National Institute Of Materials Physics - Romania

The paper presents a quantum confinement model for the electrical transport, the phototransport and the photoluminescence phenomena in nanocrystalline silicon. The infinite rectangular quantum well was proved to be the best choice for the investigated systems - nanocrystalline porous silicon and silicon nanodots embedded in an amorphous silicon dioxide matrix. Previous microstructure investigations have shown that the nanocrystalline porous silicon is formed by a nanowire network, so that the electron Hamiltonian is the sum of a one-dimensional Bloch-like Hamiltonian and a two-dimensional infinite rectangular quantum well. In the case of the silicon nanodots, the quantum well is three-dimensional. In both cases, the quantum well introduces quantum confinement levels in the band gap, the investigated phenomena being related with transitions between these levels.

Diodes processed on n-type epitaxial silicon with a thickness of 25, 50 and 75 mu m had been irradiated with reactor neutrons and high-energy protons (24 GeV/c) up to integrated fluences of Phi(eq) = 10(16) cm(-2). Systematic experiments on radiation-induced damage effects revealed the following results: in contrast to standard and oxygen-enriched float zone (FZ) silicon devices no space charge sign inversion was observed after irradiation. It is shown that the radiation-generated concentration of deep acceptors, dominating the behavior of n-type FZ diodes, is compensated by creation of shallow donors. Thus a positive space charge is maintained throughout the irradiation up to the highest fluence and even during prolonged elevated-temperature annealing cycles. Defect analysis studies using thermally stimulated current measurements attribute the effect to a damage-induced shallow donor at E-C-0.23 eV. It is argued that, as in the case of thermal donors, oxygen dimers, out diffusing from the Cz substrate during the diode processing, are responsible precursers. Results from extensive annealing experiments at elevated temperatures are verified by comparison with prolonged room-temperature annealing. These results showed that in contrast to FZ detectors, which always have to be cooled, room-temperature storage during beam off periods of future elementary particle physics experiments would even be beneficial for n-type epi-silicon detectors. A dedicated experiment at CERN-PS had successfully proven this expectation. It was verified, that in such a scenario the depletion voltage for the epi-detector could always be kept at a moderate level throughout the full S-LHC operation (foreseen upgrade of the large hadron collider). Practically no difference with respect to FZ-silicon devices was found in the damage-induced bulk generation current. The charge trapping measured with Sr-90 electrons (mip's) is also almost identical to what was expected. A charge collection efficiency of 60% (2500 e) in 50 mu m n-type epi-diodes after 24 GeV/c proton irradiation with Phi(eq) = 6.2 x 10(15) cm(-2) was reported recently, independent of the operating temperature down to -20 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

Microstructural characterization by transmission electron microscopy of the {111} planar defects induced in Si by treatment in hydrogen plasma is discussed. The {111} defects are analyzed by conventional ( TEM) and high-resolution transmission electron microscopy (HRTEM). Quantitative image processing by the geometrical phase method is applied to the experimental high-resolution image of an edge-on oriented {111} defect to measure the local displacements and strain field around it. Using these data, a structural model of the defect is derived. The validity of the structural model is checked by high-resolution image simulation and comparison with experimental images.

Theoretical studies of the coherent electronic transport in a system of coupled quantum wires show that switching on the conducting channel in one wire can be manifested in the other coupled wires. Interference processes and electronic correlations are taken into account in our studies on the same footing. The conductance changes depend on the interference conditions of a transmitted wave with that one reflected from the wires that indirectly influence the transport. We show that electronic correlations lead to a dynamical Coulomb blockade effect, which changes the conductance response quantitatively but its shape is still kept the same. Our results are discussed in correspondence with an experiment recently performed by Morimoto [Appl. Phys. Lett. 82, 3952 (2003)] on a system of coupled quantum wires.

Surface enhanced Raman scattering (SERS) performed with 676.4 and 1064 nm excitations were used to investigate single-walled carbon nanotubes (SWNTs) thin films prepared from platelets obtained by nonhydrostatic compression at 0.58 GPa. SWNTs were compressed alone or dispersed into chemical reactive and non-reactive host matrices. SERS spectra indicate that by compression, SWNTs break into fragments of different sizes, which in turn can react or not with the host matrix. In inorganic hosts (KI, Ag) donor-acceptor complexes are formed. The appearance of short fragments of SWNTs with a closed-shell fullerenes behaviour is revealed in SERS spectra. This typical signature appears in the Raman spectrum as a line at ca. 1460 cm(-1) associated with a pentagonal pinch mode. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fabrication by colloidal chemistry and an extended structural and magnetic investigation are reported for AgCo bimetallic colloidal nanoparticles using X-ray diffraction, transmission electron microscopy and superconducting quantum interference device magnetometry. The AgCo nanoparticles exhibit a core-shell structure, with a face-centred cubic Ag core and a hexagonal close-packed Co (either complete or partial) shell. A bimodal size distribution of superparamagnetic (SPNI) nanoparticles together with a small fraction of nanoparticles ferrornagnetic at room temperature has been determined and their influence on magnetic properties is discussed. The arrays of self-assembled nanoparticles exhibit a lack of saturation of magnetisation and typical SPM behaviour. The annealing of the arrays greatly enhances the ferromagnetic character of the samples. Finally, the observed magnetic behaviour of the AgCo nanoparticles is correlated with surface spin disorder induced by the particular structural features of the sample, high fraction of magnetic atoms in surface states and finite-size effects. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Using the on-set of third-harmonic susceptibility we have measured vortex melting lines of high-pressure synthesized (Cu0.6C0.4)Ba2Ca3Cu4Oy [(Cu,C):1234] and (Cu0.5C0.5)Ba2Ca2Cu3Oy [(Cu,C):1223] with preferentially oriented crystallites and having various carrier concentrations. Vortex melting lines of all (Cu,C):1223 samples and of optimum-doped (Cu,C):1234 were very well described by the commonly accepted theory of melting lines within the two-fluid model. Overdoped (Cu,C):1234 proved to have an anomalous melting line, that was phenomenologically explained by the rather special gaps evolution, that is the significant opening of a second superconducting gap due to CuO2 outer planes at a temperature lower than the critical one, where the first superconducting gap due to inner planes opens. The experimental melting lines of overdoped (Cu,C):1234 were modelled theoretically by assuming an empirical formula for the temperature dependence of anisotropy factor.

Nonalloyed ohmic contacts with lateral dimensions in the sub-100 nm range have been processed and characterized. The contacts are suitable for the fabrication of GaAs/AlGaAs nanocolumns designed in a 'top down' approach. They are realized on n-type GaAs using a thin low-temperature grown GaAs cap layer and Ti/Au metallization. For the lateral patterning of the nanoscaled ohmic contacts, electron-beam lithography based on Hydrogen Silsesquioxan (HSQ) negative resist is used, followed by sputtering and plasma etching of the metals. The I-V characteristics show ohmic contact behavior and demonstrate the scaling of the proposed contacts down to 50 nm lateral dimension. The specific contact resistance is situated in the range of 1-2 x 10(-5) Omega cm(2). (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report spectroscopic characterization of epitaxial YBCO thin films grown on LaAlO3 by pulsed laser deposition. Raman spectroscopy and spectroscopic ellipsometry were used for film characterization and the results were correlated with X-ray diffraction measurements. The mentioned techniques allowed us to analyze crystallographic, micro-structural, and morphological properties of YBCO thin films. We also demonstrated that relatively low resolution Raman spectroscopy and spectroscopic ellipsometry are reliable techniques for a rapid and non-destructive characterization of epitaxial YBCO thin films. (c) 2006 Elsevier B.V All rights reserved.

Pure and L-alanine doped Triglycine sulphate (TGS) crystals were grown in paraelectric phase (similar to 52 degrees C). Doped crystals show unequal growth rates along the ferroelectric axis. Pure TGS crystals show peculiar dielectric behavior in the ferroelectric phase, after crossing up and down the Curie point in two successive runs between room temperature and 80 degrees C. Much higher and unstable permittivity was found returning in the ferroelectric phase. At constant temperature (35 degrees C), permittivity follows a relaxation process, characterized by two relaxation times. L-Alanine doped TGS crystal shows more than one order of magnitude smaller permittivity and dielectric losses. Internal bias field of similar to 1 kV/cm, induced by the dopant, made the crystal almost monodomain and pined polarization in one direction. Pyroelectric coefficient measurements were performed at constant heating rate of the samples, using a computer controlled He cryostat and Keithley 6517 electrometer. The temperature dependence of P+ polarization component, obtained by computer integration of the pyroelectric coefficient, was measured on a large temperature interval (-20/ +80 degrees C). Pyroelectric coefficient of the doped samples was also measured by the same procedure, using a dc bias electric field, pointing in the opposite direction to the pined polarization. The polarization could be reversed, on the whole temperature range, by dc fields higher than bias or coercive field. Surprisingly, for the first time, the pyroelectric coefficient (p) was found constant on quite large temperature intervals. Doped TGS crystals show much smaller values of permittivity epsilon(r) versus the pure one and consequently, get higher figure of merit M = p/epsilon(r). The pyroelectric coefficient of this material can be tailored to become constant on a defined temperature range, under a dc field control. This characteristic makes this material valuable to be used as pyroelectric material for IR devices. (c) 2006 Elsevier B.V. All rights reserved.