Reoxidation Of Surface Research Articles

Plasma deposited amorphous silicon passivation layers on InAs surfaces

The compound semiconductors (InAs, GaSb, AlSb) having nearly matched lattice constants (≈6.1 Å) are of great interest in fabrication of infrared micro/optoelectronics, but are hampered significantly by a high density of interface states that lead to degenerately doped surface layers. Native oxides are a cause of interface states, as well as a barrier to the effective passivation of dangling bonds, and must be removed. Re-oxidation must then be prevented prior to deposition of a passivant. We demonstrate effective removal of the native oxide of InAs through utilization of an amorphous silicon plasma enhanced chemical vapor deposition process. A hydrogen-diluted silane plasma provides a reducing environment to react and remove indium and arsenic oxide species while a thin layer of amorphous silicon is grown to passivate and prevent re-oxidation of the InAs surface. The reduction of native oxide species via hydrogen-diluted argon plasma as a pretreatment prior to amorphous silicon was also explored. The surface chemistry is verified via depth profiling X-ray photoelectron spectroscopy, and the impact of a hydrogen-argon plasma pretreatment investigated to further reduce oxygen concentration at the InAs/amorphous silicon interface.

Practical Aspects of Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry Analysis Enhanced by Fluorine Gas Coinjection

The interest in using high vacuum-compatible time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors integrated within focused ion beam instruments (FIB) has increased due to the possibility of conducting correlative and/or complementary studies with other techniques such as scanning electron microscopy, energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy, electron backscatter diffraction, atomic force microscopy, and Raman spectroscopy without breaking vacuum conditions. This work discusses the practical aspects of FIB-TOF-SIMS analysis enhanced by fluorine gas. A series of systematic experiments with high-purity Al and Cu single crystals was conducted. We demonstrate that fluorine-assisted FIB-TOF-SIMS analysis yields up to several orders of magnitude higher SIMS signals due to chemically improved ionization probability of elements. The potential of fluorine gas-assisted FIB-TOF-SIMS analysis and the correlated benefits are demonstrated on a real-life sample based on comparing the results to EDS and TOF-SIMS experiments conducted without any gas. Our studies indicate that the presence of fluorine gas reduces the negative effect of preferential reoxidation of the sample surface. This enables high lateral resolution FIB-TOF-SIMS mapping because the secondary ion signals are not dominated by elements with strongly oxygen-enhanced ionization probability.

Magnetic Field-Induced Recoverable Dynamic Morphological Change of Gallium-Based Liquid Metal

We report magnetic field-driven dynamic morphological change of a liquid metal droplet: deformation and recovery; separation and merge. Low melting, non-toxic gallium-based liquid metal has been widely investigated due to its material properties and thus utilized to various applications. Morphological change of the liquid metal in required to tune characteristics of application. In order to enable the liquid metal to be non-wettable and magnetically controllable, we coated the surface with ferromagnetic iron (Fe) particles using hydrochloric acid (HCl) solution resulting in a magnetic liquid metal marble. With and without magnetic field, Fe particles-coated magnetic liquid metal marble was deformed and recovered readily. Against $\sim 8~\mu \text{L}$ magnetic liquid metal marble, depending on the coated Fe particles mass, the shape deformation under magnetic field was changed: conventional (Fe $\geqq ~4$ mg) wetting shape. Accordingly, the deformability and recovery time of the magnetic liquid metal marble were varied. With an applied magnetic field, one marble was readily separated into 2, 3, and 4 marbles. Due to re-oxidation of the magnetic liquid metal marble surface, we treated the marble with HCl solution and vapor. Split and merge occurred depending on treatment time of HCl solution or vapor. Finally, we demonstrated magnetic field-based dynamic behavior which is reversible. [2020-0218]

Process Development and DFT‐Assisted Mechanism of the Vapour Phase Ammoxidation of 2,6‐Dichlorotoluene to 2,6‐Dichlorobenzonitrile over the V2O5/γ‐Al2O3 Catalyst

AbstractAmmoxidation of 2,6‐dichlorotoluene (2,6‐DCLT) to 2,6‐dichlorobenzonitrile (2,6‐DCLBN) over V2O5/γ‐Al2O3 catalyst has been carried out in a fixed bed reactor. A maximum conversion of 96% of (2,6‐DCLT) and 79% yield to 2,6‐dichlorobenzonitrile have been obtained at a temperature of 425 °C, Air : 2,6‐DCLT mole ratio of 22 : 1 and NH3 : 2,6‐DCLT mole ratio of 11 : 1. A DFT theory computations have been performed with the objective of finding a mechanism of the reaction, activation energy and transition state for different reactions. Abstraction of C−H bond with an activation energy of 64.60 kcal mol−1 was found to be rate determining step. A new approach has been suggested for the reoxidation of the catalyst which supports reoxidation of every step associated with dehydration of species during the reaction. This finding is a contrary of the previous studies which assume reoxidation of the catalyst at the end of all reactions. Rapid reoxidation of the catalyst surface to release water is found to be the driving force for all reactions involving the cleavage of C−H or N−H bonds.

Effects of consecutive processing between cleaning and deposition on adhesion of diamond-like carbon prepared by plasma-based ion implantation and deposition

The effects of pretreatment on adhesion, which is the most important challenge of diamond-like carbon (DLC) films, were investigated. DLC films were prepared by changing pretreatment conditions on austenitic stainless steel (SUS304) substrates by hybrid process of plasma-based ion implantation and deposition (PBIID) using superimposed RF and negative high-voltage pulses. Although an oxide layer on a substrate surface was removed by Ar-plasma sputter cleaning, the substrate surface was immediately reoxidized when plasma was stopped for a moment between sputter cleaning and deposition. The DLC film on the oxide layer was poor adhesion strength less than approximately 50 MPa. Even if the oxide layer existed on the substrate surface, carbon ion implantation as an interface treatment in the PBIID process broke the oxide layer. As a result, the adhesion strength of DLC films was enhanced above the strength of epoxy resin (about 70 MPa). Furthermore, it was found that continuous generation of plasma after sputter cleaning was able to suppress reoxidation of the substrate surface. Consequently, the adhesion strength of DLC films prepared with no interface treatment was increased above 70 MPa, suggesting simplification of coating process and reduction in processing time for cost reduction.

Solder Interconnection of Aluminum Foil Rear Side Metallization for Passivated Emitter and Rear Solar Cells

Foil metallized (FolMet) solar cells combine the high-efficiency PERC technology and a cost-efficient rear side metallization based on a 9 µm thin aluminum foil. Laser fired contacts are used to attach the foil to the rear side and allow for the mechanical and electrical contact to the cell. Direct soldering on aluminum is hampered by a native oxide film formed immediately when exposed to air. To realize the cell interconnection by a standard solder process the Al foil is coated by sputter deposition or roll cladding of solderable layers. This work evaluates the solderability of 200 µm thick coated Al foils with the common solder interconnection process established in photovoltaics using standard copper ribbons for module integration. Our analysis reveals for both coating approaches strong initial mechanical adhesion of > 4 N/mm after soldering and very low contact resistivities of < 1.6 µ∙cm2. The contact resistivity shows no degradation after isothermal aging within 1000 hours at 85 °C. The mechanical adhesion of the sputter coated Al foils remains at 3 N/mm under thermal aging of more than 1000 h at 85 °C whereas the adhesion for the Al foils coated by roll cladding drops strongly to values of < 1 N/mm already after 50 hours. SEM images indicate that this effect is caused by re-oxidation of the aluminum surface supported by micro cracks in the solderable top layer.

Formation of the reaction zone between tin‐copper brazing fillers and aluminum‐silicon‐magnesium alloys: Experiments and thermodynamic analysis

AbstractA primary challenge in brazing is the controlled formation of phases resulting from interactions of elements of the liquid filler metal with those of the base material. The morphology of the brazed joint, which is decisive for the mechanical properties of the joint, is influenced by present elements and process parameters such as brazing temperature and time. Furthermore, the wetting of the base material is a crucial factor in joining of aluminum because of the low wettability of the alumina layer by molten brazing filler metals. In order to remove the alumina and prevent reoxidation of the substrate surface, the brazing process can be conducted in vacuum or inert gas atmosphere. Again, selection of process parameters is crucial for the quality of the brazed seam. In this work, we focus on the influence of the process parameters on the wetting behavior and the formation of aluminum‐copper phases theoretically by means of thermodynamic calculations using a CALPHAD database as well as by means of in‐situ observations in the large‐chamber scanning electron microscope (LC‐SEM) and by brazing experiments. Both the critical temperatures with respect to the wetting and the reaction kinetics as well as the crucial stages of the brazing process and the resulting phases were determined.

Investigation of thin oxide layer removal from Si substrates using an SiO2 atomic layer etching approach: the importance of the reactivity of the substrate

The evaluation of a plasma-based atomic layer etching (ALE) approach for native oxide surface removal from Si substrates is described. Objectives include removal of the native oxide while minimizing substrate damage, surface residues and substrate loss. Oxide thicknesses were measured using in situ ellipsometry and surface chemistry was analyzed by x-ray photoelectron spectroscopy. The cyclic ALE approach when used for removal of native oxide SiO2 from a Si substrate did not remove native oxide to the extent required. This is due to the high reactivity of the silicon substrate during the low-energy (<40 eV) ion bombardment phase of the cyclic ALE approach which leads to reoxidation of the silicon surface. A modified process, which used continuously biased Ar plasma with periodic CF4 injection, achieved significant oxygen removal from the Si surface, with some residual carbon and fluorine. A subsequent H2/Ar plasma exposure successfully removed residual carbon and fluorine while passivating the silicon surface. The combined treatment reduced oxygen and carbon levels to about half compared to as received silicon surfaces. The downside of this process sequence is a net loss of about 40 Å of Si. A generic insight of this work is the importance of the substrate and final surface chemistry in addition to precise etch control of the target film for ALE processes. By a fluorocarbon-based ALE technique, thin SiO2 layer removal at the Ångstrom level can be precisely performed from an inert substrate, e.g. a thick SiO2 layer. However, from a reactive substrate, like Si, complete removal of the thin SiO2 layer is prevented by the high reactivity of low energy Ar+ ion bombarded Si. The Si surfaces are reoxidized during the ALE ion bombardment etch step, even for very clean and ultra-low O2 process conditions.

Mechanisms of Nitrogen Incorporation at 4H-SiC/SiO&lt;sub&gt;2&lt;/sub&gt; Interface during Nitric Oxide Passivation – A First Principles Study

In this work, we investigate the behavior of Nitrogen atoms at 4H-Silicon Carbide (4H-SiC)/Silicon dioxide (SiO2) interface during nitric oxide passivation using ab-initio Density Functional Theory. Our calculations suggest different possible energetically favorable and competing mechanisms by which nitrogen atoms could a) incorporate themselves into the oxide, just above the 4H-SiC substrate, and b) substitute for carbon atoms at the 4H-SiC surface. We attribute the former process to cause increased threshold voltage instability (hole traps), and the latter to result in improved effective mobility through channel counter-doping, apart from removing interface traps in 4H-SiC power MOSFETs. These results support recent electrical and XPS measurements. Additionally, Nitric Oxide passivation is shown to energetically favor re-oxidation of the 4H-SiC surface accompanied by the generation of oxygen vacancies under the conditions considered in this work.

Effect of interface and bulk traps on the C–V characterization of a LPCVD-SiNx/AlGaN/GaN metal-insulator-semiconductor structure

Silicon nitride (SiNx) film grown by low-pressure chemical vapor deposition (LPCVD) is utilized as a gate dielectric for AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs). Trap distribution at the gate-dielectric/III-nitrides interface is characterized by a temperature-dependent ac-capacitance technique. The extracted interface state density Dit decreases from 2.92 × 1013 to 1.59 × 1012 cm−2 eV−1 as the energy level depth (EC-ET) increases from 0.29 to 0.50 eV, and then levels off to EC-ET = 0.80 eV. Capacitance-mode deep level transient spectroscopy (C-DLTS) and energy band diagram simulations reveal that deep levels with EC-ET > 0. 83 eV are responsible for the dispersion of capacitances at high temperature (>125 °C) and low frequencies (<1 kHz). A high-resolution transmission electron microscope (TEM) reveals that re-oxidation of the RCA-treated AlGaN barrier surface may be responsible for the relatively high density of shallow states at the LPCVD-SiNx/III-nitride interface.

Temperature-Scanning Method for the kinetic studies of CO oxidation over ceria–zirconia supported gold catalysts

The experimental data of CO oxidation over the series of 2wt.% Au/Ce1−xZrxO2 (x=0, 0.25, 0.5, 0.75, 1) catalyst were analyzed using a variation of the Temperature Scanning Method with the aim of catalyst’s composition optimization. The catalysts were prepared by the Direct Anionic Exchange technique. The kinetic of CO oxidation was quantified by conversion–rate–temperature (X, r, T) triplets, calculated from raw data obtained using a plug flow reactor, working at oxygen stoichiometric, rich, and lean conditions. The data were fitted to the Mars-van Krevelen adsorption-reaction model (MvK). The results showed that the MvK model is able to predict the superficial reaction and the re-oxidation of the catalyst surface. Additionally, the data at oxygen lean conditions revealed the distinctive characteristic of Ce-containing catalysts, which act as oxygen buffer by releasing-uptaking oxygen. The obtained values of the fitted parameters allowed re-mapping raw data onto the conversion–temperature–catalyst composition surface plot and their application as a tool for the optimization of catalyst composition.

Surface Interaction of PuO2, UO2+x and UO3 with Water Ice

The interaction of PuO2, UO2+x and UO3 with water ice was studied using ultraviolet photoelectron spectroscopy (UPS). Water was adsorbed at 80–120 K as thick ice multilayers. Surface modification after desorption of the ice around 200 K was investigated. Main information on the surface oxidation state was obtained by highly surface sensitive UPS-HeII spectra, probing primarily the first monolayer. The oxidation state was directly deduced from the intensity of the actinide 5f levels. The surface character of the phenomenon was further confirmed by comparing HeII spectra with the more bulk sensitive HeI spectra. Spectral interpretation was done using the cross-section variations in HeI and HeII spectra and by comparing the spectra with theoretical density of states curves, obtained by LSDA + U calculations. It was shown previously that reduction to Pu2O3 takes place, when the ice covered PuO2 films are warmed up under UV light and ice is desorbed. In this paper, this effect was investigated in further detail. It was shown that only the top surface layer is reduced. Reduction is inhibited by surface diffusion of oxygen trapped in the films during sputter deposition and not incorporated in the lattice. UO2+x and UO3 also undergo reduction, but to a significant lesser extent than PuO2. Reoxidation of surface U by bulk oxygen was much slower than that of surface Pu. It was shown that for all oxides, reduction needs the illumination of an ice overlayer by UV light. Surface reduction by atomic hydrogen was investigated to check for possible influence of ice photolysis products. UO3 was shown to be reduced to UO2 while PuO2 is not further reduced. The observations are explained by photochemical decomposition of water by the UV light (used for UPS) at the oxide-ice interface. It is thought that the oxide acts as photocatalyst, absorbing light and splitting adsorbed water. The thick ice layer traps the reaction products on the surface, thereby enabling them to react and reduce the surface. Why only the reductants (probably H) and not the concomitant oxidants react with the surface is still unknown.

Oxygen‐Functionalized Few‐Layer Graphene Sheets as Active Catalysts for Oxidative Dehydrogenation Reactions

Nanostructured graphitic forms of carbons have shown intersting potential for catalysis research and are ideal candidates to substitute the conventional metal-oxide catalysts because they can be easily disposed, which enables a greener, more sustainable catalytic process. Few-layer graphene and its functionalized form offer the opportunity to investigate the nature of graphitic active sites for oxidation reactions in well-defined carbon-based catalysts. In this paper, we report the utilization of oxygen-functionalized few-layer graphene sheets containing variable amounts of oxygen in the heterogeneous catalytic oxidative dehydrogenation (ODH) reaction of isobutane at 400ºC. Interestingly, there is poor correlation between oxygen content and catalytic performance. Carbonyl groups were found to be highly stable, and graphene that had higher sp(2) character, the lowest oxygen content, and fewer edge sites presented the lowest specific rate of isobutane reaction, although the isobutene selectivity remained high. The reoxidation of the graphene surface occurred at the same rate as the ODH reaction suggesting a Mars-van Krevelen type of mechanism, similar to that which takes place on oxide surfaces. These results appear to suggest that a higher fraction of exposed edges where oxygen active sites can be formed and exchanged should lead to more active catalysts for ODH reactions.

Highly productive iron molybdate mixed oxides and their relevant catalytic properties for direct synthesis of 1,1-dimethoxymethane from methanol

The one-step gas phase conversion of methanol to 1,1-dimethoxymethane (DMM) was studied over highly productive iron molybdate mixed oxides in a fixed-bed reactor working at atmospheric pressure. When placing the Fe–Mo–O catalyst under a reaction mixture highly concentrated in methanol, the DMM selectivity was drastically boosted. This specific and unique feature encouraged us to extensively characterize this family of particular catalysts. LEIS analysis revealed the presence of both Mo and Fe species on the outermost atomic layer of the catalysts. X-ray photoelectron spectroscopy (XPS) and in situ EPR measurements showed that the Fe centres are responsible for the redox properties. The acidic properties of the FeMo mixed oxides were then attributed to anionic vacancies acting as Lewis acid sites produced by dehydroxylation of the catalyst surface. XPS analysis also showed that oxygen from the gas phase was responsible for reoxidation of the catalyst surface with regeneration of the active sites, which suggests a Mars–van Krevelen mechanism. The good catalytic performances were then attributed to a synergistic effect between Mo and Fe species. A Mo/Fe molar ratio of 3.2 for an optimal remarkable yield in DMM of 50% was found.

Alkane and Alkanethiol Passivation of Halogenated Ge Nanowires

The ambient stability and surface coverage of halogen (Cl, Br, and I) passivated germanium nanowires were investigated by X-ray photoelectron and X-ray photoelectron emission spectroscopy. After exposure to air for 24 h, the stability of the halogen-terminated Ge nanowire surfaces toward reoxidation was found to improve with the increasing size of the halogen atoms, i.e., I > Br > Cl. Halogen termination was effective in removing the native Ge oxide (GeOx) and could also be utilized for further functionalization. Functionalization of the halogenated Ge nanowires was investigated using alkyl Grignard reagents and alkanethiols. The stability of the alkyl and alkanethiol passivation layers from the different halogen-terminated surfaces was investigated by X-ray photoelectron spectroscopy and attenuated total reflectance infrared spectroscopy. Alkanethiol functionalized nanowires showed greater resistance against reoxidation of the Ge surface compared to alkyl functionalization when exposed to ambient conditi...

Hydrogen termination for extended queue times for low temperature epitaxy

We investigated the oxidation of hydrogen terminated Si (1 0 0) surfaces stored in Fab air environment. Hydrogen termination for extended queue times with wet and dry clean techniques is investigated. Comparison of X-ray photoelectron spectroscopy (XPS) data and empirical results from Si capping layer are used to determine re-oxidation of the Si surface over extended queue times. H 2 pre-bake efficiency with the best clean method, 100:1 HF, in situ rinsed with degasified DI water was tested. 2 min 850 °C and 900 °C H 2 bakes, at 10 Torr pressure, both resulted in atomically clean surfaces below SIMS detectable for up to 96 h queue time in lab ambient (controlled 35–40% relative humidity and temperature range of 22–25 °C).

Photochemically Controlled Electrochemical Deposition and Dissolution of Ag0 Nanoclusters on Au Electrode Surfaces

A photoisomerizable thiolated nitrospiropyran SP, (1a), monolayer is assembled on a Au electrode by the primary deposition of thiolated nitromerocyanine isomer 1b as a monolayer on the electrode, followed by the irradiation of the surface with visible light, lambda > 475 nm. The surface coverage of nitrospiropyran units (1a) on the electrode is 2 x 10-10 mole cm-2. Irradiation of the electrode with UV light, 320 nm < lambda < 360 nm, results in the nitromerocyanine, MR, monolayer on the electrode that binds Ag+ ions to the phenolate units. The Ag+ ions associated with the MR monolayer undergo cyclic reduction to surface-confined Ag0 nanoclusters, and reoxidation and dissolution of the Ag0 nanoclusters to Ag+ ions associated with the monolayer are demonstrated. The electron-transfer rate constants for the reduction of Ag+ to Ag0 and for the dissolution of Ag0 were determined by chronoamperometry and correspond to ketred = 12.7 s-1 and ketox = 10.5 s-1, respectively. The nanoclustering rate was characterized by surface plasmon resonance measurements, and it proceeds on a time scale of 10 min. The size of the Ag0 nanoclusters is in the range of 2 to 20 nm. The electrochemically induced reduction of the MR-Ag+ monolayer to the MR-Ag0 surface and the reoxidation of the MR-Ag0 surface control the hydrophilic-hydrophobic properties of the surface. The advancing contact angle of the MR-Ag0-functionalized surface is 59 degrees , and the contact angle of the MR-Ag+-monolayer-functionalized surface is 74 degrees . Photoisomerization of the Ag0-MR surface to the Ag0-SP state, followed by the oxidation of the Ag0 nanoclusters, results in the dissolution of the Ag+ ions into the electrolyte solution.

Gold catalysts supported on ceria and ceria–alumina for water-gas shift reaction

Gold-supported catalysts on ceria and ceria–alumina with different ratios in the water-gas shift reaction were studied. It was shown that the addition of alumina leads to the deeper oxygen vacancies formation, detected by Raman and TPR measurements. It was established an increase of the ceria dispersion and as well as slightly increase of the gold particle size. In the presence of alumina after catalytic operation the dispersion of gold and ceria practically does not change. The lower WGS activity on gold/ceria–alumina can be explained on the basis of reduction–oxidation cycle of the reaction. It was established no efficiency re-oxidation of the catalyst surface at the temperatures of the reaction test by the water vapour and we consider this as a main reason of the lower catalytic activity.

Methanol Chemistry on Cu and Oxygen-Covered Cu Nanoclusters Supported on TiO2(110)

The thermal decomposition of methanol has been studied on TiO2(110) as well as on Cu and oxygen-covered Cu nanoclusters supported on TiO2(110) using temperature programmed desorption (TPD). The sizes of the Cu clusters were characterized by scanning tunneling microscopy (STM). Methanol chemistry on the vacuum-annealed, reduced TiO2 surface itself produces ethylene as the main desorption product. Reoxidation of the TiO2 surface quenches the production of ethylene but also results in a new formaldehyde desorption peak at 870 K. The reactivity of methanol on small Cu nanoclusters (40.2 ± 7.0 A diameter, 12.7 ± 2.4 A height) is minimal, but trace amounts of formaldehyde, CO2, methane and H2 are detected in TPD experiments, demonstrating that the Cu nanoclusters are more active than bulk single-crystal Cu surfaces. On oxygen-covered Cu nanoclusters, methanol reaction produces formaldehyde and CO2 as the major gaseous products as well as H2, water, and methane. The yields of formaldehyde and CO2 increase as the...

Studies of the mechanism of ammonia oxidation into nitrous oxide over MnBiO/ α-Al 2O 3 catalyst

A complex of kinetic and physicochemical methods: temperature-programmed surface reaction (TPSR), pulsing NH 3 or NH 3/ 16O 2 ( 18O 2) reaction mixture, and infrared and photoelectron spectroscopies, was used for characterization of the highly selective supported manganese–bismuth oxide catalysts and for the study of the mechanism of the ammonia oxidation. Ammonia oxidation was demonstrated to proceed via alternating reduction and reoxidation of the catalyst surface with participation of the lattice oxygen. NH 3 interacts with weakly bonded oxygen species through hydrogen atom abstraction to form adsorbed [N] species, which are localized on Mn 2+ and Mn δ+ (2< δ<3). Manganese ions with different oxidation degrees (Mn 3+ (Mn 4+) and Mn δ+ ) serve as active sites of the catalyst surface. The correlation between the selectivity toward N 2O and the portion of manganese in the Mn 3+ (Mn 4+) state was established. Bismuth oxide plays an important role by increasing the quantity, mobility, and thermal stability of the subsurface oxygen. The reaction kinetic scheme is suggested based on the experimental results. Numerical simulation of TPSR data confirms the reliability of the proposed reaction mechanism.