Initial Stage Of Oxygen Adsorption Research Articles

Initial stages of oxygen adsorption onIn/Si(111)-4×1

The In/Si(111)4$\ifmmode\times\else\texttimes\fi{}1$ surface with In wires is a prototypical one-dimensional electron system showing a temperature-induced structural phase transition. While most impurities are known to decrease the transition temperature $({T}_{c})$, oxygen was reported to increase the ${T}_{c}$. In order to understand the exceptional influence of oxygen, we have examined the initial stages of oxygen adsorption on the surface by scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. Oxygen molecules were found to dissociate and adsorb as atoms on the surface. STM revealed three distinct O adsorption features as well as occasional transformations from one to another. The high-resolution images showed that all three adsorbed O features were located at the center of the In trimer at both edges of the In wire. This registry is in contradiction with the previous theoretical predictions [S. Wippermann et al., Phys. Rev. Lett. 100, 106802 (2008)]. The present DFT calculations identified two of the features as O atoms incorporated in the interstitial region between the In wire and subsurface Si layer.

Thermodynamics of initial stage of oxygen adsorption on surface of amorphous aluminum-cerium alloy

An equation is proposed to connect the characteristics of the energy and the surface level population on pure and oxidized metal surface with the value of heat of the physical absorption of oxygen. The electron structure is studied of pure and oxidized surface of the amorphous Al-8 at. % Ce alloy by analyzing the results of modeling the optical conductivity obtained from the optical experiment data. The conductivity band of the alloy is shown to be split. The heat value is calculated from the oxygen absorption on the amorphous alloy surface.

Bias-dependent scanning tunneling microscopy study of the oxygen-adsorbed Si(111)-(7×7) surface: Observation of metastable molecular oxygen

We have observed the initial stage of oxygen adsorption on a Si(111)-(7×7) surface using scanning tunneling microscopy. Among the bright sites observed after exposing the surface to oxygen in occup ...

Oxygen adsorption and reaction on alpha-silicon

It is important to understand the initial stages of oxygen adsorption and reaction on alpha-silicon because of its importance in the development of nanometre silicon-based microelectronic devices. This was achieved effectively using precise background corrections applied to Si 2p x-ray photoelectron spectroscopy (XPS) measurements based on effective energy-loss functions derived from reflection election energy-loss spectroscopy analysis. It was also essential to determine O 1s XPS source functions based on the optical bulk energy-loss function. Significant correlations between the chemical shifts of the Si 2p and O 1s XPS source functions with oxygen exposure using this approach were observed. It enabled us to develop a model for oxygen adsorption and reaction at oxygen exposures of 0–1000 L at room temperature from detailed analysis of the changes in the fine spectral features revealed by this analytical approach. Features of the application of this procedure are illustrated in this paper and its application to developing a coherent understanding of the initial stages of oxygen adsorption and reaction on alpha-Si at room temperature is reported. Copyright © 1999 John Wiley & Sons, Ltd.

X-ray photoelectron spectroscopy investigation of the initial oxygen adsorption sites on the LaB6(100) surface

X-ray photoelectron spectroscopy was used to investigate the initial stages of oxygen adsorption on the (100) surface of a single crystal of lanthanum hexaboride. Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxygen adsorption markedly alters the La3d lineshapes, whereas the B1s peak is unaffected. On the clean surface the La3d3/2 peak is split into two components at binding energies of 854.7 and 851.8eV, a splitting that is typical of rare-earth metals and their compounds. The two components are associated with two different final states. In one final state the 3d core hole is poorly screened (854.7eV) and in the other it is well-screened (851.8eV). The relative intensity of the two components is known to be very sensitive to the chemical environment of the rare earth atom and a 10L O2 exposure at room temperature produces a large increase in the relative intensity of the well-screened component. Annealing the surface to 600°C and then to 700°C produces sharp c(2×2) and p(2×1) LEED patterns respectively. The La3d peaks associated with the two LEED patterns are similar to those observed after the initial 300K 10L O2 exposure, indicating oxygen bonding to La in both overlayer structures. Thus while the XPS data clearly reveal oxygen adsorption at La sites, there is no indication of adsorption at boron sites for low O2 exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides.

A metastable de-excitation spectroscopy (MDS) study on oxygen adsorption on a polycrystalline zirconium surface

The adsorption of oxygen on a polycrystalline zirconium surface at room temperature has been studied by metastable de-excitation spectroscopy (MDS) in conjunction with UPS and AES. From the analysis of the measured spectra, we have shown the following. (1) At the initial stage of oxygen adsorption (exposure <1.2 L), the surface density of states (SDOS) of zirconium changes little at around the Fermi level ( E F), while it decreases appreciably at 1–2 eV below E F ( E B=1–2 eV) by oxygen adsorption. (2) The SDOS at E B=0–2 eV decreases with increasing oxygen exposure at >1.2 L and disappears at >8 L. (3) The oxygen 2p states ( E B=5–8 eV) are localized at the subsurface region at oxygen exposure 0–2 L. (4) The ZrO 2 phase appears at the outermost zirconium surface at around 2 L, then grows with increasing exposure, and finally dominates at >8 L. It is suggested that two different phases (ZrO 2 phase and that in which oxygen occupies subsurface sites) coexist at the outermost surface at 2–8 L.

AN STM STUDY OF THE INITIAL STAGES OF OXIDATION OF Ni3Al(110)

The initial stages of oxygen adsorption on Ni 3 Al (110) at 9700 K have been studied by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). We report the observation of two different reconstructed surface phases. First, on the well-ordered surface, after exposure to O 2 at 970 K for doses &lt;5L, a saturation coverage (1/4 ML) of Ni 3 Al {110} c(2×2)- O is observed. Secondly, we report an oxygen-induced reconstruction which is incommensurate with the substrate, resulting after an incomplete annealing of the suface following Ar + bombardment. A structure model for this phase is proposed.

Angle-resolved UPS study of the oxygen-induced 2 × 1 surface of Cu(110)

The two-dimensional oxygen-induced energy bands of the Cu(110)/2 × 1−O surface were investigated by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). A new finding in the present report is that the oxygen-derived O 2p y and O 2p z bands cross and hybridize between λ and Ȳ along the [001] direction. A tight-binding band calculation assuming the missing-row reconstruction model was successfully made to reproduce the dispersion of the oxygen-derived bands, including band crossing as well as symmetry characteristics. An ARUPS study on a surface with a very small amount of adsorbed oxygen of about 0.01 ML was also made. An interesting result is that almost the same dispersion as that of the well-ordered 2 × 1−O surface of 0.5 ML is observed. This result indicates that CuO chains directed along the [001] direction grow long enough to apparently give the same dispersion, even in the initial stage of oxygen adsorption.

Initial Stage of Oxygen Adsorption onto a Si(111)–7×7 Surface Studied by Scanning Tunneling Microscopy

The initial stage of oxygen adsorption onto a Si(111)-7×7 surface has been studied at room temperature by scanning tunneling microscopy. The surface was exposed to oxygen under a partial pressure of 1×10-9 Torr. Initial exposure led to bright and dark features at adatom sites. This dark feature, which was previously reported as a main channel for oxidation, only appeared in the faulted half of the dimer adatom stacking-fault (DAS) structure. The images suggest that the adsorption onto the un-faulted half starts after the faulted half are occupied with oxygen atoms. This clearly indicates that oxygen adsorption occurs at adatom sites with higher energy states.

Studies of surface composition, structure and oxygen adsorption of Ni3Al(110) by Li+ and He+ ion scattering

The surface composition, structure and initial stages of oxygen adsorption of Ni3Al(110) have been investigated by ISS. The surface composition of the Ni3Al(110) surface was determined by Li+ and He+ ion scattering using selected scattering geometries. These results consistently show a surface composition of 50% Ni-50% Al in the outermost layer with some small percentage of Ni enhancement (perhaps 1–2%), while the second layer is basically Ni. The surface structure has been probed by measuring the intensity of Li+ ion scattering as a function of incident angle along the [001], [112], [111], [332] and [110] azimuthal directions. The interpretation of experimental data is based on the calibrated shadow cone and chain model simulations. The results show that there is no displacement of first-layer Al relative to first-layer Ni, but there is a contraction in the first-to-second layer spacing of 0.11 ± 0.08 A. The adsorption of oxygen at low exposure (∼8L) and pressures in the range 2 × 10−8−1 × 10−6 mbar at a temperature of 700°C shows the disappearance of the Ni ISS signal, and the results suggest the formation of a monolayer of an AlOx overlayer. Using a model similar to that previously studied for the formation of AlOx islands on the Ni3Al(001) surface, this oxide overlayer has been shown to be AlOx islands with no influence on remaining areas on the crystal. The mechanism for apparent interchange of the Al atoms to the surface to replace the Ni atoms is also discussed.

Initial stages of oxygen adsorption on Si(111). II. The molecular precursor.

In the initial stage of oxygen adsorption on Si(111) surfaces, a metastable molecular state is found which precedes dissociation and insertion of oxygen atoms in bridging positions between first- and second-layer silicon atoms. This precursor can be observed in conventional photoemission experiments at low coverages and---for optimum retention---at low substrate temperatures. Combining the results from several high-resolution electron spectroscopies such as x-ray photoemission (XPS), polarization- and photon-energy-dependent uv photoemission, x-ray-induced Auger spectroscopy, and near-edge x-ray absorption, we are able to characterize the precursor as a negatively charged molecular species chemisorbed in a bridge configuration between dangling bonds of the surface. Its decay and conversion into the stable dissociated state could be monitored by XPS up to room temperature, showing a thermally or electronically activated behavior. For the reaction of ${\mathrm{O}}_{2}$ with Si(111), a two-stage adsorption model is proposed. It explains a number of observations and features of oxygen adsorption on differently prepared Si(111) surfaces. The present model may be applicable to other semiconductor surfaces as well.

Adsorption of molecular oxygen on Si(111)

Abstract It has been found previously that the initial stages of oxygen adsorption on Si(lll) occur via a metastable molecular precursor which decays to dissociated oxygen in a SiO.Si bridge configuration. In the present study using synchrotron radiation, surface techniques such as UPS, XAES, and NEXAFS were employed for a detailed characterization of this intermediate state which is stabilized at LN2 temperature and at low coverages. The UPS data showing a pronounced σ-shape resonance and the high resolution O KVV Auger data corroborate our earlier suggestion of a molecular species and exclude dissociative oxygen, e.g. in an on-top position, for the precursor. The polarization dependent NEXAFS data show π- and σ-resonances and unambiguously prove the presence of an oxygen molecule with its molecular axis parallel to the surface. From the energy position of the a-shape resonance we derive an increase of the O-O bond length as compared to isolated O2.

Adsorption of molecular oxygen on Si(111)

It has been found previously that the initial stages of oxygen adsorption on Si(lll) occur via a metastable molecular precursor which decays to dissociated oxygen in a SiO.Si bridge configuration. In the present study using synchrotron radiation, surface techniques such as UPS, XAES, and NEXAFS were employed for a detailed characterization of this intermediate state which is stabilized at LN 2 temperature and at low coverages. The UPS data showing a pronounced σ-shape resonance and the high resolution O KVV Auger data corroborate our earlier suggestion of a molecular species and exclude dissociative oxygen, e.g. in an on-top position, for the precursor. The polarization dependent NEXAFS data show π- and σ-resonances and unambiguously prove the presence of an oxygen molecule with its molecular axis parallel to the surface. From the energy position of the a-shape resonance we derive an increase of the O-O bond length as compared to isolated O 2.

Initial stages of oxygen adsorption on Si(111): The stable state.

We present new experimental results from a comprehensive study of the chemisorption of oxygen on differently prepared Si(111) surfaces, using low-energy electron diffraction and high-resolution electron spectroscopies such as x-ray photoemission spectroscopy, x-ray-initiated Auger-electron spectroscopy, and polarization-dependent ultraviolet photoemission spectroscopy. In the monolayer regime two different states of oxygen have been observed: a molecular precursor and a stable dissociated configuration. The latter is the subject of this paper. Its electronic structure and local symmetry are compatible only with a bridge-bonded atom between Si atoms in the first and second layer, respectively, with relatively little influence on the dangling bonds. The presence of a second dissociated oxygen adsorbate, e.g., a diatomiclike monoxide, is excluded by our data. The coverage dependent O 1s and Si 2p data indicate that the observed satellite structures in the submonolayer regime are due to excitations rather than to chemically shifted peaks from atoms in different bonding situations.

Oxygen adsorption on Cu 3Au(100) above 350 K

The initial stage of oxygen adsorption on Cu 3Au(100) above 350 K has been investigated using Auger electron spectroscopy. In the temperature range 350–550 K the adsorption isotherms tend to saturate at levels which increase with temperature. Initial sticking coefficients in this temperature range are at least an order of magnitude less than those on Cu(100) and saturation levels vary from ≲ 5% of a monolayer at 350 K to ≲ 15% of a monolayer at 550 K. As the temperature is increased to within 100 K of the bulk order-disorder transformation, the rate of adsorption increases dramatically accompanied by apparent adsorption-induced rearrangement of the alloy surface. The degree of annealing following Ar + bombardment is observed to affect the initial uptake of oxygen throughout the entire temperature range.

Oxygen adsorption on Cu 3Au(100) above 350 K

The initial stage of oxygen adsorption on Cu 3Au(100) above 350 K has been investigated using Auger electron spectroscopy. In the temperature range 350–550 K the adsorption isotherms tend to saturate at levels which increase with temperature. Initial sticking coefficients in this temperature range are at least an order of magnitude less than those on Cu(100) and saturation levels vary from ⪅5% of a monolayer at 350 K to ⪅15% of a monolayer at 550 K. As the temperature is increased to within 100 K of the bulk order-disorder transformation, the rate of adsorption increases dramatically accompanied by apparent adsorption-induced rearrangement of the alloy surface. The degree of annealing following Ar + bombardment is observed to affect the initial uptake of oxygen throughout the entire temperature range.