Report on the 42nd IUVSTA workshop ‘Electron scattering in solids: from fundamental concepts to practical applications’

A summary is given of the workshop entitled ‘Electron Scattering in Solids: from fundamental concepts to practical applications,’ which was held in Debrecen, Hungary, on July 4–8, 2004, under the sponsorship of the International Union of Vacuum Science, Technique, and Applications (IUVSTA). This workshop was held to review the present status and level of understanding of electron‐scattering processes in solids, to identify issues of key importance (hot topics) in the light of the most recent scientific results, and to stimulate discussions leading to a deeper understanding and new solutions of current problems. This report contains summaries of presentations and discussions in sessions on elastic scattering of electrons by atoms and solids, inelastic scattering of electrons in solids, modeling of electron transport in solids and applications, and software. The principal areas of application include Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy, elastic‐peak electron spectroscopy (EPES), reflection electron energy‐loss spectroscopy (REELS), secondary‐electron microscopy, electron‐probe microanalysis (EPMA), and the use of coincidence techniques in electron‐scattering experiments. A major focus of the workshop was determination of the inelastic mean free path of electrons for various surface spectroscopies, particularly corrections for surface and core‐hole effects. Published in 2005 by John Wiley & Sons, Ltd.     

Studies of AuNi alloys by electron spectroscopies with the aid of the line shape analysis by the pattern recognition method

The spectrum of electrons elastically backscattered from the surface and within its vicinity reflects the probability of electron elastic backscattering on the surface atoms, quasi‐elastic scattering and the inelastic scattering visible in the low energy side of the elastic peak. The method for investigating the processes of electron elastic backscattering on the surface atom is called the elastic peak electron spectroscopy (EPES). In the present work, AuNi alloys of different compositions are investigated using X‐ray photoelectron spectroscopy (XPS) and the EPES method with the aid of the line shape analysis called the fuzzy k‐nearest neighbour (fkNN) rule. The line shape analysis was found to be applicable for EPES spectroscopy. It allows distinguishing the surfaces exhibiting various surface roughness, texture and grain size, and quantifying the selected information depths. The quantitative results obtained from the XPS analysis and the EPES spectra line shape analysis indicated Au surface segregation with Au surface enriched profile. Quantitative discrepancies are discussed within the non‐uniform concentration profiles of constituents due to sputter cleaning and annealing, different diffusion coefficients for Au and Ni, differences in the information depths sampled by XPS and EPES methods and differences in electron elastic backscattering cross‐sections for Ni and Au. Copyright © 2006 John Wiley & Sons, Ltd.     

Quantitative surface analysis by Auger and x-ray photoelectron spectroscopy

Recent developments in quantitative surface analysis by Auger (AES) and x-ray photoelectron (XPS) spectroscopies are reviewed and problems relating to a more accurate quantitative interpretation of AES/XPS experimental data are discussed. Special attention is paid to consideration of elementary physical processes involved and influence of multiple scattering effects on signal line intensities. In particular, the major features of core-shell ionization by electron impact, Auger transitions and photoionization are considered qualitatively and rigorous approaches used to calculate the respective transition probabilities are analysed. It is shown that, in amorphous and polycrystalline targets, incoherent scattering of primary and signal Auger and photoelectrons can be described by solving analytically a kinetic equation with appropriate boundary conditions. The analytical results for the angular and energy distribution, the mean escape depth, and the escape probability as a function of depth of origin of signal electrons as well as that for the backscattering factor in AES are in good agreement with the corresponding Mote Carlo simulation data. Methods for inelastic background subtraction, surface composition determination and depth-profile reconstructions by angle-resolved AES/XPS are discussed. Examples of novel techniques based on x-ray induced photoemission are considered.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Calculation of Surface Excitation Parameters by a Monte Carlo Method

Electron inelastic mean free path (IMFP) is an important parameter for surface chemical quantification by surface electron spectroscopy techniques. It can be obtained from analysis of elastic peak electron spectroscopy (EPES) spectra measured on samples and a Monte Carlo simulation method. To obtain IMFP parameters with high accuracy, the surface excitation effect on the measured EPES spectra has to be quantified as a surface excitation parameter (SEP), which can be calculated via a dielectric response theory. However, such calculated SEP does not include influence of elastic scattering of electrons inside samples during their incidence and emission processes, which should not be neglected simply in determining IMFP by an EPES method. In this work a Monte Carlo simulation method is employed to determine surface excitation parameter by taking account of the elastic scattering effect. The simulated SEPs for different primary energies are found to be in good agreement with the experiments particularly for larger incident or emission angles above 60° where the elastic scattering effect plays a more important role than those in smaller incident or emission angles. Based on these new SEPs, the IMFP measurement by EPES technique can provide more accurate data.     

New developments in theory of fast electron scattering in solids: Applications to microbeam analysis

The study of fast electron interaction with solids in the energy range from 100 eV to several tens of keV is prompted by quickly developing microbeam analysis techniques such as electron probe microanalysis, scanning electron microscopy, electron energy loss spectroscopy and so on. It turned out that for random solids the electron transport problem might be solved on the basis of the generalized radiative field similarity principle. The latter states that the exact differential elastic cross section in the kinetic equation may be replaced by an approximate one provided the conditions of radiative field similarity are fulfilled. Application of the generalized similarity principle to electron scattering in solids has revealed many interesting features of electron transport. Easy to use and effective formulae have been obtained for the angular and energy distribution of electrons leaving a target, total yields of characteristic photons and slow electrons escaping from a sample bombarded by fast primaries, escape probability of Auger electrons as a function of depth etc. The analytical results have been compared with Monte Carlo calculations and experiments in a broad range of electron energies and scattering properties of solids and good agreement has been observed.     

Morphology,surface roughness,electron inelastic and quasielastic scattering in elastic peak electron spectroscopy of polymers

In elastic peak electron spectroscopy (EPES), the nearest vicinity of elastic peak in the low kinetic energy region reflects electron inelastic and quasielastic processes. Incident electrons produce surface excitations, inducing surface plasmons, with the corresponding loss peaks separated by 1–20 eV energy from the elastic peak. In this work, X‐ray photoelectron spectroscopy (XPS) and helium pycnometry are applied for determining surface atomic composition and bulk density, whereas atomic force microscopy (AFM) is applied for determining surface morphology and roughness. The component due to electron recoil on hydrogen atoms can be observed in EPES spectra for selected primary electron energies. Simulations of EPES predict a larger contribution of the hydrogen component than observed experimentally, where hydrogen deficiency is observed. Elastic peak intensity is influenced more strongly by surface morphology (roughness and porosity) than by surface excitations and quasielastic scattering of electrons by hydrogen atoms. Copyright © 2007 John Wiley & Sons, Ltd.     

On the use of elastic peak electron spectroscopy (EPES) to measure the H content of hydrogenated amorphous carbon films

Quasi‐elastic scattering of 1–2 keV electrons is considered with respect to measuring the H content in hydrogenated amorphous carbon (a‐C:H) materials. Interest in the technique lies in the fact that H cannot be typically detected by electron spectroscopic means (AES or XPS for instance). The feasibility of the approach is demonstrated and a quantification procedure is proposed. At the same time however, limitations of the technique (electron stimulated H desorption, low intensity of the H related signal and its spectral interference with the π‐plasmon peak) are discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Data Preprocessing in Peak Shape Analysis of Auger Electron Spectra

 Factor analysis is an established method of peak shape analysis in Auger electron spectrometry. The influence of different commonly used data preprocessing tools onto the results of factor analysis is demonstrated on AES depth profiles of multilayers and implantation profiles. For the analysis of Auger electron spectra it has been traditional to differentiate spectra by Savitzky and Golay’s method to remove background and to elucidate changes in peak shape. For phosphorus implanted in titanium it is shown that background removal works not ideal so that inelastic losses of the Ti(LMM) Auger peak can affect the result of factor analysis for the P(LVV) peak located at ca. 250 eV lower in kinetic energy. The contribution of such losses to the background can be corrected by shifting the spectra so that the high energy side above the peak equals zero. Numerical differentiation can introduce correlated error into the data set. To diminish edge effects the reduction of filter width at the edges and cutting off the outermost data points is recommended. The precision of spectrum reproduction is considered as a crucial test for the number of principal components. The reliability factor is investigated as a measure for the goodness of spectrum reproduction.     

Free-electron model for the inelastic collision of fast electrons with benzene

Using a free-electron wave function the cross section for the excitation of the first excited state and that for the ionization of benzene in the inelastic collision with fast electrons have been estimated. In the ionization process the cross section has been found to be maximum when the secondary electron moves away with a kinetic energy of about 3 eV. For incident-electron energy above 20 eV the cross section for ionization is larger than that for excitation, while below it the reverse is the case.     

X-ray and electron induced Auger processes for I2 adsorption on uranium

The accurate determination of the kinetic energy of X-ray induced Auger electrons, which is necessary in XPS experiments, e.g. for calculating the Auger parameter, is sometimes hampered by peak interferences or by the high secondary electron background. The latter is of special importance for low kinetic energy electrons like e.g. the U(OPV) and U(OVV) Auger electrons. These problems can be circumvented by using electron induced Auger transitions (AES). However, since XPS and AES use different reference points for the energy scales, both scales have to be matched. This can be done by measuring the kinetic energy of an appropriate Auger transition in XPS and relating this value to the maximum of the second derivative of the same peak in AES.     

Quantification of surface roughness effect on elastically backscattered electrons

Electron inelastic mean free path can be obtained from a measured elastic peak electron spectroscopy spectrum combined with a Monte Carlo simulation. It is thus necessary to know the influence of various experimental factors to the measured and calculated results. This work investigates the effect of the surface roughness or the surface topography on the intensity of the elastic peak. A Monte Carlo simulation, by taking into account of realistic surface roughness for both Gaussian and non‐Gaussian type rough surfaces experimentally prepared, has been employed to study the surface topography effect. The simulations of elastic peak electron spectroscopy were performed for both planar and rough Al and Cu surfaces and for varied primary energies ranging from 200 to 2000 eV. To quantify the surface roughness effect, the surface roughness parameter is introduced according to the ratio of elastic peak intensities between a rough surface and an ideal planar surface. Simulation results have shown that surface roughness parameter is important in a certain range of emission angle and particularly for large emission angles. For grazing emission, the elastic peak intensity can be largely enhanced by roughness even at nanometer scale. Copyright © 2014 John Wiley & Sons, Ltd.     

Analytical applications of elastic electron backscattering from surfaces

Measurements of probability of elastic electron backscattering from surfaces can provide information on physical properties of the surface region with thickness comparable to the inelastic mean free path (IMFP) of electrons. The analytical technique, based on such measurements, is known as elastic peak electron spectroscopy (EPES). The most frequent application of EPES is the determination of the IMFP in solids. However, this technique can also be used to measure overlayer thickness, or to determine surface composition.

Quantitative applications of EPES, addressed here, require a reliable theoretical model describing the elastic backscattering probability from surfaces with a given structure and composition. Unfortunately, there is no simple analytical model which describes the elastic backscattering probability with an acceptable accuracy. Values of the elastic backscattering probability are usually estimated from Monte Carlo (MC) simulations of elastic backscattering events, since the theoretical model implemented in the MC scheme seems closest to reality, as compared with models leading to different analytical expressions. It is shown that the reliability of the theory is associated with accuracy of the parameters needed in the calculations. The most important parameters are the differential elastic scattering cross-sections which are presently known, especially in some angular regions, with limited accuracy. The IMFP values, determined in different laboratories via EPES, exhibit a considerable scatter, which may be due to the fact that different experimental geometries are used in measurements. Other sources of errors are briefly discussed.     

Monte‐Carlo simulation of secondary electron emission by x‐ray irradiation—an application of x‐ray absorption near‐edge structure (XANES)

A Monte‐Carlo simulation program has been developed for describing x‐ray absorption near‐edge structure (XANES) observed by synchrotron radiation. The Monte‐Carlo simulation was applied for interpreting XANES spectroscopy on a polycrystalline Ag specimen under synchrotron irradiation with photon energy 3340–3390 eV around the absorption edge of the Ag L_α line at 3352 eV. The results clearly indicate that Monte‐Carlo simulation describes the experimental results with considerable success. Dependence of secondary electron yield on the incident angle of synchrotron radiation was also studied. Copyright © 2004 John Wiley & Sons, Ltd.     

Electron scattering in Pt(PF3)4: elastic scattering, vibrational, and electronic excitation

Experimental absolute differential cross sections for elastic scattering, and for vibrational and electronic excitation of Pt(PF(3))(4) by low-energy electrons are presented. The elastic cross sections have a deep angle-dependent Ramsauer-Townsend minimum (E(min) = 0.26 eV at θ = 135°). The angular distributions of the elastic cross section at and above 6.5 eV show an unusually narrow peak at an angle which decreases with increasing energy (it is at 40° at 20 eV). Wavy structure is observed at higher angles at 15 and 20 eV. Vibrational excitation cross sections reveal five shape resonances, at 0.84, 1.75, 3.3, 6.6, and 8.5 eV. The angular distributions of the vibrational cross sections have a strong forward peak and are nearly isotropic above about 60°. Electronically excited states are characterized by electron energy-loss spectra. They show a number of unstructured bands, the lowest at 5.8 eV. They are assigned to Rydberg states converging to the 1st and 2nd ionization energies. The cross sections for electronic excitation have very high forward peaks, reaching the value of 50 A?(2) at 50 eV and 0° scattering angle. Purity of the sample was monitored by the very low frequency (26 meV) Pt-P stretch vibration in the energy-loss spectra.     

Obtaining quantitative information on surface excitations from reflection electron energy‐loss spectroscopy (REELS)

A new analysis of reflection electron energy‐loss spectroscopy (REELS) spectra is presented. Assuming inelastic scattering in the bulk to be quantitatively understood, this method provides the distribution of energy losses in a single surface excitation in absolute units without the use of any fitting parameters. For this purpose, REELS spectra are decomposed into contributions corresponding to surface and volume excitations in two steps: first the contribution of multiple volume excitations is eliminated from the spectra and subsequently the distribution of energy losses in a single surface scattering event is retrieved. This decomposition is possible if surface and bulk excitations are uncorrelated, a condition that is fulfilled for medium‐energy electrons because the thickness of the surface scattering layer is small compared with the electron elastic mean free path. The developed method is successfully applied to REELS spectra of several materials. The resulting distributions of energy losses in an individual surface excitation are in good agreement with theory. In particular, the so‐called begrenzungs effect, i.e. the reduction of the intensity of bulk losses due to coupling with surface excitations near the boundary of a solid‐state plasma, becomes clearly observable in this way. Copyright © 2003 John Wiley & Sons, Ltd.     

Scattering of N(2)O on electron impact over an extensive energy range (0.1 eV-2000 eV)

We report electron impact total cross sections, Q(T), for e-N(2)O scattering over an extensive range of impact energies approximately from 0.1 eV to 2000 eV. We employ an ab initio calculation using R-matrix formalism below the ionization threshold of the target and above it we use the well established spherical complex optical potential to compute the cross sections. Total cross section is obtained as a sum of total elastic and total electronic excitation cross sections below the ionization threshold and above the ionization threshold as a sum of total elastic and total inelastic cross sections. Ample cross section data for e-N(2)O scattering are available at low impact energies and hence meaningful comparisons are made. Good agreement is observed with the available theoretical as well as experimental results over the entire energy range studied here.     

Monte Carlo simulation of low–medium energy electrons backscattered from C/Al double layer thin films

The backscattering coefficient of low–medium energy electron beams (from 250 to 10 000 eV) impinging on C/Al double layered thin films was investigated by a Monte Carlo simulation. The aim of the research was to study the behaviour of the backscattering coefficient as a function of the beam primary energy and the thicknesses of the two layers. The backscattering coefficient as a function of the primary energy presents features that can be used to evaluate the thicknesses of the two layers. Copyright © 2006 John Wiley & Sons, Ltd.