Angle-resolved elastic peak electron spectroscopy: Role of surface excitations

We analyze the possibility of determining the surface excitation parameter (SEP) from the dependence of the elastic backscattering signal intensity on the emission angle. It has been found that the shape of this dependence is reasonably well described by the theoretical model implemented in a typical Monte Carlo simulation strategy. As shown recently, the mean percentage deviation between the experimental angular dependence and the theoretical dependence is equal to 8.82% at 200 eV, 6.28% at 500 eV and 4.69% at 1000 eV. In the theoretical model used, the surface energy losses were ignored. Close inspection of the deviations between theory and experiments indicates systematic trends that can be ascribed to the surface energy losses. We found here that taking into the account the surface energy losses further improves the agreement between theory and experiment. The total mean percentage deviation, equal to 6.65%, decreases to 5.59% if the mathematical form of the Chen formula for SEP is used, or to 5.16% if the Oswald expression is used. The material dependent coefficients in the expression of SEP derived from the emission angle dependence of the elastic peak intensity differ from these coefficients resulting from other methods. We conclude that the determination of SEP from shape of the angular dependence requires the experimental data of high quality, and the reliable theoretical model describing elastic electron backscattering.     

Surface excitation effects in elastic peak electron spectroscopy

Surface electron inelastic excitations, a consequence of electron-surface interaction, effect the measured intensities in surface-sensitive electron spectroscopic methods and distort the quantitative information. This phenomenon is more pronounced at low electron energy and glancing emission angles. In this work we investigate quantitatively the influence of the surface excitation effects on the measured electron elastic backscattering probability. As a model system we used Si, Cu and Al, i.e. materials with different surface excitation properties. Results obtained show that properly corrected measured elastic electron backscattering probabilities lead to inelastic mean free path values which compare well with the theory.     

High-resolution study of quasi-elastic electron scattering from a two-layer system

In large-angle elastic scattering events of keV electrons a significant amount of momentum is transferred from the electron to a nucleus in the target. As a consequence kinetic energy is transferred from the energetic electron to the nucleus, and hence these processes can be referred to as ‘quasi-elastic’. How much energy is transferred depends on the mass of the nucleus. In this paper, we present measurements from a two-layer system (a germanium layer and a carbon layer), and at high energies the quasi-elastic peaks of Ge and C are clearly resolved. It is demonstrated that the sample geometry has a huge effect on the observed relative intensities. It is shown that the intensities are influenced by the elastic scattering cross-section of the atoms in the film, film composition and selective attenuation, due to varying amount of inelastic scattering for layers of the film. However truly quantitative agreement is not obtained.     

Quantitative interpretation of the low-loss electron signal

Low-loss electron spectra from clean elemental standards of C, Si, Cr, Fe, Cu, Ag, and Au are presented and theoretically interpreted with the aid of two basic theories. One of these assumes a simple elastic scattering and Bethe loss regime in which the low-loss signal arises from primary electrons that have undergone a single large-angle scattering event and whose energy loss is described by the continuous slowing down approximation. However, better qualitative agreement with experiment is obtained when multiple elastic scattering is considered via the transport approximation for electron deflection. The simple low-loss electron detector used to obtain the data is also described.     

Simulations of electron transfer in grazing incidence ion–surface collisions

During the interaction of a low energy ion beam with a metallic surface in grazing incidence, electron transfer plays an important role in the final state of the scattered beam. In this work we compare two approaches—rate equations resolution and a Monte Carlo numerical code named ETISC1D—to describe the beam evolution and to compute the final populations of the various atomic and ionic species as well as the corresponding angular distributions. Results of both methods are compared to experimental data obtained by Hecht et al. for 2.3 keV He⁺(1s) and He*(1s2s, ³S₁) beams impinging on an Al(1 1 1) surface under 0.79° of incidence. The limitations of the rate equations method and the advantages of Monte Carlo simulations are pointed out. In particular, we show that although the rate equation approach can give a fast and rather accurate evaluation of the populations, it is unable to provide correct calculations of more sensitive quantities like angular distributions of scattered species as obtained with the ETISC1D code.     

Interface effect for EPES sampling depth for overlayer/substrate systems

We found that the interface effects for the sampling depth of elastic peak electron spectroscopy (EPES) for the overlayer/substrate system, first noticed by Zommer and Jablonski for the Rh/Al and Au/Ni systems, differ profoundly in their magnitude and dependence on the overlayer thickness when the overlayer and substrate materials are swapped. Monte Carlo calculations performed for the Al/Rh and Ni/Au systems show that their sampling depths, in contrary to the Rh/Al and Au/Ni, can be substantially greater than those of the elements constituting the respective system. The magnitude of the interface effect increases with the energy of the primary electron beam. It is interesting to mention that the sampling depth can be equivocal for a system with overlayer.     

Energy loss spectra of low primary energy (E0 ? 1 keV) electrons backscattered by silicon dioxide

A Monte Carlo simulation is described and utilized to calculate the energy distribution spectra of the electrons backscattered by silicon dioxide. Spectra are presented for incident energies of 250 eV, 500 eV, and 1000 eV. Spectra interpretation is based on a semiquantitative valence-band structure model for SiO₂ crystals.     

A universal algorithm for calculating the backscattering factor in Auger-electron spectroscopy

We describe a universal Monte Carlo algorithm that can be used for the efficient calculation of backscattering factors (BFs) for quantitative Auger-electron spectroscopy (AES). This algorithm makes use of the continuous slowing-down approximation and the electron stopping power instead of simulation of individual inelastic-scattering events. This approach is attractive because it can be applied to any material with an empirical formula for the stopping power, available data for differential elastic-scattering cross sections, and an empirical formula for inner-shell ionization cross sections. We report BFs for the Si KL₂₃L₂₃, Cu L₃M₄₅M₄₅, Ag M₅N₄₅N₄₅, and Au M₅N₆₇N₆₇ Auger transitions in the corresponding elemental solids. These BFs were calculated for normal incidence of the primary beam, primary energies from near threshold for ionization of the relevant core levels to 20 keV, and Auger-electron emission angles of 10°, 60°, and 80°. We found satisfactory agreement between these BFs and values obtained from a more accurate algorithm in which individual inelastic-scattering events were simulated. Percentage deviations between BFs from the two algorithms were <2% for Au, <5% for Ag, <7% for Cu, and <10% for Si for primary energies likely to be used in practical AES. These deviations arise mainly from our use of stopping powers from the empirical formula rather than more reliable values calculated from experimental optical data.     

Empty electronic states in low-energy absorbed current spectroscopy of NbSe2(0 0 0 1) surface

The fine structure of the experimental absorbed current spectra along the normal to a clean NbSe₂(0 0 0 1) surface is interpreted theoretically. It is shown that the fine structure of absorbed current spectra is mainly due to the electronic structure of unoccupied high-level electronic states (above the vacuum level), which become occupied by electrons entering a solid. The predominant role of the bulk energy-band structure effects in the spectra formation is shown. In addition, a comparison to existing theoretical and experimental data is given.     

On line shape analysis in X-ray photoelectron spectroscopy

Any solid state X-ray photoelectron spectrum (XPS) contains contributions due to multiple inelastic scattering in the bulk, surface excitations, energy losses originating from the screening of the final state hole (intrinsic losses), and, for non-monochromatized incident radiation, ghost lines originating from the X-ray satellites. In the present paper it is shown how all these contributions can be consecutively removed from an experimental spectrum employing a single general deconvolution procedure. Application of this method is possible whenever the contributions mentioned above are uncorrelated. It is shown that this is usually true in XPS to a good approximation. The method is illustrated on experimental non-monochromatized MgK spectra of Au acquired at different detection angles but for the same angle of incidence of the X-rays.     

Surface and bulk plasmon coupling observed in reflection electron energy loss spectra

Reflection electron energy loss spectra have been measured for a semiconductor and some metals (Si, Cu, Ag and Au). A novel procedure is presented to rigorously decompose the spectra into contributions corresponding to surface and volume excitations. The resulting distributions of energy losses in an individual surface loss are in good agreement with theory. In particular, the begrenzungs effect occurring at the boundary of a solid state plasma, i.e. the reduction of the intensity of bulk modes due to the coupling with surface modes, can be clearly observed in the retrieved energy loss distribution.     

Electron IMFPs in bulk Cd0.88Mn0.12Te crystals determined by EPES

The inelastic mean free path (IMFP) of electrons is a basic parameter for surface-sensitive electron spectroscopies (AES, XPS, EELS) in quantitative analyses.Cd_1−xMn_xTe mixed crystals are currently of great interest due to their magnetic and magneto-optical properties. Since information on electron transport processes in these semimagnetic compounds is scarce, their systematic studies are highly desirable.In the present work, the IMFPs in Cd_0.88Mn_0.12Te (1 1 0) crystal samples were obtained from EPES with use of the Ni standard in the electron energy range 500-2000 eV. In addition, we also explored the effect of bulk Mn content in the determination of the IMFP. Relative EPES measurements were carried out using the MICROLAB 350 spectrometer. The sample surface was sputter cleaned and amorphized by Ar⁺ ions. Surface composition of the samples was monitored in situ by XPS and AES. The measured IMFPs were uncorrected for surface excitations and compared with those predicted from the TPP-2M and G-1 formulae. Also, the values of the IMFPs determined here were compared with those evaluated from the expression of Sekine et al. However, accuracy of this expression is rather poor except the case of pure CdTe (x = 0). In general, good agreement was found between the measured IMFPs in Cd_0.88Mn_0.12Te and the corresponding predicted IMFPs. The root-mean-square deviation from IMFP values predicted from the TPP-2M formula was 1.2 Å. The mean percentage deviation from the TPP-2M IMFPs was 9.3%.     

The backscattering factor for systems with a non-uniform surface region: Definition and calculations

It has been recently shown that the backscattering factor (BF) in Auger electron spectroscopy (AES) noticeably depends on the in-depth structure of the surface region. This is a particularly important problem in sputter depth profiling monitored by AES since the signal intensity cannot be described with a single BF value. The BF depends on the removed amount of material and thus varies with sputtering time.In the present work, the definition of the BF is generalized to extend its applicability to systems with an in-depth composition profile. The generalized definition of the BF was applied to the special case of a depth profile, i.e. a buried thin layer. It has been shown that the BF for this case is expressed by the excitation depth distribution function (EXDDF) which is equivalent to the “Phi-Rho-Z” function used in electron probe microanalysis (EPMA). Different algorithms for calculating the BF are discussed. Calculations of the BF for buried layer were performed for the Ag M₄N₄₅N₄₅ Auger transition in a thin layer of silver located at different depths in three matrix materials: Si, Cu, and Au. It was found that, indeed, the BF noticeably varies with the depth of the layer in the analyzed volume, although the extent of this variation depends on the matrix material. For Si, the variation is observed for the lowest primary beam energies considered, i.e., 1 and 2 keV. For Cu, a distinct depth dependence of the BF is visible at 10 keV and lower energies, while for Au, the BF varies with depth even at the highest considered energy, i.e. 30 keV.     

Zone specificity in low energy electron stimulated desorption of Cl from reconstructed Si(1 1 1)-7 × 7:Cl surfaces

Diffraction in electron stimulated desorption has revealed a propensity for Cl⁺ desorption from rest atom vs. adatom areas and unfaulted vs. faulted zones of Cl-terminated Si(1 1 1)-(7 × 7) surfaces. We associate the 15 eV ± 1 eV threshold with ionization of Si-Cl σ-bonding surface states and formation of screened two-hole states with Si 3s character. Similar specificity is observed from A and B reconstructions. This can be due to reduced screening in unfaulted regions and increased hole localization in Si back-bonds within faulted regions.     

EELS studies of surface phonons on Ag(111)

High-resolution electron energy loss spectroscopy is used to study surface phonons in the direction of qG on Ag(111). The gap mode (S₂) at in the surface Brillouin zone is measured for the first time. We have also measured the Rayleigh mode and the resonance mode up to the zone boundary. The results are complementary to the earlier He atom scattering measurements and are in good agreement with the first-principles calculations performed recently.     

Modeling of electron-stimulated mobility and disordering of adsorbed particles

The electron-stimulated mobility and the electron-stimulated disordering of adsorbed particles is studied for a two-dimensional lattice gas model on a square lattice using kinetical Monte Carlo simulations. Pairwise nearest-neighbor repulsive interactions are considered which induce c(2 × 2) ordering of the lattice gas at low temperatures around half coverage. Adsorbed particles are allowed to perform thermally activated as well as electron-induced jumps to nearest-neighbor sites. The calculations are performed taking full advantage of the numerical power of a supermassive parallel computer.

It was found that the electron-induced mobility of adatoms causes the complete breakdown of the c(2 × 2) ordering at low temperatures if the fraction of electron-induced jumps exceeds a critical value. The breakdown of the ordering is accompanied by substantial changes of the chemical and tracer surface diffusion coefficients.     

The role of absorption in large-angle elastic scattering

The measurements of energy loss distributions obtained in electron scattering experiments at high momentum transfer are presented for Xe, Ar and methane. The spectra show a large variety of intensity distributions, clearly different from those obtained in measurements at the dipole limit. The fraction of the intensity present in the energy loss distribution compared to the elastic peak is significant, but is in line with the reduction of the elastic cross section due to absorption. It is argued that, if similar effects are present in the condensed phase, they should be dealt with in any quantitative analysis of electron transport in matter, as is often done using Monte Carlo simulations. This argument is worked out in some detail for Reflection Electron Energy Loss Spectroscopy.     

Multi-site kinetic Monte Carlo simulations of thermal desorption spectroscopy data

Presented is a kinetic Monte Carlo simulation (KMCS) algorithm for simulating experimental thermal desorption spectroscopy (TDS) data. The KMCS is based on the Master equation approach and applies a first-passage-time analysis, i.e., the time dependence of the kinetics is matched correctly. The KMCS–TDS scheme used here includes multiple kinetically distinct adsorption sites and the effect of lateral interactions as required for fitting experimental data. After the results of extensive tests of the algorithm by means of synthetic data are discussed, experimental TDS curves are reproduced by the KMCS. Two applications are demonstrated: iso-butane adsorption on ZnO(0 0 0 1)–Zn and n-pentane adsorption on carbon nanotubes.     

Diffraction from reconstructed surfaces with incommensurate domain walls

The kinematic approximation method has shown that the peak intensity and the full width at half maximum (FWHM) of stepped surfaces exhibit an oscillatory behavior for changing incident energy. This paper generalizes the kinematic approximation to an (N × 1) reconstructed surface with a distribution of various types of lateral displacements at a step. A particular solution of this model we call the fixed point solution, yields a clear intuitive understanding of these oscillations as well as an exact solution for the step density of any surface. The specific examples of (5 × 1) and (2 × 1) reconstruction are examined to show the striking differences between the reconstructed surface diffraction patterns. These differences make an examination of the half-maximum (HM) intensity position a powerful tool to determine the surface structure for any incommensurate stepped surface.     

Atomistic modeling of segregation and bulk ordering in Ag-Au alloys

The bulk and surface properties of Ag-Au alloys, for the whole range of concentration and as a function of temperature, is studied by means of a simple modeling scheme using the Bozzolo-Ferrante-Smith method for alloys. Evidence for short-range order is found and explained, as well as its relationship with the experimentally observed segregation behavior.