Werner Smekal

Surface excitation probability of medium energy electrons in metals and semiconductors

Abstract Reflection energy electron loss spectra (REELS) have been measured for several metals and semiconductors (Be, Al, Si, V, Fe, Co, Ni, Cu, Ge, Mo, Pd, Te, Ta, W, Au, Pb) in the medium energy range (150–3400 eV) for normal incidence and an emission direction of 60° with respect to the surface normal. The ratio of the number of electrons that induced a surface excitation to the intensity of the elastic peak was extracted from each spectrum providing the so-called surface excitation parameter (SEP). This quantity is equal to the average number of surface excitations an electron experiences when it crosses the surface once. For the nearly free-electron materials the results agree reasonably with free-electron theory while significant deviations are observed for the other materials. In all cases the SEP is found to be inversely proportional to the speed of the probing particle. It is generally found that the surface excitation parameter decreases with the generalized plasmon energy. A simple predictive formula to estimate the surface excitation parameter for medium energy electrons entering or leaving an arbitrary material is proposed.

Angular dependence of the surface excitation probability for medium energy electrons backscattered from Al and Si surfaces

Reflection electron energy loss spectra have been measured for medium energy electrons backscattered from Al and Si surfaces. Angular distributions were obtained for emission angles between 15° and 90° with respect to the surface normal as well as for incidence angles in the same range. The surface excitation parameter (SEP), i.e., the average number of surface excitations an electron experiences when it crosses a surface once, was extracted from each spectrum by fitting the raw data to theory and determining the ratio of the surface loss peak to the elastic peak intensity. No difference in the SEP for incoming and outgoing electrons could be detected in the data. The SEP was found to depend linearly on the time an electron spends in the vicinity of the surface. Both the energy and angular dependence of the SEP can be accurately described by free-electron theory when the electron momentum is rescaled by a material-dependent parameter. The value of the scaling parameter is given for Al and Si so that the S...

Distinguishability of N composition profiles in SiON films on Si by angle-resolved x-ray photoelectron spectroscopy

Photoelectron intensities of N 1s and O 1s peaks at selected emission angles are reported for a SiON film on Si with different assumed amounts and distributions of N in the film. The intensities were determined from an efficient simulation tool for x-ray photoelectron spectroscopy (XPS) that incorporates appropriate values of elastic- and inelastic-scattering parameters in each region of the specimen as well as the finite angular acceptance of the analyzer. Appreciable dispersion of the intensities was found only for the N 1s peak at an emission angle of 75° (with respect to the surface normal). Conventional analyses of angle-resolved XPS data that include such large emission angles are unlikely to be valid due to angle-dependent changes of the attenuation length.Photoelectron intensities of N 1s and O 1s peaks at selected emission angles are reported for a SiON film on Si with different assumed amounts and distributions of N in the film. The intensities were determined from an efficient simulation tool for x-ray photoelectron spectroscopy (XPS) that incorporates appropriate values of elastic- and inelastic-scattering parameters in each region of the specimen as well as the finite angular acceptance of the analyzer. Appreciable dispersion of the intensities was found only for the N 1s peak at an emission angle of 75° (with respect to the surface normal). Conventional analyses of angle-resolved XPS data that include such large emission angles are unlikely to be valid due to angle-dependent changes of the attenuation length.

Angular distribution of surface excitations for electrons backscattered from Al and Si surfaces

Abstract Spectra of electrons reflected from non crystalline Aluminum and Silicon surfaces have been measured for energies between 300 and 3400 eV. Angular distributions of the elastic peak as well as multiple surface and bulk excitations were recorded for two geometrical arrangements, being each others mirror image with respect to the trajectory of each detected electron. These geometries allow to distinguish between incoming and outgoing electrons in the case such a difference depends on the direction of the electrons with respect to the surface normal. Theoretical calculations [Surf. Interf. Anal. 26 (1998) 682] suggest that such a difference exists in the ratio of the elastic peak and the intensity of the first surface plasmon. The present paper compares experimental results on this quantity that is closely related to the so-called surface excitation parameter for Aluminum and Silicon with theory. The angular distribution of the elastic peak was found to closely follow the single deflection model. Within the experimental accuracy of ∼5% no difference regarding the surface excitation parameter for incoming and outgoing electron trajectories was observed.

Interpretation of nanoparticle X-ray photoelectron intensities

X-ray photoelectron (XPS) intensities have been simulated for spherical core-shell nanoparticles (NPs) in different geometrical arrangements in order to investigate the validity of commonly made assumptions for the interpretation of XPS NP intensities. The single-sphere approximation is valid for a powder sample when all spatial coordinates of the NP positions are uncorrelated. Correlations along either the depth coordinate or the lateral coordinates lead to features in the angular distribution that provide information on these correlations. The XPS intensity is proportional to the surface-to-volume ratio of nanoparticles but only for NP sizes exceeding the inelastic mean free path of the photoelectrons.

Refined calculations of effective attenuation lengths for SiO2 film thicknesses by x-ray photoelectron spectroscopy

Electron effective attenuation lengths (EALs) for SiO2 films on Si are reported that were obtained from Monte Carlo simulations of substrate Si 2p3∕2 photoelectron transport for x-ray photoelectron spectroscopy (XPS) with AlKα and MgKα x rays and SiO2 films of varying thicknesses. These EALs show a stronger dependence on SiO2 thickness than previous values found from an approximate algorithm. Since recent XPS data for SiO2 can be analyzed satisfactorily with thickness-independent EALs, the results indicate that intrinsic excitations and/or variations of inelastic-scattering probabilities near surfaces and interfaces appear to be significant in quantitative XPS.

Contribution of surface plasmon decay to secondary electron emission from an Al surface

Spectra of secondary electrons (SE) emitted from a polycrystalline Al surface have been measured in coincidence with 500 eV‐electrons for energy losses between 10 and 155 eV. The spectra for a given energy loss are qualitatively similar, consisting of surface and volume plasmon decay and a contribution attributable to direct electron–electron scattering. The similarity of the contribution of surface and volume plasmon decay in the SE spectra proves directly that electron multiple scattering is governed by a Markov‐type process. The average value of the surface plasmon decay contribution to the SE spectrum amounts to ∼25%.

Sample-morphology effects on x-ray photoelectron peak intensities

The authors have used the National Institute of Standards and Technology Database for the Simulation of Electron Spectra for Surface Analysis to simulate photoelectron spectra from the four sample morphologies considered by Tougaard [J. Vac. Sci. Technol. A 14, 1415 (1996)]. These simulations were performed for two classes of materials, two instrument configurations, and two conditions, one in which elastic scattering is neglected (corresponding to the Tougaard results) and the other in which it is included. The authors considered the Cu/Au morphologies analyzed by Tougaard and similar SiO2/Si morphologies since elastic-scattering effects are expected to be smaller in the latter materials than the former materials. Film thicknesses in the simulations were adjusted in each case to give essentially the same chosen Cu 2p3/2 or O 1s peak intensity. Film thicknesses with elastic scattering switched on were systematically less than those with elastic scattering switched off by up to about 25% for the Cu/Au morp...

Sample-morphology effects on x-ray photoelectron peak intensities. II. Estimation of detection limits for thin-film materials

The authors show that the National Institute of Standards and Technology database for the simulation of electron spectra for surface analysis (SESSA) can be used to determine detection limits for thin-film materials such as a thin film on a substrate or buried at varying depths in another material for common x-ray photoelectron spectroscopy (XPS) measurement conditions. Illustrative simulations were made for a W film on or in a Ru matrix and for a Ru film on or in a W matrix. In the former case, the thickness of a W film at a given depth in the Ru matrix was varied so that the intensity of the W 4d5/2 peak was essentially the same as that for a homogeneous RuW0.001 alloy. Similarly, the thickness of a Ru film at a selected depth in the W matrix was varied so that the intensity of the Ru 3p3/2 peak matched that from a homogeneous WRu0.01 alloy. These film thicknesses correspond to the detection limits of each minor component for measurement conditions where the detection limits for a homogeneous sample var...

Angular dependence of electron induced surface plasmon excitation

The angular dependence of the probability for electron induced surface plasmon excitation has been measured on semi-infinite planar polycrystalline Al and Au surfaces for energies between 500 and 4000 eV. The results agree accurately with the simple model in which the surface excitation probability is proportional to the surface penetration time. However, the penetration time differs from the rectilinear motion model due to deflections during elastic collisions within the surface scattering zone. A simple formula to account for the effect of elastic scattering in the surface scattering zone is given.