Native Oxide Layer Research Articles

Quasi In Situ XPS on a SiOxFy Layer Deposited on Silicon by a Cryogenic Process

A silicon oxyfluoride layer was deposited on a-Si samples using SiF4/O2 plasma at different temperatures between −100°C and −40 °C. In situ X-ray photoelectron spectroscopy measurements were then performed to characterize the deposited layer. The sample was then brought back to room temperature and analyzed again. It has been shown that a temperature below −65 °C is needed to significantly enhance the physisorption of SiFx species. Hence, in this condition, a F-rich oxyfluoride layer, stable at low temperature only, is physisorbed. Above this threshold temperature, the native silicon oxide layer is fluorinated and the proportion of O in the deposited layer is higher and remains stable even when the sample is brought back to room temperature.

Взаимосвязь электронной и атомной структуры пассивированных поверхностей n-InP(100)

Photoluminescence, Raman spectroscopy and X-ray photoelectron spectroscopy are used to study electronic and atomic structure of n-InP(100) surfaces treated with different sulfide solutions. It is shown that the sulfide treatment causes removal of the native oxide layer from the semiconductor surface and formation of the passivating layer consisting of In−S chemical bonds with the structure dependent on the solution composition and atomic arrangement at the initial surface of the semiconductor. This is accompanied by an increase in photoluminescence intensity and narrowing of the surface depletion layer. Atomic structure of the passivating layer determines the total dipole that modifies the depth distribution of the bands potentials and thus the surface electronic structure.

Designing fluorine-free electrolytes for stable sodium metal anodes and high-power seawater batteries via SEI reconstruction

F-free, cost-effective 1 M NaBH4/glyme electrolytes induce SEI reconstruction, which converts the native oxide layer on sodium metal to a NaH-based SEI layer. With 1 M NaBH4/DEGDME, we achieved long-term cycling, high-power seawater batteries.

Correlation of the electronic and atomic structure at passivated n-InP(100) surfaces

Photoluminescence, Raman spectroscopy and X-ray photoelectron spectroscopy are used to study electronic and atomic structure of n-InP(100) surfaces treated with different sulfide solutions. It is shown that the sulfide treatment causes removal of the native oxide layer from the semiconductor surface and formation of the passivating layer consisting of In-S chemical bonds with the structure dependent on the solution composition and atomic arrangement at the initial surface of the semiconductor. This is accompanied by an increase in photoluminescence intensity and narrowing of the surface depletion layer. Atomic structure of the passivating layer determines the total dipole that modifies the depth distribution of the bands potentials and thus the surface electronic structure. Keywords: surface modification, sulfur passivation, Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy.

Screen-printed Ga2O3 thin film derived from liquid metal employed in highly sensitive pH and non-enzymatic glucose recognition

A new scaffold was introduced prefacing a screen-printed native gallium oxide (Ga2O3) layer derived from galinstan and pure metallic gallium, employed in Extended Gate Field Effect Transistor (EGFET) along with electrochemical measurement for pH and non-enzymatic glucose detection. Screen-printed Ga2O3 from galinstan exhibited a better pH sensitivity (79. 46 mV/pH) with 99% linearity and improved reliability compared to Ga2O3 derived from pure gallium. The highly selective Ga2O3 (from galistan) based glucose sensor exhibited high sensitivity (22.01 mV/mM) in EGFET and a minimum limit of detection (20 μM) in electrochemical measurement. The commercial applicability of the galinstan-based EGFET glucose sensor was examined by detecting glucose in human serum. Material characterization supported the electrical results. This approach is an innovative state-of-the-art for a stable, inexpensive, and reliable pH and glucose detection method at room temperature.

Effects of cobalt deposition and pretreatment process on electrical properties of products

The preferable conditions for formation of high quality CoSi2 films and effect of process parameters on properties of products were investigated. The pretreatment should not only remove the natural oxide layer completely, but also could not damage Si substrate. The good static random access memory (SRAM) proportion of products is high when pretreatment thickness is 20 Å, reached 96.5%. The radio frequency (RF) bias power process parameter should also take an optimal value. When RF bias power is 150 W, the good SRAM proportion of products is greater than 98%. The 100 Å Co can just completely react with Si substrate after twice annealing (500 °C 30 s and 750 °C 30 s), and if it exceeds 100 Å, Co will be residual. Decreasing Co thickness leads to contact resistance (RC) increase whatever in N-well or P-well. The overall standby current (Isb) of product is least when Co thickness is 80 Å. Finally, the products achieved good electrical properties when Co thickness is 80 Å, pretreatment thickness is 20 Å and RF bias power is 150 W.

Surface Properties and Mechanical Performance of Ti-Based Dental Materials: Comparative Effect of Valve Alloying Elements and Structural Defects

Two approaches can be taken when designing properties of the native oxide layers formed on Ti-based biomedical materials: (i) changing the chemical composition of the substrate by adding biocompatible, valve alloying elements, and (ii) changing the microstructure of the substrate—especially its level of defectiveness—through large plastic deformation. However, especially in the aggressive fluoridated oral environment, it is still unknown what factor is more effective in terms of enhancing oxide layer protectiveness against biocorrosion: (i) the presence of valve alloying elements, or (ii) a high number of structural defects. To gain knowledge about the separate influence of both of these factors, surface properties were examined for commercially pure Ti and Ti–Nb–Ta–Zr alloy in microcrystalline state as well as after multiple-pass cold rolling, a process that can be readily scaled up to the industrial level. This study showed that while valve-alloying elements and structural defects individually have a beneficial effect on Ti oxide layer properties in fluoridated medium, they not have to act in a synergistic manner. These findings have to be taken into account when designing future Ti-based dental materials together with analyzing their mechanical performance with respect to mechanical strength and elastic properties.

Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Effect of heat and surface pre-treatment processes on the nitriding ability of stainless steel SUS420

As-supplied SUS420 stainless steel was annealed at 880 °C for1 h and quenched (1040 °c, 30 min), followed by tempering at 530 °c for 1 h. The sample surface after tempering was polished with different roughness levels by mechanical grinding. Samples in the states of as-annealed, as-quenched and as-tempered samples as well as ground tempered ones with different surface roughness were gas nitrided with NH3 gas at 520 °c for 5 h. The microstructure and mechanical properties of SUS420 steel before and after the heat and surface treatments were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction, and micro-hardness testing. The results show that if the natural oxide layer on the surface of the SUS420 samples was not removed, the nitriding process was very difficult or even imposible. The annealed steel gave the highest nitriding depth but low surface hardness. The samples after quenching and/or tempering had lower nitriding depth but higher hardness. The surface hardness of the as-supplied steel (333 HV) decreased with annealing (181 HV). After quenching, tempering, and gas nitriding, the values were 632, 560 and > 1000 HV, respectively. The samples after nitriding showed the appearance of fine CrN phase in the nitrided layer. The highest nitriding depth (125 pm) was obtained for the annealed samples, and subsequently decreased for the quenched samples and tempered samples. The lower the roughness of the sample, the higher is the hardness. The nitrided layer thickness tended to increase as the roughness decreased.

Study of wall re-deposition on DC-grounded ITER-relevant mirrors with RF plasma in a first mirror unit

In ITER, several first mirrors (FMs) are expected to be DC-grounded with the water cooling lines being implemented as a quarter wavelength (λ/4) RF-filter. DC-grounding of the FMs can significantly increase the plasma potential V p, which could trigger an increased wall sputtering and associated re-deposition on the FMs during plasma cleaning. To understand the scope of this impact, helium discharges were excited with DC-grounded FMs in an ITER-sized mock-up of a first mirror unit (FMU) using wall materials with different sputtering energy thresholds (E th). Additionally, a part of the FM was electrically isolated from the RF to study its impact on the erosion/re-deposition properties on the surface. The E th of the wall materials, as well as its native oxide layers, had a significant influence on the re-deposition observed on the FMs. With high E th where walls were unsputtered, both the DC-grounded and electrically isolated parts of the FM were free of deposits. However, with low E th where the walls were sputtered, there was a net wall re-deposition on the DC-grounded parts of the FM, while electrically isolated parts were still relatively clean. Further, to study the impact of floating wall components, Cu walls in the FMU were isolated from the ground. Here the walls developed a floating potential V f and the ion energy at the walls was lowered to e(V p − V f). The floating walls, in this case, were relatively unsputtered and the FMs experienced a net cleaning with total reflectivity of the mirror preserved at pristine mirror levels. This work shows that electrically isolating the FM as well as the wall surface minimizes wall re-deposition in presence of λ/4 filter and therefore are promising techniques for effective FM cleaning in ITER.

Ion tracks in ultrathin polymer films: The role of the substrate

Surface damage produced by single MeV-GeV heavy ions impacting ultrathin polymer films has been shown to be weaker than those observed under bulk (thick film) conditions. The decrease in damage efficiency has been attributed to the suppression of long-range effects arising from excited atoms lying deeply in the solid. This raises the possibility that the substrate of the films itself is relevant to the radiation effects seen at the top surface. Here, the role of the substrate on cratering induced by individual 1.1 GeV Au ions in ultrathin poly(methyl methacrylate) (PMMA) layers is investigated. Materials of different thermal and electrical properties (Si, SiO2, and Au) are used as substrates to deposit PMMA thin films of various thicknesses from ∼1 to ∼300 nm. We show that in films thinner than ∼40 nm craters are modulated by the underlying substrate to a degree that depends on the transport properties of the medium. Crater size in ultrathin films deposited on the insulating SiO2 is larger than in similar films deposited on the conducting Au layer. This is consistent with an inefficient coupling of the electronic excitation energy to the atomic cores in metals. On the other hand, the damage on films deposited on SiO2 is not very different from the Si substrate with a native oxide layer, suggesting, in addition, poor energy transmission across the film/substrate interface. The experimental observations are also compared to calculations from an analytical model based on energy addition and transport from the excited ion track, which describe only partially the results.

In situ x-ray analysis of misfit strain and curvature of bent polytypic GaAs–In x Ga1−x As core–shell nanowires

Misfit strain in core–shell nanowires can be elastically released by nanowire bending in case of asymmetric shell growth around the nanowire core. In this work, we investigate the bending of GaAs nanowires during the asymmetric overgrowth by an In x Ga1−x As shell caused by avoiding substrate rotation. We observe that the nanowire bending direction depends on the nature of the substrate’s oxide layer, demonstrated by Si substrates covered by native and thermal oxide layers. Further, we follow the bending evolution by time-resolved in situ x-ray diffraction measurements during the deposition of the asymmetric shell. The XRD measurements give insight into the temporal development of the strain as well as the bending evolution in the core–shell nanowire.

Electrochemical Complexation of Polyatomic Aluminum Cations in Quinone-Type Organic Battery Electrodes Revealed By Solid-State NMR

Rechargeable aluminum batteries represent a significant area of opportunity for beyond-lithium-ion energy storage due to the high theoretical capacity (2980 mA∙h∙g-1, 8046 mA∙h∙mL-1), large earth abundance (8.23 wt.%), low cost, and safety of aluminum metal. Due to the stable native oxide layer that forms on aluminum, very few electrolytes are able to reversibly plate and strip aluminum at room temperature. Lewis acidic chloroaluminate ionic liquid electrolytes have demonstrated success in this regard; however, any cathode materials must also be (electro)chemical stable in this electrolyte. Electrode materials based on organic molecules have garnered attention due to their tunable structures and properties, enabling optimization for specific energy storage systems. Furthermore, diverse functional groups enable a range of possible charge storage mechanisms, many of which have yet to be explored in these systems. Like aluminum, organic materials are highly abundant and can be derived from various sources without geopolitical constraints. However, few Al-organic battery studies exist, and none so far have performed mechanistic analyses based on experimental data to identify conclusively the electroactive ions and their binding sites.Here, we present a rigorous analysis of charge storage mechanisms in quinone-type organic molecules for the first time, established through multi-dimensional solid-state nuclear magnetic resonance (ssNMR) spectroscopy in concert with quantum chemical calculations and electrochemical measurements. Indanthrone quinone (INDQ) is investigated for the first time as a model system due to its two distinct quinone functionalities and low solubility in many solvents. Through-space 27Al{1H} and 13C{1H} dipolar-mediated ssNMR techniques are used to establish the sub-nanometer proximities between polyatomic aluminum cations and specific moieties in the organic frameworks. Furthermore, dipolar-mediated ssNMR measurements are sensitive to both molecular dynamics and internuclear distances, which enables us to selectively filter out signals from residual liquid electrolyte, eliminating the need for solvent rinsing that can perturb the sample. Al-INDQ cells achieve reversible capacities of up to 240 mA∙h∙g-1 and maintain capacities >100 mA∙h∙g-1 at rates up to 2400 mA∙g-1. The enolization reaction and charge-balancing AlCl2 + cation complexation was established via dipolar-mediated ssNMR, findings that are corroborated through density functional theory (DFT) calculations. Other quinone-based organic structures with modified structures and compositions were also investigated using similar methods and will briefly be compared to INDQ. In aggregate, the quinone-based model systems serve as a basis to better understand how different functional groups, heteroatoms, and organic structures affect the electrochemical properties of rechargeable Al-organic batteries. The results yield strategies towards creating ‘designer’ organic electrode materials for rechargeable aluminum batteries.

Reduction of copper surface oxide using a sub-atmospheric dielectric barrier discharge plasma

Oxide layers on metal surfaces adversely affect processability and material properties in many industrial applications. Although several plasma-based approaches for deoxidation were investigated in the past, oftentimes they either work under conditions expensive to create or require long processing times. In this study, the deoxidation effect of a non-thermal dielectric barrier discharge plasma in an Ar/H2 gas mixture at 100 hPa and 20 °C was investigated on copper surfaces with a native oxide layer. The chemical structure of surfaces before and after deoxidation was analyzed by X-ray photoelectron spectroscopy (XPS). The results revealed that ∼98% of the surface lattice oxide Cu2O was reduced to Cu after around 20 s of plasma treatment, whereas all oxygen contaminants were almost completely removed from Cu surface after around 50 s. Additionally, the kinetics of the reduction of surface oxide was studied and a Johnson-Mehl-Avrami-Erofeev-Kholmogorov kinetic model was proposed. The analysis of the morphology of surfaces was performed with atomic force microscopy (AFM), showing minor changes in the roughness after deoxidation. Moreover, optical emission spectroscopy (OES) showed atomic hydrogen radicals in the plasma phase, which likely causes deoxidation effect.

Ultra‐Shallow All‐Epitaxial Aluminum Gate GaAs/AlxGa1−xAs Transistors with High Electron Mobility

AbstractThe electron mobility in shallow GaAs/AlxGa1−xAs heterostructures is strongly suppressed by charge wafer surface, which arises from native surface oxide layers formed when the wafer is removed from the crystal growth system. Here an in situ epitaxial aluminum gate, grown as part of the wafer, is used to eliminate surface charge scattering. Transmission electron microscope characterization shows that the in situ epitaxial aluminum is crystalline, and the wafer surface is free of native oxide. The influence of Al thickness and the use of different semiconductor wetting layers at the semiconductor‐aluminum interface are examined and correlated with electron mobility. The electron mobility is found to strongly depend on aluminum thickness. For 8 nm thick aluminum, the electron mobility is also influenced by the wetting layer, with aluminum grown on GaAs producing higher mobility compared to AlAs or Al0.33Ga0.67As wetting layers. The suppression of surface charge scattering in these all‐epitaxial devices allows for high mobilities across a wide density range despite the shallow conduction channel (35 nm below the gate). These measurements also provide a uniquely sensitive method of determining the electrical quality of the semiconductor–metal interface, relevant to the formation of hybrid semiconductor–superconductor devices.

Formic acid reduction and CO2 activation at Mo2C: The important role of surface oxide

AbstractSmall organic acids, such as formic or acetic acid, which are well‐known products of the electrochemical CO2 reduction reaction, are often not further reduceable to their respective alcohols. Alcohols are well‐desired chemicals and useful as fuels, since they can be easily purified and provide a high energy density. Here, we present a combined electrochemical, differential electrochemical mass spectrometry (DEMS), and nuclear magnetic resonance spectroscopy (NMR) study, providing insight in the electro‐reduction of formic acid on Mo2C electrodes, and the corresponding formation of notable amounts of methanol, with a Faraday efficiency of ∼27 % at a high steady state current density of –0.3 mA/cm2 compared to other formic acid electroreduction catalysts, such as Pb or Sn. Intriguingly, we find that formic and acetic acid readily form on native oxide covered Mo2C surfaces in air and especially in humid CO2 atmosphere. We realize that the thin native oxide layer that is ubiquitously present on these electrodes is the key factor for the activation of CO2 and the corresponding formation of formic and acetic acid at the solid/gas interface. Subsequent electrochemical reduction of these two acids at the solid/liquid interface proceeds through direct formation of their respective alcohols. The activity toward organic acid reduction on Mo2C catalysts can therefore be enhanced by their adsorption properties at the surface and not necessarily through inhibition of the HER, which is exceptional and points to a general strength of compound catalysts for organic acid reduction and CO2 activation.

Impermeable Graphene Oxide Protects Silicon from Oxidation.

The presence of a natural silicon oxide (SiOx) layer over the surface of silicon (Si) has been a roadblock for hybrid semiconductor and organic electronics technology. The presence of an insulating oxide layer is a limiting operational factor, which blocks charge transfer and therefore electrical signals for a range of applications. Etching the SiOx layer by fluoride solutions leaves a reactive Si-H surface that is only stable for few hours before it starts reoxidizing under ambient conditions. Controlled passivation of silicon is also of key importance for improving Si photovoltaic efficiency. Here, we show that a thin layer of graphene oxide (GOx) prevents Si surfaces from oxidation under ambient conditions for more than 30 days. In addition, we show that the protective GOx layer can be modified with molecules enabling a functional surface that allows for further chemical conjugation or connections with upper electrodes, while preserving the underneath Si in a nonoxidized form. The GOx layer can be switched electrochemically to reduced graphene oxide, allowing the development of a dynamic material for molecular electronics technologies. These findings demonstrate that 2D materials are alternatives to organic self-assembled monolayers that are typically used to protect and tune the properties of Si and open a realm of possibilities that combine Si and 2D materials technologies.

How water wets and self-hydrophilizes nanopatterns of physisorbed hydrocarbons

HypothesisWeakly bound, physisorbed hydrocarbons could in principle provide a similar water-repellency as obtained by chemisorption of strongly bound hydrophobic molecules at surfaces. ExperimentsHere we present experiments and computer simulations on the wetting behaviour of water on molecularly thin, self-assembled alkane carpets of dotriacontane (n-C32H66 or C32) physisorbed on the hydrophilic native oxide layer of silicon surfaces during dip-coating from a binary alkane solution. By changing the dip-coating velocity we control the initial C32 surface coverage and achieve distinct film morphologies, encompassing homogeneous coatings with self-organised nanopatterns that range from dendritic nano-islands to stripes. FindingsThese patterns exhibit a good water wettability even though the carpets are initially prepared with a high coverage of hydrophobic alkane molecules. Using in-liquid atomic force microscopy, along with molecular dynamics simulations, we trace this to a rearrangement of the alkane layers upon contact with water. This restructuring is correlated to the morphology of the C32 coatings, i.e. their fractal dimension. Water molecules displace to a large extent the first adsorbed alkane monolayer and thereby reduce the hydrophobic C32 surface coverage. Thus, our experiments evidence that water molecules can very effectively hydrophilize initially hydrophobic surfaces that consist of weakly bound hydrocarbon carpets.

Solid-state polymer adsorption for surface modification: The role of molecular weight

HypothesisSolid-state polymer adsorption offers a distinct approach for surface modification. These ultrathin, so-called Guiselin layers can easily be obtained by placing a polymer melt in contact with an interface, followed by a removal of the non-adsorbed layer with a good solvent. While the mechanism of formation has been well established for Guiselin layers, their stability, crucial from the perspective of materials applications, is not. The stability is a trade-off in the entropic penalty between cooperative detachment of the number of segments directly adsorbed on the substrate and consecutively pinned monomers. ExperimentsExperimental model systems of Guiselin layers of polystyrene (PS) on silicon wafers with native oxide layer on top were employed. The stability of the adsorbed layers was studied as a function of PS molecular weight and polydispersibility by various microscopic and spectroscopic tools as well as quasi-static contact angle measurements. FindingsAdsorbed layers from low molecular weight PS were disrupted with typical spinodal decomposition patterns whereas high molecular weight (>500 kDa) PS resulted in stable, continuous layers. Moreover, we show that Guiselin layers offer an enticing way to modify a surface, as demonstrated by adsorbed PS that imparts a hydrophobic character to initially hydrophilic silicon wafers.

X‐ray photoelectron spectroscopy characterization of band offsets of MgO/Mg2Si and SiO2/Mg2Si heterojunctions

MgO and SiO2 are the natural oxide layer on the surface of Mg2Si and the excellent gate dielectric layer materials. MgO/Mg2Si and SiO2/Mg2Si heterojunctions were successfully prepared by magnetron sputtering. The valence band offset (VBO) and conduction band offset (CBO) of the two heterojunctions were measured by X‐ray photoelectron spectroscopy (XPS) method, and the values were 2.64 and 3.90 eV for MgO/Mg2Si and 3.24 and 4.43 eV for SiO2/Mg2Si, respectively. Different degree of band bending occurs on the two interfaces. As a result, the band diagrams of the both interfaces are all Type‐I (Straddled) band alignment. Such high VBO and CBO can provide suitable barrier heights for both electrons and holes (>1 eV); therefore, both MgO and SiO2 can be suitable candidates for the preparation of Mg2Si‐based thin film transistors.