Robert Svagera

Calculation of escape depths from inelastic mean free paths

Abstract Monte-Carlo calculations are used to quantify the influence of elastic scattering processes on the shape of electron trajectories in solid matter. Cross-sections for elastic scattering depending on the atomic number Z and the electron kinetic energy are calculated by the partial wave expansion method and the use of Thomas-Fermi-Dirac potentials. As a consequence of elastic scattering, the escape depth decreases to about 80% of the inelastic mean free path.

Imaging XPS with a hemispherical analyzer and multichannelplate detection

Abstract A description of the principle of imaging by means of a combination of hemispherical analyzer with a two dimensional electron detector (multichannelplate) is given together with details on data handling. Examples of applications in the field of pixel-analysis of thin films and of elemental distributions demonstrate new horizons in quantitative XPS.

Surface modification of polystyrene by photoinitiated introduction of cyano groups

The surface of polystyrene was modified by the introduction of CN groups. This was achieved by irradiating the polymer with UV light (254 nm) in the presence of gaseous cyanogen bromide (BrCN). X-ray photoelectron spectroscopy (XPS) and FTIR analysis showed that after irradiation both Br and CN groups were covalently bound to the polymer surface. Quantitative XPS analysis revealed that upon prolonged irradiation every second monomer unit at the surface was modified with a CN group. It was demonstrated that photochemical techniques can be employed to attach specific functional groups onto polymer surfaces.

On the energy dependence of attenuation lengths in hydrocarbon contaminations

Abstract An experimental procedure for determination of the reduced thicknesses of thin films of contamination layers is described. It becomes possible to determine the energy dependence of the attenuation length in contamination layers from the angular distribution of photoelectron and Auger signals from the substrate at different kinetic energies. Within a restricted interval of electron energies, the attenuation length is proportional to E kin m . The evaluation of our experimental results gave a value of m = 0.64 in the energy range 0.4 keV ⩽ E kin ≤ 1.4 keV. From considerations of the influences of surface roughness and the finite solid angle of acceptance on measured m values it must be expected that the true value of m is larger than 0.64. Since these two influences on measured characteristic photoelectron signals will occur in practice too, we propose an effective value m = 0.64 for correction of the energy dependent attenuation in contaminations.

Investigation of concentration depth profiles by means of angle-resolved XPS : a polynomial model

Abstract This paper deals with the reconstruction of concentration depth profiles by means of angle resolved X-ray photoelectron spectrometry. A polynomial model is introduced which describes the concentration distribution ci(x) of the element i as a polynomial of third order within the range 0 ⩽ x ⩽ xm; x gives the sample depth measured from the surface. The further constraints ci(xm) = bi (with bi as the bulk concentration of the element i) and dci(x)/dx = 0 for x = xm reduce the number of unknown parameters to two per element with the additional parameter xm and the unknown photon flux. By means of computer simulation it can be shown that the results of the reconstructed depth profiles are stable and not very sensitive to statistical errors of measured count rates. Experimentally measured count rates of an Al-Li alloy before and after segregation were evaluated. The calculation time per depth profile was approximately 10 min.

A comparison of two XPS methods for quantification of concentration profiles

Abstract Binary AlLi alloys are known for well pronounced segregation at ambient temperatures. Literature provides a good knowledge of segregation behaviour of these alloys. An exponential concentration profile with depth can be assumed. We compare the concentration profiles obtained from SIMS measurements with results from XPS. SIMS measurements [M. Vonbank and P. Varga, Vak.-Tech., 37 (1988) 220.] performed on a specimen with 9.1 at.% Li after storage at room temperature for more than 48 h gave a surface composition of 50 at.% Li and a gradient of −20 at.% Li nm −1 . We investigated the same specimen after heat treatment for 2 h 20 min at 150°C. For comparison, XPS with variable take-off angle gives 75 at.% Li and −17.8 at.% Li nm −1 assuming an exponential depth profile, 70 at.% Li and −9.9 at.% Li nm −1 with a linear response and our new imaging XPS method with a linear response gives 92 at.% Li and −15.4 at.% Li nm −1 . The standard deviations of surface concentration and gradient are approximately 9 at.% Li and 8 at.% Li nm −1 . The differences are due to: the statistical significance of measured data (8% relative error in case of variable take-off angle experiments and 25% for imaging experiments) especially due to weak Li1s signals; the necessity of a specimen transfer in ambient atmosphere; and uncertainties in sputter depths of about 10%. In principle the imaging method is similar to methods in which the surface is etched with a constant flux Ar ion beam and changes in the composition of the surface with time are monitored using XPS.

Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter λ_{SOC} in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling J_{K} and the chemical potential μ-both essential parameters determining the ground state of the material-and thus possible λ_{SOC} tuning effects have remained unnoticed. Here, we present the successful growth of the substitution series Ce_{3}Bi_{4}(Pt_{1-x}Pd_{x})_{3} (0≤x≤1) of the archetypal (noncentrosymmetric) Kondo insulator Ce_{3}Bi_{4}Pt_{3}. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and it therefore is likely to leave J_{K} and μ essentially unchanged. By contrast, the large mass difference between the 5d element Pt and the 4d element Pd leads to a large difference in λ_{SOC}, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing x (decreasing λ_{SOC}), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce_{3}Bi_{4}Pd_{3} shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

Determination of Layer Thicknesses by Total Electron Yield Measurements—Substrate Method

The application of total electron yield (TEY) measurements to the determination of layer thicknesses is described, presenting an introduction to the principles of TEY measurements, the instrumentation and the evaluation of measured signals. The formulation of the theoretical correlation between measured TEY jumps from substrate elements and the thickness of an overlayer is presented. The concept of quantitative thickness determinations by TEY is demonstrated on thin layers of AlxGa1-xAs on GaAs and on thin Ag layers on GaAs substrates. The relative error in layer thickness is found to be less than 10% in the thickness range 1–100 nm.

Multiband Transport in CoSb3 Prepared by Rapid Solidification

Abstract Nano‐grained CoSb3 was prepared by melt‐spinning and subsequent spark plasma sintering. The phonon thermal conductivity of skutterudites is known to be sensitive to the kind and the amount of guest atoms. Thus, unfilled CoSb3 can serve as model compound to study the impact of a nanostructure on the thermoelectric properties, especially the phonon thermal conductivity. Therefore, a series of materials was prepared differing only by the cooling speed during the quenching procedure. In contrast to clathrates, the microstructure of meltspun CoSb3 was found to be sensitive to the cooling speed. Although the phonon thermal conductivity, studied by means of Flash and 3ω measurements, was found to be correlated with the grain size, the bulk density of the sintered materials had an even stronger impact. Interestingly, the reduced bulk density did not result in an increased electrical resistivity. The influence of Sb and CoSb2 as foreign phase on the electronic properties of CoSb3 was revealed by a multi‐band Hall effect analysis. While CoSb2 increases the charge carrier density, the influence of the highly mobile charge carriers introduced by elemental Sb on the thermoelectric properties of the composite offer an interesting perspective for the preparation of efficient thermoelectric composite materials.

Mechanism of Rare Earth Incorporation and Crystal Growth of Rare Earth Containing Type-I Clathrates

Type-I clathrates possess extremely low thermal conductivities, a property that makes them promising materials for thermoelectric applications. The incorporation of cerium into one such clathrate has recently been shown to lead to a drastic enhancement of the thermopower, another property determining the thermoelectric efficiency. Here we explore the mechanism of the incorporation of rare earth elements into type-I clathrates. Our investigation of the crystal growth and the composition of the phase Ba8–xRExTMySi46–y (RE = rare earth element; TM = Au, Pd, Pt) reveals that the RE content x is mainly governed by two factors, the free cage space and the electron balance.