Publications

The results of an in situ transmission electron microscopy study of the formation of Co-silicides on patterned (001) Si substrates are discussed. It is shown that the results of the in situ heating experiments agreed very well with the data based on standard rapid thermal annealing experiments. Fast heating rates resulted in better definition of the silicide lines. Also, better lines were obtained for samples that received already a low-temperature ex situ anneal. A Ti cap layer gave rise to a higher degree of epitaxy in the CoSi2 silicide.

It is shown that the figure of merit of a thermoelectric couple can be appreciably improved by using a pulsed current transferred from the thermoelectric circuit. In the frame of this theory un unusual drop of temperature in a thermoelectric circuit is possible. An ultrafast process of thermoelectric conduction is described, whose increased performance is realized by minimizing the effects of thermal dissipation.

We propose an improved method of thermally stimulated currents (TSC) spectra analysis in the case of diodes having a concentration of traps higher than that of doping impurities. Beside the calculation of trap concentrations from TSC peaks analysis, the method allows us to evaluate the density and the type of the very deep trapping level which, due to the contribution of leakage current, can not be detected in a real TSC experiment. The proposed method is applied to a p(+)-n Silicon diode irradiated with 1.82x10(13)neutrons/cm(2). (C) 2001 American Institute of Physics.

We investigate the transport properties of quantum dots placed in a strong magnetic field using a quantum-mechanical approach based on the two-dimensional tight-binding Hamiltonian with direct Coulomb interaction and the Landauer-Buttiker formalism. The electronic transmittance and the Hall resistance show Coulomb oscillations and also prove multiple addition processes. We identify this feature as the "bunching" of electrons observed in recent experiments and give an elementary explanation in terms of spectral characteristics of the dot. The spatial distribution of the added electrons may distinguish between the edge and bulk states and it has specific features for bunched electrons. The dependence of the charging energy on the number of electrons is discussed for a strong magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.

We have studied the nonlinear current transport in Ga0.47In0.53As/Al0.48In0.52As quantum-well structures near filling factor 1. A strictly bistable switching between the nearly nondissipative quantum Hall conduction and dissipative conduction was observed at filling factors between 1 and 1.5 at the critical current. The steepness of the transitions was found to be sharper than 4 ppm (limited by the resolution of the setup) with respect to the filling factor. In contrast to thermal or density-distribution instabilities, which can lead to similar bistabilities at any integer filling factor, the effect observed in this study occurred exclusively around filling factor 1. Consequently, we attribute the switching to a feedback effect between the tunneling rate between different spin levels and the effective spin gap, which depends on the degree of spin polarization. The hysteresis of the switching is accompanied by a partially ferromagnetic memory of the spin polarization when sweeping the external magnetic field. We have confirmed this assumption by calculations applying a screened Hartree-Fock model. The calculated results are in good agreement with the data obtained experimentally for different orientations of the magnetic field. A possible conjunction of the effect with nuclear spin magnetization was excluded by investigating the response of the bistable switching to exposure to radiation at the frequencies of the corresponding nuclear magnetic resonances.

Not only (Ag-)(2) and Ag-(Na+) centres but also various Ag--Na+ aggregate centres have been produced in Ag+-,Na+- and Ca2+-codoped KCl crystals by unusual electrolytic colouration such as colouration at a relatively low temperature of 280 degreesC and a high electric field of 5000 V/cm. Absorption and magnetic circular dichroism spectra due to these centres have been investigated. (C) 2001 Elsevier Science B.V. All rights reserved.

Silver layers were grown in quartz crystals by electrodiffusion from an anodic layer of AgNO3. Different experimental arrangements were employed in order to obtain such layers in a reproducible way. We performed DC ionic conductivity measurements on Z-cut and X-cut samples as a function of temperature in the heating up phase and as a function of time at the electromigration temperature. The metal structures and the quartz samples were analyzed by electron microscopy and electron diffraction. Based on the experimental results and on the observations a model, for the metal structure formation is proposed. (C) 2001 Elsevier Science B.V. All rights reserved.

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO2 laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an a-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a Mossbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)(5)/C2H4 mixtures is discussed.

Intermetallics of the type NdCo9-xFexSi2Cy (x = 3; 5; 6; 7 and y = 0; 1) and Nd2Co15-xFexSi2Cy (x = 7; 9; 12 and y = 0; 1) were prepared by are melting in order to study the competing effects of Fe and Co substitutions on the magnetic hyperfine fields as well as the effects of interstitial carbon additions on the symmetry and local magnetic properties in these systems. The investigations were performed by X-ray diffraction, Mossbauer spectroscopy and hysteresis loop measurements. Both site and average hyperfine magnetic field values were analyzed in order to discriminate between the Fe and Co contributions to the local atomic environments in these systems. While a higher Fe content as well as a higher Co content can both lead to an increase in the site magnetic fields, it was found that the model based on hyperfine field distributions provides a better description of the trends observed in hyperfine magnetic fields in these compounds. (C) 2001 Elsevier Science Ltd. All rights reserved.