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Physics-based compact models for redox-based resistive switching memory (ReRAM) devices are used to increase the physical understanding of the complex switching process as well as to allow for accurate circuit simulations. This includes that models have to cover devices showing bipolar switching (BS) and complementary switching (CS). In contrast to BS devices, which store the information in (at least) one high and one low resistance state, CS devices use (at least) two high resistance states. Applications of CS devices range from passive crossbar arrays to novel logic-in-memory concepts. The coexistence of CS and BS modes in one device has been shown experimentally. Here, a physics-based compact model describing BS and CS consistently is presented. Besides modeling CS devices, the model improves the description of BS as it allows to reproduce and explain anomalies in the BS RESET process. The model includes ion drift and diffusion along the filament. The influence of different parameters on the drift–diffusion balance is discussed.

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.

Exsolution phenomena are highly debated as efficient synthesis routes for nanostructured composite electrode materials for the application in solid oxide cells (SOCs) and the development of next-generation electrochemical devices for energy conversion. Utilizing the instability of perovskite oxides, doped with electrocatalytically active elements, highly dispersed nanoparticles can be prepared at the perovskite surface under the influence of a reducing heat treatment. For the systematic study of the mechanistic processes governing metal exsolution, epitaxial SrTi0.9Nb0.05Ni0.05O3-δ thin films of well-defined stoichiometry are synthesized and employed as model systems to investigate the interplay of defect structures and exsolution behavior. Spontaneous phase separation and the formation of dopant-rich features in the as-synthesized thin film material is revealed by high-resolution transmission electron microscopy (HR-TEM) investigations. The resulting nanostructures are enriched by nickel and serve as preformed nuclei for the subsequent exsolution process under reducing conditions, which reflects a so far unconsidered process drastically affecting the understanding of nanoparticle exsolution phenomena. Using an approach of combined morphological, chemical, and structural analysis of the exsolution response, a limitation of the exsolution dynamics for nonstoichiometric thin films is found to be correlated to a distortion of the perovskite host lattice. Consequently, the incorporation of defect structures results in a reduced particle density at the perovskite surface, presumably by trapping of nanoparticles in the oxide bulk.

Journal of materials chemistry / A (2021). doi:10.1039/D1TA03205D, Published by Royal Society of Chemistry, London [u.a.]

IEEE transactions on electron devices 68(3), 1024-1030 (2021). doi:10.1109/TED.2021.3049765, Published by IEEE, New York, NY

Efficient catalysts made of cheap and abundant metal ions are in need to overcome the sluggish kinetics of the anodic water oxidation reaction. The development of an inexpensive catalyst with improved performance is key to produce hydrogen by the electrolytic water splitting reaction. The user friendly chemical solution deposition method was applied to prepare films with predecided mole concentrations of copper and nickel. The mixed metal oxide with equal mole ratio of Cu:Ni (1:1) achieved the maximum activity with the onset of water oxidation at overpotential of 0.40 V (1.63 V vs RHE) and achieved the current density of 1 mA/cm2 at an overpotential of 0.420 V at pH 13. The activity of this combination is attributed to the copper facilitated in-situ formation of a layered nickel oxy-hydroxide structure. The presence of both metal ions was found to be necessary indicating a synergy between Cu and Ni for the oxygen evolution reaction. The present work representing the simple synthesis process of the catalyst with the improved water oxidation activity is a promising step to develop electrodes for water electrolysis.

Advanced electronic materials 1900808 (2019). doi:10.1002/aelm.201900808, Published by Wiley, Chichester

In the present work we have demonstrated CuO-ZnO composite thin film with optimized CuO content selectively sense carbon monoxide gas. The ‘n’-type gas sensing characteristics of these composite films are changed to ‘p’-type beyond 8.0 mol% CuO contents. Through the analyses of photoluminescence spectra in conjunction with XPS we have demonstrated that up to 33.0 mol% CuO contents the gas sensing characteristics of the composite films are controlled by the type of point defects (oxygen vacancies, zinc interstitials, and copper vacancies) in CuO and ZnO grains. We have hypothesized that these point defects help to chemi-absorb oxygen and CO preferentially on ZnO and CuO grains respectively to yield selective carbon monoxide sensing of ZnO-CuO composite film with 7.0 mol% CuO contents. Beyond 33.0 mol% CuO contents the effect of point defects diminishes and the gas sensing characteristic are grossly controlled by the nature of the CuO grains covering ZnO grains in the composite films. Our work provides comprehensive insight towards the understanding of the gas sensing characteristics of hetero-composite thin films deposited using a mixed precursor sol.

The complex oxide compound praseodymium barium cobalt oxide (PBCO) is an efficient catalyst for the oxygen evolution reaction (OER) during electrochemical water splitting, with an activity that is mainly ascribed to PBCO’s inherent atomic structure and band alignment. Here, we report on epitaxial PBCO thin films showing electrocatalytic properties, with current densities of up to 10 mA/cm2 at 1.8V vs. RHE. Dense PBCO thin films are synthesized in a disordered perovskite phase as well as in an coherently oxygen vacancy-ordered (double-)perovskite phase, in which oxygen vacancies are incorporated in every second CoO2-δ atomic plane along the out-of-plane direction. The transition from disordered to ordered growth occurs temperature-controlled during the growth process and can be directly monitored in-situ by means of reflection high-energy electron diffraction. The epitaxial fabrication process allows the control of structure and phase of the oxide catalysts, providing model systems for exploring structure-...

Micro solid oxide fuel cells (μ-SOFC) were manufactured with perovskite type proton conductors on silicon substrates and with structured Pt-grid electrodes. In order to miniaturize the μ-SOFCs and to shorten the ion path through the electrolyte, the thin film proton conductors were only ~ 510 nm thick. The thin films consist of 10 mol% yttrium-doped BaZrO3 (BZY10) and they were deposited by means of chemical solution deposition (CSD). CSD was applied, because it represents a highly attractive fabrication method, considering the relatively low investment costs and flexibility with regard to stoichiometry. The backsides of the μ-SOFCs on the substrates were opened by wet chemical and plasma etching to form the freestanding membranes. The completed μ-SOFCs resist up to a temperature of 450 °C. Their electrical properties, such as permittivity, and resistivity were investigated. By means of electrochemical impedance spectroscopy (EIS) in the temperature range of 100 °C to 450 °C, the resistivity properties and the activation energies of the model μ-SOFC were studied with humid hydrogen in nitrogen at the anode and different oxygen partial pressures at the cathode. The results provide a clear hint for a dominating protonic defect transport mechanism in the electrolyte. In the 450 °C measurement, the model μ-SOFCs reached an open circuit voltage of 600 mV with 100% oxygen at the cathode and humid hydrogen in nitrogen at the anode.

The performance of molecule based electronic devices can be improved by use of transition metal oxides (TMO) as charge injection buffer layers between electrodes and organic semiconductors. It is known that the appropriate energy level alignment at the molecule/TMO interface determines the efficiency of the respective TMO. Herein, scanning tunneling microscopy is employed to characterize the interface formed by fullerenes (C60) deposited on a titanium oxide (TiO) ultrathin film, which is created by oxidation of a Pt3Ti(111) surface. Individual C60 are identified with orbital resolution, and the interfacial and intermolecular interactions are characterized in detail. Furthermore, the energy level alignment at the C60/TiO ultrathin film interface is deduced based on scanning tunneling spectroscopy data. The results demonstrate that the C60/TiO interface corresponds to a type-I heterojunction and, thus, is useful to decouple C60 molecules from the metallic alloy surface.

Current–voltage characteristics of oxide-based resistive switching memories often show a pronounced asymmetry with respect to the voltage polarity in the high resistive state (HRS), where the HRS after the RESET is more conducting than the one before the SET. Here, we report that most of this HRS asymmetry is a volatile effect as the HRS obtained from a read operation differs from the one taken from the switching cycle at identical polarity and voltages. Transitions between the relaxed and the volatile excited states can be achieved via voltage sweeps, which are named subloops. The excited states are stable over time as long as a voltage is applied to the device and have a higher conductance than the stable relaxed state. Experimental data on the time and voltage dependence of the excitation and decay are presented for Ta/TaOx/Pt and Ta/ZrOx/Pt devices. The effect is not limited to one oxide or electrode material but is observed with different magnitudes (up to 10× current change) in several oxide systems. These observations describe an additional state variable of the memristive system that is controlled in a highly polarity dependent manner.

The formation mechanism of 2-dimensional electron gases (2DEGs) at heterointerfaces between nominally insulating oxides is addressed with a thermodynamical approach. We provide a comprehensive analysis of the thermodynamic ground states of various 2DEG systems directly probed in high temperature equilibrium conductivity measurements. We unambiguously identify two distinct classes of oxide heterostructures: For epitaxial perovskite/perovskite heterointerfaces (LaAlO3/SrTiO3, NdGaO3/SrTiO3, and (La,Sr)(Al,Ta)O3/SrTiO3), we find the 2DEG formation being based on charge transfer into the interface, stabilized by the electric field in the space charge region. In contrast, for amorphous LaAlO3/SrTiO3 and epitaxial γ-Al2O3/SrTiO3 heterostructures, the 2DEG formation mainly relies on the formation and accumulation of oxygen vacancies. This class of 2DEG structures exhibits an unstable interface reconstruction associated with a quenched nonequilibrium state.

Donor-doped transition metal oxides such as donor-doped strontium titanate ($n\ensuremath{-}{\mathrm{SrTiO}}_{3}$) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere ($p{\mathrm{O}}_{2}$). We correlate the $p{\mathrm{O}}_{2}$ dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$ surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry.

In this work, thin film (down to 10 nm) $(\mathrm{V}_{1-\mathrm{x}}\text{Cr}_{\mathrm{x}})_{2}\mathrm{O}_{3}$ Mott-oxide based nano-devices (electrode width down to 120 nm) are fabricated for the first time. The devices show volatile threshold switching and NDR caused by thermal feedback. Fast ( $(\mathrm{V}_{1-\mathrm{x}}\text{Cr}_{\mathrm{x}})_{2}\mathrm{O}_{3}$ films, enabling predictions for further scaled device geometries.

Lanthanum doped barium titanate was nitridated by thermal ammonolysis with a newly designed reactor, which performs extended efficiency to supply ammonia with less decomposition at elevated temperatures above 600 °C. The kinetics of nitrogen-uptake was studied systematically at elevated temperatures. Nitrogen-content, thermal stability and crystallographic phase of nitridated lanthanum doped barium titanate were characterized by several techniques. The final nitrogen-content of nitridated lanthanum doped barium titanate was much higher than the content of lanthanum, resulting in the formation of oxygen vacancies compensating the additional negative electrical charge of nitrogen anions occupying regular oxygen sites in the lattice. Through thermal ammonolysis, strongly reducing conditions because of the decomposition of ammonia generating hydrogen gas causes the formation of barium orthotitanate as a secondary phase. Ammonolysis temperature below 800 °C and suppression of thermal decomposition of ammonia gas are essential to realize a nitridated lanthanum doped barium titanate as a single phase.

The local electronic properties of tantalum oxide (TaOx, 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO3) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaOx films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO3, we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaOx and SrRuO3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.

Interface reactions constitute essential aspects of the switching mechanism in redox-based resistive random access memory (ReRAM). For example, the modulation of the electronic barrier height at the Schottky interface is considered to be responsible for the toggling of the resistance states. On the other hand, the role of the ohmic interface in the resistive switching behavior is still ambigious. In this paper, the impact of different ohmic metal-electrode (M) materials, namely W, Ta, Ti, and Hf on the characteristics of Ta2O5 ReRAM is investigated. These materials are chosen with respect to their free energy for metal oxide formation and, associated, their impact on the formation energy of oxygen vacancy defects at the M/Ta2O5 interface. The resistive switching devices with Ti and Hf electrodes that have a negative defect formation energy, show an early RESET failure during the switching cycles. This failure process with Ti and Hf electrode is attributed to the accumulation of oxygen vacancies in the Ta2O5 layer, which leads to permanent breakdown of the metal–oxide to a low resistive state. In contrast, the defect formation energy in the Ta2O5 with respect to Ta and W electrodes is positive and for those highly stable resistive switching behavior is observed. During the quasi-static and transient-pulse characterization, the ReRAM devices with the W electrode consistently show an increased high resistance state (HRS) than with the Ta electrode for all RESET stop voltages. This effect is attributed to the faster oxygen exchange reaction at the W-electrode interface during the RESET process in accordance to lower stability of WO3 than Ta2O5. Based on these findings, an advanced resistive switching model, wherein also the oxygen exchange reaction at the ohmic M-electrode interface plays a vital role in determining of the resistance states, is presented.

The properties of oxide-based resistive switching ReRAM devices depend critically on the thickness of the different layers of the device stack. Here we focus on the variation of the pristine resistance and the forming voltage with the thickness of the reactive oxygen exchange electrode layer. By means of electrical capacitance and X-ray photoelectron spectroscopy characterization, we show how this variation of the pristine properties is linked to the formation of (sub)stoichiometric oxide layers at the metal oxide/oxygen exchange layer interface. This insight provides guidelines for further device optimization

Ion transport in ceramics of the low-temperature phase of tantalum pentoxide, L-Ta2O5, was examined by means of diffusion experiments and subsequent analysis of diffusion profiles with time-of-flight secondary ion mass spectrometry (ToF-SIMS). 18O/16O isotope anneals were used to investigate oxygen diffusion, and oxygen tracer diffusion coefficients were obtained for the temperature range of 623 ≤ T/K ≤ 873 at an oxygen partial pressure of pO2 = 0.2 bar and for the oxygen partial pressure range of 10-2 ≤ pO2/bar ≤ 100 at a temperature of T = 723 K. Cation diffusion in Ta2O5 was probed by using chemically similar niobium as the diffusant (in the absence of stable tantalum isotopes). Thin films of Nb2O5 were deposited onto Ta2O5 ceramics; diffusion anneals yielded niobium diffusion coefficients for the temperature range of 1073 ≤ T/K ≤ 1223 at an oxygen partial pressure of pO2 = 0.2 bar. Comparison of the measured diffusion coefficients strongly suggests that oxygen is many orders of magnitude more mobile than niobium in L-Ta2O5 at these temperatures and at pO2 = 0.2 bar. The electrical conductivity was also determined in the range 950 ≤ T/K ≤ 1200 and 10-23 ≤ pO2/bar ≤ 10-2. Considered together with the measured diffusion coefficients, the conductivity data indicate that under oxidising conditions conduction is due to oxygen ions above T = 1090-1130 K and due to electron holes below this temperature range. Point-defect models are presented that are consistent with these transport data and with conductivity data in the literature. They suggest that under oxidising conditions oxygen interstitials are the majority ionic charge carriers in L-Ta2O5. The implications for resistive switching devices are discussed.

Cu and Ag redox reactions at the interfaces with Ta2O5 and the impact of Ta2O5 film density on the forming process of Cu,Ag/Ta2O5/Pt atomic switch structures are investigated. Cyclic voltammetry measurements revealed that under positive bias to the Cu (Ag) electrode, Cu is preferentially oxidized to Cu2+, while Ag is oxidized to Ag+ ions. Subsequent negative bias causes a reduction of oxidized Cu (Ag) ions at the interfaces. The diffusion coefficient of the Cu and Ag ions in the Ta2O5 film is estimated from the results from different bias voltage sweep rates. It is also found that the redox current is enhanced and the forming voltage of the Cu/Ta2O5/Pt cell is reduced when the density of the Ta2O5 film is decreased. This result indicates the importance of the structural properties of the matrix oxide film in understanding and controlling resistive switching behavior.

Dislocations in perovskite oxides have been found to have important impacts on their electronic and ionic transportation properties, which are believed to be related to the structure and chemistry of dislocation cores. For dislocation cores at a 6° low-angle [0 0 1] tilt grain boundary in SrTiO3, an embedded TiOx rocksalt-like structure has been suggested, consistent with a deficiency of Sr. However, direct evidence supporting these suggestions has not been obtained up to now. In this work, we reveal the atomic structure and chemistry of edge dislocation cores at a low-angle [0 0 1] symmetric tilt-boundary in SrTiO3 bicrystals by imaging techniques of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) with an FEI Titan cube3 60–300 (PICO) microscope operated at 80 kV. The experimental results demonstrate direct evidence for a local coordination of edge-sharing TiO6 octahedra at the dislocation cores, which can be understood as the result of strain. The local coordination of edge-sharing TiO6 octahedra is associated with the face-centered cubic (FCC) NaCl-type TiO phase. The present study therefore provides a solid structural and chemical basis for understanding the properties of dislocations.

We present a numerical drift-diffusion model that combines bulk ionic transport with electrode kinetics for oxygen exchange to describe the electroforming process in resistive- switching devices. Simulations of electroforming by voltage ramps are conducted and the conductive filament formation is analyzed in dependence of various kinetic parameters. It is shown how the filament growth direction and the post-forming resistance state are governed by the rate of oxygen exchange compared to the rate of ionic transport inside the oxide layer.

The existence of non-ferroelectric regions in ferroelectric thin films evokes depolarization effects leading to a tilt of the P(E) hysteresis loop. The analysis of measured hysteresis of lead zirconate titanate (PZT) thin films is used to determine a depolarization factor which contains quantitative information about interfacial layers as well as ferroelectrically passive zones in the bulk. The derived interfacial capacitance is smaller than that estimated from conventional extrapolation techniques. In addition, the concept of depolarization is used for the investigation of fatigue behavior of PZT thin films indicating that the mechanism of seed inhibition, which is responsible for the effect, occurs in the entire film.

Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta2O5-based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 106 pulse cycles and/or 103I–V sweeps maintaining a good memory window with a low dispersion in ROFF and RON values, nanosecond fast switching, and data retention of at least 104 s. Multilevel programing capability is presented with both I–V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.

International audience; The control and rational design of redox-based memristive devices, which are highly attractive candidates for next-generation nonvolatile memory and logic applications, is complicated by competing and poorly understood switching mechanisms, which can result in two coexisting resistance hystereses that have opposite voltage polarity. These competing processes can be defined as regular and anomalous resistive switching. Despite significant characterization efforts, the complex nanoscale redox processes that drive anomalous resistive switching and their implications for current transport remain poorly understood. Here, lateral and vertical mapping of O vacancy concentrations is used during the operation of such devices in situ in an aberration corrected transmission electron microscope to explain the anomalous switching mechanism. It is found that an increase (decrease) in the overall O vacancy concentration within the device after positive (negative) biasing of the Schottky-type electrode is associated with the electrocatalytic release and reincorporation of oxygen at the electrode/oxide interface and is responsible for the resistance change. This fundamental insight presents a novel perspective on resistive switching processes and opens up new technological opportunities for the implementation of memristive devices, as anomalous switching can now be suppressed selectively or used deliberately to achieve the desirable so-called deep Reset.

The resistive switching (RS) properties of a thin Al2O3 layer and TiOx/Al2O3 bilayers integrated into TiN/metal oxide/Pt crossbar devices are investigated for future memristive device (ReRAM) applications. The oxide bilayer stack is realized in consecutive atomic layer deposition (ALD) processes at 300 °C without any post-annealing step. Stoichiometric Al2O3 and oxygen-deficient TiOx thin films are grown from dimethylaluminum isopropoxide [DMAI: (CH3)2AlOCH(CH3)2] and tetrakis-dimethlyamido-titanium [TDMAT: Ti(N(CH3)2)4], respectively, as the metal sources, and water as the oxygen source. High insulating characteristics are confirmed for as-grown amorphous Al2O3 films with a dielectric permittivity of 8.0 and disruptive field strength of about 7 MV cm−1, whereas the oxygen-deficient TiOx shows semiconducting behavior. The bipolar-type RS characteristics of TiN/TiOx/Al2O3/Pt cells show a strong dependence on both oxide layer thicknesses. A stable OFF/ON state resistance ratio of about 105 is obtained for the bilayer structure of 5 nm TiOx and 3.7 nm Al2O3.

We investigate the effect of acceptor-dopant cations on oxygen-vacancy migration in the perovskite oxide SrTiO3 by static lattice simulation techniques. We focus on two themes: dopant cations modifying the activation energies for vacancy migration, and dopant cations binding oxygen vacancies in binary associates. In both cases a variety of defect configurations exceeding the scope of first nearest neighbor (1NN) interaction is examined, i.e. 1NN, 2NN, 3NN, 4NN, and 5NN. The behavior of four divalent dopants (Ni2+, Fe2+, Co2+, and Mn2+) and one trivalent dopant (Al3+) is compared. The simulations predict that the binding energy of an oxygen vacancy to any of these dopants is negative at 1NN sites, but only converges to zero for 4NN sites. In addition the simulations show that the migration energy of a vacancy is affected by an acceptor-dopant cation over a length scale of several unit cells. A simple analytical model is used, together with the calculated site and activation energies, to predict how in gene...

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Cation-based resistive-switching memories rely on the injection and drift of metal ions in nanoscale thin films. In insulators that do not initially contain mobile cations, such as SiO 2 , Ta 2 O 5 , and so forth, water redox reactions occurring at the counter electrode (CE) were found to be essential in enabling the dissolution of the active electrode and to keep electroneutrality. In this study, we report on the impact of the CE on redox processes prior to resistive switching. Potentiodynamic measurements for various electrode materials revealed that the catalytic activity of the CE towards the water redox process determines the concentration of dissolved ions within the oxide and influences the rate of the total cell reaction. This trend can be used as an indicator for the design of both cation- and anion-conducting oxide-based resistive-switching random-access memories.

We propose a new method for obtaining forming-free ReRAM devices by oxygen ion implantation (O 2 IIP) in the metal oxide film during the device fabrication process. By tuning the implantation dose, as-fabricated devices can be transformed into the ON state. Subsequent standard RESET and SET switching cycles reveal that the forming-free devices switch in a similar way to reference (formed) devices. The devices also show good R OFF /R ON ratio (>200), retention (104 sec@125°C) and endurance reliability (106 cycles), showing the absence of any device degradation caused by the O 2 IIP process. This method is applied on both (PVD) Ta 2 O 5 and (ALD) HfO 2 nanoscale ReRAM devices, demonstrating the versatile applications of the technique.

Thin ZrO2 films and ZrO2/TiO2 bilayers grown by atomic layer deposition (ALD) are integrated into metal–oxide–metal (MOM) structures for investigation of resistive switching (RS) properties. The films have different microstructure depending on the used ALD oxygen source and stacking sequence for the bilayers. Pt/ZrO2/Ti/Pt devices show unipolar RS for oxide thicknesses of 11–18 nm. The devices with O3 grown ZrO2 show higher yield in comparison to the ones with H2O processed oxide. The switching polarity of the Pt/ZrO2/Ti/Pt cells depends on the thickness of the Ti electrode layer. The increase of the Ti layer thickness leads to a change in switching polarity from unipolar to bipolar. The formation of ZrO2/TiO2 bilayers results in changes in the RS behavior of the MOM cells depending on the stacking sequence.

Strontium titanate, SrxTiyOz (STO), thin films with various cation stoichiometries were deposited by plasma-assisted atomic layer deposition (ALD) using cyclopentadienyl-based metal precursors and oxygen plasma as counter-reactant. [Sr]/([Sr] + [Ti]) compositions ranging from 0.46 to 0.57 were obtained by changing the (SrO)/(TiO2) ALD cycle ratios. As-deposited 15–30 nm thick SrxTiyOz films prepared at 350 °C on Pt-coated silicon substrates were amorphous. Post-annealing at 600/650 °C for 10 min under N2 gas resulted in a crystallization into the perovskite phase. Stoichiometric STO and Sr-rich STO films exhibited a certain degree of (111) texture while the Ti-rich STO films showed a lower degree of crystallization. Crystallized layers exhibited a smaller band gap Eg than amorphous ones, while within the stoichiometry series the value of Eg increased with increasing Sr-content. Within the stoichiometry series Pt/STO/Pt structures with Sr-rich STO films showed the lowest leakage current densities. At 1.0 V values of about 2 × 10−8 and 5 × 10−6 A cm−2 were obtained for the as-deposited and the annealed films, respectively. Highest capacitance density was obtained for 15 nm polycrystalline stoichiometric SrTiO3 films resulting in a capacitor equivalent thickness CET of about 0.7 nm. Pt/SrxTiyOz/Pt capacitors with the STO being in amorphous state exhibited a positive voltage nonlinearity factor α of about 400 ppm V−2, while the negative α-values for crystallized films showed a systematic variation with the stoichiometry, the degree of crystallization and the thickness of the STO layer. This demonstrates that a broad performance range of MIM capacitors is accessible by controlling the stoichiometry and the degree of crystallization of plasma-assisted ALD SrxTiyOz thin films.

Variations in tetragonal distortion and the degree of crystallinity of ultrafine BaTiO3 particles—both industrially applied criteria qualifying their polar characteristics—upon growth during annealing were studied on powders, prepared by the hydrothermal synthesis and by the oxalate route respectively. Regardless of the synthetic route both these properties were found to develop approximately to the same extent with increasing primary particle size. The degree of crystallinity was detectable only above a critical value of c/a=1.007 at an average particle size of around 120 nm. Intragranular porosity in hydrothermal powders and the stoichiometric Ba/Ti-ratio are of minor influence. However, distinct differences in particle growth behavior for both powders were observed. This discrepancy is ascribed to a special morphology of the hydrothermal product, where enhanced particle growth was found to be governed by oriented attachment of individual crystallites and by the motion of solid–solid interfaces.

We studied the growth of TiO2 by liquid injection atomic layer deposition (ALD) utilizing two different amide-based titanium sources, tetrakis-dimethylamido-titanium [(NMe2)4-Ti, TDMAT] and its rec...

The significance of atomic layer deposition (ALD) for the growth of transition metal oxide thin films which are to be integrated into metal-oxide-metal structures for future nonvolatile resistive switching memory devices is illustrated. Focus is put to the requirements on transition metal oxide layers with respect to the control of morphology, structure, and defect state. For the representative example of ALD TiO2-x thin films grown from different titanium sources possibilities for a design of thin film properties by means of precise process control are exemplified. The success of this strategy is highlighted by the resistive switching behavior of 8 nm ALD TiO2-x thin films integrated into Pt/ TiO2-x/Ti/Pt cross point junctions of 100 nm x 100 nm lateral size by which it could be demonstrated that a stable SET operation in TiO2 films is possible for compliance currents of about 20 µA.

The electrical properties of donor-doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ $(n$-STO) are profoundly affected by an oxidation-induced metal-insulator transition (MIT). Here we employ dynamical numerical simulations to examine the high-temperature MIT of $n$-STO over a large range of time and length scales. The simulations are based on the Nernst-Planck equations, the continuity equations, and the Poisson equation, in combination with surface lattice disorder equilibria serving as time-dependent boundary conditions. The simulations reveal that $n$-STO, upon oxidation, develops a kinetic space charge region (SCR) in the near-surface region. The surface concentrations of the variously mobile defects (electrons, Sr vacancies, and O vacancies) are found to vary over time and to differ considerably from the values of the new equilibrium. The formation of the SCR in which electrons are strongly depleted occurs within nanoseconds, i.e., it yields a fast MIT in the near-surface region during the oxidation process. As a result of charge (over-)compensation by Sr vacancies incorporated at the surface of $n$-STO, this SCR is much more pronounced than conventionally expected. In addition, we find an anomalous increase of O vacancy concentration at the surface upon oxidation caused by the SCR. Our simulations show that the SCR fades in the long term as a result of the slow in-diffusion of Sr vacancies. We discuss implications for the electrical conductivity of $n$-STO crystals used as substrates for epitaxial oxide thin films, of $n$-STO thin films and interfaces, and of polycrystalline $n$-STO with various functionalities.

Resistive random access memories based on redox phenomena (ReRAM) combine several advantages. Beside their good scalability, high endurance and fast switching speed they are also very energy efficient. This work presents a study of the SET kinetics of SrTiO 3 -based resistive switches covering the timescale from

Redox-Based Resistive Switching Memories (ReRAM), also called nanoionic memories or memristive elements, are widely considered to provide a potential leap beyond the limits of Flash (with respect to write speed, write energies) and DRAM (with respect to scalability, retention times) as well as energy-efficient approaches to neuromorphic concepts[1].

Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase was presumably present mainly along the grain boundaries of SrTiO3 after the unipolar set switching in Pt/TiN/SrTiO3/Pt structure. A detailed electrical characterization revealed that the samples showed typical bipolar and complementary RS after the memory cell was unipolar reset.

Compared to conventional NAND flash resistive switching metal-oxide cells show a number of advantages, like an increased endurance, lower energy consumption, and superior switching speed. Understanding the role of defects for the resistive switching phenomenon in metal oxides is crucial for their improvement and thereby also for their acceptance as a next generation data storage device. Strontium titanate (STO) is considered a model material due to its thoroughly investigated defect chemistry. This paper presents a comparative study of the switching kinetics for three different compositions [Sr]/([Sr]+[Ti]) of 0.57 (Sr-rich), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO). The STO films, deposited by atomic layer deposition, were integrated in Pt/STO/TiN nanocrossbars with a feature size of 100 nm. By analysis of the transient currents, the switching kinetics are investigated between 10 ns and 104 s for the SET and 10 ns and 100 s for the RESET. A clear influence of the composition on the degree of nonlinearity of the switching kinetics was observed. Applying an analytical model for the oxygen vacancy migration, we were able to explain the differences in the SET kinetics by composition-dependent changes in the thermal conductivity and by a lower activation energy for the Ti-rich sample. This might be utilized in design rules of future ReRAM devices.I. INTRODUCTION

Resistive switching memories based on the formation and dissolution of a metal filament in a simple metal/oxide/metal structure are attractive because of their potential high scalability, low-power consumption, and ease of operation. From the standpoint of the operation mechanism, these types of memory devices are referred to as gapless-type atomic switches or electrochemical metallization cells. It is well known that oxide materials can absorb moisture from the ambient air, which causes shifts in the characteristics of metal-oxide-semiconductor devices. However, the role of ambient moisture on the operation of oxide-based atomic switches has not yet been clarified. In this work, current–voltage measurements were performed as a function of ambient water vapor pressure and temperature to reveal the effect of moisture on the switching behavior of Cu/oxide/Pt atomic switches using different oxide materials. The main findings are: i) the ionization of Cu at the anode interface is likely to be attributed to chemical oxidation via residual water in the oxide layer, ii) Cu ions migrate along grain boundaries in the oxide layer, where a hydrogen-bond network might be formed by moisture absorption, and iii) the stability of residual water has an impact on the ionization and migration processes and plays a major role in determining the operation voltages. These findings will be important in the microscopic understanding of the switching behavior of oxide-based atomic switches and electrochemical metallization cells.

Atomic Force Microscopy (AFM) measurements have been performed for Fe-doped SrTiO3 thin films with an Fe concentration of 1 and 2 at%. Thin films with a thickness of 15–50 nm were grown by pulsed laser deposition on single crystalline SrTi0.99Nb0.01O3 substrates. Low-energy electron diffraction examination showed that the films are single crystalline. The regions treated with the AFM tip (applied dc voltage up to 6 V) showed inhomogeneity of the electrical conductivity. Resistive switching was studied by the use of local conductive atomic force microscopy and was present in all samples. Fe doping was found to influence the character of switching.

Reverse micellar microemulsions were utilised to synthesise stable lead titanate colloids from typical sol–gel type molecular precursors. The particles of a few nanometres in diameter that formed by hydrolysis in the micelle cores demonstrated a nanocrystalline texture without the need for thermal treatment. In a chemical solution deposition routine based on the nanoparticle dispersions, ferroelectric PbTiO3 thin films showing excellent remanent polarisation of more than 50 μC cm−2 were derived. The electrical characteristics of the layers were correlated to their microstructure which was controlled by the precursor composition and the annealing conditions. A columnar morphology that was realised by spinning on coatings of up to 100 nm in individual thickness proved most favourable with respect to the ferroelectric performance of the films.

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.

Resistive switching based on transition metal oxide memristive devices is suspected to be caused by the electric-field-driven motion and internal redistribution of oxygen vacancies. Deriving the detailed mechanistic picture of the switching process is complicated, however, by the frequently observed influence of the surrounding atmosphere. Specifically, the presence or absence of water vapor in the atmosphere has a strong impact on the switching properties, but the redox reactions between water and the active layer have yet to be clarified. To investigate the role of oxygen and water species during resistive switching in greater detail, isotope labeling experiments in a N2 /H218 O tracer gas atmosphere combined with time-of-flight secondary-ion mass spectrometry are used. It is explicitly demonstrated that during the RESET operation in resistive switching SrTiO3 -based memristive devices, oxygen is incorporated directly from water molecules or oxygen molecules into the active layer. In humid atmospheres, the reaction pathway via water molecules predominates. These findings clearly resolve the role of humidity as both oxidizing agent and source of protonic defects during the RESET operation.

APL materials 6(4), 046106 (2018). doi:10.1063/1.5026063, The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum., Published by AIP Publ., Melville, NY

This overview describes the present understanding of resistive switching phenomena encountered in chalcogenide-based cells which may be utilized in energy-efficient non-volatile memory devices and in array-based logic applications. 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 transition metal oxides which switch by the thermochemical and the valence change mechanism. Phase transformations are not only discussed for phase change alloys but also for redox-related switching processes. The switching kinetics as well as the ultimate scalability of switching cells are related to structural defects in the materials.