Kurt Richter

Determination of short-range order and interlayer extension in Fe/Al multilayers using XAFS

Short-range order in Fe/Al multilayers has been determined using Fe K fluorescence x-ray absorption fine structure (XAFS) experiments. The two multilayers studied (prepared by pulsed laser deposition) exhibited different Fe single layer thickness (4.3 nm and 30 nm, respectively) but comparable total amount of Fe (six thin and one thick layers, respectively). From XAFS signals arising at different regions of the sample it was concluded that a solid solution of -Fe had been formed in the vicinity of the Fe/Al interface, the thickness of which was estimated to be 3.8 nm. The average Fe neighbourhood in these interface regions contained about 50% Al substituting for Fe.

DAFS Experiments with Non-Centrosymmetric Single Crystals

Diffraction anomalous fine structure (DAFS) experiments were applied to an epitaxially grown (Ga,In)P layer on a [001] GaAs substrate as a single-crystalline model substance. The requirements for the reliable measurement of reflection intensities as a function of photon energy, as well as the quantitative DAFS analysis resulting in the complex-valued fine-structure function of the scattering factor, are described. In the case of single crystals, effort had to be put into performing the DAFS measurements in order to hold the position of the Bragg reflection exactly during the energy scan. Using the zinc-blende-type structure as an example, it is shown for the first time that, similar to single-crystal structure analysis, the lack of inversion symmetry has a significant impact on the DAFS signal, so that DAFS may contribute to structure analysis as well.

Short-range order in β-NiAl microalloyed with iron

Abstract Results of a quantitative analysis of fluorescence extended X-ray absorption fine structure are presented obtained at the Fe and Ni K absorption edges of a β-NiAl sample microalloyed with iron. The concentration of Fe was 0.2 at.% substituting for Ni. The quantitative analysis results in a suggestion of a local structure model for the neighbourhood of Ni as well as of Fe atoms. Results show that Fe atoms at this low content do not occupy Ni sites in the B2 structure nor sites in clusters of α-Fe. They rather indicate that small clusters of Fe-Ni are formed which crystallize similar to either the Cu type or to the ordered phase FeNi3 in the Cu3Au type.

Software based digital signal processing and spectrum deconvolution in X-ray spectroscopy

Approaches for software based digital signal processing and numerical deconvolution of measured signals which overcome limitations of state-of-the-art systems are described. The basic technical equipment for digital signal processing consists of an energy resolving detector with a preamplifier followed by a fast sampling analogue-to-digital converter (ADC). The main idea is the numerical decomposition of the measured signal into contributions caused by single photon absorption using standard pulses. The latter can be obtained by measurements under definite conditions. The maximum pulse rate is then limited only by the ratio of sampling time to the time between two pulses which should be attributed to single events. Thus pulse overlaps do not require pulse rejection. At sampling rates of 10(8) samples per second theoretically a comparable photon rate can be detected at throughputs of 100%. Beyond that it is outlined that in a comparable manner a numerical deconvolution of measured energy spectra (statistic distribution functions of single events) into combinations of standard spectra, which can likewise be determined by measurement, offers outstanding possibilities, too. On the one hand the energy resolution attainable for individual events for a given detector can be improved drastically by the statistical treatment of spectra. On the other hand an energy resolving work principle becomes possible for certain detectors, which do not permit this conventionally due to their poor signal to noise ratio.

DAFS experiments for phase-sensitive determination of local structure around Zr in Co/Zr multilayers

Diffraction Anomalous Fine Structure (DAFS) experiments were applied to support EXAFS analysis for studying amorphisation behaviour of Co/Zr multilayers stimulated by annealing below the crystallisation temperature. The layers, prepared by electron beam vaporisation, initially polycrystalline, exhibited hcp structure. The solid state reaction (SSR) during annealing was indicated by decreasing X-ray reflection intensities. This information and the knowledge of short-range order in Zr crystallites determined by DAFS enabled contributions of amorphous and crystalline phases to the average EXAFS signals to be separated. DAFS experiments were carded out at the Zr-K absorption edge using Zr 0002 reflection in symmetric geometry. A strong [0002] fibre texture was observed which did not change during annealing. After more than 1 hour annealing at 250°C Co atoms have been detected in the Zr crystallites at a Zr-Co distance of 3.03A.

Determination of polarity in noncentrosymmetric layer/substrate systems

Energy-dispersive anomalous X-ray scattering has been used for the determination of the polarity of a noncentrosymmetric layer/substrate system. The method was applied to an epitaxically grown (Ga,In)P layer on a (001) GaAs substrate as an example to show its applicability as a routine procedure for noncentrosymmetric thin-film systems. In the simplest case, four energy spectra of various orders of the 111 reflections were sufficient to identify polarity, without the need for intensity corrections.

X-Ray Analysis of the Short-Range Order in the Ordered-Alloy Domains of Epitaxial (Ga,In) P Layers by DAFS of Superlattice Reflections

Ordered-alloy domains of epitaxially grown (Ga,In)P layers have been observed elsewhere using transmission electron microscopy and transmission electron diffraction. We used diffraction anomalous fine-structure (DAFS) experiments at superlattice reflections occurring in several directions to explore the short-range order around Ga atoms in such ordered domains in epitaxial (Ga,In)P layers grown on (0 0 1) GaAs substrates. The requirements for a reliable measurement of the reflection intensity depending on the photon energy are described. A quantitative DAFS analysis resulting in the short-range order parameters is explained in detail. The local structure around Ga in the whole (Ga,In)P layer \( \left( {F\bar 43m} \right) \) can be understood by a local structure model, whereas in contrast, the local structure around Ga atoms in the ordered regions (R 3 m) can be described by the values expected on the basis of the virtual-crystal model.

Apparative Grundlagen der RFA

Nach der Art der Selektion der charakteristischen Strahlung unterscheidet man heute »wellenlangendispersive« (oder »kristalldispersive«) und »energiedispersive« Gerate. Mit diesen nicht ganz treffend gewahlten Begriffen bezeichnet man abkurzend die Zerlegung mittels Interferenz zum einen und mit Hilfe der energieproportionalen Amplituden der Spannungsimpulse von energieauflosenden Detektoren zum anderen. Die Hauptbaugruppen der Gerate sind folgende: eine Primarstrahlungsquelle zur Anregung der Fluoreszenzstrahlung in der Probe, ein Spektrometer zur Aufnahme des Spektrums der von der Probe emittierten Strahlung, eine leistungsfahige Nachweis- und Auswerteelektronik zur Bestimmung der Intensitat der charakteristischen Linien sowie der daraus folgenden Elementkonzentrationen.

Konzentrationsbestimmung mittels RFA

Das Hauptanwendungsgebiet der RFA ist die quantitative Elementanaiyse, wobei der Zusammenhang zwischen der Intensitat ausgewahlter Rontgenspektrallinien und der Konzentration ausgenutzt wird.

Physikalische Grundlagen der RFA

Bei der RFA trifft eine primare Rontgenstrahlung, emittiert von einer Rontgenrohre oder einer radioaktiven Quelle Q, auf die zu analysierende Probe P (Bild 2–1). Als Folge dieser Wechselwirkung geht von der Probe eine sekundare Rontgenstrahlung aus, die in einzelnen Komponenten charakteristisch fur die chemische Zusammensetzung der Probe ist.