Oxide Stoichiometry Research Articles

Interface Characterization of Plasma‐Treated InAs Electrodes for Resistive Random‐Access Memories Using Capacitance–Voltage Methods

InAs nanowires have been used as selectors and electrodes in a one‐transistor one‐resistor resistive random‐access memory (RRAM) configuration for increased memory scalability. Such oxide‐based memories not only show promise both in conventional memory technology but also as analog memories for neural networks. The small areas of the oxide elements, however, result in challenges for electrical characterization of the oxide–semiconductor interface. By analyzing larger test structures, it is possible to perform impedance spectroscopy on the oxide–InAs interface. In this article, HfO2‐based RRAMs on InAs are fabricated using plasma‐enhanced atomic layer deposition. The effect of the plasma‐length is investigated using capacitance–voltage and DC measurements. It is found that an increased plasma‐length results not only in a higher total electron trapping but also in the formation of a more stable interfacial oxide, possibly related to the stoichiometry of the complex InAs oxide. The findings give us a better understanding of the noise‐ and switching‐performance of scaled nanowire devices.

X-ray photoelectron spectroscopy of metal oxide nanoparticles: chemical composition, oxidation state and functional group content.

Surface chemistry drives the interaction of a material with its surroundings, therefore it can be used to understand and influence the fate of nanomaterials when used as functional materials or when released to the environment. Here we have used X-ray photoelectron spectroscopy (XPS) to probe the chemical composition, oxidation state and functional group content of the near surface region of four families of commercially available metal oxide nanoparticles from several different suppliers. The analyzed nanoparticles varied in size and surface functionalization (unfunctionalized vs. amine, stearic acid, and PVP-coated samples). Survey and high-resolution scans have provided information on the atomic composition of the samples, including an estimate of the stoichiometry of the metal oxide, the presence of functional groups and the identification and quantification of any impurities on the surface. The presence of significant impurities for some samples and the variation from the expected oxidation state in other cases are relevant to studies of the environmental and health impacts of these materials as well as their use in applications. The functional group content measured by XPS shows a similar trend to earlier quantitative nuclear magnetic resonance (qNMR) data for aminated samples. This indicates that XPS can be a complementary probe of surface functional group content in cases where the functional group contains a unique element not otherwise present on the nanoparticles.

Artificial synapses based on HfOx/TiOy memristor devices for neuromorphic applications

As a result of enormous progress in nanoscale electronics, interest in artificial intelligence (AI) supported systems has also increased greatly. These systems are typically designed to process computationally intensive data. Parallel processing neural network architectures are particularly noteworthy for their ability to process dense data at high speeds, making them suitable candidates for AI algorithms. Due to their ability to combine processing and memory functions in a single device, memristors offer a significant advantage over other electronic platforms in terms of area scaling efficiency and energy savings. In this study, single-layer and bilayer metal-oxide HfOxand TiOymemristor devices inspired by biological synapses were fabricated by pulsed laser and magnetron sputtering deposition techniques in high vacuum with different oxide thicknesses. The structural and electrical properties of the fabricated devices were analysed using x-ray reflectivity, x-ray photoelectron spectroscopy, and standard two-probe electrical characterization measurements. The stoichiometry and degree of oxidation of the elements in the oxide material for each thin film were determined. Moreover, the switching characteristics of the metal oxide upper layer in bilayer devices indicated its potential as a selective layer for synapse. The devices successfully maintained the previous conductivity values, and the conductivity increased after each pulse and reached its maximum value. Furthermore, the study successfully observed synaptic behaviours with long-term potentiation, long-term depression (LTD), paired-pulse facilitation, and spike-timing-dependent plasticity, showcasing potential of the devices for neuromorphic computing applications.

Well‐defined synthesis of crystalline MnO, Mn2O3, and Mn3O4 phases by anodic electrodeposition and calcination

AbstractTailoring of the stoichiometry, crystallinity, and microstructure of manganese oxides (MnOx) is of utmost importance for technological applications in the field of catalysis, energy storage, and water splitting. In this work, α‐Mn2O3, α‐Mn3O4, and MnO thin films with defined stoichiometric compositions and crystal structures were prepared by calcination of an anodically electrodeposited Mn‐oxyhydroxide precursor film in different gas atmospheres (air, inert, or reducing gas). The crystal structure and composition of the precursor and product films were determined by combining X‐ray diffraction, transmission electron microscopy, Raman spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis. The anodically electrodeposited precursor film consists of nanocrystals of α‐Mn3O4 dispersed in an amorphous MnOOH matrix phase, and can be fully transformed into either crystalline α‐Mn2O3, α‐Mn3O4, or MnO upon calcination in an oxidizing, inert or reducing atmosphere, respectively. In situ high‐temperature X‐ray diffraction was applied to derive the phase transformation kinetics, resulting in a corresponding activation energy which decreases in the order α‐Mn2O3 (268 kJ/mole) > MnO (102 kJ/mole) > α‐Mn3O4 (60 kJ/mole). The disclosed synthesis routes for the preparation of single‐phase MnOx films with a defined crystal structure and stoichiometry can be exploited for a wealth of applications.

Considerations for using Calibration Curves to Infer Oxide Stoichiometry from Atom Probe Tomography Data

Considerations for using Calibration Curves to Infer Oxide Stoichiometry from Atom Probe Tomography Data

Enhanced oxygen evolution reaction by modulating the electronic states of a flexible van der Waals membranous catalyst with infinite-layer phase

The precise stoichiometry and lattice ordering of perovskite transition metal oxide (TMO) thin films enable the atomic-level catalytic mechanisms in the process of oxygen evolution reaction (OER), accordingly facilitating the creation and refinement potentially effective catalysts. Nevertheless, the fundamental understanding of the interaction between the electronic and structural properties of infinite-layer structured electrocatalysts, as well as their catalytic efficiency, remains lacking. Herein, the successful acquisition of the membranous LaNiO2 catalysts with infinite-layer phase were achieved via topological reconstruction. Subsequent electrochemical testing revealed a decrease in the OER performance of nickelate after reduction, attributed to the diminished bonding strength between catalyst and oxygen intermediates. Given these unexpected findings, this study aimed to leverage surface strains to manipulate electronic states, thereby modulating the OER performance of LaNiO2. Applying either compressive or tensile strains to LaNiO2 enabled the restoration of its performance to pre-reduction levels induced by hydrogen, owing to the reinforced bond strength between catalyst and oxygen intermediates. This research provides both experimental evidence and theoretical guidance for understanding the importance of defect and stress engineering in OER, offering valuable insights for improving OER performance in flexible nano-membranous electrocatalysts with infinite-layer phase for practical applications.

Surface chemistry of ion beam modified native titania/Ti interfaces examined using X-ray photoelectron spectroscopy

It is often assumed in X-ray Photoelectron Spectroscopy that binding energy shifts are synonymous with changes in the chemical state of an atom and the chemical state can be described in terms of oxidation state and stoichiometry of the elements in a material that correlates with photoemission peaks. However, when an atom is bonded into a crystal lattice, the shapes and binding energy of photoemission may not match the expected stoichiometry when measured by XPS, even though shifts in binding energy suggest new oxidation states. In this work, a set of experiments is presented, in which Ar+ and He+ ions of different energies modify the native oxide on a titanium foil which yields XPS spectra that are not easily open to analysis by conventional peak models. In the course of analyzing these complex photoemission data by linear algebraic methods, the prospect emerged that XPS is suggesting that sputtering a native oxide may be providing insight into structural perturbation in addition to the stoichiometry-type changes to the native oxide.

Effect of hydrogen on the chemical state, stoichiometry and density of amorphous Al2O3 films grown by thermal atomic layer deposition

Amorphous oxide thin films grown by thermal atomic layer deposition (ALD) typically contain high impurity concentrations of hydrogen, which affects both chemistry and structure and thereby the functional properties, such as the barrier properties in, for example, microelectronic and photovoltaic devices. This study discloses the effect of H incorporation in amorphous Al2O3 ALD oxide films on the local chemical binding states of Al, O and H, as well as the oxide density and stoichiometry, by a combined analytical approach using elastic recoil detection analysis, Rutherford backscattering spectroscopy and full chemical state analysis by dual‐source X‐ray photoelectron spectroscopy (XPS)/hard X‐ray photoelectron spectroscopy (HAXPES). The experimental findings are compared with crystalline anhydrous α‐Al2O3 and hydroxide α‐Al (OH)3 reference phases and further supported by molecular dynamic simulations. It is shown that H preferably forms covalent –OH hydroxyl bonds with O in the nearest‐neighbour coordination spheres of interstitial Al cations, which affects both the ligand electronic polarizability and the bond length of the randomly interconnected [AlOn] polyhedral building blocks in the amorphous ALD oxide films.

Deposition of Composite Nanoparticle-Based Thin Films for Gas Sensing

Working principle of conductometric gas sensors is based on modulation in electrical conductivity of the sensing material due to adsorption-desorption reactions between the target gas and the sensor surface. Nano-structuring the sensing material increases the effective surface area for interaction between the target gas and the sensing material thereby enhancing the sensorial response. Metal oxide semiconductors (MOS) have shown remarkable sensing performance to oxidizing and reducing gases. The combination of different MOS has often demonstrated to synergistically improve the gas sensing performance via the formation of highly sensitive heterojunctions at their interface.In this work, we deposited thin films composed of nanoparticles (NPs) of p-type CuOx and n-type WOx to enhance the effective surface area of the material and to realize the maximum number of nano p-n heterojunctions in the material. The films were prepared using the Nanogen-Trio NP source equipped with three 1′′ magnetrons, mounted on a custom-built deposition chamber and sputtered by a DC power supply. Depositions were executed in Ar+O2 gas mixture. The O2 admixture may significantly enhance the mass flux of NPs. Furthermore, the O2 admixture allows one to tune the desired stoichiometry of the respective metal oxides. On the other hand, the pronounced hysteresis complicates the deposition of WOx NPs. The deposition process was controlled using an in-house developed software to prepare thin films composing a mixture of NPs with a defined volumetric ratio of the individual materials.This work demonstrates the versatility of our custom-built gas aggregation source that facilitates the preparation of multi-composite NP-based thin film giving the best sensorial response.

Fabrication and characterization of TiOx based single-cell memristive devices

Nowadays, remarkable progress has been observed in research into neuromorphic computing systems inspired by the human brain. A memristive device can behaviorally imitate the biological neuronal synapse therefore memristor-based neuromorphic computing systems have been proposed in recent studies. In this study, the memristive behaviors of titanium dioxide sandwiched between two platinum electrodes were investigated. For this purpose, three SiO2/Pt/TiOx/Pt thin films with 7.2 nm, 40 nm, and 80 nm TiOx metal-oxide layers were fabricated using a pulsed laser deposition technique. The fabrication process, structural properties, photoluminescence properties and electrical transport characterization of each thin film have been investigated. All thin films were analyzed in terms of the film stoichiometry and degree of oxidation using high-resolution x-ray photoelectron spectroscopy. By measuring the layer thickness, density, and surface roughness with the x-ray reflectivity technique, by analyzing the structural defects with photoluminescence spectroscopy and by characterizing the quasi-static electrical properties with the conventional two probes technique, we have shown that the fabricated memristive devices have bipolar digital switching properties with high ROFF/RON ratio. This type of switching behavior is applicable in random access memories. Experimental current–voltage behavior in the form of pinched hysteresis loop of the films have been modelled with generalized memristor model.

Engineering stoichiometry of high entropy fluorite oxide to obtain optimized thermophysical property and improved corrosion resistance

Engineering stoichiometry of materials is an effective strategy to optimize the comprehensive properties. Here we prepare high-entropy Er2(Y0.2Yb0.2Nb0.2Ta0.2Cex)2Oδ with single-phase fluorite structure by a solid-state reaction. The effects of stoichiometry of cerium on thermophysical properties and corrosion resistance to calcium−magnesium−alumina−silicate (CMAS) of the high-entropy oxides have been investigated. By engineering the cerium stoichiometry, thermal conductivity increases slightly from 1.21 to 1.36 W·m−1·K−1 and thermal expansion coefficients also increases from 10.56 × 10−6 to 11.61 × 10−6 °C−1 at 1000 °C. Furthermore, corrosion resistance of the high-entropy Er2(Nb0.2Ta0.2Y0.2Yb0.2Cex)2Oδ improves with the increase of cerium content. The effects of cerium content on CMAS resistance have been discussed in detail. Our work reveals that engineering stoichiometry of high-entropy oxides is expected to be an effective strategy to broaden the compositional space and optimize the thermophysical properties and CMAS corrosion resistance.

Optimization of deposition conditions to tune optoelectronic properties of MoO3-x films prepared by RF-sputtering technique

Tunable optoelectronic properties of Molybdenum oxide (MoO3-x) make it a suitable applicant for its use as a catalyst and a hole transport layer. Oxygen vacancies play a vital role in deciding the properties of MoO3-x. In this article, the influence of deposition conditions like substrate temperature and gaseous environment on optoelectronic properties of MoO3-x films are investigated. Increase in substrate temperature have formed oxygen deficient MoO3-x films having lesser transmission (∼30%) and higher conductivity (15-200 Ω−1cm−1). Films prepared at the substrate temperature of 400 °C and rf-power 80 W are mostly crystalline and have led to the formation of MoO2 along with MoO3-x which results in highly conducting less transparent film suitable for catalysis application. Further, introduction of oxygen gas during deposition led to the formation of highly transparent (∼80%) and resistive (10−6 Ω−1cm−1) thin films suitable for hole transport layer application. An increase in oxygen concentration and decrease in oxygen vacancy is confirmed by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy measurement respectively for films deposited in presence of oxygen gas. These studies imply that stoichiometry of molybdenum oxide can be controlled by varying the film preparation conditions and thus optoelectronic properties can be tuned accordingly.

Simulation of the effect of material properties on yttrium oxide memristor-based artificial neural networks

This paper reports a simulation study concerning the effect of yttrium oxide stoichiometry on output features of a memristor-based single layer perceptron neural network. To carry out this investigation, a material-oriented behavioral compact model for bipolar-type memristive devices was developed and tested. The model is written for the SPICE (Simulation Program with Integrated Circuits Emphasis) simulator and considers as one of its inputs a measure of the oxygen flow used during the deposition of the switching layer. After a thorough statistical calibration of the model parameters using experimental current–voltage characteristics associated with different fabrication conditions, the corresponding curves were simulated and the results were compared with the original data. In this way, the average switching behavior of the structures (low and high current states, set and reset voltages, etc.) as a function of the oxygen content can be forecasted. In a subsequent phase, the collective response of the devices when used in a neural network was investigated in terms of the output features of the network (mainly power dissipation and power efficiency). The role played by parasitic elements, such as the line resistance and the read voltage influence on the inference accuracy, was also explored. Since a similar strategy can be applied to any other material-related fabrication parameter, the proposed approach opens up a new dimension for circuit designers, as the behavior of complex circuits employing devices with specific characteristics can be realistically assessed before fabrication.

Thermodynamic Theory of Phase Separation in Nonstoichiometric Si Oxide Films Induced by High-Temperature Anneals

High-temperature anneals of nonstoichiometric Si oxide (SiOx, x < 2) films induce phase separation in them, with the formation of composite structures containing amorphous or crystalline Si nanoinclusions embedded in the Si oxide matrix. In this paper, a thermodynamic theory of the phase separation process in SiOx films is proposed. The theory is based on the thermodynamic models addressing various aspects of this process which we previously developed. A review of these models is provided, including: (i) the derivation of the expressions for the Gibbs free energy of Si oxides and Si/Si oxide systems, (ii) the identification of the phase separation driving forces and counteracting mechanisms, and (iii) the crystallization behavior of amorphous Si nanoinclusions in the Si oxide matrix. A general description of the phase separation process is presented. A number of characteristic features of the nano-Si/Si oxide composites formed by SiOx decomposition, such as the local separation of Si nanoinclusions surrounded by the Si oxide matrix; the dependence of the amount of separated Si and the equilibrium matrix composition on the initial Si oxide stoichiometry and annealing temperature; and the correlation of the presence of amorphous and crystalline Si nanoinclusions with the presence of SiOx (x < 2) and SiO2 phase, respectively, in the Si oxide matrix, are explained.

Characterization of electrochemical deposition of copper and copper(I) oxide on the carbon nanotubes coated stainless steel substrates

The nanocomposite coatings composed of carbon nanotubes and various forms of copper were prepared in the two-step process. Firstly, carbon nanotubes were coated on stainless steel substrate using electrophoretic deposition at constant current. Then, the process of electrochemical deposition using copper(II) sulphate solutions was performed under high overpotential conditions. The modification of the copper(II) cations concentration in the solution and the deposition time provided the formation of various forms of crystals. The samples and their cross-sections were observed and examined using scanning electron microscope equipped with electron dispersive spectroscopy system. The analysis of chemical composition revealed that in addition to the pure copper crystals, the crystals characterized by the presence of copper and oxygen were formed. Therefore, Raman spectroscopy was applied to determine the unknown stoichiometry of this copper oxide. The point and in-depth analysis identified copper(I) oxide in the form of different size crystals depending on the concentration of the copper(II) sulphate solution. To confirm these findings, grazing incidence X-ray diffraction measurements were also performed. the combination of the applied methods has provided the detailed description of the preparation of the nanocomposite coatings with the proposed mechanism of copper(I) oxide formation.

Effect of ammonium halide salts on wet chemical nanoscale etching and polishing of InGaAs surfaces for advanced CMOS devices

Wet surface treatment of InGaAs is crucial for high-performance complementary metal-oxide semiconductor (CMOS) devices as it reduces material loss and oxide formation in the InGaAs layer. In this present study, the surface chemistries of In0.53Ga0.47As during wet chemical etching and the chemical-mechanical planarization (CMP) process in acidic (HCl/H2O2/H2O mixture) and alkaline (NH4OH/H2O2/H2O mixture) solutions were investigated. Elemental oxide formation, surface termination, and stoichiometry after the chemical etching/CMP process were investigated through X-ray photoelectron spectroscopy and atomic force microscopy. In wet chemical etching, the dissolution of the InGaAs elemental oxide was more prominent in acidic HCl mixture solutions compared to alkaline NH4OH mixture solutions. It was observed that the overall surface etching was more aggressive in acidic solution, whereas the alkaline solution provided higher surface roughness (Ra, ∼7.19 nm, scanning area of 5 μm × 5 μm) due to the lower dissolution capacity of indium oxide. The effects of the oxidizer and the slurry pH on the removal rate, Ra, and surface contamination were evaluated after the InGaAs CMP process. A high removal rate (∼110 nm/min) and considerably lower surface contamination were obtained using an alkaline slurry (pH 10). A novel InGaAs CMP slurry is developed by adding different ammonium halide salts to the alkaline slurry for improving surface roughness and removal rate. The addition of ammonium halide salts to an alkaline slurry was very effective in achieving a higher removal rate (∼175 nm/min) and smoother surface (Ra < 0.6 nm) with lower contamination during the InGaAs CMP process.

Controlling the optical properties of hafnium dioxide thin films deposited with electron cyclotron resonance ion beam deposition

The effects of reactive and sputtering oxygen partial pressure on the structure, stoichiometry and optical properties of hafnium oxide (HfO2) thin films have been systematically investigated. The electron cyclotron resonance ion beam deposition (ECR-IBD) technique was used to fabricate the films on to JGS-3 fused silica substrates. The amorphous structure of HfO2 films were determined by X-ray Diffraction. Energy-dispersive X-ray Spectroscopy and Rutherford Backscattering Spectrometry were carried out for the composition and stoichiometry analysis, where this suggests the formation of over-stoichiometric films. The data suggests that the O:Hf ratio ranges from 2.4 – 4.45 to 1 for the ECR-IBD fabricated HfO2 films in this study. The transmission and reflectance spectra of the HfO2 films were measured over a wide range of wavelengths (λ = 185 – 3000 nm) by utilizing a spectrophotometer. The measured spectra were analyzed by an optical fitting software, which utilizes the model modified by O'Leary, Johnson and Lim, to extract the optical properties, refractive index (n) and the bandgap energy (E0). By varying the reactive and sputtering oxygen partial pressure, the optical properties were found to be n = 1.70 – 1.91, and E0 = 5.6 – 6.0 eV. This study provides a flexible method for tuning the optical properties of HfO2 coatings by controlling the mixture of reactive and sputtering gas.

In Situ Neutron Reflectometry Study of a Tungsten Oxide/Li-Ion Battery Electrolyte Interface.

The solid electrolyte interface/interphase (SEI) is of great importance to the viable operation of lithium-ion batteries. In the present work, the interface between a tungsten oxide electrode and an electrolyte solution consisting of LiPF6 in a deuterated ethylene carbonate/diethyl carbonate solvent was characterized with in situ neutron reflectometry (NR) at a series of applied electrochemical potentials. NR data were fit to yield neutron scattering length density (SLD) depth profiles in the surface normal direction, from which composition depth profiles were inferred. The goals of this work were to characterize SEI formation on a model transition-metal oxide, an example of a conversion electrode, to characterize the lithiation of WO3, and to help interpret the results of an earlier study of tungsten electrodes without an intentionally grown surface oxide. The WO3 electrode was produced by thermal oxidation of a W thin film. Co-analysis of NR and X-ray reflectivity data indicated that the stoichiometry of the thermal oxide was WO3. As the electrode was polarized to progressively more reducing potentials, starting from open circuit and down to +0.25 V versus Li/Li+, the layer that was originally WO3 expanded and increased in lithium content. The reduced electrode consisted of two to three layers: an inner layer (the evolving conversion electrode) which may have been mixed W and Li2O and unreacted WO3 or LixWO3, a layer rich in protons and/or lithium, possibly corresponding to LiOH or LiH (the inner SEI), and an outermost layer adjacent to the solution with an SLD close to that of the solution, possibly consisting of lower SLD species with solution-filled porosity or deuteron-rich species derived from the solvents (the outer SEI), though the presence of this layer was tenuous. For the steps in the direction of more oxidizing potentials, the evolution of the layer structure was qualitatively the reverse of that seen when stepping toward more negative potentials, though with hysteresis. The SLD gradient suggested that the reaction was not limited by diffusion within the film. No clear phase boundary was evident in the evolving conversion electrode.

The application of the EQCM data analysis for the examination of surface oxide formation on Pt-Pd-Ru ternary alloys

The stoichiometry of surface oxides, formed on Pt-Pd-Ru alloys in 0.5 M H2SO4 aqueous solutions, was postulated on the basis of cyclic voltammetric (CV) and the electrochemical quartz crystal microbalance (EQCM) measurements. A procedure of data analysis was elaborated to extract the information on surface oxide properties from the correlations between the EQCM frequency changes and electric charge. The average values of metal oxidation state in the oxides depend on potential of their formation, alloy composition and degree of alloy homogeneity. The highest metal oxidation state (+8) was reached for 10 %Pt-25 %Pd-65 % Ru alloy, while the lowest one (+4) for 35 %Pt-26 %Pd-38 %Ru alloy. Preliminary hydrogen electrosorption on Pd-rich (63 %) or Ru-rich (65 %) Pt-Pd-Ru alloys resulted in a decrease in the average metal oxidation states in surface oxides, whereas for other alloys the influence of hydrogen treatment was negligible.

Carbonate dimorphism, and the reinterpretation of rates of lattice and excess oxygen-driven catalytic cycles

Catalytic implications of local deviations from nominal oxide stoichiometry remain critical to consider yet challenging to elucidate in the context of bulk oxide-mediated light alkane oxidation reactions, in part due to a lack of a priori knowledge of surface active oxygen site counts. Carbonate dimorphism, i.e., differences in carbonate speciation resulting from CO2 adsorption onto unsupported nickel oxide surfaces, can be exploited toward quantifying the surface density of lattice and excess oxygens, and to further deconvolute their respective contributions toward specific steps within the ethane oxidation reaction network. Density functional theory, volumetric gas sorption, in situ titration, in situ spectroscopy, and transient kinetic data interpreted in light of quantitative estimates of surface excess oxygen density confirm the involvement of excess oxygen in partial oxidative turnovers producing ethene from ethane and O2. These collective studies cast a more nuanced interpretation of site requirements pertaining to nickel oxide-mediated alkane oxidation. Our findings reveal that excess oxygen atoms can be titrated under reaction conditions, and that these sites are (on average) more selective to ethene than lattice oxygens. Tools and methodologies employed herein connect seemingly disparate elements of bulk oxide catalysis research – the need for quantitative estimates of oxide surface non-stoichiometry on the one hand, and the propensity of unsupported oxides toward carbonate formation on the other – and provide a template for the possible broader application of quantitative analyses of probe-molecule binding characteristics toward elucidation of active site density and speciation in catalytic partial oxidation reaction systems of commercial relevance.