G. Gergely

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.

The elastic peak in AES and EELS

Abstract Elastic peak electron spectroscopy (EPES) determines the spectra of secondary electrons in absolute units in the vicinity of the elastic peak including also loss peaks. The percentage of elastically reflected electrons N e is determined by comparing the peak to total spectrum area. Quantitative data can be deduced from the measurement of the coefficient of secondary emission or electron reflection. N e is mainly determined by the atomic number for medium energy excitation (1–3.2 keV). It can be used as reference for quantitative AES or EELS. This paper deals with the application of EPES. Results obtained on the backscattering cross section of C, Si, Ge, Mo, W and Au are presented. The K core level ionization cross section of carbon was determined. The volume plasmon loss process is discussed. EPES was used for evaluating some experimental results on the excitation cross section of a silicon surface state and vibrations of CO adsorbed on Pt have also been determined.

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

Experimental determination of the inelastic mean free path (IMFP) of electrons in Cr, Mo, Ge and Si based on the elastic peak intensity ratio with a Ni reference sample

Abstract The inelastic mean free path (IMFP) of electrons has been determined for selected elemental solids using elastic peak electron spectroscopy (EPES) and a Ni standard. The IMFP was evaluated for the range of 500–3000 eV on Cr, Mo, Ge and Si materials. The Ni standard surface has been prepared by electrolysis and HV vapour deposition. Its quality was verified by AES and STM. The theoretical model relating the elastic peak intensity to the value of the IMFP was based on relativistic scattering cross sections and the multiple elastic scattering events were simulated by a Monte Carlo procedure. Reasonable agreement of the obtained IMFP values and their dependence on the energy with the data by Tanuma et al. and by Ashley et al. was found.

Experimental determination of the inelastic mean free path for Cu, Ag, W, Au and Ta, in the energy range 500-3000 eV by elastic peak electron spectroscopy and using Ni reference sample

Abstract The inelastic mean free path (IMFP) is the most important electron transport parameter. The value of IMFP can be determined by electron reflection experiments (EPES) supported by the Monte Carlo theory with the multiple elastic scattering events considered. The values of IMFP for the high atomic number elements obtained from such a model and applying AI standard were found to be in reasonable agreement with the theoretical values. Recently, Ni has been proved to be a more adequate reference sample due to the lack of inelastic losses appearing in the vicinity of the elastic peak. In the present work the energy dependence of the IMFP for Cu, Ag, W, Ta and Au using Ni standard was evaluated by EPES in the energy range 500–3000 eV. The Ni standard of high quality was used, where its surface roughness was verified by scanning tunnelling microscopy. Reasonable agreement with Tanuma et al. results was obtained in the energy range 500–2000 eV. Above 2000 eV the obtained results were in agreement with the values by Ashley and Tung.

Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

Abstract The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin (d2 = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick (d3 = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Ψ and Δ values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for λ = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72, 6.40, 4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n2 = 1.77 (alumina) and n3 = 1.58 (hydrated oxide). Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the d2 and d3 values of the samples was found, owing to hydration.

Surface morphology development during ion sputtering: roughening or smoothing?

We report on STM studies of ion-sputtered surfaces, applying sputtering conditions which were shown to produce a relatively smooth surface. The height correlation function was calculated for the nickel layer in both the as-received and sputtered conditions. The large-scale roughness of the as-received specimen was reduced by ion sputtering according to expectations derived from Auger depth profiling. On the other hand, the small-scale roughness was increased due to sputtering. Self-affine scaling regions are identified, and the exponents are compared to theoretical and numerical results.

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.