Thin Native Oxide Research Articles

Native Oxide Layer Role during Cryogenic‐Temperature Ion Implantations in Germanium

Herein, the structural properties and chemical composition of Ge samples implanted with tellurium at cryogenic temperatures are analyzed, focusing on the role of the native oxide. For germanium, cryogenic‐temperature implantation is a requirement to achieve hyperdoped impurity concentrations while simultaneously preventing surface porosity. In this work, the critical role of the thin native germanium oxide is demonstrated when performing ion implantations at temperatures close to the liquid nitrogen temperature. The structural and chemical composition of tellurium‐implanted samples as a function of the implanted dose from 5 × 1014 to 5 × 1015 cm−2 is analyzed. After a laser melting process, the incorporated oxygen is diffused to the surface forming again a GeOx layer which retains a large fraction of the total implanted dose. These detrimental effects can be eliminated by a selective etching of the native oxide layer prior to the ion implantation process. These effects have been also observed when implanting on Si substrates. This work identifies key aspects for conducting implantations at cryogenic temperatures, that are otherwise negligible for ion implanting at room temperature.

Investigating the influence of pore-widening time control in electrochemical anodization for the formation of hybrid titanium nanostructures with enhanced superhydrophobicity for improved corrosion resistance efficiency

Titanium (Ti) alloys possess remarkable strength, high-temperature stability, and processability, making them indispensable in biomaterials, aircraft engines, fuselage manufacturing, and ship propulsion shafts. However, when exposed to potential differences or minor defects in contact with other alloys, the thin natural oxide film on their surface is vulnerable to damage, leading to accelerated corrosion or diminished stability. This study investigated the morphology of the final oxide film formed with varying pore-widening (PW) process time (10–60 min with 10-minute intervals) after the anodization of Ti-Grade 4 material. The oxide films formed from the anodization and the PW process for 10 to 40 min exhibited a porous structure, while a hybrid pillar and pore structure were observed after a PW process of 50 min. As the PW time increases, the pore diameter expands, forming hybrid pillar and pore nanostructures, and superhydrophilicity is observed. After FDTS (1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane) coating, the phenomenon elucidates the transition from hydrophobicity to superhydrophobicity and the enhancement of water droplet mobility, with the lowest contact angle hysteresis measured at PW of 60 min and demonstrating improved corrosion resistance, achieving a corrosion inhibition efficiency of over 99% compared to the untreated titanium alloy surface. Notably, the structure encompassing pillar and pore features exhibits exceptional suitability for applications on material surfaces and interfacial phenomena, such as cell adhesion. Furthermore, surfaces characterized by excellent superhydrophobicity because of coating can find multifunctional applications encompassing anti-corrosion, anti-fouling, anti-icing, and self-cleaning capabilities.

Dependence of the Metal-Insulator-Semiconductor Schottky Barrier Height on Insulator Composition.

The effects of different high-κ tunnel oxides on the metal-insulator-semiconductor Schottky barrier height (ΦB) were systematically investigated. While these high-κ interlayers have been previously observed to affect ΦB, there has never been a clear consensus as to why this ΦB modulation occurs. Changes in ΦB were measured when adding 0.5 nm of seven different high-κ oxides to n-Si/Ni contacts with a thin native silicon oxide also present. Depending on the high-κ oxide composition and ΦB measurement technique, increases in ΦB up to 0.4 eV and decreases up to 0.2 eV with a high-κ introduction were measured. The results were compared to several different hypotheses regarding the effects of tunnel oxides on ΦB. The experimental data correlated most closely with the model of a dipole formed at the SiOx/high-κ interface due to the difference in the oxygen areal density between the two oxides. Knowledge of this relationship will aid in the design of Schottky and ohmic contacts by providing criteria to predict the effects of different oxide stacks on ΦB.

Photoswitching effect of graphene photogated diodes with interfacial layers

Graphene/semiconductor heterojunction-based photodetectors are expected to achieve high performance in various wavelength regions. Graphene photogated diodes (GPDs) based on such heterojunctions are promising for the fabrication of high-performance photodetectors, which cannot be achieved using conventional technologies. However, their action mechanism has not yet been theoretically demonstrated. In this study, we investigated the mechanism of GPDs with and without an interfacial layer. We demonstrated that the thin interfacial native oxide layer is responsible for the reduction in the Fermi level induced by photogating, which is similar to “photoswitching.” This effect induces an extraordinarily large output current in GPDs with a barrier layer.

Simulation of Native Oxide‐Passivated CsSn0.5Ge0.5I3 Highly Stable Lead‐Free Inorganic Perovskite Solar Cell

The search for a lead‐free, nontoxic, highly stable perovskite phase could end at the native oxide‐passivated composition of CsSn0.5Ge0.5I3. Mainly its remarkable stability is due to a thin native oxide layer (Sn‐doped GeO2) formation over CsSn0.5Ge0.5I3, which suppresses the Sn2+ oxidation to Sn+4. Herein, the performance of this mixed Sn–Ge perovskite and the role of native oxide is analyzed. The experimental device with 7% efficiency is numerically replicated, utilizing reasonable device configuration, material parameters, and defect model. The low efficiency of CsSn0.5Ge0.5I3 is due to high bulk defects. The simulation result shows that Sn oxidation suppressing the native oxide layer imparts stability but does not significantly impact device efficiency. The optimization provides passivated design for absorbers with high bulk defects. Low thickness at high defect density is preferable, and higher thickness at lower defect density is optimal for device design. The final optimized device achieves an efficiency of 21%. The results summarized here can provide a guideline for high‐efficiency, lead‐free, nontoxic stable perovskite solar cells.

A protective SiAlN coating on topographic surface of laser powder bed fusion manufactured Ti alloy

Protective SiAlN coating with Mo interlayer has been directly deposited onto as-built topographic surface of laser-based powder bed fusion manufactured Ti6Al4V alloy where high surface roughness, even with irregular geometrical features and native oxide scale on it. The topographic surface of Ti6Al4V is well protected by SiAlN/Mo coatings without spallation, cracks or even oxidation after 50 h of exposure at 800 °C for 10 cycles in air. The conformal nature of sputtered SiAlN/Mo coatings enables topographic surface to be well covered, and Mo diffuses through thin native oxide scale on topographic surface into Ti alloy, thereby forming strong chemical bonding between coatings and Ti substrates.

Tip-Enhanced Raman Imaging of Plasmon-Driven Coupling of 4-Nitrobenzenethiol on Au-Decorated Magnesium Nanostructures.

Magnesium nanoparticles (MgNPs) exhibit localized surface plasmon resonances across the ultraviolet, visible, and near-infrared parts of electromagnetic spectrum and are attracting increasing interest due to their sustainability and biocompatibility. In this study, we used tip-enhanced Raman spectroscopy (TERS) to examine the photocatalytic properties of MgNP protected by a thin native oxide layer and their Au-modified bimetallic analogs produced by partial galvanic replacement, Au-MgNPs. We found no reduction of 4-nitrobenzenethiol (4-NBT) to p,p'-dimercaptoazobisbenzene (DMAB) when a Au-coated tip was placed in contact with a self-assembled monolayer of 4-NBT molecules adsorbed on MgNPs alone. However, decorating Mg with Au made these bimetallic structures catalytically active. The DMAB signal signature of photocatalytic activity was more delocalized around AuNPs attached to Mg than around AuNPs on a Si substrate, indicating coupling between the Mg core and Au decorations. This report on photocatalytic activity of a bimetallic structure including plasmonic Mg paves the way for further catalyst architectures benefiting from Mg's versatility and abundance.

Enhancement of the corrosion properties of selective laser melted Co-Cr-Mo alloy by selective oxidation annealing

This study investigated the surface modification effect of the selective oxidation annealing (SOA) on the corrosion properties of cobalt‑chromium‑molybdenum (Co-Cr-Mo) alloy fabricated by selective laser melting (SLM). For the SOA, each element's oxidation driving force was calculated, and the condition in which only Cr was selectively oxidized was thermodynamically analyzed. The oxidation potential was controlled by the partial pressure ratio of hydrogen gas and water vapor, and the alloy samples were annealed following the condition of the selective oxidation of Cr. The changes in the oxide layer and corrosion properties according to the SOA time were examined. On the as-built sample's surface, a very thin native oxide film was formed. After the SOA, a dense chromium oxide (Cr 2 O 3 ) layer was formed on the surface, and its thickness increased along with the SOA time. These changes in the oxide layer had an impact on corrosion properties. Even though all samples before and after the SOA were under the passivation state, the corrosion performances in terms of corrosion potential and corrosion current density were improved by the SOA. The electrochemical impedance spectroscopy (EIS) results further confirmed that the improved corrosion resistance mainly came from the increased oxide layer resistance ( R o ) because of the thickened oxide layer. • The surface of the selective laser melted Co-Cr-Mo alloy was modified by the selective oxidation annealing. • The Cr 2 O 3 oxide layer was formed on the alloy surface by the selective oxidation of Cr. • The thickness of the Cr 2 O 3 oxide layer increased with the increasing the selective oxidation annealing time. • The corrosion potential and corrosion current density improved by adopting the selective oxidation annealing.

Si nanocone structure fabricated by a relatively high-pressure hydrogen plasma in the range of 3.3–27 kPa

In this study, we prepared a silicon nanocone structure using a relatively high-pressure H2 plasma in the range of 3.3–27 kPa. The silicon sample with the prepared nanocone structure exhibited a black surface. We investigated the dependence of the silicon nanocone formation behavior on various experimental parameters such as H2 pressure, processing time, substrate temperature, input power, and substrate bias. A small amount of air feed and a thin native oxide layer are desirable for the nanocone formation. Furthermore, the silicon temperature during plasma exposure plays an important role in increasing the silicon nanocone height. In addition, the polarity of the substrate bias drastically changes the surface structure from the nanocone in the case of a negative bias to a low-aspect-ratio pyramidal structure in that of a positive bias. This result implies that the anisotropic ion incidence is important for nanocone formation, despite the relatively high process pressure.

A study on repeatable and universal growth morphology optimization for nanowires grown on Si substrates

In this work, a series of NWs growths with different treatments for substrate surface have been carried out to study growth morphology optimization for NWs grown on Si substrates. A substrate treatment method of high-temperature annealing combined with thin oxide layer formed on the substrate surface has been developed. It is found that a lot of crystal defects exist in random locations of common substrates surface which make it difficult to grow NWs, but easy to form parasitic islands. XRD and AFM show that high-temperature annealing process can effectively eliminate most of the crystal defects in the substrates, inhibit the formation of parasitic islands. Combined with forming thin natural oxide layer on the substrate surface, the density of InAs NWs can be increased, the growth morphology uniformity of InAs NWs on Si substrates can be improved and parasitic islands are rarely formed on the substrates surface at all. In order to verify the effectiveness and universality of this method, another experiment of InGaAs NWs grown at different temperatures obtained excellent growth results as well. Therefore, this substrate treatment method has good effectiveness, repeatability and universality in optimizing the growth morphology of NWs.

Wetting of aluminium-based complex metallic alloys

Abstract Many complex metallic alloys are known to form in aluminium-based systems containing transition metals like Cu, Pd, Fe or Cr, the most famous one being the stable icosahedral quasicrystal discovered in the Al –Cu –Fe system. Although covered by a thin native oxide layer, adhesion of water onto the complex compounds is very different from that onto the oxide or onto oxidised aluminium. We show here how this atypical behaviour is related to the structural complexity of the compound. We then produce data that allows us to estimate the actual surface energy of the same compounds, a property that is also a fingerprint of structural complexity in Al-based intermetallics.

Nonenzymatic electrochemical sensors via Cu native oxides (CuNOx) for sweat glucose monitoring

Diabetes is a chronic disease, which is the seventh leading cause of death worldwide. The increased risk to the health of diabetic patients, with comorbidities in harsh situations like COVID-19 pandemics, necessitates real-time monitoring of blood glucose. Monitoring of blood glucose is invasive, hence, sweat glucose monitoring can be an alternative approach to address the invasive issue in blood glucose monitoring. This work reports a facile, low-cost, high-performance copper (Cu) native oxide (CuNOx)-based nonenzymatic electrochemical sensor for sweat glucose sensing. We utilized a very thin Cu native oxide of ~10 nm on Cu thin film for the sensing because of the excellent catalytic oxidation behavior of cuprous oxide (Cu2O) to glucose. The anodic sweep of cyclic voltammetry of glucose showed that the hydroxyl ions from sodium hydroxide convert the electrode surface into different oxides [Cu(I), Cu(II) Cu(III)], which electro-oxidize glucose to gluconolactone, then eventually to gluconic acid resulting in oxidation current. The CuNOx sensors exhibited a sensitivity of 603.42 μA mM−1 cm−2, a linear range beyond the desired limit of 7.00 mM with excellent linearity, and a low limit of detection of 94.21 μM. Excellent repeatability and stability (stable >1 year) with relative standard deviation (RSD) of 2.67%, and 2.70%, respectively were achieved for 1 mM glucose. The selectivity with common interferants of glucose in human sweat and blood showed an RSD of 3.56%. We believe the electrocatalytic efficacy of the CuNOx sensors for glucose sensing can open a new prospect in the fabrication of wearable sweat glucose sensors.

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.

Influence of Intrinsic Silicon Layer and Intermediate Silicon Oxide Layer on the Performance of Inline PECVD Deposited Boron-Doped TOPCon

In this article, the development and optimization of carrier-selective and passivating contacts by industry-scale inline plasma-enhanced chemical vapor deposition and their successful integration into solar cells are reported. Amorphous Si thin films with varying carbon content (SiC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) were deposited on a thermally grown ultrathin tunnel oxide (TOPCon) and electrically characterized. Furthermore, the impact of a vacuum break (VB) during the deposition of a layer stack consisting of intrinsic amorphous Si [a-Si:H(i)] and boron-doped SiC <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> was investigated. That is, samples that were processed with VB were exposed to ambient air, and hence, a thin native oxide was formed on the a-Si:H(i) layer, which affected the boron diffusion into the absorber resulting in a distinct anneal behavior of the contacts. Upon optimization, these layers provided an excellent surface passivation quality, which was reflected in an implied open-circuit voltage of 733 mV for n-type and 716 mV for p-type TOPCon structures, respectively. In addition, very low contact resistivities of 0.3 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for n-type and 0.5 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for p-type TOPCon were measured, respectively. These optimized TOPCon structures were implemented into both sides contacted p-type laboratory solar cells. After a two-step furnace anneal, these cells achieved a maximum energy conversion efficiency of 22.7% with evaporated contacts.

Ferroelectric α‐In2Se3 Wrapped‐Gate β‐Ga2O3 Field‐Effect Transistors for Dynamic Threshold Voltage Control

AbstractIndium selenide (α‐In2Se3), which is a recently emerging ferroelectric semiconductor, can solve a major hindrance to applications of an ultra‐wide bandgap beta‐gallium oxide (β‐Ga2O3) semiconductor. Here, ferroelectric α‐In2Se3 wrapped‐gate β‐Ga2O3 field‐effect transistors (FETs) for dynamic threshold voltage (VTH) control is demonstrated. The dry‐transferred α‐In2Se3 layer is wrapped around β‐Ga2O3 channel, which allows efficient electrostatic gate modulation. Thus, the ferroelectricity of α‐In2Se3 and a thin native oxide interlayer formed at the interface between β‐Ga2O3 and α‐In2Se3 can provide effective VTH control. Applying a positive voltage pulse to the gate electrode induces positive VTH shift; hence, the device can be even changed from depletion to enhancement (E‐) mode. The E‐mode β‐Ga2O3 FET exhibits steep‐subthreshold slope with a negligible hysteresis. The VTH of E‐mode can be further modulated by applying back‐gate bias, and electrical performance can be enhanced via dual‐gate operation. The approach demonstrates an energy efficient β‐Ga2O3‐based switching device architecture integrated with ferroelectric van der Waals 2D materials.

Surface roughness and substrate induced symmetry-breaking: influence on the plasmonic properties of aluminum nanostructure arrays.

The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.

Deposition of Titanium Dioxide Coating by the Cold-Spray Process on Annealed Stainless Steel Substrate

The surface of most metals is covered with thin native oxide films. It has generally been believed that to achieve bonding, the oxide covering the surface of metallic particles or metal substrates must be broken and removed by adiabatic shear instability (ASI), whether induced at the particle–substrate interface or at the particle–particle interface. The aim of the present research is to investigate the correlation between the remaining oxide amorphous layer and substrate-deformation with the adhesion strength of cold-sprayed TiO2 coatings towards the bonding mechanism involved. Relevant experiments were executed using stainless steel (SUS 304), subjected to various annealing temperatures and cold-sprayed with TiO2 powder. The results indicate an increasing trend of coating adhesion strength with increasing annealed substrate temperature. The influence of substrate plastic deformation and atomic intermixing at the remaining amorphous oxide layer is discussed as the factors contributing to the increasing adhesion strength of cold-sprayed TiO2 coatings.

3D-graphene-laser patterned p-type silicon Schottky diode

The influence of the laser patterning (LP) process on the quality of graphene (Gr) film and Schottky diode characteristics was researched in this study. First of all, p-type silicon (Si) was patterned by homemade femtosecond laser source. To compare the resulting effect, non-patterned n-Si and p-Si were used as substrates. To achieve vertically oriented three-dimensional (3D) Gr nanosheets (VGNs) onto the laser patterned p-type Si, non-patterned n-Si, and p-Si substrates, we used Radio-Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) technique. Then, Raman analyses for VGNs obtained on patterned and non-patterned p-Si and n-Si substrates were conducted. All results indicate that the Gr obtained on all substrates is vertically oriented. Scanning electron microscopy (SEM) analyses were also performed to obtain information about the morphological properties of the 3D Gr structure. In addition, the influence of the laser patterning at the Gr-Si interface was evaluated by comparing two sets of devices which are junctions of Gr-Laser Patterned Si (Gr-LPSi) and Gr-Insulator-Silicon (Gr-I-Si) at 300 K in the dark medium. D1 device consists of a Gr-LPSi junction that involves RF-PECVD grown Gr on the silicon, whose surface exposed the laser beam for patterning. D2 device consists of a Gr-I-Si junction that involves RF-PECVD grown Gr onto the silicon, whose surface involves a thin native oxide layer (~2 nm). The results show that the laser treatment causes an increment in Schottky barrier height (SBH) and decreases leakage currents under a reverse bias voltage of the diode. Notably, we have seen the influence of the laser patterning process on 3D Gr nano-sheets which is that the entire surface of the substrate has now reformed new structures in the nano-sphere and nano-rose morphology, so we believe that this is the first work that shows these structures in this form.

Environmental fatigue of superelastic NiTi wire with two surface finishes

Surface finish of NiTi is widely perceived to affect its biocompatibility and corrosion fatigue performance. The aim of this work was to find out, whether a carefully engineered surface oxide shows any beneficial effect over electropolished surface on the fatigue performance of superelastic NiTi wire mechanically cycled in simulated biofluid. Series of corrosion and environmental fatigue tensile tests was performed on superelastic NiTi wire with two different surface finishes frequently used in medical device industry. Open Circuit Potential reflecting the activity of chemical reactions on the surface of the wire cycled in electrochemical cell was continuously monitored during the fatigue tests. Microcracks at the surface of the fatigued NiTi wires were characterized by SEM and TEM. It was found that the carefully engineered 70 nm thick TiO2 oxide provides the NiTi wire with similar level of protection against the static corrosion as the less than 10 nm thin natural oxide on the electropolished wire and that it does not have any positive effect on its performance in environmental fatigue tests, whatsoever. On the contrary, the wire covered by the carefully engineered 70 nm thick TiO2 oxide displayed systematically poorer fatigue performance upon tensile cycling under specific critical loading conditions (strain amplitude <0.5% at large mean strains 1–7%).

Permeation and diffusion of hydrogen in vanadium using deoxidation and glow-discharge surface cleaning techniques

Vanadium (V) is an attractive hydrogen separation membrane material. However, its diffusion behavior under hydrogen permeation is not well understood mainly because of surface oxidation. Here, we have attempted to observe the permeation of hydrogen through V by subjecting V samples to different surface treatments e.g., electropolishing and plasma-induced cleaning using a glow-discharge technique. We have achieved a high flux of hydrogen controlled by long-range diffusion under a low hydrogen potential at 723–973 K. The diffusion coefficients of hydrogen in defect-free and high-purity samples analyzed from the permeation curves at 823–973 K are almost consistent with the literature data obtained by Gorsky effect and absorption measurements. However, the activation energy of diffusion (19 kJ mol−1) is 8–14 kJ mol−1 higher than the literature values. This difference in activation energy suggests that permeation through the active surface of V is highly sensitive to trapping by oxygen—a slight amount of oxygen dissolved from a very thin native oxide layer would segregate near the surface to trap hydrogen. Furthermore, we have found that residual strain, grain boundaries, and lattice defects, induced by mechanical deformation, reduce the flux and retard the diffusion considerably.