The interference of an electron beam with the surface reaction between oxygen and germanium

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10^?7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm^?2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10^?7 Torr min, the maximum oxide thickness was about 18 Å. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 Å), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.     

Effect of electron irradiation on the oxidation of indium

The oxidation kinetics of indium is studied for two cases, namely, during continuous electron irradiation (E _p = 1800 eV) and without electron irradiation, as a function of the time of exposure to an oxygen medium at a partial oxygen pressure of 10⁻⁴ Pa and room temperature. The initial oxygen exposure was 50 L. The kinetic curves recorded upon continuous electron irradiation have two inflection points, and they can be attributed to the following three states of oxidation: physical adsorption, chemisorption with the formation of a nonstoichiometric oxide layer, and the growth of a homogeneous oxide layer. Only the first inflection point is observed during oxidation without electron irradiation, and further exposure does not lead to the second inflection point within the experimental time.     

Studies of clean and oxidized tellurium surfaces

Clean and oxidized surfaces of tellurium films have been studied using electron-excited Auger electron spectroscopy, X-ray photoelectron spectroscopy, energy loss spectroscopy, and electron-stimulated desorption. The results for clean tellurium are in general agreement with previous studies, but the oxidation studies have provided new information. Reaction between oxygen gas and tellurium was found to be an activated process requiring tellurium temperatures in excess of 60°C to produce detectable oxide for 30 min exposures to ~800 Torr of oxygen. Increasing the temperature to 200°C produced a 10.6 A layer of TeO₂. This layer was rapidly removed by electron irradiation with a cross-section of ~3 × 10^?18 cm² for electron-stimulated desorption of oxygen by 2 keV electrons; however, there was evidence for reduced cross sections for thinner TeO₂ layers.     

Quantitative Auger electron spectroscopy analysis with co-evaporated AuCu films

A method for the quantitative Auger electron spectroscopy (AES) analysis by using a co-evaporation technique is extended to the AuCu system following the previous work. The calibration curves for lower Auger energy have peaks at 60 eV for Cu and at 69 eV for Au, and for higher Auger energy peaks at 239 eV for Au and at 920 eV for Cu. It is found that a simple linear relation does not exist in the results for AES measurements and the bulk analysis by atomic absorption spectroscopy (AAS) because of the back-scattering effect and the overlap of the spectra at lower energies in the Au-Cu system. It is also found that the adsorption of oxygen caused by electron beam bombardment has a significant influence on the AES results. The calibration curves obtained after a correction for oxygen adsorption are successfully applied to the determination of the composition at the surface of a sputtered AuCu alloy.     

Enhancement of oxygen adsorption on silicon following electron irradiation

While electron-stimulated adsorption of oxygen on silicon during electron irradiation is well known, we here report enhanced adsorption of oxygen on both crystalline and hydrogenated amorphous silicon following electron irradiation. The adsorption enhancement is proportional to the energy dissipation at the irradiated surface, and thus is inversely related to the beam energy. Electronic excitation processes are thought to cause bonding changes and thus enhanced adsorption.     

An aes study of a copper-iron sulphide mineral

Synthetic bornite, Cu₅FeS⁴ has been studied by Auger electron spectroscopy. Sputtercleaned bornite shows a sulphur spectrum with three peaks at 138, 147 and 149 eV. These Auger transitions are different from those observed when sulphur is adsorbed on metal surfaces, where the peaks are at 139, 149 and 154 eV. The adsorption of oxygen on the surface of bornite at room temperature results in the formation of a layer of iron oxide and, in addition, the sulphur spectrum loses its fine structure and shows only a single peak at 148 eV. Under the influence of both the ion sputter beam and the electron beam, the surface composition of bornite shows large and rapid changes which are due mainly to movement of mobile Cu⁺ ions through the lattice, this movement being caused by surface charging effects.     

Electron-stimulated desorption of sodium atoms from oxidized molybdenum

The time-of-flight technique combined with a surface-ionization-based detector has been used to investigate the yield and energy distribution of sodium atoms escaping in electron-stimulated desorption (ESD) from adlayers on the surface of molybdenum oxidized to various degrees and maintained at T=300 K as functions of incident electron energy and surface coverage by sodium. The sodium-atom ESD threshold is about 25 eV, irrespective of sodium coverage and extent of molybdenum oxidation. Molybdenum covered by an oxygen monolayer exhibits secondary thresholds at ∼40 eV and ∼70 eV, as well as low-energy tailing of the energy distributions, its extent increasing with surface coverage by sodium Θ. The most probable kinetic energies of sodium atoms are about 0.23 eV, irrespective of the degree of molybdenum oxidation and incident electron energy at Θ=0.125, and decrease to 0.17 eV as the coverage grows to Θ=0.75. The results obtained are interpreted within a model of Augerstimulated desorption, in which adsorbed sodium ions are neutralized by Auger electrons appearing as the core holes in the 2sO, 4sMo, and 4pMo levels are filled. It has been found that the appearance of secondary thresholds in ESD of neutrals, as well as the extent of their energy distributions, depend on surface coverage by the adsorbate. Fiz. Tverd. Tela (St. Petersburg) 40, 768–772 (April 1998)     

The adsorption of oxygen on gold

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in 10^?5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacuo, has been shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen adsorption was detected on a clean sample for oxygen pressures up to 10^?4 Torr and sample temperatures between 300–600 K. Chemisorption of oxygen atoms could be induced by placing a hot platinum filament close to the sample during exposure to oxygen. The activation energy for desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163 kJ mol^?1. The chemisorbed oxygen atoms and the oxygen associated with silicon were distinguished by different O(1s) binding energies (529.2 and 532.3 eV respectively) and by different O(KVV) Auger fine structure.     

Electron-stimulated desorption of lithium atoms from oxidized molybdenum surface

The yield and energy distributions of lithium atoms upon electron-stimulated desorption from lithium layers adsorbed on the molybdenum surface coated with an oxygen monolayer have been measured as functions of the impact electron energy and lithium coverage. The measurements are performed using the time-of-flight technique and a surface ionization detector. The threshold of the electron-stimulated desorption of lithium atoms is equal to 25 eV, which is close to the ionization energy of the O 2s level. Above a threshold of 25 eV, the yield of lithium atoms linearly increases with an increase in the lithium coverage. In the coverage range from 0 to 0.45, an additional threshold is observed at an energy of 55 eV. This threshold can be associated with the ionization energy of the Li 1s level. At the electron energies above a threshold of 55 eV, as the coverage increases, the yield of lithium atoms passes through a maximum at a coverage of about 0.1. Additional thresholds for the electron-stimulated desorption of the lithium atoms are observed at electron energies of 40 and 70 eV for the coverages larger than 0.6 and 0.75, respectively. These thresholds correlate with the ionization energies of the Mo 4s and Mo 4p levels. Relatively broad peaks in the range of these thresholds indicate the resonance excitation of the bond and can be explained by the excitation of electrons toward the band of free states above the Fermi level. The mean kinetic energy of the lithium atoms is equal to several tenths of an electronvolt. At electron energies less than 55 eV, the energy distributions of lithium atoms involve one peak with a maximum at about 0.18 eV. For the lithium coverages less than 0.45 and electron energies higher than 55 eV, the second peak with a maximum at 0.25 eV appears in the energy distributions of the lithium atoms. The results obtained can be interpreted in the framework of the Auger-stimulated desorption model, in which the adsorbed lithium ions are neutralized after filling holes inside inner shells of the substrate and lithium atoms.     

Electron-stimulated desorption of potassium and cesium atoms from an oxidized molybdenum surface

The yield and energy distributions of potassium and cesium atoms emitted in electron-stimulated desorption (ESD) from a molybdenum surface, oxidized to different extent and maintained at 300 K, have been measured by the time-of-flight technique with a surface ionization detector. The ESD threshold for potassium and cesium atoms lies around 25 eV, irrespective of molybdenum oxidation state. In the case of molybdenum coated by an oxygen monolayer, secondary thresholds at ∼40 and ∼70 eV have been observed, as well as atomic energy distribution tailing down to very low energies. The most probable kinetic energies of the atoms are a few tenths of one eV. The results are explained within a model involving Auger neutralization of the adsorbed alkali metal ions after the filling of the 2s O, 4s Mo, and 4p Mo core holes. The possibility of ESD of a neutral species as a result of oxide-cation core-level ionization has been demonstrated for the first time. Fiz. Tverd. Tela (St. Petersburg) 39, 758–761 (April 1997)     

Spectroscopic study of plasmons in ion-irradiated single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes was experimentally investigated using x-ray photoelectron spectroscopy, reflection electron energy-loss spectroscopy, and Auger electron spectroscopy. A shake-up satellite structure observed near the C 1s core-level lines in the x-ray photoelectron spectra at high binding energies in the range 284–330 eV due to excitation of π and π + σ plasmons was studied. The effect of irradiation by 1-keV argon ions on the shape of the spectra was analyzed. The shape of the C 1s satellite spectra was found to be sensitive to Ar⁺ irradiation in the electron energy loss range 10–40 eV corresponding to excitation of π + σ plasmons. Auger spectroscopy revealed the presence of argon on the surface of ion-irradiated samples. The argon content increased to ～4 at. % with increasing irradiation dose. An analysis of the results obtained and their comparison with the data available in the literature led to a qualitative conclusion that the bond angles of the carbon atoms making up the walls of single-walled carbon nanotubes are distorted at sites exposed to Ar⁺ irradiation.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

Electron-stimulated desorption of rare-earth metal atoms

The yield of europium and samarium atoms in electron-stimulated desorption from layers of rare-earth metals (REMs) adsorbed on the surface of oxidized tungsten has been measured as a function of the incident electron energy, surface coverage by REMs, degree of tungsten oxidation, and substrate temperature. The measurements were performed using the time-of-flight method with a surface-ionization-based detector within the substrate temperature interval 140–600 K. The yield studied as a function of electron energy has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, and those of Eu atoms, at 36 and 41 eV. These energies correlate well with the REM 5p and 5s core-level excitation energies. The REM yield is a complex function of the REM coverage and substrate temperature. The peaks due to REM atoms are seen at low REM coverages only, and their intensity usually passes through a maximum with increasing coverage and substrate temperature. The concentration dependence of the REM atom yield is affected by the deposition of slow Ba⁺ ions, but only if they are deposited after the REM adsorption. At higher REM coverages, additional peaks are observed at electron energies of 42, 54, and 84 eV, which originate from excitation of the 5p and 5s tungsten levels and result from desorption of SmO and EuO molecules. The temperature dependence of the intensity of these peaks is explained to be due to the order-disorder phase transition. The desorption of REM atoms is the result of their reversed motion through the adsorbed REM layer, and the SmO and EuO molecules desorb due to the formation of an antibonding state between the REM oxide molecules and the tungsten ions.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Growth and properties of oxide films on Zn(0001)

The initial stages of the formation of ZnO films on cleavage planes of Zn single crystals have been studied. The growth rate is found to be linear in pressure and independent of temperature over the range from 77 to 425 K. The growth law is consistent with the rate limiting step being the adsorption of oxygen rather than ion transport through the oxide layer. Both the atomic and electronic structure have been monitored during oxygen exposure. The LEED patterns and intensity-voltage data indicate that at low temperatures the oxide is either amrophous or a fine grained polycrystal while above room temperature it is single crystal ZnO epitaxially oriented on Zn(0001). The changes during oxygen exposure of the Zn M₂₃M₄₅M₄₅, M₂₃M₄₅V, M₂₃VV and corresponding M₁ Auger spectra have been studied in some detail. The details of the spectra are identified through comparison with calculated and measured band structures and with previous observations of LMM transitions. The relaxation energy associated with the two d-band holes in the final state (i.e., δE(M₄₅M₄₅)] is found to be ～9 eV in good agreement with a recent calculation. The corresponding relaxation energy for the oxide is ～13.5 eV. The development of the electron energy loss spectra with oxygen exposure has been followed for a range of primary energies. A growth model which is consistent with the LEED observations, Auger peak heights and lineshapes and energy loss spectra is that the oxide grows heterogeneously on the Zn surface with no distinguishable precursor adsorbed state. However, contrary to previous models, it is shown that the surface is completely covered by oxide at a mean thickness of 2–3 monolayers. LEED, Auger and energy loss data for the two polar cleavage faces of ZnO are presented for comparison with those from the oxide overlayer.     

Mechanisms for oxygen adsorption on the Si(110) surface studied by Auger electron spectroscopy

Oxygen adsorption on the Si(110) surface has been studied by Auger electron spectroscopy. For a clean annealed surface chemisorption occurs, with an initial sticking probability of ～6 × 10^?3. In this case the oxygen o_kll signal saturates and no formation of SiO₂ can be detected from an analysis of the Si L_2,3VV lineshape. With electron impact on the surface during oxygen exposure much larger quantities are adsorbed with the formation of an SiO₂ surface layer. This increased reactivity towards oxygen is due to either a direct effect of the electron beam or to a combined action of the beam with residual CO during oxygen inlet, which creates reactive carbon centers on the surface. Thus in the presence of an electron beam on the surface separate exosures to CO showed adsorption of C and O. For this surface subsequent exposure in the absence of the electron beam resulted in additional oxygen adsorption and formation of SiO₂. No adsorption of CO could be detected without electron impact. The changes in surface chemistry with adsorption are detectable from the Si L_2,3VV Auger spectrum. Assignments can be made of two main features in the spectra, relating to surface and bulk contributions to the density of states in the valence band.     

Oxidation kinetics of thin titanium films grown on tungsten

Growth of thin Ti films on (100)W and the kinetics of their oxidation are studied using thermal-desorption spectroscopy and Auger electron spectroscopy. Titanium films grow nearly layer by layer on the (100)W face at room temperature. The activation energy for desorption of Ti atoms decreases from 5.2 eV for coverage θ=0.1 to 4.9 eV in a multilayer film. Oxidation of a thin (θ=6) titanium film starts with dissolution of oxygen atoms in its bulk to the limiting concentration for a given temperature, after which the film oxidizes to TiO, with the TiO₂ oxide starting to grow when exposure of the film to oxygen is prolonged. The thermal desorption of oxides follows zero-order kinetics and is characterized by desorption activation energies of 5.1 (TiO) and 5.9 eV (TiO₂).     

The reactive adsorption of CCl4 on GaAs(100) surfaces at 300 K

Auger electron spectroscopy has been used to monitor the adsorption of CCl₄ on an As-rich GaAs(100) surface at 300 K. Intensities of the Ga (55 eV), As (34 eV), C (270 eV) and Cl (181 eV) transitions have been used to estimate surface number densities at saturation and relative C : Cl stoichiometry of the surface species. Number densities of (4.3 ± 0.2) × 10¹⁴ and (2.0 ± 0.2) × 10¹⁴ cm ⁻² are obtained for carbon and chlorine respectively, suggesting that coverage saturates near one theoretical monolayer and that the C : Cl stoichiometry is approximately 2:1. These data are discussed in terms of a reactive adsorption mechanism.     

Energies and chemical shifts of the sulphur 1s level and the KL2L3(1D2) Auger line in H2S,SO2 and SF6

The sulphur 1s binding energies and KL₂L₃(¹D₂) Auger energies have been measured in gaseous H₂S, SO₂ and SF₆. The experimental data, including the chemical shifts, are compared with various theoretical ab initio results. Theoretical and experimental values agree within 1-2 eV for the chemical shift and the binding energy of the 1s level, provided in the latter case relaxation, relativistic and correlation corrections are applied. Likewise, Shirley's method²⁰, which uses empirical energies, predicts the Auger energies satisfactorily. The measured S 1s binding energies are 2478.5(1) eV, 2483.7(1) eV and 2490.1(1) eV, and KL₂L₃(¹D₂) Auger energies are 2098.7(1) eV, 2095.5(2) eV, 2092.6(1) eV for H₂S, SO₂ and SF₆, respectively. The chemical shift for the 1s electron is found to be greater than for the 2s or 2p electron and in better accord with the prediction of the potential model. Data suggest the molecular relaxation energy to be small compared with the atomic relaxation energy.