Klaus Wetzig

XPS investigations of surface segregation of doping elements in SnO2

Abstract Various doped tin oxide (dopants Sb, Nb, In) were prepared. By means of X-ray photoelectron spectroscopy (XPS) the dependence of the surface concentration on doping element, doping concentration and preparation technique was determined. Simultaneously, the electrical and morphological properties are strongly influenced even by low doping concentration. The dopant distribution was studied by XPS and SIMS depth profiling. A model for dopant distribution in the fine powders was proposed. At low concentrations, the doping element is build into the lattice of SnO 2 partially and the residue substitutes Sn atoms in the topmost layer. Particles of the second phase are found at higher doping concentrations. Additionally, the analysis of Sb 3d 3/2 peak position and shape in Sb doped samples shows a decrease of oxidation state of antimony with increasing doping concentration.

Experimental Studies on the Mechanism of Wet Chemical Etching of Silicon in HF ∕ HNO3 Mixtures

A detailed kinetic study was performed to elucidate the mechanism of wet chemical etching of silicon in a HF-rich HF/HNO 3 mixture. In contrast to earlier studies, the etch rates were determined by dissolution of only a few milligrams of silicon in carefully thermostatted acid mixtures in order to avoid a change in composition during the experiments and an uncontrolled warming of the etchant. All etch experiments were followed by chemical analytics. The etch rates were studied as a function of temperature, silicon content of the etchant (utilization), and stirring speeds. By choosing proper reaction conditions, intermediates of the reduction process of HNO 3 , such as N 2 O 3 , were stabilized and spectroscopically identified. Furthermore, it was found that the nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a parameter for the reactive N(III) species in the concentrated etchant. Two different etch regimes were identified. In the region of high nitrite concentrations, the etch rate is apparently independent on the nitrite concentration. At lower nitrite concentrations, the etch rate decreases linearly with the nitrite concentration. Kinetic examinations showed that the reaction mechanism remains unchanged in both regimes. Furthermore, the kinetic parameters of nitrite decays were determined. The obtained results provide the first explanation of why an etch mixture of constant HF-HNO 3 ratio at given Si content can exhibit different etch behavior. A mechanistic model on the role of N(III) species and dissolved gases in the etching process including a suggestion of the rate-limiting step is presented. Consequences for technical applications are discussed.

Emission spectra of copper and argon in an argon glow discharge containing small quantities of hydrogen

The influence of hydrogen contained in a sample or otherwise introduced into a glow discharge source (GDS) is here extensively experimentally presented for the case of copper as a sample by means of the addition of small quantities of molecular hydrogen (<1% relative partial pressure) to the argon carrier gas. The progressive addition of molecular hydrogen causes different intensity changes particular to the individual lines of different species such as atomic (Cu I) and ionic (Cu II) copper, and also atomic (Ar I) and ionic (Ar II) argon. Some interesting features of the emission spectrum of hydrogen such as its line, band and even continuum spectrum are observed. It was also found that the depth resolution becomes worse even at very low concentrations of hydrogen.

Microstructure and Giant Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers

Direct current plating, pulse plating, two-pulse plating, and reverse pulse plating were used to produce electrodeposited Co-Cu alloys and Co-Cu/Cu multilayers under galvanostatic control from an electrolyte containing CoSO 4 and CuSO 4 . Atomic force microscopy, X-ray diffraction, and transmission electron microscopy were used to study the sample structure and morphology. Direct current plating resulted in a Co 95 Cu 5 alloy with nearly equal amounts of face-centered cubic (fcc) and hexagonal close packed phases, while all pulsed current methods yielded multilayers with fcc structure, Giant magnetoresistance (GMR) behavior was observed in the multilayers with a maximum magnetoresistance (MR) ratio of about 9% as measured at 8 kOe. The shape of the MR curves and the magnitude of the GMR were very similar, regardless of the sign of the current between the Co deposition pulses. The results of structural studies also confirmed the formation of a multilayer structure for each pulsed electrodeposition mode. The conclusion was that the spontaneous exchange reaction between Co and Cu 2+ is responsible for the formation of a pure Cu layer even under reverse pulse plating conditions. The GMR of the multilayer deposits decreased with increasing bilayer number, due to the deterioration of the microstructure as the deposit grew.

Comparison of depth resolution for direct current and radiofrequency modes in glow discharge optical emission spectrometry

The correlation between depth resolution and the crater formation process was investigated for depth profiling in two glow discharge modes, viz., direct current (dc) and radiofrequency (rf) glow discharge optical emission spectrometry. The rf system described functions without a matching unit and shows a very high reproducibility and stability (within several tens of milliseconds), which are comparable to those obtained in the dc mode. By using conductive samples with a well known multilayer structure (five double layers with 100 nm Cu and 100 nm CrNi each), the discharge parameters for maximum depth resolution were determined and crater formation during the sputtering process was investigated. The best depth resolution was found at the same values of pressure and power for both modes. Under these optimized conditions the depth resolution increases linearly with the sputtered depth and amounts to 5–10% of the latter. Finally, the CRAS model was applied to simulate an intensity–time profile in order to discuss the depth resolution achieved.

Influence of hydrogen on the analytical figures of merit of glow discharge optical emission spectroscopy—friend or foe?

It is well known that the presence of small quantities of hydrogen in an argon glow discharge (GD) causes serious alterations to the excitation and ionisation mechanisms in the GD plasma and hence to the analytical signals. This so-called “hydrogen effect” leads also to a change in the shape of the sputtering crater and its roughness. The present work shows how the manifold effects of hydrogen can be exploited in glow discharge optical emission spectroscopy (GD-OES) in order to improve analytical figures of merit such as analytical sensitivity, detection limits and depth resolution. Other problems caused by the presence of hydrogen, e.g. the occurrence of hydride bands, are demonstrated and discussed. It is shown that start-up phenomena leading to a falsification of the initial part of GD-OES depth profiles can be partially inhibited by controlling the GD source cleanness, and possibly by adding hydrogen.

Depth profiling of electrically non-conductive layered samples by RF-GDOES and HFM plasma SNMS

Abstract The work is intended to compare the capabilities of two similar depth profiling techniques to analyse electrically non-conductive samples. In order to get a better evaluation of the depth resolution, various multilayer sandwiches, such as SiO2/TiO2 and Si3N4/SiO2 deposited on glass substrates have been investigated. Optimised depth profiles are presented for both methods, glow discharge optical emission spectrometry (GDOES) and radiofrequency mode (known as “HFM” in the SNMS literature) of plasma secondary neutral mass spectrometry (SNMS). The optimisation procedure, necessary to get the best set of plasma parameters, which result in the optimal depth resolution, is also described for one selected sample. Additionally, sputtering crater profilometry was carried out in order to check out the flatness of the sputtered crater. The influence of the thickness of the sample substrate on the sputtering rate is discussed. Finally, advantages and disadvantages of the use of these two depth profiling methods, especially for the non-conductive samples, are concluded from this comparative study. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of a cross-sectioned sample was carried out in order to get supplementary information.

A small-angle neutron scattering model for polydisperse spherical particles with diffusion zones and application to soft magnetic metallic glass

An analytical expression for the small-angle neutron scattering intensity of diluted systems of polydisperse spherical particles, with diffusion zones, embedded in a matrix is presented. It is used within a nonlinear regression procedure to analyse small-angle neutron scattering experiments with polarized neutrons on an Fe73.5Si15.5B7CuNb3 alloy. The results for the nuclear and magnetic scattering length densities allow verification of the inhibitor concept introduced for the explanation of the limited sizes of precipitates developing during nanocrystallization. In the case of amorphous Fe73.5Si15.5B7CuNb3 alloy, the observed nanocrystals of the Fe3Si type are surrounded by an Nb-enriched shell, which stops the growth of the precipitates. With the results of polarized neutron scattering experiments, it is shown that magnetic and nuclear small-angle neutron scattering signals have the same origin. Additionally, the precision of the fits is improved by complementary use of polarized neutrons.

An approach to the reconstruction of true concentration profile from transient signal in spatially resolved analysis by means of laser ablation ICP MS

A transient analytical signal caused by continuous ablation with moving (line scan) or permanent (depth profiling) beam location was approximated by means of superposition of single-shot responses. The evolution of the parameters of single-shot response was used to reveal the true concentration profile of spatially inhomogeneous samples. The role of an ablation cell as an integrator of ablation shots has been considered and the effect of ablation cell volume on the analytical characteristics has been elucidated.

Correlation between interface structure and giant magnetoresistance in electrodeposited Co–Cu/Cu multilayers

Abstract An attempt has been made to understand the correlation between the interface structure and the giant magnetoresistance (GMR) properties of electrodeposited Co–Cu/Cu multilayers by measurements performed on a series produced by galvanostatic electrodeposition under the application of different capacitances connected parallel to the electrochemical cell, and this was expected to increase the width of the chemically intermixed interface between the magnetic and non-magnetic layer. In contrast to expectation, the GMR values of multilayers electrodeposited in the presence of a capacitance remained nearly unchanged even at the highest applied capacitance value, as a consequence of immiscibility of alloying elements.