B. Lesiak

Experimental determination of the inelastic mean free path of electrons in solids

Abstract The method based on the elastic peak electron spectroscopy was used to determine the energy dependence of the IMFP for different carbon samples and selected metals. This method provides the values of the IMFP corresponding to the definition of the E-42 Committee of the ASTM. The measurements were made with respect to the aluminium standard. The main features of the method were extensively discussed: the choice of the aluminium standard, quantification of the elastic peak intensity, and the validity of the theoretical model relating the elastic peak intensity to the IMFP. The values of the IMFP were obtained for energies ranging from 270 to 2350 eV. The results are in good agreement with the theoretical data available in the literature. Attention is drawn to the fact that the density of the sample should be known, since this parameter influences greatly the resulting values of the IMFP.

Derivation of the electron inelastic mean free path from the elastic peak intensity

Measurements of the elastic peak intensity make possible estimation of the true inelastic mean free path (IMFP). Corresponding experiments are relatively simple and can be performed for any sample. The theoretical models can be used to derive the relation between elastic peak intensity and IMFP for single and multiple elastic scattering. In the present work both models were based on the differential scattering cross-sections calculated within the partial wave expansion method. Examples of calculations of the IMFP using both theoretical models are presented.

Multiple scattering analysis for determining the electron inelastic mean free path (IMFP) by elastic peak electron spectroscopy

Abstract The IMFP of electrons in high atomic number solids was determined using elastic peak electron spectroscopy. The Monte Carlo calculations based on the differential elastic scattering cross-sections resulting from the partial wave expansion method made possible the accurate analysis of experimental results. Multiple elastic scattering of electrons is playing a dominant role in high atomic number elements. Experiments provided the ratio of elastic peak intensities for a given sample and the standard. The CMA was used in the measurements. In this work the IMFP of Ta, W and Au was studied, using Al as a standard. Multiple elastic backscattering probability. The ration of angular distribution of elastically backscattered electrons and the elastic backscattering probability. The ratio of elastic peak intensity of the sample and of the standard was calculated for the CMA detecting angle (θ = 138°) and fitted to the experimental results. Such procedure provides the true value of the IMFP. Results were obtained in the energy range between 500 and 3000 eV. They are compared to the available literature data.

Determination of the inelastic mean free path (IMFP) of electrons in germanium and silicon by elastic peak electron spectroscopy (EPES) using an analyser of high resolution

Abstract The IMFP of electrons is a fundamental material parameter of surface analysis by AES, XPS, EPES and EELS. In surface analysis calculated IMFP values are used. Their experimental determination is rather difficult. The IMFP of amorphous Ge and polycrystalline Si was determined by comparing the elastic peak intensity ratios with electrolytic Ni reference sample of 1 nm surface roughness, achieved by dedicated Ar+ ion bombardment cleaning and examined by STM. Experimental results obtained with a hemispherical analyser type ESA 31 developed by ATOMKI Debrecen have been evaluated by Monte Carlo analysis, based on Jablonskis differential elastic scattering cross sections and elaborated for the HSA analyser angular window. Due to the 5 × 10−5 energy resolution of the ESA 31 no spectrometer correction was needed. The ratio of the background to the elastic peak was

Inelastic mean-free path of electrons at nanocrystalline diamond surfaces

Surprisingly large difference between inelastic mean-free path (IMFP) values of signal electrons calculated from optical data and from the predictive TPP-2M formulae for diamond or graphite has motivated us to verify both sources of IMFP by measuring these values using different method. Thin, perfectly transparent nanocrystalline diamond films were characterized by x-ray induced photoelectron and Auger electron spectroscopy. Elastic peak electron spectroscopy measurements and the Monte Carlo calculations of electron transport were applied to determine the IMFPs for medium electron energy. The surface energy losses were also considered when evaluating these IMFPs.

Surface composition of the CoPd alloys studied by electron spectroscopies

Palladium-based alloys and bimetallic systems are of importance in heterogeneous catalysis. Recently, attention was devoted to studies of the catalytic properties of the Co-Pd binary system. An important issue to consider is the surface composition of this alloy. The aim of the present work was to determine the surface composition of the polycrystalline CoPd alloys in a wide range of constituent concentrations (Co30Pd70 at.%, Co50Pd50 at.% and Co70Pd30 at.%) at temperatures up to 900°C. Quantitative AES and XPS analyses were used for this purpose. To determine the surface composition, the values of the inelastic mean free path (IMFP) for the studied alloys are necessary. These were derived from the elastic peak intensity. The reported values of the IMFP for Pd are in agreement with the data available in the literature. Reasonable consistency of the IMFP values for CoPd alloys and Co was also observed. The surface compositions of the CoPd alloys at room temperature determined by AES and XPS are in good agreement. It has been found that Pd segregates to surfaces of the Co30Pd70 and Co50Pd50 alloys at temperatures above 300 °C.

Surface excitation effects in elastic peak electron spectroscopy

Abstract 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.

Determination of the inelastic mean free paths of electrons in copper and copper oxides by elastic peak electron spectroscopy (EPES)

Theoretical values of the inelastic mean free path (IMFP) and their electron-energy dependence are available in tbe literature from predictive formulae for various categories of materials, such as elemental solids, inorganic and organic compounds. in contrast, the experimental IMFP values were determined for a more limited number of materials. This refers especially to multicomponent solids. We have measured the IMFP dependence on energy for Cu and two copper oxides, CuO and Cu 2 O, using elastic peak electron spectroscopy. The experiment consisted of measuring the backscattering probabilities for the investigated materials and an Ni standard in the energy range 400-1600 eV. The IMFP values were determined using a theoretical model describing the phenomenon of elastic backscattering. The IMFP values determined in this work are in reasonable agreement with experimental and theoretical values published in the literature.

Recoil effect in carbon structures and polymers

Abstract The recoil effect on quasi-elastic scattering of electrons has been described by Boersch. The Rutherford type scattering on the nucleus produces a shift of the elastic peak maximum of Δ E em , proportional to 1/ M (atomic mass) and electron energy. Laser et al. studied the recoil effect for exact calibration of the energy scale for XPS. The other effect is broadening of the elastic peak. Reflection electron energy loss spectra were studied by an electron spectrometer ESA 31 of high energy resolution. Various modifications of C (electrode and reactor graphite, glassy C, powder) and conducting polymers (polyacetylene, polyaniline, polythiophene) have been studied with varying composition of C, O, H, S, Pd etc. In the case of the samples containing different elements the complex nature of the elastic peaks (consisting of the sum of components having different intensities and shifts) was clearly observed. Possible explanations for the recoil effects detected are reviewed.

Determination of the inelastic mean free path of electrons in GaAs and InP after surface cleaning by ion bombardment using elastic peak electron spectroscopy

Abstract The inelastic mean free path (IMFP) of electrons is an important material parameter needed for quantitative AES, EELS and non-destructive depth profiling. The distinction between the terms for IMFP and the attenuation length (AL) has been established by ASTM standards. A practical experimental method for determining values of the IMFP is elastic peak electron spectroscopy (EPES). In this method, experimentally determined ratios of elastically backscattered electrons from test surfaces and from a Ni reference standard are compared with the values evaluated theoretically. The present paper reports systematic measurements of the IMFP by EPES for GaAs and InP. They are carried out in two laboratories using two different electron spectrometers: a CMA in Budapest and DCMA in Warsaw. Prior to measurements, the samples were amorphized by high-energy Ar + ions (100–400 keV), and the surface composition was determined by quantitative XPS. Argon cleaning produces enrichment of samples in the surface layer in Ga (80%) and In (70%), respectively. The experiments refer to a such modified sample surface that was considered in Monte Carlo calculations. The experimental data were analyzed using calibration curves from Monte Carlo calculations which account for multiple elastic scattering events. This approach has been used previously for elemental solids and is now extended to amorphized binary compounds. The experimental values of IMFP obtained in both laboratories exhibited a reasonable agreement with the available literature data in the 0.1–3.0 keV energy range. With respect to the information depth of EPES, the experimental results refer to the bulk composition within a reasonable extent.