KVV radiative Auger emission spectrum of metallic silicon

The KVV radiative Auger region in the K X-ray emission spectrum of silicon in metallic powder form has been ionometrically recorded and compared with the expected spectrum calculated by using the measured L_2,3VV Auger spectrum available in the literature. It is found that the experimental KVV radiative Auger spectrum has some additional features. An attempt has been made to identify these as being due to various VV levels as final states.     

Transition density of states for Si(100) from L1L23V and L23VV Auger spectra

The L₁L₂₃V and L₂₃VV Auger spectra of sputtered and annealed Si(100) have been measured and the transition density of states extracted. The line shapes for the two transitions differ, indicating the importance of matrix element effects. Whereas the L₁L₂₃V line shape closely resembles results of other measurements of the Si density of states, the L₂₃VV line shows a strong emphasis on p-like states.     

SiO2 and Al2O3 valence band density of states from self-deconvolution of L23VV Auger spectrum

The self-deconvolution of L₂₃VV Auger spectra of SiO₂ and Al₂O₃ has been carried out. The transition density functions obtained are compared with the local density of states (LDOS) of the valence band near the surface, as given by other techniques (XPS, UPS, XES) and also by theory. A fair agreement in the number and peak positions of valence band is produced. These compounds with MgO constitute an oxide series of increasing ionicity and the effects of initial hole localization in the transition density function are discussed.     

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.     

Hydrogen chemisorption on silicon (100) 2 × 1

Changes in the valence band density of states (DOS) of a (100) silicon surface that accompany he chemisorption of atomic hydrogen onto that surface are deduced from a study of the changes in the L_2,3VV Auger lineshape. Complementary changes in the conduction band DOS are inferred from changes in L_2,3VV-core-level characteristic loss spectra (CLS). The chemisorbed hydrogen layer is identified as the dihydride phase from low energy electron diffraction measurements. Upon hydrogen adsorption the DOS at the top of the valence band decreases and new energy levels associated with the Si-H bonds appear lower in the band. Assuming that the Auger signal from the hydrogen covered sample consists of a superposition of a signal from silicon atoms bonded to hydrogen in the dihydride layer and an elemental-Si signal from the substrate, a N(E) difference spectrum with features due only to the dihydride is obtained by subtracting the background corrected, loss deconvoluted L_2,3VV signal for a clean (100)Si surface rom the corresponding signal for the hydrogen covered surface. Comparisons of the energy position of the major peak in this difference spectrum with that of the main peak in a gas phase silane Si-L_2,3VV spectrum, and of the corresponding Auger energy calculated empirically, indicate a hole—hole interaction energy of ~8 eV for the two-hole final state in the gaseous system and zero for the dihydride surface system. Hydrogen induced changes in the conduction band DOS are less apparent than those of the valence band DOS with only the possibility of a decrease in the DOS at the bottom of the conduction band being inferred from the CLS measurements. Electron stimulated desorption of hydrogen from the dihydride layer is adduced from changes in the Auger lineshape under electron beam irradiation of the surface. Hydrogen induced changes in the near-elastic electron energy loss spectra (ELS) are also reported and compared with previously published ELS results.     

An auger electron spectroscopic investigation of the electron states of copper alloys

L₃VV Auger transitions of copper alloys show a feature due to a band-like states, the shape and intensity of which depend on the composition. The energy separation between this feature and the L₃M_4·5M_4·5 peak increases progressively with Cu concentration.     

The participant Coster–Kronig preceded Auger transition in the resonant L2,3-M2,3V Auger electron spectrum of Ti metal

The L_2,3-M_2,3V resonant Auger electron spectroscopy (RAES) spectrum of Ti metal measured by Le Fêvre et al. [P. Le Fêvre, J. Danger, H. Magnan, D. Chandesris, J. Jupille, S. Bourgeois, M.-A. Arrio, R. Gotter, A. Verdini, A. Morgante, Phys. Rev. B69 (2004) 155421] is analyzed in the light of relaxation and decay of the resonantly excited L_2,3-hole states. The relaxation time of the resonantly excited L_2,3-hole state to the fully relaxed (screened) one is much shorter than the L_2,3-hole Auger decay time, whereas the participant Coster–Kronig (CK) decay time of the resonantly excited L₂-hole state to the fully relaxed L₃-hole state at the L₂ resonance is as short as the relaxation time of the resonantly excited L₂-hole state to the fully relaxed one. The excited electron is predominantly either rapidly decoupled from the L_2,3-hole decay or annihilated by the participant CK decay. Thus, near the L_2,3 edges the L_2,3-M_2,3V RAES spectral peak appears at constant kinetic energy. The L_2,3-M_2,3V RAES spectrum shows a normal L_2,3-M_2,3V Auger decay profile not modulated by the density of empty d states probed by the resonant excitation. Not only the relaxation time but also the participant CK decay time depends on photon energy because they depend on the density of empty d states probed by the resonant excitation. As a result, the L_2,3 X-ray absorption spectroscopy spectral line broadening depends on photon energy.     

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.     

The lineshape of the L2,3 VV Auger spectrum of silicon

An attempt is presented to understand the details of the lineshape of the Si L_2,3 VV Auger spectrum from the (111) surface in the 7 × 7 superstructure. In the experiments we have followed the variation of the lineshape induced by adsorption of O₂, H₂O, CO and by bombardment with 3 keV Ar⁺ ions, over a range from a small perturbation of the surface to major changes in surface structure. For small perturbations from the clean surface we were able to resolve changes in the local density of states at surface silicon atoms. By unfolding the experimental spectra, effective transition densities of states result, which compare quite closely with calculated densities of states, apart from a certain enhancement of surface features in the experiments. All peaks in the experimental spectra can be explained, based on densities of states at the surface of pure Si(111) (7 × 7) (91.8 and 84.8 eV), Si(111) + adsorbed oxygen (70.6 eV), SiO₂, (78.9 and 64.5 eV) and plasmon losses, at 71.0 and 57.5 eV for the clean surface.     

On the Auger-photoelectron coincidence spectroscopy spectra of Cu(100), Cu metal and CuOx/Cu surface

The M₃–VV Auger-photoelectron coincidence spectroscopy (APECS) spectrum of Cu(100) and the L₃–VV APECS spectra of Cu metal and CuOx/Cu surface are analyzed in detail. The narrowing and energy shift of the photoelectron line in the M₃–VV APECS spectrum is well predicted by the present theory. The spectrum shows the presence of the M₂–M₃(V)–VV(V) decay in which a hole in the 4s band hops away prior to the decay of M₃ hole. The analysis of the L₃ photoelectron spectra of Cu metal measured in coincidence with the ³F or ¹G Auger line raises a question concerning the presence of two different core–hole states upon the L₃ level ionization recently proposed by Thurgate and Jiang [Surf. Sci. 466 (2000) L807]. The analysis of the L₃–VV APECS spectrum of CuOx/Cu shows that the final-state charge–transfer interaction plays an important role in CuO.     

The structure of theL 2,3 VV-auger line of silicon and silicon compounds

TheL _2,3 VV Auger transitions of Si, SiO₂, and SiC have been measured and compared with the self-fold electron density of states. The data indicate that Auger matrix effects must be included to explain the structure of the Auger lines. A comparison with soft X-ray measurements of Wiech shows, that the measured Auger line shape is nearly identical with the self-foldK _β emission band. The selection rules for X-ray emission lead then to the conclusion that mostlyp-like valence electrons are involved in the Auger transition. This result indicates the relative importance ofs andp states in Auger transitions which is in accordance with theoretical calculations of Feibelman et al.     

Effect of surface plasmon oscillations on Auger electron emission

The integrated areas of the Al L₂₃VV and O KL₂₃L₂₃ Auger peaks and the Al surface plasmon energy ?ω_S are reported for the Al(001) surface as a function of exposure to O in the exposure range 0–114 L(1 L=1langmuir=10^?6Torr sec). It is shown that for exposures below a critical value of 15 L, ?ω_S is constant within experimental error while the O Auger peak area increases linearly. For exposures above 15 L, ?ω_S decreases linearly from 10.5 eV to 8.5 eV and the O Auger peak area undergoes relatively slow linear increases correspondingly. The Al Auger peak area decreases by 30% per 1 eV decrease of ?ω_S. The results are discussed with reference to theory relating Auger transition intensities to the spectral density function.     

Changes in Auger spectra of Mg and Fe due to oxidation

Auger electron spectra of clean Mg and Fe surfaces have been investigated under UHV conditions. The main Auger peaks in the low energy Auger spectra of these elements are identified as due to L_2,3VV and M_2,3VV transitions for Mg and Fe respectively. Changes in the low energy spectra of these clean surfaces of Mg and Fe due to chemisorption of residual oxygen in the UHV system, were also studied. The results indicate that for each oxidised surface new larger Auger peaks appear at energies lower than the original main peaks in the clean spectra. The changes in the spectra are believed to be due to the energy shifts of inner energy levels and valence bands involved in the Auger transitions as an oxide is formed.     

Solid state effects in the Auger spectrum of zinc and oxidised zinc

Measurements of the zinc L_2,3M_4,5M_4,5 Auger spectra are reported. The line shapes in solid zinc are similar to those in zinc vapour but the Auger energies have increased by about 15 eV and the line breadths have broadened from 0.5 eV to 1.0 eV fwhm. The ratio of the L₂:L₃ groups differ from the vapour suggesting that L₂L₃M_4,5 Coster-Kronig transitions occur in the solid but not in the vapour. Changes in the spectra with oxidation have been observed. The Auger lines broaden on oxidation and a line breadth of 3.2 eV fwhm gives the best fit to the spectrum of almost fully oxidised zinc. The oxide L₃M_4,5M_4,5¹G₄ peak progressively shifts from 2.6 eV to 4.2 eV below the metal peak as the oxide thickness increases, the latter value being close to the measured shift in crystalline zinc oxide. Similar energy variation is reported for solid Argon condensed onto clean silver and the shifts are explained in terms of variation in “extra electron relaxation” with film thickness.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

On the intensity ratio I(KL2,3L2,3)/I(KL1L1) of the auger carbon lines in series of d-metal carbides

C KVV Auger spectra have been obtained for series of nd-metal carbides (n = 3,4 and 5). The numbers of electrons participating in the C KVV Auger processes for these compounds are estimated by considering the intensity ratio I(KL_2,3L_2,3)/I(KL₁L₁). It is concluded that to explain the high intensities of the KL_2,3L_2,3 Auger lines for the d-metal carbides, it is necessary to consider the participation of conduction-band electrons (and/or interatomic transitions) in the C KVV Auger decay process.     

Surface valence band and plasmon features on clean cleaved silicon

The L₁L_2,3V Auger transition from vacuum cleaved surfaces of silicon has been studied in detail. Features in the distribution which are not due to plasmon losses are shown to correspond with primary features in the valance band density of states. However, effects of transition probability energy dependence apparently smear the structure, which may also be due in part to particular surface region properties. Bulk plasmon losses up to 9 in number are clearly resolved and the relative intensities compared with simple theory. A mean free path for bulk plasmon emission is deduced which is approximately twice the m.f.p. for other loss processes. Background contamination produces a new peak at approximately 4.5 eV.     

Oxidation and thermal reduction of the Cu(1 0 0) surface as studied using positron annihilation induced Auger electron spectroscopy (PAES)

Changes in the surface of an oxidized Cu(1 0 0) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the annihilation induced Cu M_2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The intensity then decreases monotonically as the annealing temperature is increased to ∼600 °C. Experimental probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons are estimated from the measured intensities of the positron annihilation induced Cu M_2,3VV and O KLL Auger transitions. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. The effects of oxygen adsorption on localization of positron surface state wave function and annihilation characteristics are also analyzed. Possible explanation is proposed for the observed behavior of the intensity of positron annihilation induced Cu M_2,3VV and O KLL Auger peaks and probabilities of annihilation of surface-trapped positrons with Cu 3p and O 1s core-level electrons with changes of the annealing temperature.     

Spectres auger et sections efficaces d'ionisation de la couche L23 du magnésium et de l'aluminium sous impact de protons et d'ions lourds (10–100 keV)

A comparison of Auger structures observed on the energy distributions of secondary electrons emitted from Mg and A1 solid targets bombarded by either light particles (H⁺ and He⁺) or heavy ions (Ne⁺, Ar⁺, …) is presented. With incident protons, it essentially appears a broad peak corresponding to a L₂₃VV transition and a weak shoulder due to the surface and bulk plasmon excitation. The Auger structures obtained with heavy ions are richer and the peaks which compose it are sharper. Such atomic-like structures correspond to Auger transitions from excited (with one or two L₂₃ holes) moving recoiling atoms. The experimental L₂₃ Mg and A1 ionization cross sections were determined from Auger spectra. In H⁺?Mg (or A1) collisions our results are in good agreement with the theoretical values calculated in a PWBA model. In the case of heavy ion-target interactions, we compared the experimental measurements with ionization cross section calculations obtained in a Landau-Zener model.     

On the L3 level photoelectron spectrum of Cu metal measured in coincidence with the L3–VV Auger-electron spectrum

We provide an answer to the question why the L₃ photoelectron line of Cu metal measured in coincidence with the L₃–VV (³F or ¹G) Auger-electron line, does not line up with the L₃ single photoelectron line. We provide also an answer to the question why the L₃ coincidence photoelectron line is unshifted when the Auger-electron analyzer is moved away from the Auger-electron line. We show that it is the initial core–hole self-energy by the monopole excitation (screening) and the density of final states which play an important role in the shift and narrowing of Auger-photoelectron coincidence spectroscopy (APECS) spectral line. To explain the shifted APECS spectral line, Thurgate and Jiang (Surf. Sci. 466 (2000) L807) recently proposed the presence of two different core–hole states in the main-line state upon the L₃ level ionization in Cu metal. However, their explanation appears to be incorrect.