Search

In this account the application of synchrotron radiation based X-ray photoelectron spectroscopy (XPS) for the investigation of electrochemically active gas-solid and liquid-solid interfaces will be discussed. The potential of Near Ambient Pressure XPS (NAP-XPS) for the estimation of the surface electronic structure of electrochemically active interfaces will be described by two examples. Thereto the oxygen evolution reaction (OER) over Pt and IrO x anodes will be introduced. In particular the analysis of XP core level spectra of IrO x requires the development of an appropriate fit model. Furthermore the design of reaction cells based on proton exchange membranes (PEM) and on electron transparent graphene membranes, which enables the investigation of liquid-gas and liquid-solid interfaces under electrochemical relevant conditions will be discussed. In the last part of this article a perspective to the EMIL project at the synchrotron radiation facility BESSY will be given. The purpose of this project is the implementation of two new beamlines enabling X-ray photoelectron spectroscopy in the X-ray regime from 80 eV–8 keV under reaction conditions. The extension to the so called tender X-ray regime will allow the release of higher kinetic energy photoelectrons which have a higher inelastic mean free path compared to photoelectrons excited by soft X-ray radiation and therefore will enable the investigation of solid-liquid interfaces under electrochemical reaction conditions.

It is investigated how the model used to describe a dielectric function (i.e. a Mermin, Drude, Drude–Lindhard, Levine–Louie with relaxation time dielectric function) affects the interpretation of a REELS experiment, the calculation of the electron inelastic mean free path as well proton stopping and straggling. Three dielectric functions are constructed that are based on different models describing a metal, but have identical loss functions in the optical limit. A loss function with the same shape, but half the amplitude, is used to derive four different model dielectric functions for an insulator. From these dielectric functions we calculate the differential inverse mean free path, the mean free path itself, as well as the stopping force and straggling for protons. The similarity of the underlying physics between proton stopping, straggling and the electron inelastic mean free path is stressed by describing all three in terms of the differential inverse inelastic mean free path. To further highlight the reason why observed quantities depend on the model dielectric function used we study partial differential inverse inelastic mean free paths, i.e. those obtained by integrating over only a limited range of momentum transfers. In this way it becomes quite transparent why the observable quantities depend on the choice of model dielectric function.

Short-channel AlGaAs/GaAs High Electron Mobility Transistors (HEMTs) with gate lengths ranging up to 10 nm were fabricated using electron-beam lithography process. Transconductance starts to rise rapidly as the gate length becomes on the order of the inelastic mean free path of electrons in case of our devices below 80 nm. After reaching a maximum at around 40 nm, it is observed that both the measured transconductance and the subsequently extracted effective electron velocity drop rapidly with further reduction in gate length. We investigated this behavior with a transient transport model based on the retarded Langevin equation (RLE), which indicates the existence of a minimum acceleration length needed for the carriers to reach the overshoot velocity. Also, it clearly shows a degradation of the overshoot. This approach confirms the overshoot is limited by having a minimum acceleration length to reach the peak value of the overshoot due to a source resistance.

Six models, including a single-scattering model, four hybrid models, and one dielectric function model, were evaluated using Monte Carlo simulations for aluminum and copper at incident beam energies ranging from 0.5 keV to 10 keV. The inelastic mean free path, mean energy loss per unit path length, and backscattering coefficients obtained by these models are compared and discussed to understand the merits of the various models. ANOVA (analysis of variance) statistical models were used to quantify the effects of inelastic cross section and energy loss models on the basis of the simulated results deviation from the experimental data for the inelastic mean free path, the mean energy loss per unit path length, and the backscattering coefficient, as well as their correlations. This work in this study is believed to be the first application of ANOVA models towards evaluating inelastic electron beam scattering models. This approach is an improvement over the traditional approach which involves only visual estimation of the difference between the experimental data and simulated results. The data suggests that the optimization of the effective electron number per atom, binding energy, and cut-off energy of an inelastic model for different materials at different beam energies is more important than the selection of inelastic models for Monte Carlo electron scattering simulation. During the simulations, parameters in the equations should be tuned according to different materials for different beam energies rather than merely employing default parameters for an arbitrary material. Energy loss models and cross-section formulas are not the main factors influencing energy loss. Comparison of the deviation of the simulated results from the experimental data shows a significant correlation (p

A general procedure for peak intensity evaluation on the inelastic background of X-ray photoelectron spectra (XPS) has been developed, which is made up of tasks in three nested levels, level I, II and III. The task in level I is a simple background optimization for the case when the inelastic mean free path (IMFP, denoted λ(E)) and other constraints are given in advance [M. Jo, Surf. Interface Anal., 2003, 35, 729–737]. In order to find appropriate constraints, a batch job, which is a series of single jobs with systematically varied constraints, is constructed and iteratively solved in level II by improving the constraints from the viewpoint of how successfully the background is subtracted. It is found that, by the task in level II, for polycrystal Cu metal, the spectrum after inelastic background subtraction still contains a baseline component proportional to E −k, where E (>50 eV) is the electron kinetic energy and the exponent k around 2.4. This baseline is attributed to the high-energy tail of primary excitation spectrum of true secondary electron emission. The relative core intensities thus determined are in good agreement with the Scofield’s photoexcitation cross sections. The relation of core and Auger peak intensities is consistent as demonstrated by the fact that the sum of Cu 2p and 2s equals that of all Cu LMM peaks. The level III calculation is designed for the general case where λ is not available before the analysis. The relative energy dependence of λ except for the absolute value can be estimated by iterating the level II tasks with updated λ(E), based on the assumption that the peak intensities should be proportional to Scofield’s values. The relative deviation of thus determined λ(E) in level III from those by the well-known calculation (Tanuma, Powell, and Penn, TPP) is less than 1.7% between E = 600 and 2000 eV if their values at 800 eV are assumed to coincide. The normalized peak shapes and relative intensities after background subtraction in this level are also very similar to those obtained in level II starting with λ(E) by TPP that is currently known to be reliable. These findings indicate that an almost knowledge-free analysis, which is free from the detail of the solid’s properties such as chemical composition and nevertheless retains the material-dependent results, became possible for the first time, as long as one keeps away from the discussion of absolute intensity.

For a better control and understanding of the effects of radiation damage in living tissues by using the irradiation of biological systems by energetic ion beams have multiple applications in medical physics and space radiation health, such as hadron therapy for cancer treatment and it is necessary to advance an accurate description of the energy loss from the ion beam to the target. In the present work the dielectric formalism like Drude has been used to calculate the probability for an energetic proton and to produce electronic excitations in five targets of high biological interest, namely, liquid water, DNA, PMMA, Adenine and Guanine. Also, the effects of ionization fraction on the mean excitation energy, average energy and Inelastic mean free path studied taking in to consideration electronic excitation in the target. Good agreement achieved with previous work.

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials. Unfortunately this information is scarce when investigating solid/liquid interfaces and chemical or photochemical reactions under ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples present in the atmosphere or in the presence of a solvent. One promising approach is the use of graphene (Gr) windows transparent to both photons and electrons. This paper proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest during the fabrication stage. The GNBs were successfully employed to follow in-operando the thermal-induced reduction of FeCl3 to FeCl2 in aqueous solution. The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopy methods (XPS and XAS) in different phases of the process. The interaction of various components in solution with solid surfaces constituting the cell was obtained, also highlighting the formation of a covalent C–Cl bond in the Gr structure. For the easiness of GNB fabrication and straightforward extension to a large variety of solutions, we envisage a broad application of the proposed approach to investigate in detail electronic mechanisms that regulate liquid/solid electron transfer in catalytic and energy conversion related applications.

This paper studies the absolute thickness measurement of pyrolytic graphite spheroids (GSs) by using STEM-EELS mode with log-ratio method and Kramers-Kroning (K-K) method, taking the measured thickness from TEM image as reference that is the diameter of GSs ranging from 60 to 250 nm. The effect of collection semi-angle (β) on thickness measurement has been investigated. It is found that in general the thickness obtained by K-K analysis with surface effect corrected shows the best accuracy, followed by K-K sum rule and then log-ratio method for the three different collection semi-angles of 12.4, 17.3 and 21.1 mrad applied. Of these angles, the smallest one gives an overestimated result and the largest one gives an underestimated result, whereas between the two, the angle of 17.3 mrad that is about 2x convergence semi-angle (9.0 mrad) is identified as more appropriate for K-K analysis. The surface-scattering correction, inelastic mean free path of GS and effect of refractive index n on thickness measurement for different β angles are also investigated. Moreover, the optical property deduced from the data collected at the center of graphite spheroid, which is related to its microstructure, is characterized by K-K analysis.

In the present study, two kind of samples were used: (i) a monolayer graphene film with a thickness of 0.345 nm deposited by the CVD method on Cu foil, (ii) graphene flakes obtained by modified Hummers method and followed by reduction of graphene oxide. The inelastic mean free path (IMFP), characterizing electron transport in graphene/Cu sample and reduced graphene oxide material, which determines the sampling depth of XPS and AES were evaluated from relative Elastic Peak Electron Spectroscopy (EPES) measurements with the Au standard in the energy range 0.5–2 keV. The measured IMFPs were compared with IMFPs resulting from experimental optical data published in the literature for the graphite sample. The EPES IMFP values at 0.5 and 1.5 keV was practically identical to that calculated from optical data for graphite (less than 4% deviation). For energies 1 and 2 keV, the EPES IMFPs for rGO were deviated up to 14% from IMFPs calculated using the optical data by Tanuma et al. [1] . Before EPES measurements all samples were characterized by various techniques like: FE-SEM, AFM, XPS, AES and REELS to visualize the surface morphology/topography and identify the chemical composition.

The elastic and inelastic mean free paths of three metallic glass alloys, Ni60Nb40, Pd82Si18 and Ni80P20, have been measured from focused ion beam prepared thin samples with measured thickness gradients. The elastic/inelastic mean free paths of the three alloys are 35±0.5/97±3, 26±0.5/148±3 and 40±0.5/129±2.5nm, respectively. Elastic mean free paths predicted from atomic scattering cross sections consistently underestimate the experimental data. A model based on the Wentzel atomic model was developed and the fit to available data is in much better agreement with experiments, with a maximum deviation of ~4nm. The parametrized model is thus capable of predicting the elastic mean free path for other amorphous materials. Existing models for the inelastic mean free path are not in good agreement with the experimental data.

Electron transport properties in praseodymium (Pr) foil samples were studied by elastic-peak electron spectroscopy (EPES). Prior to EPES measurements, the Pr sample surface was pre-sputtered by Ar ions with ion energy of 2–3 keV. After such treatment, the Pr sample still contained about 10 at.% of residual oxygen in the surface region, as detected by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses. The inelastic mean free path (IMFP), characterizing electron transport within this region (4 nm-thick), was evaluated from EPES using both Ni and Au standards as a function of energy in the range of 0.5–2 keV. Experimental IMFPs, λ , were approximated by the simple function λ = kE p , where E is energy (in eV), and k = 0.1549 and p = 0.7047 were the fitted parameters. These values were compared with IMFPs for the praseodymium surface in which the presence of oxygen was tentatively neglected, and also with IMFPs resulting from the TPP-2M predictive equation for bulk praseodymium. We found that the measured IMFP values to be only slightly affected by neglect of oxygen in calculations. The fitted function applied here was consistent with the energy dependence of the EPES-measured IMFPs. Additionally, the measured IMFPs were found to be from 2% to 4.2% larger than the predicted IMFPs for praseodymium in the energy range of 500–1000 eV. For electron energies of 1500 eV and 2000 eV, there was an inverse correlation between these values, and then the resulting deviations of −0.4% and −2.7%, respectively, were calculated.

Electron transport is of fundamental importance and has application in a variety of fields. Different scattering mechanisms affect electron transport in the condensed phase; hence, it is important to comprehensively understand these mechanisms and their scattering cross sections to predict electron transport properties. Whereas electron transport is well understood for high kinetic energy (KE) electrons, there is a discrepancy in the experimental and theoretical values for the Inelastic Mean Free Path (IMFP) in the low KE regime. In this work, velocity map imaging soft X-ray photoelectron spectroscopy is applied to unsupported organic nanoparticles (squalene) to extract experimental values of inelastic and elastic mean free paths (EMFPs). The obtained data are used to calculate corresponding scattering cross sections. The data demonstrate a decrease in the IMFP and increase in the EMFP with increasing electron KE between 10 and 50 eV.

GaN is a promising material for power semiconductor devices used in next-generation vehicles. Its electrical properties such as carrier mobility and threshold voltage are affected by the interface between the oxide and the semiconductor, and identifying the interface states is important to improve these properties. A surface-sensitive measurement of Ga K-edge extended X-ray absorption fine structure (EXAFS) by detecting Ga LMM Auger electrons that originate from Ga K-shell absorption is proposed for GaN. LMM Auger electrons with low energies were detected and the EXAFS oscillation was confirmed, providing information on the Ga atoms at the surface. Investigation of thermally oxidized GaN with an oxide film of defined thickness showed that the analysis depth was less than 10 nm, which is consistent with the inelastic mean free path of 2.3 nm estimated for LMM Auger electrons in GaN.

The energy loss functions (ELFs) of three transition metals (Cr, Co and Pd) have been derived from reflection electron energy loss spectroscopy spectrum with a theoretical analysis of the measured data [Xu et al., J. Appl. Phys. 123, 043306 (2018)]. In this work, we update our previous ELFs in a wider photon energy region (0–200 eV) with a better accuracy, which is verified by sum rules and a root-mean-square deviation. The electron inelastic mean free paths (IMFPs) of Cr, Co and Pd have been calculated with the obtained ELFs by adopting a dielectric response theory. We employ both the single-pole approximation and full Penn algorithm for the calculation of IMFPs, and the calculated results are compared with other references.

The TPP-2M formula is the most popular empirical formula for the estimation of the electron inelastic mean free paths (IMFPs) in solids from several simple material parameters. The TPP-2M formula, however, poorly describes several materials because it relies heavily on the traditional least-squares analysis. Herein, we propose a new framework based on machine learning to overcome the weakness. This framework allows a selection from an enormous number of combined terms (descriptors) to build a new formula that describes the electron IMFPs. The resulting framework not only provides higher average accuracy and stability but also reveals the physics meanings of several newly found descriptors. Using the identified principle descriptors, a complete physics picture of electron IMFPs is obtained, including both single and collective electron behaviors of inelastic scattering. Our findings suggest that machine learning is robust and efficient to predict the IMFP and has great potential in building a regression framework for data-driven problems. Furthermore, this method could be applicable to find empirical formula for given experimental data using a series of parameters given a priori, holds potential to find a deeper connection between experimental data and a priori parameters., GRAPHICAL ABSTRACT

The using of radiation in cancer therapy is covered a large kind of these treatments, that can by swift protons. The mean excitation energy, the effective number of electrons, probability per unit path length, the average energy of secondary ejected electrons, single differential cross section and inverse inelastic mean free path have been calculated in the energy range (0-10 MeV). Those were done in the scope of dielectric formalism, the energy loss function is characterized by the single Drude function for outer electron effects and by general oscillator strength GOS for inner electrons (k-shell), that for five compounds liquid water H2O, DNA, Adenine, Guanine and PMMA. Our result give value of ionization potential I(w) = 79.21 eV, 86.75 eV, 73.13 eV, 79.24 eV and 71.06 eV for DNA, liquid water, Adenine, Guanine and PMMA. Most probability and single differential cross section happened at lower incident energy proton, secondary electrons ejected with apart of maximum average energy in DNA that about 70 MeV and higher peak in description of invers inelastic mean path length also were in DNA around 0.2 MeV. Good agreement is achieved with available previous work. The work has been programing analytical by Fortran 90.

We review new self-consistent models of inelastic electron scattering in condensed matter systems for accurate calculations of low-energy electron inelastic mean free paths (IMFPs) for XAFS and low energy diffraction. The accuracy of theoretical determinations of the electron IMFP at low energies is one of the key limiting factors in current XAFS modeling and Monte Carlo transport. Recent breakthroughs in XAFS analysis show that there exist significant discrepancies between theoretical and experimental IMFP values, and that this can significantly impact upon extraction of other key structural parameters from both XANES and XAFS. Resolution of these discrepancies is required to validate experimental studies of material structures, and is particularly relevant to the characterization of small molecules and organometallic systems for which tabulated electron scattering data is often sparse or highly uncertain. Novel models implement plasmon coupling mechanisms for the first time, in addition to causally-constrained lifetime broadening and high-precision density functional theory, and enables dramatic improvements in the agreement with recent high profile IMFP measurements. We discuss a theoretical approach for IMFP determination linking the optical dielectric function and energy loss spectrum of a material with its electron scattering properties and characteristic plasmon excitations. We review models inclusive of plasmon coupling, allowing us to move beyond the longstanding statistical approximation and explicitly demonstrate the effects of band structure on the detailed behaviour of bulk electron excitations in a solid or small molecule. This interrogates the optical response of the material, which we obtain using density functional theory. We find that new developments dramatically improve agreement with experimental electron scattering results in the low-energy region (30 eV → 200 eV) where plasmon excitations are dominant. Corresponding improvements are therefore made in Low Energy Electron Transport, LEEM, theoretical XAFS spectra and detector modelling.

Based on a recent experimental data of elastic transmissivity and the elastic reflectivity measured on graphene, we have performed a theoretical study on electron inelastic mean free path (IMFP) and elastic mean free path (EMFP) by an improved approximation in a classical electron trajectory framework. A hump feature, which is considered to be owing to the out-of-plane mode of the π + σ plasmon, is clearly shown in our IMFP results of multilayer graphene while it is not seen in that of monolayer graphene. The obtained EMFPs are one order of magnitude greater than previously reported results. This work shows that the classical electron trajectory framework still works for revealing the physics picture of low-energy electron interaction with graphene, even for the transverse direction of monolayer graphene, which is the thinnest material.

Although precision in XPS can be excellent, allowing small changes to be easily observed, obtaining an accurate absolute elemental composition of a solid material from relative peak intensities is generally much more problematical, involving many factors such as background removal, differing analysis depths at different photoelectron kinetic energies, possible angular distribution effects, calibration of the instrument transmission function, and variations in the distribution of photoelectron intensity between “main” peaks (those usually used for analysis) and associated substructure following the main peak, as a function of the chemical bonding of the elements concerned. The last item, coupled with the use of photoionization cross sections and/or relative sensitivity factors (RSFs), is the major subject of this paper, though it is necessary to consider the other items also, using LiF as a test case. The results show that the above issues, which are relevant to differing degrees in most XPS analyses, present significant challenges to highly accurate XPS quantification. LiF, using the Li1s and F1s XPS peaks, appears, at first sight, to be an ideal case for high accuracy. Only 1s core levels are involved, removing any possible angular effects, and it is a wide bandgap material, resulting in the main Li1s and F1s peaks being well separated from the following scattered electron backgrounds. There are, however, two serious complications: (1) the main F1s and F2s levels have a major loss of intensity diverted into satellite substructure spread over ∼100 eV KE from the main line, whereas the Li1s level has very much less diversion of intensity; (2) there is serious overlap of the substructure from F2s (∼30 eV BE) with the main line of Li1s at ∼56 eV. We report here a detailed analysis of the LiF XPS, plus a supporting theory analysis of losses of intensity from Li1s and F1s to satellite structure, based on the cluster models of LiF. We conclude that, if the overlap from the F2s substructure is correctly subtracted from Li1s, and the intensity from satellites for F1s and Li1s properly estimated, the atomic composition of the single crystal LIF may be recovered to within 5%, using the photoionization cross sections of Scofield, inelastic mean free path lengths based on Tanuma, Powell, and Penn, and the calibrated instrument transmission function. This refutes the claim by Wagner et al., based on their empirical determination of RSFs, (which applied only to the instruments and the analysis procedure they used, in 1981) that Scofield values are too low in general and, for Li1s in particular, are low by a factor of ∼2. This is important because Wagner-based RSFs (sometimes modified and sometimes not) are still embedded in quantification software on modern commercial instruments, and so analysts need to be aware of how those RSFs were obtained/modified. Incorrect use can lead to large quantification errors.

We investigate band bending effects occurring at the interface between atomically clean Ge(001) and molecular beam epitaxy (MBE) deposited copper and platinum. Low energy electron diffraction (LEED) confirmed the crystallinity of the surface, evidenced the formation of (2 × 1) and (1 × 2) reconstructions, and revealed that it is strongly affected with metal deposition. X-ray photoelectron spectroscopy (XPS) data let us assume a Stranski–Krastanov growth mechanism and confirmed that the observed band bending is associated to an ohmic contact in both cases. For the platinum contact, the high values of the apparent inelastic mean free path (IMFP) derived from the evolution of the XPS intensities indicate a prevalence of mixture of Pt with Ge nearby the interface. Pt deposited on Ge(001) does not behave like a Schottky contact, as one may have expected due to the higher work function of platinum. The observed effect is similar to the case where interfacial Pt had a lower work function by 2.25/1.96 eV than that of metallic Pt. We propose a model to explain this fact by the effective mass variation or to the conduction band broadening due to the strong intermixing of platinum with germanium under the surface.

Composition dependence of the dielectric and optical properties of (HfZrO 4 ) (1 − x ) (SiO 2 ) ( x ) (0 ≤ x ≤ 0.2) gate dielectric thin films, grown on Si(100) by the atomic layer deposition method, was investigated by means of reflection electron energy loss spectroscopy (REELS). The quantitative analysis of REELS spectra was carried out by using the QUASES-XS-REELS and QUEELS-e( k , ω )-REELS software, which determine the dielectric function and optical properties through an analysis of experimental REELS in terms of a simulated energy loss function (ELF). For HfZrO 4 , the ELF showed peaks in the vicinity of 10.5, 15.6, 18, 21.5, 26.7, 34.5, 39.5, 46.5 and 57 eV. For HfZr-silicates with low SiO 2 concentration (x = 0.10 and 0.15), the peak positions were similar to those of HfZrO 4 , but for x = 0.20, the number of peaks were reduced and are at 14.8, 23.5, 34.5, 40.5, and 46.5 eV. These peaks originate from the superposition among d electron states of Zr, f electron states of Hf and p electron states of Si. The strength of the peak at 46.5 eV decreased as we increased the amount of SiO 2 in the compounds, which indicates that it is due to excitation between Hf 4f and Zr 3d electron states. Changes in the complex dielectric function, optical properties and band gap related to the SiO 2 concentration in the films were discussed systematically. In addition, the inelastic mean free path (IMFP) was also calculated from the determined dielectric function. The IMFP of the HfZr-silicates increased with increasing SiO 2 content and with increasing primary energy. This method is of high importance in terms of determining the dielectric and optical properties and inelastic mean free path from REELS spectra. The advantage of the method applied here is that no information on the morphology and components of the sample is required. This means that this method presented here turned out to be a convenient and efficient tool for investigation of optical properties and inelastic mean free path of ultrathin high- k alloy materials.

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.

New theoretical model describing the sampling depth of elastic peak electron spectroscopy (EPES) has been proposed. Surface sensitivity of this technique can be generally identified with the maximum depth reached by trajectories of elastically backscattered electrons. A parameter called the penetration depth distribution function (PDDF) has been proposed for this description. Two further parameters are descendant from this definition: the mean penetration depth (MPD) and the information depth (ID). From the proposed theory, relatively simple analytical expressions describing the above parameters can be derived. Although the Monte Carlo simulations can be effectively used to estimate the sampling depth of EPES, this approach may require a considerable amount of computations. In contrast, the analytical model proposed here (AN) is very fast and provides the parameters PDDF, MPD and ID that very well compare with results of MC simulations. As follows from detailed comparisons performed for four elements (Al, Ni, Pd and Au), the AN model practically reproduced complicated emission angle dependences of the MPDs and the IDs, correctly indicating numerous maximum and minimum positions. In the energy range from 200 eV to 5 keV, the averaged percentage differences between MPDs obtained from the MC and the AN models were close to 4%. An important conclusion resulting from the present studies refers to the procedure of determination of the inelastic mean free path (IMFP) from EPES. Frequently, the analyzed sample is deposited as a thin overlayer on a smooth substrate. From an analysis of the presently obtained IDs, is follows that 99% of trajectories in analyzed experimental configurations reaches depth not exceeding 2.39 in units of IMFP. Thus, one can postulate that a safe minimum thickness of an overlayer should be larger than about 3 IMFPs. For example, the minimum thickness of an Al overlayer shoud be about 8 nm at 5000 eV.

The energy dependence of the electron inelastic mean free path, λ(E), in silicon oxide and silicon nitride is experimentally determined via Auger electron spectroscopy according to Auger signal attenuation with varying film thickness. Silicon-oxide- and silicon-nitride films are formed on different metal substrates by means of plasma-enhanced chemical vapor deposition. Analysis of the results and their comparison with theoretical data indicate that, in the chosen material, variations in the electron mean free path versus their energy are estimated more reliably by means of experiments than through the use of universal theoretical curves. It appears that the results obtained in this work can help in the more accurate determination of the width and location of interfaces in multilayer structures.

Two-dimensional dopant mapping using secondary electrons (SEs) in the scanning electron microscope (SEM) is a technique under intense research and development due to improvements in instrumental resolution and its potential to enable rapid low-cost diagnostics in process optimization for microelectronic and optoelectronic fabrication. However, an initial drawback of the technique is the lack of a complete quantitative model to obtain accurate information on dopant profiles. This paper focuses on detailed studies of dopant profiling quantification on a wide range of doped silicon homojunctions in an SEM under various operating conditions for the first time. Doping type and geometry-dependent effects from the specimen are taken into account, and improved sensitivity, resolution, and quantification accuracy due to energy filtering of the SEs are demonstrated. The resulting SE intensities under varying operating conditions are surveyed with an indication of aspects of the mechanism responsible for doping contrast, including the patch fields, surface band bending, and inelastic mean free path effects in the specimen, as well as the angular velocities of the SEs. Although all samples satisfied the classical logarithmic doping dependence of contrast approximation, some of them required specialized energy-filtering techniques. The theoretical model based on energy filtering to determine the built-in voltage across the $p\!-\!n$ junction that is in the literature does not accurately apply to $n^{+}\!-i-n$ homojunctions. The surface band bending and inelastic mean free path dependence of the SE kinetic energy models have been derived in this paper, which accurately describe the quantitative contrast characteristics from donor distributions, thereby producing more accurate quantification of this technique in semiconductor research and industry.

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.

Probability of elastic electron backscattering from surfaces is typically calculated from theoretical models implemented in Monte Carlo simulation strategies since this approach is considered to be the most accurate. However, an analytical model proposed by Oswald et al. [J. Electron Spectrosc. Relat. Phenom. 61 (1993) 251], after later modifications, has been found to be of similar accuracy. The relevant analysis was performed for selected elemental solids. In the present work, a possibility to use the analytical formalism in elastic peak electron spectroscopy for determination of the IMFP has been presently studied. For this purpose, two further modifications of the analytical theory were made: (i) creation of a database of parameters facilitating calculations of the angular distribution of electrons after multiple collisions, and (ii) extension of the formalism to solids constituted of different atomic species, i.e. alloys and compounds. Comparison of experimental data (angular distribution of backscattered electrons, ratios of elastic peak intensities) with predictions of both theoretical models proves that these models are of similar accuracy. The IMFPs for elemental solids derived from experimentally measured ratios using the analytical formalism and the Monte Carlo simulations are practically identical. On analysis of IMFPs obtained for 13 elemental solids in the energy range from 200 eV to 5000 eV, it has been estimated that the IMFPs obtained from the analytical formalism deviate on average from IMFPs from Monte Carlo calculations by 3.09%. Similar agreement was found for an alloy (Al0.48Ni0.52) and compounds (GaSb, InSb, GaN and SiC) considered here.

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.

The effective attenuation length (EAL) is a useful parameter in quantitative applications of x-ray photoelectron spectroscopy (XPS). This parameter is used in place of the inelastic mean free path (IMFP) in expressions for different XPS applications to correct those expressions for elastic scattering of the photoelectrons. We consider expressions used to determine (i) the thickness of an overlayer film on a planar substrate, (ii) the surface composition, (iii) the depth of a thin marker or delta layer, and (iv) the shell thickness of a core–shell nanoparticle. An EAL can be used for each of these applications. In general, the EAL depends on the particular defining equation as well as on the XPS configuration. Many attempts were made in the 1970s and 1980s to measure EALs for the determination of overlayer-film thicknesses, but there were often wide scatters in the reported results due to the difficulty in preparing uniform films with known thicknesses. We have therefore been motivated to calculate EALs for each application. The SRD 82 database from the National Institute of Standards and Technology (NIST) provides EALs for the measurement of overlayer-film thicknesses and of marker-layer depths. These EALs can be determined for photoelectron energies between 50 eV and 2 keV and for user-specified XPS configurations. We review EAL predictive equations for the determination of overlayer-film thicknesses on a planar substrate for XPS with unpolarized x rays and with linearly polarized x rays as well as an EAL predictive equation for quantitative analysis by XPS. These equations are simple analytical expressions that are valid for well-defined ranges of experimental conditions and for useful ranges of electron energies. We also point out that EALs for the determination of overlayer-film thicknesses can be derived from the simulated photoelectron intensities obtained from the NIST Database for the Simulation of Electron Spectra for Surface Analysis (SRD 100). Where possible, we make comparisons of the calculated EALs with illustrative experimental results. A key parameter in the EAL predictive equations is the so-called albedo, a useful measure of the strength of elastic-scattering effects in a material. The albedo is a simple function of the IMFP and the transport mean free path (TRMFP). We provide a tabulation of albedo and TRMFP values in the supplementary material for 41 elemental solids and 42 inorganic compounds for photoelectron energies between 50 eV and 30 keV. For other materials, albedo values can be determined from IMFP and TRMFP data available in the NIST SRD 82 and SRD 100 databases.

Mo disulfide overlayers with the thickness exceeding 1.77 nm were obtained on Si substrates through mechanical exfoliation. The resulting Mo disulfide flakes were then analyzed ex situ using combination of Auger electron spectroscopy (AES), elastic-peak electron spectroscopy (EPES) and scanning electron microscopy (SEM) in order to characterize their surface chemical composition, electron transport phenomena and surface morphology. Prior to EPES measurements, the Mo disulfide surface was sputter-cleaned and amorphized by 3 kV argon ions, and the resulting S/Mo atomic ratio varied in the range 1.80&ndash, 1.88, as found from AES measurements. The SEM images revealed single crystalline small-area (up to 15 &mu, m in lateral size) Mo disulfide flakes having polygonal or near-triangular shapes. Such irregular-edged flakes exhibited high crystal quality and thickness uniformity. The inelastic mean free path (IMFP), characterizing electron transport, was evaluated from the relative EPES using Au reference material for electron energies E = 0.5&ndash, 2 keV. Experimental IMFPs, &lambda, determined for the AES-measured surface compositions were approximated by the simple function &lambda, = kEp, where k = 0.0289 and p = 0.946 were fitted parameters. Additionally, these IMFPs were compared with IMFPs resulting from the two methods: (i) present calculations based on the formalism of the Oswald et al. model, (ii) the predictive equation of Tanuma et al. (TPP-2M) for the measured Mo0.293S0.551C0.156 surface composition (S/Mo = 1.88), and also for stoichiometric MoS2 composition. The fitted function was found to be reasonably consistent with the measured, calculated and predicted IMFPs. We concluded that the measured IMFP value at 0.5 keV was only slightly affected by residual carbon contamination at the Mo disulfide surface.

The electron inelastic mean free path (IMFP) is an important quantity for electron spectroscopy and microscopy techniques. At present, there are very few data of IMFPs for two-dimensional (2D) materials. Here, we determine the IMFP at energies below 100 eV for monolayer graphene both experimentally and theoretically. The experimental determination is based on a data-driven spectral analysis technique, including the virtual substrate method and the reverse Monte Carlo method. The theoretical determination is performed within the dielectric formalism, using the energy-loss function calculated in the time-dependent density functional theory. The experimental and theoretical results show that the IMFP for monolayer graphene is almost constant (about 1 nm) in the energy range of 6–100 eV. This study suggests a general and reliable approach to determine low-energy IMFPs for 2D materials.

The study of the chemical composition of buried interfaces by X-ray photoelectron spectroscopy (XPS) is limited by the inelastic mean free path of emitted photoelectrons (PE). Soft X-ray sources (AlKα) are generally suitable for careful probing of surfaces or very thin films. Here we applied gas cluster ion beam sputtering in combination with in-situ XPS (GCIB-XPS) to analyze buried GaP/Si(0 0 1) heterointerfaces. The GCIB method was used to dig a crater into the 20 nm thick GaP(0 0 1) film. We found optimal parameters for GCIB sputtering and achieved interface layers without severe damage. Destructive effects, i.e. broadening of core level peaks, could not be completely avoided, however, and the formation of metallic Ga on the GaP surface was observed. PE spectra of the sputtered heterostructures were compared with corresponding reference spectra of sputtered bulk crystals. Interface contributions to the intensity of phosphorus and sillicon core level peaks were revealed: interface components are shifted to high binding energies of P 2p Si 2p core levels. Similar results were obtained on 4 nm thick GaP/Si(0 0 1) by XPS. Finally, a top-to-bottom concept for buried semiconductor interfaces studies by GCIB-XPS is demonstrated.

Since the emission of secondary electrons for any incident energy always involves the formation and emission of a cascade of slow electrons (

Inelastic mean free path (IMFP) was determined by electron energy loss spectroscopy (EELS) to estimate the accurate thickness of TEM thin foil using needle-shaped specimen. From EELS measurements performed for 99.99% Al, Si wafer and 99.99% Fe, linear relationships were confirmed between the thickness of the TEM thin foil and the ratio of the total intensity of EELS spectrum to the total intensity of zero-loss spectrum for all samples. By weighted least-square fitting, the IMFP was estimated to be 143−150 nm for Al, 159−165 nm for Si with amorphous layer, and 92−94 nm for Fe with the collection semi-angle β = 11.9 − 35.7 mrad, and accelerated voltage of 200 kV. Thus, the dependence of IMFP on β is not dominant. The accuracy depends on the roundness of the cross-section of the needle-shaped specimen, and is observed to be low in terms of percentage in this work.

An investigation of the optical and electronic properties of amorphous silicon dioxide by means of a combination of reflection electron energy loss spectroscopy (REELS) and secondary electron–electron energy loss coincidence spectroscopy (SE2ELCS) is presented. Optical constants for a-SiO2 were extracted from the REELS measurements and a band gap of 9.1 eV was determined by deconvolution of multiple scattering and fitting the differential inverse inelastic mean free path with a model energy loss function (ELF). The coincidence measurements allow to determine the surface barrier height and the electron affinity was determined to be 0.8 eV. Furthermore, the coincidence measurements show that even in the case of an insulator, plasmon decay is the main mechanism for generation of secondary electrons.

Information is provided on four terms that are used for different purposes in x-ray photoelectron spectroscopy (XPS): the inelastic mean free path (IMFP), the effective attenuation length (EAL), the mean escape depth (MED), and the information depth (ID). While the IMFP is a parameter that depends on both the material and electron energy, the other three terms depend on the IMFP, the instrumental configuration, and the magnitude of elastic-scattering effects in the sample material. In addition, different EALs can be defined for different XPS applications, and the numerical values for each application can differ. Guidance is given on sources of IMFP and EAL data and on predictive equations for IMFPs, EALs, MEDs, and IDs. This guide is one of a series intended to highlight best practices in the use of XPS.

The emerging field of electronics based on ferro-functional materials relies on driving effectively and predictably a ferroelectric system between different polarization states through bias applied to metallic contacts. This requires detailed understanding of the growth mechanisms and electronic properties of the interface, including ferroelectric and material – dependent band alignment and Schottky barrier heights. Whether the major contribution at the interface band alignment comes from the work function difference or from the ferroelectric state is still under debate. Here, using X-ray photoemsion and ab-initio calculations, we derive the complex microscopic picture of metal/ferroelectric interface formation, including growth mechanism, valence alteration, ferroelectric-dependent electrostatic potential and thickness – dependent compensation mechanisms of ferroelectricity, starting from the ultrathin growth of Cu up to 100 A on BaTiO3. One establishes the evolution of the band bending and of the build-in potential from the initial probed thickness of the ferroelectric in the range of 3 λ ( λ – the inelastic mean free path) while gradually approaching the contact region with the metal at higher thickness of the top layer. We find that the well-defined orientation of the ferroelectric polarization lead to a band bending at the interface, which add at the bending expected from the work function difference of the two joining materials.