VV Auger Peak Research Articles

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.

Growth and isothermal desorption of ultrathin silver layers on the Ni(111) face at the substrate temperature from 180 to 900 K

The growth mode of silver on the Ni(111) face was investigated by using AES in a substrate temperature range of 180 K< T<750 K, and at a deposition rate of 0.0008 ML s −1< F<0.0048 ML s −1. In the measured kinetics of the Ni M 2,3VV Auger peak, apart from segments corresponding to the growth of the first and second layer, additional linear segments appear around coverages Θ=1 and 2. Their presence and dimensions depend on the deposition temperature but not on the deposition rate. This is interpreted under the supposition that the Schwoebel barrier at the edges of first-layer islands is negligible. During annealing at a constant temperature between 820 and 900 K silver is desorbed and the Ni M 2,3VV Auger peak height increases linearly with time, showing different slopes for the desorption of the second- and first-layer silver atoms. From these slopes, the binding energies of silver atoms at island edges of the second and first layer were determined. LEED patterns indicate the presence of the (111) oriented silver domains rotated by ±2° with respect to the substrate.

Nondestructive determination of the concentration profile in the surface layer of AuCu alloy with the use of directional AES and Monte-Carlo simulations

Ratios of M 2,3VV Auger peak heights of gold and copper for the Au 0.6Cu 0.4 alloy expected for particular compositions of two surface atomic layers and for two incidence angles of the primary electron beam in an RFA spectrometer were calculated with the use of Monte-Carlo computer simulation of the process of Auger electron transport to the sample surface, including elastic and inelastic scattering of those electrons. Those ratios were compared with the corresponding ratios obtained experimentally which gave the relative concentration of gold atoms in the first and second atomic layers equal to 0.79 ± 0.05 and 0.60 ± 0.20, respectively.

Growth of thin Ti films on Si(111)-(7×7) surfaces

A study of the room-temperature growth of ultrathin Ti films [up to 7 monolayers (ML)] on clean and atomically flat Si(111)-(7×7) surfaces using Auger electron spectroscopy and low-energy electron diffraction (LEED) is presented. The variations in the Auger signal due to Si L2,3VV with binding energy of 92 eV are used to model the growth morphology of this system. These measurements indicate the growth of an initial disordered and continuous Ti film of up to 1.6 ML in thickness where the LEED pattern completely disappears and the Si Auger signal is strongly attenuated. As more Ti is deposited, this is followed by the disintegration of the continuous film and the formation of an intermixed Ti/Si film. This is evidenced by a change in the slope of the Auger signal time plot, and the reappearance of the LEED pattern. The modification in the overlayer composition for films thicker than 1.6 ML is confirmed by a change in the Si L2,3VV Auger peak that resembles the peak shape due to TiSi2.

The Growth Of Thin Ti Films on Si(111)-(7×7) Surfaces

AbstractA study of the room-temperature growth of ultrathin Ti films (up to 7 ML) on clean and atomically flat Si(111)- (7×7) surfaces using Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) is presented. The variations in the Auger signal due to Si L2.3VV with binding energy of 92 eV are used to model the growth morphology of this system. These measurements indicate the growth of an initial disordered and continuous Ti film of up to 1.6 ML in thickness, where the LEED pattern completely disappears and the Si Auger signal is strongly attenuated. As more Ti is deposited, this is followed by the disintegration of the continuous film and the formation of an intermixed Ti/Si film. This is evidenced by a change in the slope of the Auger signal time (AST) plot, and the reappearance of the LEED pattern. The modification in the overlayer composition for films thicker than 1.6 ML is confirmed by a change in the Si L2.3VV Auger peak that resembles the peak shape due to TiSi2.

Low energy Auger electron spectroscopy of iron oxide: oxidation of one monolayer of iron on Cu(100)

Abstract The peak shape of the iron M 2,3 VV Auger peak during the oxidation of one monolayer of iron on Cu(100) was studied. Peak shape analysis indicated that all iron is oxidised to γ-Fe 2 O 3 before oxygen adsorbs on the Cu(100) surface. The oxidation of iron probably proceeds at once from Fe 0 to Fe 3+ though initially adsorbing oxygen causes a different ratio between the cross transition and the auto-ionisation peak than later adsorbing oxygen.

Low energy Auger electron spectroscopy of iron oxide: oxidation of one monolayer of iron on Cu(100)

The peak shape of the iron M 2,3VV Auger peak during the oxidation of one monolayer of iron on Cu(100) was studied. Peak shape analysis indicated that all iron is oxidised to γ-Fe 2O 3 before oxygen adsorbs on the Cu(100) surface. The oxidation of iron probably proceeds at once from Fe 0 to Fe 3+ though initially adsorbing oxygen causes a different ratio between the cross transition and the auto-ionisation peak than later adsorbing oxygen.

Positron states on the Cs/Cu(100) surface.

The attenuation of the Cu ${\mathit{M}}_{2,3}$VV Auger peak with Cs coverage on Cu(100) was measured using positron-annihilation-induced Auger electron spectroscopy (PAES) and conventional (electron-induced) Auger electron spectroscopy. The data (obtained at both 303 and 163 K) indicate that below a critical coverage (\ensuremath{\sim}0.7 monolayer) the positrons are trapped at the Cu/Cs interface in agreement with theoretical calculations. We observe a sharp (less than 0.02 monolayer wide) drop in the normalized PAES intensity at the critical coverage indicating a transition to another position state in which the positrons are localized on the vacuum side of the Cs overlayer.

An Auger and EELS study of oxygen adsorption on FeS2

AES and EELS are used to study the effect of oxygen on the FeS2 surface. The main Auger Fe M2.3VV attenuates with increased oxygen exposures and shifts to lower kinetic energy while the high kinetic energy feature due to an 'autoionization process' remains unaltered in energy. The EELS spectra measured as a function of oxygen exposure clearly indicate the formulation of iron oxide similar to that of alpha -Fe2O3. For the FeS2+O2 system, the authors show that Ueff is negligibly small and is in good agreement with the estimated Ueff for Fe2O3. A slight change in the S L2.3VV Auger peak is observed on adsorption of oxygen and the formation of sulphur oxides is ruled out.

A soft X-ray study of a Li [sbnd]S surface compound

We have studied a “Li 2S x” compound (x ≈ 0.79to0.95) with soft X-ray excited valence and core-level photoemission and Auger electron emission. The Lils core level is found at a binding energy of 56.9 eV, shifted by ∼ 1.9 eV relative to the bulk ls level of Li metal. The Li KVV Auger transition gives two peaks at kinetic energies 44.9 and 41.2 eV with respect to the Fermi level, corresponding to final states with two valence holes on different sites or on the same site, respectively. The S2p photoemission doublets are observed at 162.7 and ∼ 163.7 eV, their de-excitation giving rise to the SL 2.3VV Auger peak at 149.2 eV. We discuss the electronic interactions in the Li 2S x surface compound, and make a comparison with those in a Li O surface compound.

Sensitivity of positron-annihilation-induced Auger-electron spectroscopy to the top surface layer.

We present the first study of positron-annihilation-induced Auger-electron emission from a well defined overlayer-metal system. A 0.5 monolayer of S on Cu(110) produced a fourfold reduction in the positron induced Cu ${M}_{2}$,3VV Auger peak. The attenuation is accounted for by theoretical calculations indicating that the positron surface state is displaced away from the Cu surface by the S overlayer, decreasing the annihilation rate of positrons with Cu core electrons.

Characterization of Si/CoSi2/Si(111) heterostructures using Auger plasmon losses

The cobalt LMM Auger spectra are observed to undergo significant shape changes as a function of Si overlayer thickness and annealing temperature in Si/CoSi2 /Si heterostructures prepared by codeposition and solid phase epitaxy on Si(111) substrates. The changes are dominated by strong increases of the bulk plasmon loss intensity and shifts in the plasmon loss energy with increasing Si overlayer thickness. These effects can be used to probe the overlayers with electrons generated in the underlying layers. They are used here to measure the thickness of Si overlayers on CoSi2 . We find a linear relationship between the ratio of the plasmon loss peak associated with the Co-L23 VV Auger peak to the Auger peak itself and the known thickness of deposited Si overlayers for thicknesses up to 30 Å. Using this calibration, we monitor island formation in annealed Si/CoSi2 /Si and diffusion of Si in CoSi2 /Si. We deduce the formation of islands in the deposited Si overlayers at temperatures of 550 °C for thicknesses less than 30 Å. We observe Si diffusion in CoSi2 /Si at temperatures as low as 400 °C. We measure activation energies of 0.52–0.60 eV for the diffusion, as determined from Arhennius plots of the plasmon/Auger data, and conclude that the diffusion most likely proceeds through residual defects in the CoSi2 .

Diffusion of Si in thin CoSi2 layers

We present evidence of Si diffusion in 100 Å layers of CoSi2 grown by room-temperature codeposition and annealing on Si(111) substrates. By monitoring the intensity of the Co MVV and Si LVV Auger peaks, we find a Si-rich surface layer after annealing, in agreement with the results of others. We find that this layer can be removed by chemical etching and re-formed by subsequent annealing. By measuring the intensity of the plasmon energy loss peak associated with the Co L23 VV Auger peak, we conclude that the Si must exist on top of the CoSi2 and we obtain the effective Si overlayer thickness as a function of annealing temperature by calibrating the plasmon loss data against known overlayer thicknesses on unannealed samples. We find similar results on samples grown both with and without the addition of a 10 Å Si cap to prevent pinhole formation in the CoSi2 and we have indications that the same type of diffusion occurs also beneath the native oxide layer on samples that have not had the surface Si removed by chemical etching. In all of the samples studied, Si diffusion was observed to be non-negligible at temperatures on the order of 400 °C, which is well below the point where pinhole formation is first observed. This result suggests that the diffusion does not depend on the presence of observable pinholes as previously thought.

Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the Mo M 4,5NN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the Mo N 2,3VV Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molydenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.