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.     

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.     

Electron inelastic mean free path (IMFP) values of Kapton,polyethylene (PE), polymethylmethacrylate (PMMA), polystyrene (PS) and polytetrafluoroethylene (PTFE) measured with elastic peak electron spectroscopy (EPES)

Elastic peak electron spectroscopy (EPES) was employed to measure the inelastic mean free path (IMFP) for energies between 500 and 1600 eV for five insulating organic compounds: Kapton, polyethylene (PE), poly(methyl methacrylate) (PMMA), polystyrene (PS) and polytetrafluoroethylene (PTFE). A Ni and a Si sample were used as reference materials to avoid measurement of the elastic reflection coefficient in absolute units. Correction of experimental elastic peak intensities for surface excitations was performed which turned out to be essential. The results are compared with recent evaluations of optical constants to yield the IMFP in the literature giving satisfactory agreement, with deviations generally below 20%. Investigation of the kinematics in an electron reflection experiment shows that the dispersion coefficient used in REELS data analysis cannot be identified with the true plasmon dispersion.     

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.     

Imaging and Analysis of Surfaces with a Scanning Electron Microscope and Electron Spectrometer

The topography and the composition of a surface are in many cases of equal importance (catalysis, electroplating, pretreatment of foils and sheet metal, corrosion, passivation, adsorption, coating of fibers, etc.), and this explains the great interest in methods of investigation that reveal both. If the demands on the resolving power, the analytical possibilities, and the thickness of the surface layer are not too exacting, combined devices like the scanning electron microscope and its analytical accessories can be used. When it is necessary to avoid the compromises involved in simultaneous imaging and analysis, the investigations must be carried out with separate equipment. As an example of a method for the analysis of surfaces we consider briefly photo- and Auger electron spectroscopy (ESCA).     

Variation of the electron inelastic mean free path during depth profiling of the Fe/Si interface as determined by quantitative REELS

The aim of this work is to determine the dependence of the electron inelastic mean free path (IMFP) at the Fe/Si interface during depth profiling by sputtering with 3 keV Ar⁺ ions. In order to estimate the variation of the IMFP at the interface, reflection electron energy‐loss spectroscopy (REELS) measurements were performed after different sputtering times at the Fe/Si interface with three different primary electron energies (i.e. 0.5, 1 and 1.5 keV). Even though it is highly likely that a compound (i.e. Fe_xSi) is formed at the interface, all the experimental REELS spectra could be analysed as a linear combination of those corresponding to pure Si and Fe. Using the model developed by Yubero and Tougaard for quantitative analysis of these REELS spectra we could estimate the IMFP values along the depth profile at the interface. The resulting IMFPs are observed to vary linearly with the average composition (as determined by REELS) at the Fe/Si interface as it is sputter depth profiled. The energy dependence of the IMFP for different compositions is presented and discussed. For completeness, we have determined the energy‐loss functions as well as the IMFPs of the pure elements (i.e. Fe and Si). Copyright © 2004 John Wiley & Sons, Ltd.     

Application of backscattering spectrometry for composition and thickness determination of zirconium oxide layers on autoclaved zircaloy

Summary The determination of atomic composition and thickness of zirconium oxide layers on zirconium based alloys is of practical interest in nuclear industry. This paper describes the application of backscattering spectrometry for the non-destructive determination of composition and thickness of zirconium oxide layers on autoclaved zircaloy coupons. The spectrometry used here is the Rutherford backscattering spectrometry (RBS) with 2.5 MeV α-particles, 3.05 MeV ¹⁶O(α,α)¹⁶O resonance scattering and ¹⁶O(p,p)¹⁶O elastic scattering with 2.5 MeV proton beam. Proton backscattering is found to be the most suitable technique for the overall compositional analysis of the layers whereas 3.05 MeV ¹⁶O(α,α)¹⁶O resonance scattering, for depth profiling of oxygen. The former technique is simple and provides rapid measurements. The lower stopping cross sections of protons and enhanced scattering cross section for oxygen over a wide range of proton energy enable the analysis of oxide layers of larger thicknesses. The thicknesses of these layers determined by backscattering are in good agreement with cross-sectional micrographs taken by scanning electron microscope (SEM).     

Monte Carlo Simulation of Electron Transport and X-Ray Generation. I. Electron Elastic and Inelastic Scattering

We present different theoretical approaches to determine differential cross sections for elastic and inelastic interactions of electrons. These cross sections are the basic ingredients for accurate Monte Carlo simulation of electron transport in matter. The considered models range from simple analytical approximations employed in early calculations to purely numerical differential cross sections described by large databases calculated with state-of-the-art theory.     

Investigation of thin surface films by microprobe analysis

The electron microprobe has been applied to study thin films on metallic substrates. The intensities of the characteristic X-rays emitted by thin films of various elements and thicknesses on sublayers of different materials were measured. Two different theoretical approaches (Bishop and Poole, as well as Yakowitz and Newbury) were applied to interpet the X-ray intensities and to determine the film thickness from the intensity measurements. The effect of backscattering from the substrate, resulting in an increase of the intensity of characteristic X-rays of the film, is being described on the basis of a theory given by Hutchins. The corresponding equation for the backscattering factor has been modified to take into account the transmission of the electrons through the film, depending on the mass thickness of the film and the electron energy. The results obtained from theory and experiment are in good agreement for the different experimental parameters applied, except for very thick layers of high atomic numbers measured at low energies where the absorption of electrons in the film plays a dominant role.     

Study of Thin Oxide Films by Electron, Ion and Synchrotron Radiation Beams

 Titanium oxide and zirconium oxide thin films deposited on silicon substrates were characterised using electron probe microanalysis (EPMA), Rutherford backscattering spectroscopy (RBS), time-of-flight elastic recoil detection analysis (TOF-ERDA) and scanning photoelectron microscopy (SPEM). The composition and mass thickness of the films were determined and the results of different methods compared. It was revealed that the synchrotron radiation used for SPEM studies caused considerable modification of zirconia films grown at low temperatures.     

The configurational backscattering of beta-particles

A closed analytical formula of the backscattering coefficient is described, in which the scattering function is calculated with regard to the electron configuration and chemical bond of the target. The theoretical evaluations of the backscattering coefficient have been compared with available experimental data and good agreement has been found.     

Experimental determination of the effective attenuation length of palladium 3d 5/2 photoelectrons in a magnetron sputtered Pd nanolayer

An electron effective attenuation length (EAL) of 1.68 nm for Al Kα excited Pd 3d 5/2 photoelectrons with a kinetic energy of 1.152 keV has been determined experimentally using a sputtered Pd film deposited on an ultra flat fused quartz substrate. The film thickness was reduced by Ar ion sputtering several times in order to obtain different Pd film thicknesses which are used to determine experimental EAL values. These results are compared to data generated by using a Simulation of Electron Spectra for Surface Analysis (SESSA) simulation using an inelastic mean free path (IMFP) calculated with the Tanuma–Powell–Penn (TPP)‐2M formula and with ‘elastic scattering on and off’. Contributions to the uncertainty budget related to the experimental approach are discussed in detail. Proposals on how to further improve the approach are suggested. Copyright © 2016 John Wiley & Sons, Ltd.     

Inelastic mean free path of low‐energy electrons in condensed media: beyond the standard models

The most established approach for ‘practical’ calculations of the inelastic mean free path (IMFP) of low‐energy electrons (~10 eV to ~10 keV) is based on optical‐data models of the dielectric function. Despite nearly four decades of efforts, the IMFP of low‐energy electrons is often not known with the desired accuracy. A universal conclusion is that the predictions of the most popular models are in rather fair agreement above a few hundred electron volts but exhibit considerable differences at lower energies. However, this is the energy range where their two main approximations, namely, the random‐phase approximation (RPA) and the Born approximation, may be invalid. After a short overview of the most popular optical‐data models, we present an approach to include exchange and correlation (XC) effects in IMFP calculations, thus going beyond the RPA and Born approximation. The key element is the so‐called many‐body local‐field correction (LFC). XC effects among the screening electrons are included using a time‐dependent local‐density approximation for the LFC. Additional XC effects related to the incident and struck electrons are included through the vertex correction calculated using a screened‐Hubbard formula for the LFC. The results presented for liquid water reveal that XC may increase the IMFP by 15–45% from its Born–RPA value, yielding much better agreement with available experimental data. The present work provides a manageable, yet rigorous, approach to improve upon the standard models for IMFP calculations, through the inclusion of XC effects at both the level of screening and the level of interaction. Copyright © 2015 John Wiley & Sons, Ltd.     

Simulation of positron and electron elastic mean free path and diffusion angle on DNA nucleobases from 10 eV to 100 keV

Positron and electron interaction collisions in living cells are efficiently simulated by Monte Carlo (MC) codes where huge data tables are needed. Present study provides detailed results of charged particles elastic interactions needed in MC on DNA nucleobases (adenine, thymine, cytosine and guanine, deoxyribose, and phosphoric acid). Indeed, electron and positron elastic cross sections, elastic mean free paths, and elastic angular distributions P(θ) are calculated from 10 eV to 100 keV using a corrected form of the independent atom method taking into account the geometry of the biomolecule. Our calculated results are compared with theoretical data available in the literature in absence of experimental data, in particular for positron. Moreover, our numerical results are presented in analytic format modeled to be used for fast sampling in the MC simulation of elastic collisions; particularly, we provide a useful analytic expression for sampling the elastic diffusion angle. For positron collisions on adenine, the relative error between numerical and analytic elastic diffusion angles is not exceeding 2% in the energy full range 10 eV to 100 keV.     

Backscattered electrons from gold surface films deposited on silicon substrates: a joint experimental and computational investigation to add new potentiality to electron microscopy

This paper addresses the problem of the thickness determination of thin gold overlayers deposited on silicon bulk substrates by looking at the electron backscattering coefficient involved in scanning electron microscopy (SEM). A Monte Carlo code, used to calculate the backscattering coefficient, together with a simple experimental setup, which uses a conventional SEM, allow to determine thin film thickness (in the range 25–200 nm) with an estimated accuracy of 20%. This adds obviously new potentiality to SEM. Copyright © 2012 John Wiley & Sons, Ltd.     

Quantitative analysis of silicon-oxynitride films by EPMA

Thin films of silicon oxynitride with diverse compositions were prepared by de-magnetron sputtering of silicon, utilising oxygen and nitrogen gas flows and the sputtering power to vary the composition. In order to investigate the composition of these films, a method of analysis by electron probe micro analysis with energy dispersive detection was developed and the figures of merit were compared to the wavelength dispersive method used by other authors. The precision and repeatability of the results are evaluated and the accuracy is checked by comparison with Rutherford backscattering and nuclear reaction analysis. Energy dispersive X-ray spectrometry was proven to be applicable to analyse silicon oxynitride films of any composition yielding quantitative results for nitrogen and oxygen as well as silicon. Besides the good analytical performance, electron probe micro analysis with energy dispersive X-ray spectrometry has turned out to be a non-destructive, quick, easy to use and cost effective tool for the routine analysis of light elements in thin films.     

Characterization of molecular linkages placed between zeolite microcrystal monolayers and a substrate with X-ray reflectivity

We prepared silicalite-1 microcrystal (MC) monolayers on a Si wafer using two different types of molecular linkages, namely, through chloropropyl (CP) groups and through CP/polyethylene imine/CP groups. Whereas the scanning electron microscope images of the two MC monolayers look very much alike but hardly give any information on the nature of molecular linkage between the monolayers and the substrate, their reflectivity curves are distinctively different, despite the fact that the thicknesses of the molecular linkage layers ( approximately 10-20 A) are negligibly small compared to the thicknesses of MC monolayers, ( approximately 3200 A). On the basis of the atomic force microscopic images of the MC surfaces, a rough surface layer with the thickness of approximately 160 A was introduced onto the surface of each MC to conduct a meaningful simulation of the curves with the recursive Parratt formalism. The obtained thickness, roughness, and density of each layer were reasonable, indicating that X-ray reflectivity is a very useful tool for the characterization of very thin layers of molecular linkages existing between much thicker MC monolayers and the substrate.