Interfacial Suboxide Research Articles

Metal-free boron-assisted Fe(III)/peracetic acid system for ultrafast removal of organic contaminants: Role of crystalline nature and interfacial suboxide boron intermediates

Peracetic Acid (PAA) has emerged as an alternative candidate oxidant for H2O2 and persulfate-mediated catalytic oxidation. However, the sluggish kinetics of Fe3+/Fe2+ recycling still severely causes its limited applications. In this work, boron, for the first time, a metal-free and inexpensive co-catalyst, was introduced into the Fe3+/PAA Fenton-like system to remarkably accelerate the Fe2+ regeneration, further facilitating organic wastewater decontamination (an almost 145.0-fold kinetic increase on BPA removal). More excitingly, the crystalline nature of boron significantly modulated the activation system, and the crystalline boron and amorphous boron exhibited contrasting Fe3+ reduction characteristics. The amorphous boron-contained system enabled ultra-fast deterioration of various micro-pollutants. Validation experiments and theoretical calculations demonstrate that the surface B-B bonds with the formed interfacial suboxide boron continuously donated electrons to boost the rapid reduction of Fe3+. This work opens a new research avenue to promote the application of Fe3+-activated PAA systems with high activity in continuity and also guides the development of a new series of co-catalysts in wastewater treatment.

Crystalline boron significantly accelerates Fe(III)/PMS reaction as an electron donor: Iron recycling, reactive species generation, and acute toxicity evaluation

Crystalline boron (C-boron) could provide redox active sites in Fenton-like reactions due to the surface B-B bonds and interfacial suboxide boron in the surface structure of B12 icosahedra. However, related research on boron chemistry is limited. We demonstrate here that addition of C-boron to the Fe(III)/peroxymonosulfate(PMS) system effectively improves the degradation of flunixin meglumine, aspirin, nitrobenzene, and benzoic acid at pH 4.0–10.0 (without buffer). Compared with the Fe(III)/PMS system, the kobs of the C-boron/Fe(III)/PMS system for degradation of these four contaminants at pH 7.0 was approximately 8.7–65.1 times higher. C-boron enhanced the homogeneous Fe(II) concentration in the Fe(III)/PMS system from 5.4 to 16.2 mg/L in a 20-min reaction, whereas Fe(IV) was generated from the Fe(II) produced in the system.Electron paramagnetic resonance and quenching experiments revealed the presence of ·OH, 1O2, and O2·− in the reaction, and the corresponding contributions of ·OH to oxidizing flunixin meglumine and aspirin were 16.7 % and 28.1 % at pH 7.0. The C-boron/Fe(III)/PMS solution significantly inhibited Microcystis aeruginosa growth, with the rate increasing in the first 24 h and then decreasing from 48 to 96 h. This toxicity was resulted by the inhibition of the chlorophyll a and chlorophyll b in the photosynthetic system. Overall, this work reported the feasibility of C-boron enhancement for pollutants degradation in Fe(III)/PMS system for the first time.

Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo.

Identifying the structure of the Al2O3/Al interface is important for advancing its performance in a wide range of applications, including microelectronics, corrosion barriers, and superconducting qubits. However, beyond the study of a few select terminations of the interface using computational methods, and top-down, laterally averaged spectroscopic and microscopic analyses, the explicit structure of the interface and the initial stages of propagation of the interface into the metal are largely unresolved. In this study, we utilize ab initio grand canonical Monte Carlo to perform a physically motivated, unbiased exploration of the interfacial composition and configuration space. We find that at equilibrium, the interface is atomically sharp with aluminum vacancies and propagates in a layer-by-layer fashion, with aluminum excess in the oxide layer at the interfacial plane. Oxygen incorporation, aluminum vacancy formation, and aluminum vacancy annihilation are the building blocks of Al2O3 formation at the interface. The localized interfacial mid-gap states from under-coordinated aluminum atoms from the oxide and the immediate depletion of aluminum states near the Fermi level upon oxygen incorporation prevent oxygen dissolution ahead of the interface front and result in the layer-by-layer propagation of the interface. This is in sharp contrast to the ZrO2/Zr system, which forms interfacial sub-oxides, and also explains the favorable self-healing nature of the Al2O3/Al system. The occupied interfacial mid-gap states also increase the calculated n-type Schottky barrier heights. Additionally, we identify that interfacial aluminum core-level shifts linearly depend on the aluminum coordination number, whereas interfacial oxygen core-level shifts depend on long-range ordering at the interface. The detailed geometric and electronic insights into the interface structure and evolution expand our understanding of this fundamental interface and have important implications for the engineering and design of Al2O3/Al-based corrosion coatings with enhanced barrier properties, controllable transistor technologies, and noise-free superconducting qubits.

Fast and Long‐Lasting Iron(III) Reduction by Boron Toward Green and Accelerated Fenton Chemistry

AbstractGeneration of hydroxyl radicals in the Fenton system (FeII/H2O2) is seriously limited by the sluggish kinetics of FeIII reduction and fast FeIII precipitation. Here, boron crystals (C‐Boron) remarkably accelerate the FeIII/FeII circulation in Fenton‐like systems (C‐Boron/FeIII/H2O2) to produce a myriad of hydroxyl radicals with excellent efficiencies in oxidative degradation of various pollutants. The surface B−B bonds and interfacial suboxide boron in the surface B12 icosahedra are the active sites to donate electrons to promote fast FeIII reduction to FeII and further enhance hydroxyl radical production via Fenton chemistry. The C‐Boron/FeIII/H2O2 system outperforms the benchmark Fenton (FeII/H2O2) and FeIII‐based sulfate radical systems. The reactivity and stability of crystalline boron is much higher than the popular molecular reducing agents, nanocarbons, and other metal/metal‐free nanomaterials.

Effect of annealing temperature on structural, electrical, and UV sensingcharacteristics of n-ZnO/p-Si heterojunction photodiodes

The aim of this study is to investigate the influences of annealing temperature on initial device characteristics and their correlations with ultraviolet (UV) radiation sensitivity of n-type zinc oxide/p-silicon (n-ZnO/p-Si) heterojunction photodiodes. Evolutions on the crystalline structure, surface morphology, ideality factor, barrier potential, interface state density, and donor concentration were systematically analyzed during the initial device characterizations before testing the UV sensitivities of the photonic devices. The results demonstrate that the sensitivity of each n-ZnO/p-Si photodiode annealed at various temperatures was linearly correlated with different UV illumination intensities. The devices annealed at 750 ºC and 550 ºC exhibit more sensitive performance than devices annealed at 150 ºC and 350 ºC owing to lower dark currents and good oxide quality. However, the presence of an interfacial suboxide or silicate oxide layer significantly decreased the responsivity of the photodiodes annealed at 750 ºC. The maximum responsivity value was 126 mA/W for the photodiode annealed at 550 ºC. It can be concluded that the photodiode annealed at 550 ºC exhibits promising performance for UV sensing applications.

Effect of nitrogen passivation on interface composition and physical stress in SiO2/SiC(4H) structures

The electron density and physical stress at the thermally oxidized SiC/SiO2 interface, and their change with nitrogen incorporation, were observed using x-ray reflectivity, Raman scattering, and in-situ stress measurement. There is no evidence for residual carbon species at the SiO2/SiC. Instead, a ∼1 nm thick low electron density layer is formed at this interface, consistent with interfacial suboxides (SiOx, 0.3 < x < 2), along with high interfacial stress. Nitrogen passivation, a known process to improve the interface state density and electronic properties, eliminates the low density component and simultaneously releases the interface stress. On the basis of these findings, a chemical interaction model is proposed to explain the effect of the nitrogen in terms of both stress reduction and elemental control of the dielectric/SiC interface, resulting in a higher quality gate stack on SiC.

Atomically Thin Interfacial Suboxide Key to Hydrogen Storage Performance Enhancements of Magnesium Nanoparticles Encapsulated in Reduced Graphene Oxide.

As a model system for hydrogen storage, magnesium hydride exhibits high hydrogen storage density, yet its practical usage is hindered by necessarily high temperatures and slow kinetics for hydrogenation-dehydrogenation cycling. Decreasing particle size has been proposed to simultaneously improve the kinetics and decrease the sorption enthalpies. However, the associated increase in surface reactivity due to increased active surface area makes the material more susceptible to surface oxidation or other side reactions, which would hinder the overall hydrogenation-dehydrogenation process and diminish the capacity. Previous work has shown that the chemical stability of Mg nanoparticles can be greatly enhanced by using reduced graphene oxide as a protecting agent. Although no bulklike crystalline MgO layer has been clearly identified in this graphene-encapsulated/Mg nanocomposite, we propose that an atomically thin layer of honeycomb suboxide exists, based on first-principles interpretation of Mg K-edge X-ray absorption spectra. Density functional theory calculations reveal that in contrast to conventional expectations for thick oxides this interfacial oxidation layer permits H2 dissociation to the same degree as pristine Mg metal with the added benefit of enhancing the binding between reduced graphene oxide and the Mg nanoparticle, contributing to improved mechanical and chemical stability of the functioning nanocomposite.

Interfacial Ga-As suboxide: Structural and electronic properties

The structural and electronic properties of Ga-As suboxide representative of the transition region at the GaAs/oxide interface are studied through density functional calculations. Two amorphous models generated by quenches from the melt are taken under consideration. The absence of As–O bonds indicates that the structure is a mixture of GaAs and Ga-oxide, in accordance with photoemission experiments. The band edges of the models are found to be closely aligned to those of GaAs. The simulation of charging and discharging processes leads to the identification of an As-related defect with an energy level at ∼0.7 eV above the GaAs valence band maximum, in good agreement with the experimental density of interface states.

On the permanent component profiling of the negative bias temperature instability in p-MOSFET devices

In this manuscript, we have investigated the negative bias temperature instability (NBTI) induced border-trap (Nbt) depth in the interfacial oxide region of PMOS transistors using multi-frequency charge pumping (MFCP) method. We emphasize on the distribution of the permanent component in the oxide near the interface, giving a clear insight on its effect on NBTI features. According to the experimental data, the extracted effective dipole moment (aeff) and field-independent activation energy (Ea) have revealed a linear relation with depth distance (Z), which consistently explain the variation of n as well as Ea,eff often reported in the literature. In fact, aeff and Ea increase with the depth, indicating the presence of the precursor defects having different effective dipole moments and activation energies. We suggest that such traps are most likely related to O3−xSixSi–H (x=1 and x=2) family defects (or Pb center hydrogen complex) located in the interfacial sub-oxide region.

Impact of incorporated oxygen quantity on optical, structural and dielectric properties of reactive magnetron sputter grown high-κ HfO2/Hf/Si thin film

High-κ hafnium-oxide thin films have been fabricated by radio frequency (rf) reactive magnetron sputtering technique. To avoid formation of an undesired interfacial suboxide layer between Si and high-κ film, prior to HfO2 deposition, a thin Hf buffer layer was deposited on p-type (100) Si substrate at room temperature. Effect of oxygen gas quantity in the O2/Ar gas mixture was studied for the optical and structural properties of grown HfO2 high-κ thin films. The grown thin oxide films were characterized optically using spectroscopic ellipsometer (SE) in detail. Crystal structure was studied by grazing incidence X-ray diffractometer (GIXRD) technique, while bonding structure was obtained by Fourier transform infrared spectroscopy (FTIR) analyses. In agreement with GIXRD and FTIR analyses, SE results show that any increment above ideal quantity of oxygen content in the gas mixture resulted in decrements in the refractive index and thickness of HfO2 dielectric film, while increments in SiO2 thickness. It is apparent from experimental results that oxygen to argon gas ratio needs to be smaller than 0.2 for a good film quality. The superior structural and optical properties for grown oxide film were obtained for O2/Ar gas ratio of about 0.05–0.1 combined with ∼30W constant rf sputtering power.

Moisture requirements to reduce interfacial sub-oxides and lower hydrogen pre-bake temperatures for RPCVD Si epitaxy

In silicon epitaxy technology, residual O/H2O contamination requires thermal reduction using hydrogen pre-bakes to achieve atomically clean Si surfaces. This H2 pre-bake leads to unwanted increase of thermal budget. The higher the level of interfacial O, the longer the pre-bake period and the higher the temperature have to be for removal of these contaminants. Moisture contamination from the pre-epi chamber etch (immediately before the wafer is loaded), with it's use of high flows of HCl, is identified as a major contributor to the area density of the interfacial O. Stringent control of moisture impurity in the HCl is required to reduce thermal budget. Ultra low temperature technology to desiccate HCl is used to achieve single digit ppb moisture resulting in lower temperature hydrogen pre-bakes than achieved with standard solid media-type HCl purification.

Interfacial chemistry and valence band offset between GaN and Al2O3 studied by X-ray photoelectron spectroscopy

The interface region between Ga-face n-type GaN and Al2O3 dielectric (achieved via atomic-layer deposition or ALD) is investigated by X-ray photoelectron spectroscopy (XPS). An increase in the Ga-O to Ga-N bond intensity ratio following Al2O3 deposition implies that the growth of an interfacial gallium sub-oxide (GaOx) layer occurred during the ALD process. This finding may be ascribed to GaN oxidation, which may still happen following the reduction of a thin native GaOx by trimethylaluminum (TMA) in the initial TMA-only cycles. The valence band offset between GaN and Al2O3, obtained using both core-level and valence band spectra, is found to vary with the thickness of the deposited Al2O3. This observation may be explained by an upward energy band bending at the GaN surface (due to the spontaneous polarization induced negative bound charge on the Ga-face GaN) and the intrinsic limitation of the XPS method for band offset determination.

Wet Chemical Oxidation of Silicon Surfaces Prior to the Deposition of All-PECVD AlO<sub>x</sub>/<i>a</i>-SiN<sub>x</sub> Passivation Stacks for Silicon Solar Cells

Aluminum oxide (AlOx) is currently under intensive investigation for use in surface passivation schemes in solar cells. AlOx films contain negative charges and therefore generate an accumulation layer on p-type silicon surfaces, which is very favorable for the rear side of p-type silicon solar cells as well as the p+-emitter at the front side of n-type silicon solar cells. However, it has been reported that quality of an interfacial silicon sub-oxide layer (SiOx), which is usually observed during deposition of AlOx on Silicon, strongly impacts the silicon/AlOx interface passivation properties [1]. The present work demonstrates that a convenient way to control the interface is to form thin wet chemical oxides of high quality prior to the deposition of AlOx/a-SiNx:H stacks by the plasma enhanced chemical vapor deposition (PECVD).

Water-Related Hole Traps at Thermally Grown GeO2–Ge Interface

The generation mechanism of positive charge present in germanium oxide film thermally grown on a germanium substrate has been investigated in this study. Water-related hole traps are generated in the interfacial germanium suboxide layer. The negative flat-band voltage shift due to the charged hole traps increases with increasing electric stress field in the oxide. Both low-temperature growth of the oxide film and postmetallization annealing have been proposed for the improvement of the flat-band voltage shift. The former is effective in minimizing the suboxide layer thickness by suppressing germanium monoxide volatilization during the oxide growth. The latter method successfully reduces the density of traps caused by water desorption from the interfacial suboxide layer.

Effect of high-pressure H2O treatment on elimination of interfacial GeOX layer between ZrO2 and Ge stack

This investigation demonstrates the effect of high-pressure H2O treatment on the elimination of the interfacial germanium suboxide (GeOX) layer between ZrO2 and Ge. The formation of GeOX interlayer increases the gate-leakage current and worsen the controllability of the gate during deposition or thermal cycles. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy reveal that high-pressure H2O treatment eliminates the interfacial GeOX layer. The physical mechanism involves the oxidation of non-oxidized Zr with H2O and the reduction of GeOX by H2. Treatment with H2O reduces the gate-leakage current of a ZrO2/Ge capacitor by a factor of 1000.

Effects of interfacial suboxides and dangling bonds on tunneling current through nanometer-thick SiO2layers

Quantum-mechanical tunneling of charge carriers through nanometer-thick SiO${}_{2}$ layers is one of the key issues in Si-based electronics. Here, we report first-principles transport calculations of charge-carrier tunneling through nanometer-thick SiO${}_{2}$ layers in Si/SiO${}_{2}$/Si structures. We find that tunneling of holes in the valence bands occurs mainly via oxygen 2$p$ orbitals perpendicular to Si--O--Si bonds, and it can be enhanced greatly by interfacial suboxides and dangling bonds in Si/SiO${}_{2}$ interfaces. Electrons in the conduction bands show tunneling behaviors sensitive to their wave vectors parallel to the interfaces, reflecting the six conduction-band minima in the bulk Si. Our results provide atomistic description of tunneling currents through SiO${}_{2}$ layers, and suggest that leakage current will be blocked more effectively if suboxides and dangling bonds are reduced.

Thermal performance of sputtered Cu films containing insoluble Zr and Cr for advanced barrierless Cu metallization

Pure Cu films and Cu alloy films containing insoluble substances (Zr and Cr) were deposited on Si(100) substrates, in the presence of interfacial native suboxide (SiO x ), by magnetron sputtering. Samples were vacuum annealed between 300°C and 500°C to investigate effects of Zr and Cr additions on the thermal performance of Cu films. After annealing, copper silicides were found in the Cu(Zr) films, while no detectable silicides were observed in Cu and Cu(Cr) films. Upon annealing, Zr accelerated the diffusion and reaction between the film and the substrate, and lowered the thermal stability of Cu(Zr) alloy films on Si substrates, which was ascribed to the ‘purifying effect’ of Zr on the Si substrates. Whereas, Cr prohibited the agglomeration of Cu films at 500°C and decreased the surface roughness. As a result, the diffusion of Cu in Si substrates for Cu(Cr) films was effectively inhibited. In contrast to the high resistivity of Cu(Zr) films, the final resistivity of about 2.76 μΩ·cm was achieved for the Cu(Cr) film. These results indicate that Cu(Cr) films have higher thermal stability than Cu(Zr) films on Si substrates and are preferable in the advanced barrierless Cu metallization.

Interface Stoichiometry and Structure in Anodic Niobium Pentoxide

High-resolution transmission electron microscopy and electron energy loss spectroscopy (EELS) were performed on electrochemically anodized niobium and niobium oxide. Sintered anodes of Nb and NbO powders were anodized in 0.1 wt% H3PO4 at 10, 20, and 65 V to form surface Nb2O5 layers with an average anodization constant of 3.6+/-0.2 nm/V. The anode/dielectric interfaces were continuous and the dielectric layers were amorphous except for occurrences of plate-like, orthorhombic pentoxide crystallites in both anodes formed at 65 V. Using EELS stoichiometry quantification and relative chemical shifts of the Nb M4,5 ionization edge, a suboxide transition layer at the amorphous pentoxide interface on the order of 5 nm was detected in the Nb anodes, whereas no interfacial suboxide layers were detected in the NbO anodes.

Preparation of ZrO 2 ultrathin films as gate dielectrics by limited reaction sputtering—On growth delay time at initial growth stage

ZrO 2 thin films were produced by limited reaction sputtering process varying the deposition parameters. An interesting growth phenomenon was observed in the initial growth stage of amorphous samples, appearing to suppress film growth for the first several minutes. The structures of such ultrathin ZrO 2 films were investigated by high-resolution Rutherford backscattering (HR-RBS) and X-ray photoelectron spectroscopy (XPS). The results suggest that the existence of interfacial suboxides due to the adsorption-induced surface reaction and diffusion-induced internal reaction, lead to the deteriorated interfacial performance. The mechanism and effects of the growth delay time on the interfacial characteristics are discussed in detail.

Charging mechanism in a SiO2 matrix embedded with Si nanocrystals

One of the applications of a Si nanocrystals (nc-Si) embedded in a SiO2 matrix is in the area of nonvolatile memory devices based on the charge storage in the material system. However, whether the charge trapping mainly occurs at the nc-Si∕SiO2 interface or in the nc-Si is still unclear. In this work, by x-ray photoemission spectroscopy analysis of the Si 2p peaks, the concentrations of both the nc-Si and the Si suboxides that exist at the nc-Si∕SiO2 interface are determined as a function of thermal annealing, and the charging effect is also measured by monitoring the shift of the surface C 1s peak. It is observed that the annealing-caused reduction of the total concentration of the interfacial suboxides is much faster than that of both the C 1s shift and the nc-Si concentration. In addition, the trend of the C 1s shift coincides with that of the nc-Si concentration. The results suggest that the Si nanocrystal, rather than the nc-Si∕SiO2 interface, plays the dominant role in the charging effect.