Initial interaction of oxygen with aluminium single crystal faces: A LEED,AES and work function study

Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function (Kelvin probe) measurements have been used to study the initial interaction of clean Al(111), (100) and (110) surfaces with oxygen at room temperature. The oxidation process was found to be surface orientation dependent, but a common feature has been always observed on the three low-index surfaces: they show two distinct phases, i.e. a chemisorbed phase followed then by an oxidized phase. From analysis of AES, LEED and Kelvin probe results, an adsorption mechanism of O on Al for each surface orientation is proposed.     

Dynamic scaling and optical properties of Zn（S,O, OH） thin film grown by chemical bath deposition

The scaling behavior and optical properties of Zn(S, O and OH) thin films deposited on soda-lime glass substrates by chemical bath deposition method were studied by combined roughness measurements, scanning electron microscopy and optical properties measurement. From the scaling behaviour, the value of growth scaling exponent β , 0.38±0.06, was determined. This value indicated that the Zn(S, O, OH) film growth in the heterogeneous process was influenced by the surface diffusion and shadowing effect. Results of the optical properties measurements disclosed that the transmittance of the film was in the region of 70%-88% and the optical properties of the film grown for 40 min were better than those grown under other conditions. The energy band gap of the film deposited with 40 min was around 3.63 eV.     

V(100)表面上S和O偏析的LEED—AES研究

用AES,LEED等表面分析手段,对V(100)表面上杂质S,O偏析作了详细研究。明确了S,O偏析关系;发现(8×1)-O,(4×1)-O两个新的表面结构;系统观察和分析了V(100)表面在不同S,O偏析量情况下的各种表面超结构,并获得这些表面超结构的相互关系。 关键词：     

KrF-excimer-laser-induced native oxide removal from Si (100) surfaces studied by Auger electron spectroscopy

The mechanism of KrF-excimer-laser cleaning of Si(100) surfaces was studied by Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) spectroscopy. The dependence of the cleaning efficiency on the laser fluence was investigated by using a mildly focused laser beam and carefully measuring the energy density distribution of the laser spot impinging on the sample. These values were compared with the AES spectra measured in different points of the irradiated area and with the morphology observed by optical microscopy. Samples as received from the manufacturer were first investigated. It was found that desorption of weakly bonded organic adsorbates occurs at energy densities as low as 0.3 J/cm², whereas significant oxide removal takes place only at an energy density above 0.8 J/cm², which produces damaged surface morphologies. The experimental findings, in agreement with the temperatures calculated for the laser-induced Si heating, indicated that a large fraction of the oxide film is dissolved in the molten silicon, leading to oxygen concentration below the AES detection limit only when the melted depth was of the order of several hundred nanometers. Atomically clean, damage-free Si(100) surfaces were obtained after irradiation of samples pre-etched for 1 min in a HF: H₂O (5%) solution, which had only a thin SiO _x (x < 2) layer and F, C and O containing adsorbed species. Complete contaminant elimination was achieved in this case with 15 pulses at 0.8 J/cm² without any damaging of the surface.     

Oxygen adsorption on (110) silver

The adsorption of oxygen on a (110)Ag surface is investigated by means of Auger electron spectroscopy, LEED and low energy helium ion scattering (IS). With LEED two ordered structures, i.e. (3×1) and (2×1) were observed at oxygen exposures of 1700 L and 7000 L respectively. The oxygen signal observed by AES and IS increases monotonically with oxygen exposure. The signals can be related to absolute coverage by comparison with Δφ measurements and by the use of the LEED data. With this calibration and with theoretical scattering cross-sections the IS measurements allow the position of the adsorbed oxygen to be estimated. The observation of a strong azimuthal anisotropy of the IS signal, e.g. a large oxygen signal if the plane of scattering is parallel to the [110] direction and a relatively small oxygen signal in the [100] direction, leads to the conclusion that the oxygen is adsorbed in a bridge position between two Ag atoms of the [110] surface channels, its centre being slightly below the centres of the Ag atoms.     

Relationships between electron inelastic mean free paths, effective attenuation lengths, and mean escape depths

The terms inelastic mean free path (IMFP), effective attenuation length (EAL), and mean escape depth (MED) are frequently used to specify the surface sensitivity of Auger-electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). These terms are different conceptually because of the effects of elastic-electron scattering, and generally have different numerical values for a specified material and electron energy. In addition, values of the EAL and MED depend on the instrumental configuration. We give an historical overview of efforts to measure EALs by the overlayer method and of work to investigate elastic-scattering effects in AES and XPS. We then apply an analytical formalism developed from a solution of the kinetic Boltzmann equation within the transport approximation to demonstrate the relationships between the IMFP, EAL, and MED for selected elemental solids and for common measurement conditions. Examples are given to show the magnitude of elastic-scattering effects on MED values for angle-resolved XPS and AES. If XPS or AES data are acquired for emission angles between zero and 60°, the ratio of the MED to that found with elastic scattering neglected is approximately constant (to within 10%), and this ratio can be used to determine an average value for the EAL. This EAL value can then be used to establish the depth scale in the data analysis. Finally, we show ratios of the EAL to the IMFP for XPS from the Au 4s subshell with Mg K X-rays as a function of emission angle and depth; this ratio has a weak dependence on emission angle from zero to 40° but a more pronounced dependence for larger emission angles.     

On Auger electron spectroscopic measurements of ZnCl2 —coal interaction

By combining electron stimulated desorption (ESD) with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and work function change (Δφ) measurements the information content of ESD with regard to surface structure and composition is examined, using the surface systems O/W(100) and O/W(110). Although it is not possible to separate the local interaction from the ion escape phase, the comparison of the ESD results with Information derived from LEED, AES and Δφ and the use of simple models of the local interaction gives a rather detailed picture of the location and environment of adsorbed atoms which provides a reasonably reliable basis for the interpretation of UPS spectra of adsorption layers.ESD is extremely sensitive to adsorbed layers. The fact that the ion signal depends not only on coverage but also on the structure and structure-dependent properties of the adsorbate makes on the one hand coverage determination difficult if not impossible, on the other hand opens the door to structure analysis. The potential for obtaining structure information can be easily assessed by comparison with electron probe results.In comparison with other ion probes such as ion scattering spectroscopy and secondary ion mass spectroscopy, ESD is at present the most promising ion probe method for obtaining information on the location of adsorbed atoms from angular and energy distribution measurement (ESDIAD and ESDIED). This is clearly seen by the comparison with the structural data derived from LEED, AES and Δφ measurements for the complex system O/W(100). The consistency of the data obtained with ESD and electron probe techniques lends strong support to the simple models on which the analysis of the ESD results from chemisorbed layers are based. The comparison of ESD results from the system O/W(100) at high coverage and from O/W(110) with 0⁺ ion emission from oxides shows, however, that caution is in place when assigning ESD features to atoms chemisorbed on the metal surface. Without a careful analysis of the ion energy, threshold and/or cross-section such ions cannot be distinguished from ions produced by dissociation of oxides which may be present on the surface only in small quantity. These ions usually are not related to the chemisorbed species which covers most of the surface and therefore dominates the signals seen with (nearly) all other surface probes.If the consistency of LEED, AES, Δφ and ESD data for O/W(100) is not fortuitous, then ESD has already given some important feed-back to the electron probe techniques: the structural models derived from vibrational ELS spectra have to be revised. Increasing accumulation of experimental data and deepening of the theoretical understanding of the physical processes involved in ion emission will have to show how much further information complementary to that from electron probes can be obtained from ion probes.     

Estimation of backscattering factor for low atomic number elements and their alloys

The single elastic scattering theory of Everhart was adapted to determine the angular and energy distribution of electrons backscattered from low atomic number solid materials (Z < 40–45). This distribution and the classical ionization cross-section expression of Gryziński were used in the calculations of the backscattering factor, r, in quantitative AES analysis. The values of r were found to be in reasonable agreement with the results of the Monte Carlo calculations and the existing experimental data. Everhart's theory was extended for the case of mixtures, and that made possible the determination of the backscattering factor for binary alloys. It was found that neglecting the concentration dependence of r in quantitative AES analysis apparently enriches the surface with the component having the lower atomic number. The experimental data on surface composition of binary alloys measured by AES are discussed in view of the presented theory.     

On the stability of Caesium-covered ZnO surfaces

The characterisation of the Cs/ZnO system using LEED and AES, results in the observation of new superstructures on the (000$\text{1}$)—O polar surface, a (2 × 2) structure and a combined (2 × 2) and (√3 × √3) configuration. A possible explanation is proposed for the existence of these superstructures in terms of electrostatic energy calculations.     

Bulk dissolved Si as a cause of the “oxide” formation on Pt(111) surfaces

The influence of silicon atoms dissolved in the bulk of a Pt(111) crystal on the high temperature “oxidation” of Pt was studied by Auger spectrometry (AES) and low energy ion scattering (IS). The “oxidation” procedure was applied on clean and Si-prepared crystals. The preparation was performed by depositing a Si layer on the Pt(111) surface and thermally diffusing it into the bulk beyond the AES detectability. The results show that even deeply diffused silicon plays a decisive part in the “oxidation” process. This seems to explain the observations of other authors concerning the unexpected high stability of the “oxide” and the poor reproducibility of its rate of formation as well as of its saturation density. There is evidence that the “oxide” is primarily a silicon-oxygen bond.     

Hydrocarbon interaction with clean and oxidised Cu(111)

The interaction of methane, propane, ethene and propene with clean and oxidised Cu(111) has been studied in the temperature range 300–750 K and at pressures up to 0.1 Pa with ellipsometry, AES and LEED. Methane and propane showed no measurable interaction under our conditions. Propene exposure on Cu(111) at T > 475 K resulted in a slow carbon deposition of the graphitic type. Propene was able to remove all oxygen from Cu(111)/O at T 625 K. The fastest part of the reduction has a first order dependence on the propene pressure; the apparent activation energy is 44.9 ± 5.2 kJ mol⁻¹. Though ethene is less reactive than propene it is also able to reduce an oxidised copper surface at T = 725 K.     

Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H₂O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a₁ ← O 1s resonant transition. The AES at the 4a₁ ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H₂O, and the normal-AES of the bulk H₂O, where the 4a₁ electron is delocalized before Auger transitions. H⁺ is found to be the only ion species in AEPICO spectra measured at the 4a₁ ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a₁ ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a₁ ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H₂O, which is extracted from the AES at the 4a₁ ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a₁ ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H⁺ desorption at the 4a₁ ← O 1s resonance. The enhancement of the H⁺ yield is ascribed to the O---H anti-bonding character of the 4a₁ orbital.     

High dose implantations of antimony for buried layer applicationsrrr

A computer simulation of low energy implantation is presented. The method of calculation takes into account the inelastic processes and vacancies in the crystal lattice.

The described simulation was used for a nitrogen implanted molybdenum single crystal. From the result of the calculation the possible positions of implanted nitrogen on the surface and in the bulk were obtained. The results of simulations are compared with those measured by SIMS and AES.     

Comparison of escape depths between Auger electron and disappearance potential spectroscopy electron

The escape depths of the characteristic electrons of the Auger electron and the quasielastically reflected electron were determined by Auger electron spectroscopy (AES) and disappearance potential spectroscopy (DAPS), respectively, for a Cr overlayer onto Ti and Fe substrates. For the case of Cr on Fe, in-situ measurements of AES and DAPS were carried out. From the results, the mean free paths of 455, 575 and 710 eV electrons through Cr were obtained as 9.6, 13 and 15 Å, respectively. The attenuation length of a 2.5 keV primary electron of AES through Cr was also obtained and the value was 62 Å. In addition, the mean free paths of electrons with the same energy depend on the scattering materials of Cr, Mo and W (material dependence). The phenomena are useful for a quantitative electron spectroscopy of surfaces.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Deposition of monolayer and bulk lead on Ag(111) studied in vacuum and in an electrochemical cell

The early stages of Pb condensation on Ag(111) have been studied in ultrahigh vacuum and in an electrochemical cell. The vacuum deposited films were investigated by AES, LEED and work function measurements, and the electrolytically deposited films in situ by cyclic voltammetry, potential step experiments and reflectance spectroscopy. Despite the very different environments a similar growth behaviour is observed. In both cases deposition starts with the formation of a uniform monolayer on Ag(111), which occurs in three steps: random adsorption at very low coverages followed by a √3 × √3 superstructure and finally, a hexagonal close packed layer. From an analysis of the AES data for the vacuum deposited film a particular case of Stranski-Krastanov type is found for the subsequent growth. The formation of ordered submonolayer structures which is clearly detectable by LEED, is inferred for the electrolytically formed layer since a similar coverage dependence in the reflectance is observed. From the underpotential shift of the electrolytically formed monolayer an excess binding energy of about 0.3 eV is deduced for the Pb adatoms on Ag(111).     

The bonding state of carbon segregated to α-iron surfaces and on iron carbide surfaces studied by electron spectroscopy

Segregated carbon on the Fe(100) surface has been studied by means of X-rayand ultraviolet photoelectron (XPS, UPS), Auger electron (AES) and electron energy loss spectroscopy (ELS). For comparison, the surfaces of polycrystalline graphite and of iron carbides stabilized by chromium or manganese additions have been investigated. On the iron surface, carbon exists as a chemisorbed state or graphitic multilayer. The two states exhibit different energy positions in XPS, and are different in energy positions and lineshapes in AES and ELS. During the transition of graphitic carbon to chemisorbed carbon on Fe(100) a novel coverage-dependent Auger feature is reported. The spectra of graphitic carbon on the iron surface always coincide with those of solid graphite. The carbon Auger transitions of chemisorbed carbon and of iron carbides exhibit very similar lineshapes, but the energy positions of both states differ in AES as well as XPS.     

J/psi production and nuclear effects for d + Au and p + p collisions at square root of S(NN) = 200 GeV

J/psi production in d + Au and p + p collisions at square root of S(NN) = 200 GeV has been measured by the PHENIX experiment at rapidities -2.2 < y < +2.4. The cross sections and nuclear dependence of J/psi production versus rapidity, transverse momentum, and centrality are obtained and compared to lower energy p + A results and to theoretical models. The observed nuclear dependence in d + Au collisions is found to be modest, suggesting that the absorption in the final state is weak and the shadowing of the gluon distributions is small and consistent with Dokshitzer-Gribov-Lipatov-Altarelli-Parisi-based parametrizations that fit deep-inelastic scattering and Drell-Yan data at lower energies.     

Electron spectroscopic studies of the adsorption of methanol on zinc oxide powder

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Auger electron spectroscopy (AES) have been used to study the adsorption of methanol on ZnO powder. The He(II) spectrum of electron-beam-eleaned ZnO was similar to that reported by Rubloff .¹ for the ZnO (11&#x0304; 0 0) surface. The spectrum of methanol condensed at ?150°C matched that of gas-phase methanol. At low methanol coverages, difference spectra showed no increase in the methanol O(2p) lone pair binding energy reported to be indicative of chemisorption. A residual carbon and oxygen surface component on the ZnO powder may have prevented chemisorption. XPS measurements were found to be more useful in observing this layer than was AES. A decrease in the ZnO valence band intensity near the valence band maximum indicated that a strong interaction with the substrate had occurred.In this system the valence bands of the adsorbate and substrate overlap, and the true position of the methanol O(2p) lone pair orbital is obscured by the effects of the substrate band modification. Also, for insulators or semiconductors, a change in the measured position of the valence band maximum relative to the Fermi level may occur upon adsorption. Since the Zn(3d) core level should not be affected, it was used as an energy reference for the present work. The problems of energy referencing in UPS studies of adsorption on semiconductors are discussed.     

On the fabrication and characterization of graded slanted chiral nano-sculptured silver thin films

Graded slanted chiral sculptured silver thin films are produced using oblique angle deposition together with rotation of substrate holder about its surface normal, plus a shadowing block, fixed at the center of the substrate holder. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used for characterization of these films. The results showed a structural gradient with distance from the edge of the shadowing block, which in turn is responsible for the decrease in the volume of void fraction and increase of grain size. Plasmon absorption peaks observed in the optical analysis of these nano-structures showed that their wavelength region and intensity depend on the polarization and the incident angle of light, as well as the distance from the edge of the shadowing block. Photoluminescence (PL) study of the samples showed that there is an inverse size dependence of the PL spectra intensity.