mA cm^–2.     

Stable resistive switching characteristics of Ce:HfOx film induced by annealing process

In this work, Ce:HfO_x films were fabricated and the resistive switching characteristics were investigated. The chemical bonding states of the films were explored by X-ray photoelectron spectroscopy. The annealing process was carried out to modulate the concentration of oxygen vacancies in the film to confirm the dominant role of oxygen vacancies on resistive switching behaviors, which resulted in the elimination of unstable oxygen vacancies and the introduction of oxygen vacancy near Ce dopants due to the reduction of Ce⁴⁺. Benefiting from the oxygen vacancies near Ce dopants, stable resistive switching performance can be achieved for the annealed Ce:HfO_x sample. A schematic diagram based on the formation and rupture of oxygen vacancy filaments was proposed to illustrate the switching behaviors of annealed Ce:HfO_x sample.     

Low-Power Wake-Up Radio for Wireless Sensor Networks

Power consumption is a critical issue in many wireless sensor network scenarios where network life expectancy is measured in months or years. Communication protocols typically rely on synchronous operation and duty-cycle mechanisms to reduce the power usage at the cost of decreased network responsiveness and increased communication latency. A low-power radio-triggered device can be used to continuously monitor the channel and activate the node for incoming communications, allowing purely asynchronous operations. To be effective, the power consumption of this wake-up device must be on the order of tens of microwatts since this device is always active. This paper presents our first attempt at designing such a low-power receiver. Very few realizations of wake-up devices are reported in the literature and none presents power dissipation below 40 μW. Our design implements a complete wake-up device and initial results indicate an average power consumption below 20 μW, which is more than 2 times lower than other reported devices.     

A novel 1,1′-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1′-bis-[4-(5,6-dimethyl-1H-benzimidazole-1-yl)-butyl]-4,4′-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4′-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200 °C, and a stable voltage window of ～4 V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605 nm), a remarkably high coloration efficiency of 725 cm² C^?1 at 605 nm and switching times of 2–3 s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000 cycles, thus ratifying its suitability for electrochromic window/display applications.     

Analysis of “on” and “off” times for thermally driven VO2 metal-insulator transition nanoscale switching devices

Vanadium oxide undergoes a sharp metal-insulator transition in the vicinity of room temperature and there is considerable interest in exploring novel device applications that utilize this phase transition. Using experimentally determined values of the thermal conductivity across the metal-insulator transition in VO₂ thin films, we estimate the switching characteristics of two-terminal VO₂ devices. The minimum switching time for both heating (“on” state) and cooling (“off” state) processes is explored using a simple resistance-capacitance thermal circuit model. The estimated minimum switching time is on the order of ∼1 ns for 20 nm VO₂ films which is comparable to experimentally observed switching times. Optimal operating temperatures to maximize switching times are estimated. Methods to further enhance the switching kinetics by device thickness, carrier doping/strain, interfacial thermal resistance and input thermal energy are discussed. The simulations are compared with a 3-D model for VO₂ devices on Si substrates utilizing COMSOL. The results are of potential relevance to the emerging field of correlated oxide electronics with fast phase transitions.     

Wavelength-tunable thulium-doped single mode fiber laser based on the digitally programmable micro-mirror array

We propose a wavelength-tunable thulium-doped single mode fiber laser with a digitally controlled micro-mirror array device. The fast and flexible lasing wavelength switching property was achieved by the pixelated spatial modulation of the micro-mirror array. The proposed laser provides a maximum output power of 160 mW with 24% slope efficiency and a narrow output linewidth of less than 0.03 nm. The operating wavelength is continuously tunable from 1863 nm to 1937 nm with a wavelength selectivity accuracy of less than 0.4 nm and a fast switching time of ∼75 μs.     

Temperature-dependent resistive switching characteristics for Au/n-type CuAlOx/heavily doped p-type Si devices

Bipolar switching phenomenon is found for Au/n-type CuAlO_x/heavily doped p-type Si devices at temperatures above 220 K. For high or low resistive states (HRS or LRS), the electrical resistance is decreased with increasing temperature, indicating a semiconducting behavior. Carrier transport at LRS or HRS is dominated by hopping conduction. It is reasonable to conclude that the transition from HRS to LRS due to the migration of oxygen vacancies (V_O) is associated with electron hopping mediated through the V_O trap sites. The disappearance of the resistive switching behavior below 220 K is attributed to the immobile V_O traps. The deep understanding of conduction mechanism could help to control the device performance.     

Development and analysis of nitrogen-doped amorphous InGaZnO thin film transistors

The nitrogen-doped (N-doped) amorphous InGaZnO thin film transistors (a-IGZO TFTs) were investigated against the undoped and oxygen doped (O-doped) devices. The N-doped a-IGZO TFTs exhibited better electrical performance and bias stress stability, and especially more stable thermal properties. The X-ray photoemission spectroscopy (XPS) measurements were carried out at different temperatures (298 K and 393 K) to examine the physical essence of the thermal instability of undoped, O-doped, and N-doped a-IGZO TFTs. The XPS characterization results indicated that nitrogen doping caused lesser oxygen vacancy variation with the temperature (0.6%) compared with undoping (7.2%) and oxygen doping (11.8%). Hence, the a-IGZO TFTs with N-doped active layers had much better stability than those with undoped and O-doped active layers.     

Enhanced photocatalytic activity of zinc oxide synthesized by calcination of zinc sulfide precursor

ZnO nanoparticles were synthesized by calcination of ZnS precursor in an air atmosphere, in which ZnS had been firstly synthesized through precipitation with sodium sulfide (Na₂S) as the precipitator. Detailed structure and morphology of the samples were characterized by X-ray diffraction, energy dispersive spectroscopy, scanning electron microscopy, and transmission electronic microscopy. Optical properties were examined by UV–vis absorption spectroscopy. Photocatalytic activities of the samples were evaluated by degradation of Reactive Blue 14 (KGL). The results indicate that ZnS precursor converted into pure ZnO stepwise via calcination at a temperature range of 400–800 °C, and pure ZnO can be achieved above 700 °C. ZnO obtained by calcination at 700 °C had an average crystalline size around 45 nm and exhibited the highest photocatalytic activity, degrading KGL by almost 97.1% after 60 min under ultraviolet irradiation, which was superior to that of the directly synthesized and commercial ZnO. The inherent correlation between different samples and their photocatalytic activities was discussed. The phase, crystalline size, specific surface area and oxygen vacancy defects of the samples were proposed to affect their photocatalytic activity.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.     

Accelerated Ionic Motion in Amorphous Memristor Oxides for Nonvolatile Memories and Neuromorphic Computing

Memristive devices based on mixed ionic–electronic resistive switches have an enormous potential to replace today's transistor‐based memories and Von Neumann computing architectures thanks to their ability for nonvolatile information storage and neuromorphic computing. It still remains unclear however how ionic carriers are propagated in amorphous oxide films at high local electric fields. By using memristive model devices based on LaFeO₃ with either amorphous or epitaxial nanostructures, we engineer the structural local bonding units and increase the oxygen‐ionic diffusion coefficient by one order of magnitude for the amorphous oxide, affecting the resistive switching operation. We show that only devices based on amorphous LaFeO₃ films reveal memristive behavior due to their increased oxygen vacancy concentration. We achieved stable resistive switching with switching times down to microseconds and confirm that it is predominantly the oxygen‐ionic diffusion character and not electronic defect state changes that modulate the resistive switching device response. Ultimately, these results show that the local arrangement of structural bonding units in amorphous perovskite films at room temperature can be used to largely tune the oxygen vacancy (defect) kinetics for resistive switches (memristors) that are both theoretically challenging to predict and promising for future memory and neuromorphic computing applications.     

Bicolor Light-Emitting Diode Based on Zinc Oxide Nanorod Arrays and Poly(2-methoxy,5-octoxy)-1,4-phenylenevinylene

The current study reports a novel inorganic/organic light-emitting diode (LED), consisting of zinc oxide (ZnO) nanorod arrays and poly(2-methoxy, 5-octoxy)-1,4-phenylenevinylene (MOPPV). ZnO nanorod arrays passivated using polyacrylamide (PAM) with 70 nm diameter were successfully prepared by a simple polymer-assisted chemical method. Enhancement of the ZnO defect emission is caused by PAM passivation, as observed in photoluminescence spectra. Infrared absorption spectra reveal that PAM is chemically or physically adsorbed on the surfaces of ZnO nanorod arrays. The electroluminescence (EL) spectrum shows bluish light at 406 nm from ZnO transition emission, and light emission with center at 600 nm from exciton emission in MOPPV. The potential EL mechanism is electron transition to zinc vacancy in PAM/ZnO nanorod arrays, and exciton radiation luminescence in MOPPV film. This novel PAM/ZnO-MOPPV device may be helpful to promote development of multicolor LEDs.     

Highly Stable Ag–Au Core–Shell Nanowire Network for ITO-Free Flexible Organic Electrochromic Device

Solid and flexible electrochromic (EC) devices require a delicate design of every component to meet the stringent requirements for transparency, flexibility, and deformation stability. However, the electrode technology in flexible EC devices stagnates, wherein brittle indium tin oxide (ITO) is the primary material. Meanwhile, the inflexibility of metal oxide usually used in an active layer and the leakage issue of liquid electrolyte further negatively affect EC device performance and lifetime. Herein, a novel and fully ITO-free flexible organic EC device is developed by using Ag–Au core–shell nanowire (Ag–Au NW) networks, EC polymer and LiBF₄/propylene carbonate/poly(methyl methacrylate) as electrodes, active layer, and solid electrolyte, respectively. The Ag–Au NW electrode integrated with a conjugated EC polymer together display excellent stability in harsh environments due to the tight encapsulation by the Au shell, and high area capacitance of 3.0 mF cm⁻² and specific capacitance of 23.2 F g⁻¹ at current density of 0.5 mA cm⁻². The device shows high EC performance with reversible transmittance modulation in the visible region (40.2% at 550 nm) and near-infrared region ( − 68.2% at 1600 nm). Moreover, the device presents excellent flexibility ( > 1000 bending cycles at the bending radius of 5 mm) and fast switching time (5.9 s).     

Enhanced photocatalytic degradation properties of zinc oxide nanoparticles synthesized by using plant extracts

ZnO nanoparticles were successfully prepared by biological synthesis using aqueous extracts of Allium sativum (garlic), Allium cepa (onion) and Petroselinum crispum (parsley). For all ZnO samples, the XRD studies reveal a hexagonal wurtzite structure, without supplementary diffraction lines. The particle size is influenced by the type of plant extract used and varies between 14 and 70 nm. The biomolecules involved in the biosynthetic procedure was evidenced by FTIR spectroscopy. The presence of Mn and Fe in ZnO powders synthesized by using plant extracts was highlighted by ICP-MS. The EPR spectroscopy confirms the presence of Fe³⁺ and Mn²⁺ ions in ZnO samples and its variation depending on the plant extract. Also, Zn vacancy complexes and oxygen vacancies are present in all analyzed samples. A narrowing of the band gap for the ZnO prepared with plant extracts was observed as compared to that of the ZnO, prepared using solely ultrapure water. The photodegradation studies conducted in the presence of UV light irradiation indicated that ZnO nanoparticles prepared using garlic extract exhibit the highest efficiency in the photodegradation of methylene blue dye.     

Electric Field Gradient-Controlled Domain Switching for Size Effect-Resistant Multilevel Operations in HfO2-Based Ferroelectric Field-Effect Transistor

The ferroelectric field-effect transistor (FeFET) is a promising memory technology due to its high switching speed, low power consumption, and high capacity. Since the recent discovery of ferroelectricity in Si-doped HfO₂ thin films, HfO₂-based materials have received considerable interest for the development of FeFET, particularly considering their excellent complementary metal-oxide-semiconductor (CMOS) compatibility, relatively low permittivity, and high coercive field. However, the multilevel capability is limited by the device size, and multidomain switching tends to vanish when the channel length of the HfO₂-based FeFET approaches 30 nm. Here, multiple nonvolatile memory states are realized by tuning the electric field gradient across the Hf_0.5Zr_0.5O₂ (HZO) ferroelectric thin film along the channel direction of FeFET. The multi-step domain switching can be readily and directionally controlled in the HZO-FeFETs, with a very low variation. Moreover, multiple nonvolatile memory states or multi-step domain switching can be effectively controlled in the FeFETs with a channel length less than 20 nm. This study suggests the possibility to implement multilevel memory operations and mimic biological synapse functions in highly scaled HfO₂-based FeFETs.     

Synergistic Engineering of Doping and Vacancy in Ni(OH)2 to Boost Urea Electrooxidation

Nickel hydroxide (Ni(OH)₂) has been identified as one of the best promising electrocatalyst candidates for urea oxidation reaction (UOR) due to its flexible structures, wide compositions, and abundant 3d electrons under alkaline conditions. However, its layered structure with limited exposed edge sites severely hinders further improvement of the UOR activity. Herein, oxygen-vacancy rich and vanadium doped Ni(OH)₂ (O_vac-V-Ni(OH)₂) catalysts are prepared and synergistically boost the urea electrooxidation. Vanadium doping contributes more exposed active sites, and simultaneously generates oxygen vacancies, switching the rate-determining step of UOR from *COOH deprotonation to the N–H bond cleavage process and lowering the thermodynamic barrier by around 1.13 eV. The novel O_vac-V-Ni(OH)₂ demonstrates good electrocatalytic performances with a working potential of 1.47 V at a high current density of 100 mA cm⁻². Synergistic engineering of doping and oxygen vacancy is a promising strategy for designing efficient UOR electrocatalysts.