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Filamentary‐type resistive switching devices based on the valence change mechanism (VCM) have been studied extensively during the past decades, due to their great performance as nonvolatile memories and their promising nature for novel computing architectures. For many materials exhibiting the VCM, two different polarities can be observed, namely, counter‐eightwise and eightwise switching. The more technologically relevant materials, such as TaOx and HfOx, usually exhibit the counter‐eightwise resistive switching mode, however, the nature of its abrupt switching limits the potential for their implementation in more analog systems. On the contrary, the eightwise resistive switching mode exhibits a more gradual switching profile, which makes it a more suitable candidate for neuromorphic and in‐memory computing applications. Contrary to its counterpart, the eightwise switching mode has not been fully understood for several years, hampering the improvement of memristive devices based on this mode. This review presents the major advancements that have been made to understand the mechanism behind the eightwise switching mode and its implications on the properties of the memristive devices. The works presented here are based on the model system of SrTiO3, however, the obtained knowledge and the strategies to tailor their properties can be transferred to other technologically relevant systems. [ABSTRACT FROM AUTHOR]

In this work, the parameters for the formation of 10% yttrium-doped barium zirconate (BZY) thin films were investigated systematically using the low-cost and scalable chemical solution deposition method. By modifying the chemistry of the precursor solution and applying different thermal treatment schedules, the key parameters of controlling microstructure and growth orientation were unraveled. Films with porous and lamellar, granular, and dense columnar structures, respectively, were obtained at a relatively low annealing temperature (950 ℃). Particularly, the film with a highly (200) preferred orientation was attained by employing a seed layer approach, in combination with a butyrate-based precursor coating solution. The decomposition of the precursors was surveyed through thermal analysis. The in-plane conductivity of BZY films was investigated by electrochemical impedance spectroscopy and the corresponding activation energy was evaluated. The relationship between microstructures and electrochemical properties was discussed and concluded that the denser BZY film with larger grains shows better performance. • High (200) oriented Ba(Zr,Y)O 3 (BZY) films with dense columnar microstructure were prepared by chemical solution deposition. • The average grain size of BZY thin films can reach about 60 nm at a relatively low annealing temperature (950 °C). • The nonorthodox butyric acid-based precursor solution shows unique decomposition properties among the precursor solutions. • High temperature impedance spectroscopy was applied to evaluate the proton-conducting properties of the BZY thin films. [ABSTRACT FROM AUTHOR]

Hysteretic resistance effects based on a correlation between ferroelectric polarization and conductivity might become of particular interest for nonvolatile memory applications, because they are not subject to the scaling restrictions of charge based memories such as the ferroelectric random access memory. Two basic concepts, a metal-ferroelectric-metal structure and a metal-ferroelectric-semiconductor structure are discussed in the literature. This contribution discusses the principle of operation of those concepts in terms of the band model. A generalized model is proposed, which is based on a conductive metal-ferroelectric-semiconductor-metal structure. Here, the existence of a low and a high conductive state originates from a switch of the polarization in the ferroelectric layer and a resulting positive or negative polarization charge at the ferroelectric-semiconductor interface. Charge carriers in the film are attracted by or depleted at the interface giving rise to different local conductivities. By simulation, the effect of internal screening caused by mobile charge carriers on the hysteretic current-voltage behavior and the depolarizing field in the ferroelectric are estimated. The simulation discloses a switching ratio up to several orders of magnitude and a conductivity window, which scales with the donor concentration. It may also explain resistive switching in systems consisting only of one ferroelectric layer by assuming the presence of nonferroelectric interface layers. [ABSTRACT FROM AUTHOR]

Focuses on an investigation of the temperature dependence of the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTio3 ceramics. Details on the simulation technique used in the study; What the investigation showed.

Proton conducting yttrium-doped barium zirconate (BZY) thin films have been successfully synthesized by chemical solution deposition (CSD). The seed layer technique is developed to obtain BZY films with different growth orientations, in combination with the layer-by-layer approach. The concentration of the precursor solution for the deposition of the seed layer is the decisive parameter to the final orientations and microstructures of BZY films. In particular, BZY thin films with the preferred (111) and (100) orientations have been fabricated on platinized silicon wafers, which are rarely reported in previous literature results. Meanwhile, the average grains size can be about 230 nm in the plane of the interface. In addition, these films with certain orientations and larger grains are obtained at a, compared to bulk BZY processing, relatively low annealing temperature (950 °C). The different nucleation behavior during the deposition of seed layers has been discussed to explain this relationship between the fabricating processes and final microstructures. For the BZY thin film with (111) orientation, annular dark field imaging in scanning transmission electron microcopy (HADDF-STEM) has been employed to reveal its porous and layered internal microstructure. In addition, proton conductivity of these thin films has been evaluated by electrochemical impedance spectroscopy (EIS) at intermediate temperatures (350–600 °C). • Key deposition parameters of seed layers to control the microstructures of Y-doped barium zirconate (BZY) films established. • BZY films with (111) orientation and large grain size (~230 nm) by chemical solution deposition at low temperature (950 °C). • Fine nanostructure and proton conductivity of different BZY thin films studied. [ABSTRACT FROM AUTHOR]

In this Letter we discuss the SET/RESET current asymmetry of the bipolar resistive switching in electrochemical metallization cells based on an analytical switching model. The model is based on the growth and dissolution of a metallic filament. It accounts for the controllability of the low resistive state and the RESET current by tuning the SET current. Based on this model a possible physical explanation for the SET/RESET asymmetry is presented. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Part of Focus Issue on “Functional Oxides” (Eds.: T. Frauenheim, J. M. Knaup, P. Broqvist, S. Ramanathan) The origin of the SET/RESET current asymmetry in electrochemical metallization memory (ECM) cells is discussed using an advanced simulation model. The simulation results suggest that the experimentally observed asymmetry can be physically understood by: (i) the geometric asymmetry between the Cu/Ag filament tip and the counter electrode in combination with (ii) the electrochemically favored Cu/Ag oxidation. Thus, this study deepens the understanding of ECM cells. [ABSTRACT FROM AUTHOR]

Abstract: A survey of non-volatile, highly scalable memory devices which utilize dedicated resistive switching phenomena in nanoscale chalcogenide-based memory cells is presented. We introduce the basic operation principle of the phase change mechanism, the thermochemical mechanism, and the valence change mechanism and we discuss the crucial role of structural defects in the switching processes. We show how this role is determined by the atomic structure of the defects, the electronic defect states, and/or the ion transport properties of the defects. The electronic structure of the systems in different resistance states is described in the light of the chemical bonds involved. While for phase-change alloys the interplay of ionicity and hybridization in the crystalline and in the amorphous phase determine the resistances, the local redox reaction at the site of extended defects, the change in the oxygen stoichiometry, and the resulting change in the occupancy of relevant orbitals play the major role in the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase-change alloys but also for both other types of switching processes. The switching kinetics as well as the ultimate scalability of switching cells is related to structural defects in the materials. [Copyright &y& Elsevier]

Abstract: The review provides a survey of non-volatile, highly scalable memory devices which are based on redox phenomena controlling the resistance of nanoscale memory cells. The classification of the memory effects, the understanding of the underlying mechanisms, and a sketch of the integration efforts will be presented. [Copyright &y& Elsevier]

Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues. [ABSTRACT FROM AUTHOR]

Abstract: The total conductivity and the partial hydrogen ion conductivity of a single crystal of SrTiO3 have been investigated by transport number measurements using a hydrogen activity concentration cell/EMF method in wet atmospheres as a function of pO2 (10−20–1 atm) at 1000 °C and as a function of temperature (350–1050 °C) in wet hydrogen. The single crystal exhibits a small but readily measurable proton conductivity contribution in wet oxidising atmospheres. Earlier indications of apparent – but unlikely – transport by negatively charged hydrogen ions under reducing conditions and high temperatures in polycrystalline SrTiO3 have been reproduced for the single crystal, thus eliminating grain boundaries and porosity as a cause of such findings. The paper proposes transport of neutral hydrogen species through the oxide as an alternative cause: This results in polarization of the hydrogen activity at the interface between the electrodes and the electrolyte, in turn giving rise to an unintended gradient in oxygen activity and an EMF resulting from the relatively high oxygen ion transport number of the material. [Copyright &y& Elsevier]

Abstract: This article reviews the current understanding of the electrical properties of the grain boundaries of acceptor-doped zirconia and ceria, however, with an emphasis on the grain-boundary defect structure. From an electrical point of view, a grain boundary consists of a grain-boundary core and two adjacent space-charge layers. The grain-boundary cores of acceptor-doped zirconia and ceria are positively charged, probably owing to the oxygen vacancy enrichment there. Oxygen vacancies are therefore depleted in the space-charge layer. The grain-boundary conductivities of acceptor-doped zirconia and ceria are at least two orders of magnitude lower than the corresponding bulk values, depending on temperature and dopant level. Such a phenomenon is due to the facts: (1) that oxygen vacancies are severely depleted in the space-charge layer, and (2) that the grain-boundary impurity phase blocks the ionic transport across the grain boundaries by decreasing the conduction path width and constricting current lines. In materials of high purity, the effect of the space-charge depletion layer is dominant; however, in materials of normal purity, the effect of the grain-boundary impurity phase is dominant. A Schottky barrier model satisfactorily explains all the phenomenological observations of the grain-boundary electrical properties of materials of high purity, and experimental evidence soundly supports the model. Various factors (alumina addition and grain size) influencing the grain-boundary electrical properties are discussed, and some special aspects of nanocrystalline materials are highlighted. [Copyright &y& Elsevier]

Abstract: The grain boundary cores of acceptor-doped zirconia and ceria are positively charged, which is probably due to the oxygen vacancy enrichment there. Oxygen vacancies are therefore depleted in the space charge layers, which is the decisive cause of the low grain boundary conductivity in acceptor-doped zirconia and ceria materials of high purity. This space charge concept is elaborated, and crucial experimental evidences are summarized. [Copyright &y& Elsevier]

In contrast to dense donor-doped SrTiO3 (STO) ceramics and single crystals, porous fine grained thick films reveal a surprisingly fast resistivity response in reply to a change of the oxygen partial pressure (pO2) within some 10 ms even at temperatures below 900 °C. Although this unexpected behavior was attributed to a grain boundary effect, the resistivity versus pO2 plot shows a similar characteristic as found for dense ceramics or single crystals, respectively. This observation suggests that cation vacancies, which significantly influence the electrical behavior of the bulk, but which are known to equilibrate only at highest temperatures, may also play a key role in the formation of resistive grain boundaries in the moderate temperature range. For mobility reasons, a change in the cation vacancy concentration might then be limited to a few monolayers on each side of the interface and a very high interface density of defect states is needed to explain the drastic resistivity change of the sensor observed in the experiment. We propose a generalized point defect model that involves the formation of a near-interface space charge region. The latter is found to affect the local defect equilibria significantly. As a consequence, the concentration of each defect type differs from the bulk value. The fast sensor response might then originate from a space charge induced increase of the cation vacancy concentration situated only near the interface that exceeds the value predicted from electroneutral defect considerations by 2 orders of magnitude. The local formation of cation vacancies causes a strong depletion of electrons. The space charge itself is formed due to the difference in mobilities of ionic and electronic species. [Copyright &y& Elsevier]

Topotactic phase transitions induced by changes in the oxygen vacancy concentration can largely alter the physical properties of complex oxides, including electronic and magnetic phases, while maintaining the structural integrity of the crystal lattice. An oxygen‐vacancy‐induced topotactic phase transition from perovskite (PV) to brownmillerite (BM) is achieved in epitaxial La0.6Sr0.4CoO3−δ (LSCO) thin films. Two novel intermediate states with different oxygen content are identified by X‐ray diffraction, which involves a single‐phase reduced PV state and a mixed state of co‐existing PV and BM. The combination of depth‐sensitive polarized neutron reflectometry (PNR) and Rutherford backscattering (RBS) allows a quantitative determination of magnetization and the mean oxygen content in all states, revealing a continuous transition from La0.6Sr0.4CoO2.97 to La0.6Sr0.4CoO2.5. BM formation is observed for an LSCO layer with an oxygen content of 2.67, while the magnetic and electronic transition already occurs for a layer with a higher oxygen content of 2.77 (and above) and in the absence of a BM signature. These results demonstrate that the physics of electronic metal‐to‐insulator transition (MIT), magnetic ferromagnet‐to‐non‐ferromagnet transition (FM‐to‐non‐FM), and structural PV‐to‐BM phase transition should be considered within the framework of separate but interrelated processes. [ABSTRACT FROM AUTHOR]

Abstract: A new memory cell concept, which is appropriate for the use with resistive hysteretic memory elements is introduced. Resistive memories are one of the main memory development streams today. The introduced concept can be used to facilitate the integration with current CMOS technology, where no plateline is required. This is accomplished by using a capacitor, which is serially connected to the resistive element. [Copyright &y& Elsevier]

We report on a measurement procedure to separate ferroelectric switching current and dielectric displacement current from the leakage current in leaky ferroelectric thin-film capacitor structures. The ac current response is determined for two adjacent frequencies. Taking advantage of the different frequency dependencies of the ferroelectric switching current, dielectric displacement current and ohmic current, the hysteresis loop is calculated without performing a static leakage current measurement, which causes a high dc field stress to the sample. The applicability of the proposed measurement procedure is demonstrated on a Pt/Pb(Zr,Ti)O3/IrO2 ferroelectric capacitor revealing a high leakage current. [ABSTRACT FROM AUTHOR]

This review addresses resistive switching devices operating according to the bipolar valence change mechanism (VCM), which has become a major trend in electronic materials and devices over the last decade due to its high potential for non-volatile memories and future neuromorphic computing. We will provide detailed insights into the status of understanding of these devices as a fundament for their use in the different fields of application. The review covers the microscopic physics of memristive states and the switching kinetics of VCM devices. It is shown that the switching of all variants of VCM cells relies on the movement of mobile donor ions, which are typically oxygen vacancies or cation interstitials. VCM cells consist of three parts: an electronically active electrode (AE), often a metal with a high work function, in front of which the switching occurs, a mixed ionic-electronic conducting (MIEC) layer consisting of a nanometer-scale metal oxide or a stack of different metal oxides, and an ohmic counter electrode (OE). After an introduction to definitions and classification, the fundamentals of solid-state physics and chemistry associated with VCM cells are described, including redox processes and the role of electrodes. The microscopic changes induced by electroforming, a process often required prior to resistive switching, are described in terms of electronic initialization and subsequent changes in chemistry, structure, and conductivity. The switching process is discussed in terms of switching polarity, geometry of the switching region, and spectroscopic detection of the valence changes. Emphasis is placed on the extreme nonlinearity of switching kinetics described by physics-based multiscale modeling, ranging from ab initio methods to kinetic Monte Carlo and finite element models to compact models that can be used in circuit simulators. The review concludes with a treatment of the highly relevant reliability issues and a description of the failure mechanisms, including mutual trade-offs. [ABSTRACT FROM AUTHOR]

Devices made of amorphous thin films of the prototypical Mott‐insulator chromium‐doped V2O3 show a threshold and negative differential resistance effect after an electroforming step. Here, it is demonstrated that this effect is caused by the formation of a crystalline filament, in which a thermal runaway effect can occur. A compact model is developed that can describe the switching behavior as well as its inherent variability. The influence of the doping concentration and oxygen stoichiometry on the switching behavior is characterized and by fitting the model to the experimental data, the underlying physical changes are extracted. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including resistively switching chalcogenides, material properties of oxides, and developments in solid-state physics and material science.

A defect chemical model based on charge neutrality and laws of mass action is proposed to clarify the details of the chemistry of point defects for donor-doped Yb x Ca 1-x MnO 3. DC-conductivity measurements were carried out in a wide range of partial pressure of oxygen p (O 2) ≈ 10−1 down to 10-19 MPa at 750⁰C for the first time without disintegrating the ceramic sample through reduction. A comparison of the experimental observations and the theoretical defect chemical models clearly shows the possibilities for controlling charge carriers in dependence of partial pressure of oxygen p (O 2) and dopant concentration. The origin of a plateau state, of a drastic decrease in conductivity in the intermediate and reduction p (O 2) regimes are figured out, respectively. In addition, the kind and concentration of the electronic and ionic majority charge carriers are determined and formulated according to the proposed defect chemical model. Furthermore, phase transitions were studied in a wide range of p (O 2) at elevated temperatures [ABSTRACT FROM AUTHOR]

A comprehensive overview is provided about the role of bulk conductivity contributions in compounds of the composition Yb x Ca 1-x MnO 3 (0–10 at. % Yb-dopant concentration). For this purpose, in-situ impedance spectroscopy was successfully employed at different temperatures (−100 up to 300 °C) and frequencies (1 Hz–1 MHz). These experiments reveal the main role of grain boundaries as well as electronic and ionic contributions in conductivity. The contribution of different resistance components in electric transport properties were proposed on the base of a double-Schottky-barrier model. Migration of oxygen vacancies and their participation in conductivity were studied and the results are confirmed by observing oxygen released using a ZrO 2 oxygen sensor during dilatometry measurements in a wide range of temperatures. [ABSTRACT FROM AUTHOR]

We report on a simulation model for bipolar resistive switching in cation-migration based memristive devices. The model is based on the electrochemical driven growth and dissolution of a metallic filament. The origin of multilevel switching is proposed to be direct tunneling between the growing filament and the counter electrode. An important result of our parameter simulation studies is that different materials show the same experimental multilevel behavior. Our model fully reproduces the experimental data and allows for an explanation of the transition from bipolar to nonpolar switching. [ABSTRACT FROM AUTHOR]

The alternation from bipolar to unipolar resistive switching was observed in perovskite Ba0.7Sr0.3TiO3 thin films. By controlling the switching voltage, either bipolar or unipolar switching was obtained. When the switching voltage is higher than a threshold voltage, the device exhibits unipolar switching while if the switching voltage is lower than a threshold voltage, the device shows bipolar switching behavior. The bipolar-to-unipolar alternation is dynamically repeatable and may be related to the local modification of broken filaments by oxygen vacancy movement. [ABSTRACT FROM AUTHOR]

The pressure dependence of pentacene (C22H14) transistors with solution-processed polyvinylphenol gate dielectric on glass substrates is investigated by applying uniaxial mechanical pressure with a needle. We found that organic thin-film transistors are sensitive to applied pressure inherently. The measurements reveal a reversible current dependence of the transfer characteristics where the drain current is switching between two states. Experimental and simulation results suggest that switch-on voltage and interface resistance are affected. The change takes seconds, hinting at trap states being responsible for the effect. [ABSTRACT FROM AUTHOR]

The possibility of preparing translucent ferroelectric heavily donor-doped BaTiO3 ceramics rich in TiO2 has been demonstrated by conventional ceramic processing based on the synthesis via the solid state route and on subsequent sintering and annealing at high levels of oxygen partial pressure. The resulting ceramics have very high densities and a very fine-grained microstructure with a remarkably narrow grain size distribution. Under oxidizing conditions large amounts of donor – in this case La – can be incorporated into the perovskite lattice during sintering, as the variation of the lattice parameters determined by combined XRD (X-ray diffraction) and Rietveld refinements suggests. The electrons released from these donors are captured most probably by vacancies of titanium, which act as acceptors. The optical properties are largely determined by defect chemistry and in particular by the concentration of free conducting electrons. Particularly the reduction of the free electron concentration is believed to be mainly responsible for the high degree of optical translucency. The normalized values of transmittance almost reach the best values reported so far for nanocrystalline barium titanate powders that have been consolidated via pressure-assisted sintering. Measurements of the refractive indices showed that they can be modified by the state of polarization of the ferroelectric BaTiO3-based ceramics. This newly developed material possibly opens a door to novel electro-optical applications. [ABSTRACT FROM AUTHOR]

Abstract A comprehensive study was performed in order to find the effect of different calcination and sintering conditions on the physical properties of calcium manganite ceramics in dependence of temperature T and partial pressure of oxygen p (O 2). The eventual formation of oxygen vacancies during sintering was investigated and the results were confirmed by monitoring the release of oxygen using a ZrO 2 oxygen sensor. The phase transition behavior was studied using thermogravimetric analysis (TGA) in a wide range of p (O 2) ≈ 10−1 MPa down to 10-19 MPa at high temperatures accompanied by dilatometry- and XRD-measurements. Furthermore, the present study reveals for the first time a way for reducing and preventing crack formation that may occur during sintering. The present systematic research provides essential fundamental information before performing electrical measurements necessary in order to understand important factors about charge carriers and electrical transport mechanism. [ABSTRACT FROM AUTHOR]

We study the voltage dependence of the SET time statistics of bipolar resistive switching in a tantalum oxide thin film. Weibull analysis reveals that the SET time statistics exhibit a crossover from deterministic to stochastic nature in a single cell as the amplitude of the applied voltage is lowered. While the Joule heating effect has a general contribution to the SET physics in both deterministic and stochastic cases, the magnitude of the positive feedback cycle of the Joule heating determines the statistical nature. Sufficient feedback effect under a voltage of large amplitude increases the SET probability with time, resulting in the SET time distribution with a deterministic nature. When amplitude of the applied voltage is small, on the other hand, the feedback effect is weak hence the SET process is controlled by randomness of the cell condition. In this case, the SET time is totally unpredictable hence its statistics has a stochastic nature. Since the crossover between the deterministic and stochastic regimes is found to occur based on the electric field rather than the current, we argue that it stems from a field-driven redox reaction at the tantalum oxide/tantalum interface. [ABSTRACT FROM AUTHOR]

We discuss electronic and ionic defect concentration profiles at the conducting interface between the two wide-band-gap insulators LaAlO3 and SrTiO3 (STO). The profiles are deduced from a thermodynamic model considering a local space charge layer (SCL) originating from charge transfer to the interface region, thus combining electronic and ionic reconstruction mechanisms. We show that the electrical potential confining the two-dimensional electron gas (2DEG) at the interface modifies the equilibrium defect concentrations in the SCL. For the n-conducting interface, positively charged oxygen vacancies are depleted within the SCL, while negatively charged strontium vacancies accumulate. Charge compensation within the SCL is achieved by a mixed ionic-electronic interface reconstruction, while the competition between 2DEG and localized ionic defects is controlled by ambient pO2. The concentration of strontium vacancies increases drastically in oxidizing conditions and exhibits a steep depth profile towards the interface. Accounting for the low cation diffusivity in STO, we also discuss kinetic limitations of cation defect formation and the effect of a partial equilibration of the cation sublattice. We discuss the resulting implications for low temperature transport. [ABSTRACT FROM AUTHOR]

Solid-state programmable metallization cells have attracted considerable attention as memristive elements for Redox-based Resistive Random Access Memory (ReRAM) for low-power and low-voltage applications. In principle, liquid-state metallization cells could offer the same advantages for aqueous systems, such as biomedical lab-on-a-chip devices, but robust resistive switching has not yet been achieved in liquid electrolytes, where electrodeposition is notoriously unstable to the formation of fractal dendrites. Here, the recently discovered physics of shock electrodeposition are harnessed to stabilize aqueous copper growth in polycarbonate nanopores, whose surfaces are modified with charged polymers. Stable bipolar resistive switching is demonstrated for 500 cycles with <10 s retention times, prior to any optimization of the geometry or materials. [ABSTRACT FROM AUTHOR]

Metal-free chemical vapor deposition (CVD) of single-layer graphene (SLG) on c-plane sapphire has recently been demonstrated for wafer diameters of up to 300 mm, and the high quality of the SLG layers is generally characterized by integral methods. By applying a comprehensive analysis approach, distinct interactions at the graphene-sapphire interface and local variations caused by the substrate topography are revealed. Regions near the sapphire step edges show tiny wrinkles with a height of about 0.2 nm, framed by delaminated graphene as identified by the typical Dirac cone of free graphene. In contrast, adsorption of CVD SLG on the hydroxyl-terminated α-Al2O3 (0001) terraces results in a superstructure with a periodicity of (2.66 ± 0.03) nm. Weak hydrogen bonds formed between the hydroxylated sapphire surface and the π-electron system of SLG result in a clean interface. The charge injection induces a band gap in the adsorbed graphene layer of about (73 ± 3) meV at the Dirac point. The good agreement with the predictions of a theoretical analysis underlines the potential of this hybrid system for emerging electronic applications. [ABSTRACT FROM AUTHOR]

The full text of the corrigendum is available in the pdf provided. [ABSTRACT FROM AUTHOR]

2018 werden bereits weltweit 1,6 Zettabytes an Daten anfallen. Würde man die explodierenden Datenmengen noch lange auf herkömmlichen Computern verarbeiten, wäre deren Energiebedarf enorm. Deswegen werden neuartige energieeffiziente Computertechnologien mit semantischen und kognitiven Fähigkeiten wie das menschliche Gehirn entwickelt. Sie sollen Informationen sortieren, analysieren und auf dieser Basis Entscheidungen treffen können. Solche Fähigkeiten könnte die hier vorgestellte Memristor-Logik ermöglichen. Mit ihr könnten neuartige Computer große Datenmengen energieeffizient analysieren. Die Programmierung der Memristor-Logik funktioniert grundsätzlich anders als bei herkömmlicher Logik-Hardware, denn sie verändert nicht nur die Software, sondern auch die Hardware. Die neuartigen Bausteine können zudem das Ergebnis einer logischen Operation nichtflüchtig speichern. [ABSTRACT FROM AUTHOR]

Owing to their unique features such as thresholding and self‐relaxation behavior diffusive memristors built from volatile electrochemical metallization (v‐ECM) devices are drawing attention in emerging memories and neuromorphic computing areas such as temporal coding. Unlike the switching kinetics of non‐volatile ECM cells, the thresholding and relaxation dynamics of diffusive memristors are still under investigation. Comprehension of the kinetics and identification of the underlying physical processes during switching and relaxation are of utmost importance to optimize and modulate the performance of threshold devices. In this study, the switching dynamics of Ag/HfO2/Pt v‐ECM devices are investigated. Depending on the amplitude and duration of applied voltage pulses, the threshold kinetics and the filament relaxation are analyzed in a comprehensive approach. This enables the identification of different mechanisms as the rate‐limiting steps for filament formation and, consequently, to simulate the threshold kinetics using a physical model modified from non‐volatile ECM. New insights gained from the combined threshold and relaxation kinetics study outline the significance of the filament formation and growth process on its relaxation time. This knowledge can be directly transferred into the optimization of the operation conditions of diffusive memristors in neuromorphic circuits. [ABSTRACT FROM AUTHOR]

The substitution in ( Ba0.70 Sr0.30) TiO3 thin films by the rare-earth element dysprosium prepared at 1000°C by chemical solution deposition on nickel foils was investigated. The relatively large thermal budget applied (via annealing temperature) is shown to enhance the solubility of the Dy3+doping ion into the crystal lattice of the perovskite films. Preference for B-site occupancy of this amphoteric cation was further promoted by the addition of BaO excess (1 mol%), which results in slightly larger grains in the films as observed by scanning electron microscopy. Despite this Ba-rich composition, the presence of secondary phases in the thin films was not detected by X-ray diffraction. Transmission electron microscopy revealed no evidence for local segregation of Dy at grain boundaries, neither the formation of NiO at the interface between the film and the metal foil was observed. The substitution of Ti4+ by Dy3+ leads to the formation of strong electron acceptors in the system, which balance the number of ionized oxygen vacancies arisen from the reductive crystallization atmosphere used during processing. As a consequence, the dielectric loss (tan σ) and leakage conduction measured in the resulting thin-film capacitors were significantly reduced with respect to nominally undoped samples. The improvement of this capacitor feature, combined with the relatively high permittivities obtained in the films (490-530), shows the effectiveness of dysprosium doping within a thin-film fabrication method for potential application into the multilayer ceramic capacitor technology. [ABSTRACT FROM AUTHOR]

Abstract: Metal–electrolyte–metal-structures with germanium sulphide (GeSx) as ion conducting solid are one of the most promising candidates for application in a fast and high density resistive switching memory. We report on the preparation of 40nm to 500nm thin films of GeSx by radio-frequency sputtering and their suitability for integration into next generation memory applications. The influence of the deposition parameters on key morphological and chemical properties of the films was thoroughly studied. We found that the films'' stoichiometry can be precisely controlled and tuned by variation of the deposition parameters. The comparative electrical characterization demonstrated the suitability of sputtered GeSx for integration in resistively switching memory cells with high reproducibility and reliable memory operation. [Copyright &y& Elsevier]

The reliable fabrication of nanoelectrode pairs with predefined separations in the few nanometer range is an essential prerequisite for future nanoelectronic devices. Here we demonstrate a fine-tuned electron-beam lithographic (EBL) fabrication route which is suitable for defining nanoelectrode pairs with a gap size down to 3 ± 1 nm and with a yield of 55%. This achievement is based on an optimized two-layer resist system in combination with an adopted developer system, as well as on an elaborated nanoelectrode pattern design taking into consideration the EBL inherent proximity effect. Thus, even a structural control in the nanometer scale is achieved in the EBL process. [ABSTRACT FROM AUTHOR]

In this article, resistive switching based on the thermochemical mechanism (TCM) is reviewed. This mechanism is observed when thermochemical redox processes dominate over electrochemical processes. As the switching is based on thermal effects, it is inherently unipolar, i.e., the transitions between the resistive states can be induced by the same bias voltage polarity. NiO has emerged as a 'model material' for resistive switching based on the TCM effect and the discussion of the resistance states and the switching processes are focused on this material with the appropriate electrodes, mainly Pt. Unipolar switching is unambiguously filamentary. Conductive filaments are formed during the electroforming process needed prior to memory switching. The SET operation is interpreted as a sequence of threshold switching and subsequent Joule heating which triggers local redox reactions in which oxygen deficient NiO and, if the amount of released oxygen exceeds a certain amount, also metallic Ni will form. The RESET transition can be described as a thermally activated solid-state process resulting in a local decrease of the metallic Ni species. In terms of operation and reliability, a trade-off between RESET current reduction and retention was experimentally found. This is due to the decreasing long-term stability of the filaments with decreasing size. In addition, the scaling projection of a TCM-based memory technology with NiO is directly related to RESET currents and the availability of appropriate select devices. [ABSTRACT FROM AUTHOR]

Spark plasma sintering (SPS) was used to densify BaTiO3-Ni composite powders to relative densities above 92.8%. With the increasing Ni content, a decrease in relative density is observed, suggesting that Ni hampers the consolidation process. The microstructures of the BaTiO3-Ni composites were of duplex character. The crystallite size of the BaTiO3-grains was around 100 nm in average. The ceramic matrix phase of BaTiO3 surrounded Ni inclusions of approximately 1 μm in diameter that were completely incorporated without the formation of any elongated metallic filaments. The ac conductivity of these BaTiO3-Ni composites increased with increasing Ni content and with temperature. The dominant conduction mechanisms in SPSed BaTiO3-Ni composites showed quite a complicated behavior. A gradual change from band conduction of trapped electrons in oxygen vacancies to a hopping-type motion of small polarons between Ti4+ and Ti3+ is suggested to occur, when the Ni content increases. The influence of oxygen vacancies and other lattice defects on the electrical properties of BaTiO3-Ni composites is discussed. [ABSTRACT FROM AUTHOR]

Several strategies were evaluated focusing on the optimum reliability of thin-film ceramic capacitors derived from different compositions based on BaTiO3 (BT) and prepared by chemical solution deposition onto nickel foils. Film crystallization was carried out at 800°C under reducing conditions to prevent the detrimental oxidation of the metal electrode while allowing the formation of the pure perovskite phase. In a first approach, compositional modifiers were introduced into the system, yielding to Ba(Ti0.7Zr0.3)O3 (BTZ) and (Ba0.7Sr0.3)TiO3 (BST) solid solutions. Despite the lower permittivities obtained with respect to pure BT, dielectric losses and leakage current densities in these films were observed to improve. With the aim of further optimizing these features, the second approach focuses on the defect chemistry of BST films by incorporation of either donor–acceptor cations (Mn2+ and Nb5+) or single rare-earth elements (Dy3+ or Y3+“magic ions”) into the perovskite. The leakage conduction in the associated capacitors is revealed to effectively decrease in the first case, whereas doping with rare-earth cations does not substantially improve the dielectric properties of the ceramics. Processing effects related to the thin-film configuration, rather distant from the classical scenario used within the technology of multilayer bulk ceramic capacitors, also account for the final properties obtained in the films of this work. [ABSTRACT FROM AUTHOR]

Abstract: The self-assembly of ω-ferrocenylalkanethiols (FcCnSH) with different alkyl-spacer lengths on Au(111) substrates has been studied by scanning tunneling microscopy (STM). Upon deposition at room temperature FcCnSH molecules tend to form multilayers, while by thermal treatment monolayer formation, a rearrangement of the molecules and the formation of ordered domains is achieved. The surface structure of the resulting full coverage self-assembled monolayers is resolved with molecular resolution by STM. The ordered monolayer structure of ω-ferrocenylpropanethiol is discussed in comparison with its bulk crystal structure, derived from single crystal X-ray analysis. Based on these results a monolayer structure of ω-ferrocenylalkanethiols with longer alkyl chains closely related to the bulk crystal structure of the shorter alkyl-spacer derivates is suggested. Our results provide detailed insight into the self-assembly of FcCnSH on gold substrates. [Copyright &y& Elsevier]

Abstract: We have investigated the electrical and optical properties of Ba(Hf x Ti1− x )O3 (x =0, 0.1, 0.2, 0.3, 0.4) (BHT) thin films deposited on platinized silicon and fused quartz substrates. Analyses of the X-ray diffraction patterns reveal that with the increase in Hf contents there is a systematic increase of the lattice constants of BHT films. Irrespective of the measurement frequencies the dielectric constants was found to be systematically decreased, whereas their frequency dispersion was found to be reduced with increasing Hf contents. The leakage current data measured using a metal-insulator–metal configuration reveal that the Schottky emission is the dominant leakage current mechanism in these films. BHT films, deposited on transparent fused quartz substrates, were also characterized in terms of their optical properties. For this purpose the transmittance of the undoped as well as Hf doped barium titanate thin films was measured as a function of wavelength in the range of 290 nm to 800 nm. The transmission spectra were analysed to estimate the wavelength dependence of the refractive indices/extinction coefficients as well as the variation of optical band gap of these films. With the increase of Hf contents, a systematic increase of the band gap [from 3.65 eV (undoped film) to 4.15 eV (40 at.% Hf doped barium titanate film)] was observed. The reduction of the leakage current with increasing hafnium substitution is discussed on the basis of an increasing Schottky barrier height and due to a simultaneous increase in the band gap of the material. [Copyright &y& Elsevier]

The synthesis of lead titanate thin films by two different chemical solution deposition routes and via nanoparticle dispersions is described. The differences in decomposition kinetics of the solutions were determined and correlated to the morphology evolution and ferroelectric characteristics of the deposited films. High remanent polarisation values of up to 55 μC cm-2 were obtained by the sol-gel method as well as the nanoparticle dispersion technique. Measurements of the dielectric properties of the films revealed low leakage current densities in the range of 10-6 A cm-2 and a maximum dielectric constant of ∼360. [ABSTRACT FROM AUTHOR]

Scalable nonvolatile memory devices that operate at low voltage and current, exhibit multilevel cell capability, and can be read nondestructively using simple circuitry, are highly sought after. Such devices are of particular interest if they are compatible with back-end-of-line processing for CMOS integrated circuits. A variety of resistance-change technologies show promise in this respect, but a new approach that is based on switching in copper-doped silicon dioxide may be the simplest and least expensive to integrate. This paper describes the characteristics of W-(Cu/SiO2)-Cu programmable metallization cell (PMC) devices formed by the thermal diffusion of Cu into deposited SiO2. PMC devices operate by the electrochemical control of metallic pathways in solid electrolytes. Both unipolar and bipolar resistive switching could be attained in these devices. Bipolar switching, which is identical to that seen in PMC devices based on other solid electrolytes, was observed for low bias (a few tenths of volts) and programming currents in the microampere range. The resistance ratio between high and low states was on the order of 103, and a multibit storage is considered possible via the strong dependence of ON-state resistance on programming current. The low and high resistance states were stable for more than 5 × 104 s. The devices could be made to exhibit unipolar switching using a negative bias on the order of –1 V combined with erase currents of hundreds of microampere to a few milliampere. In this case, the OFF/ON ratio was 106. [ABSTRACT FROM AUTHOR]

Abstract: Resistive memory (RRAM) is one of the strong emerging technologies in modern memory field. This type of memories has the potential to be the replacement of several current memory types. As any new technology, RRAM brings new challenges concerning technology and design. This work discusses some aspects concerning the design of active resistive memories and compares three possible memory architectures. [Copyright &y& Elsevier]

We report on a novel reference voltage scheme for FRAMs which solves most problems like fatigue, temperature dependency, voltage drop, etc still present in other reference schemes: dynamic reference scheme . It tracks the variations of the ferroelectric capacitors in the memory and provides a common reference voltage line for all sense amplifiers on chip or different reference voltages for different memory banks. No dedicated reference cells are required, instead the memory cells themselves are utilized to generate the reference voltage by periodical test of their voltage levels or by setting a signal that starts the test. This scheme extends life-time and reliability of the memory and enables a self test and repair capability. [ABSTRACT FROM AUTHOR]

The scaling of the ferroelectric field effect transistor (FeFET) is studied by utilizing transistor model simulation (BSIM3v3) and analytic expressions, and is compared to the scaling of the MOSFET. Two scaling approaches are discussed—‘ variable gate stack scaling,’ that requires changing the gate stack, and ‘constant gate stack scaling,’ that leaves the gate stack unchanged. The material parameters were assumed fixed throughout the scaling procedure. [ABSTRACT FROM AUTHOR]

Future improvements in the development of highly volume-efficient multilayer ceramic capacitors (MLCC) require manufacturing processes that allow for a reduced dielectric thickness well below 1 μm. Obviously, such thin dielectric layers can only be produced if nanosized dielectric powders are applied and deposited by techniques more advanced than tape casting and screen printing. These processes require high-purity, homogeneous, weakly agglomerated ultrafine powders. Over the past few years, significant progress has been made in the synthesis of nanocrystalline BaTiO3-based powders for use in MLCC. The present review article aims to summarize the present state of the art with respect to some of the technically and industrially most relevant preparation routes, including methods based on solid-state reactions and, more importantly, solution-based approaches. [ABSTRACT FROM AUTHOR]

The chemical solution deposition (CSD) technique as a highly flexible method for the fabrication of electronic oxide thin films is reviewed. Various chemical aspects of different approaches are discussed, including sol–gel, hybrid, and metallo-organic decomposition (MOD) routes, which all have been successfully applied for the deposition of this class of materials. Principles of the selection and properties of the educts, the mechanism of film crystallization, and tailoring of microstructure through manipulation of deposition parameters are reported. The role of thermodynamics on the phase transformation process is also reviewed. Finally, some of the applications for chemical solution-derived thin films currently under development are illustrated, and recent advances, such as the deposition of less than 100-nm-thick films, lateral nanostructuring, and the formation of single nano-sized grains are considered. To cite this article: R.W. Schwartz et al., C. R. Chimie 7 (2004). [Copyright &y& Elsevier]