National Institute Of Materials Physics - Romania

Consider a bunch of interacting electrons confined in a quantum dot. The later is suddenly coupled to semi-infinite biased leads at an initial instant t = 0. We identify the dominant contribution to the ergodic current in the off-resonant transport regime, in which the discrete spectrum of the quantum dot is well separated from the absolutely continuous spectrum of the leads. Our approach allows for arbitrary strength of the electron-electron interaction while the current is expanded in even powers of the (weak) lead-dot hopping constant tau. We provide explicit calculations for sequential tunneling and cotunneling contributions to the current. In the interacting case, it turns out that the cotunneling current depends on the initial many-body configuration of the sample, while in the non-interacting case, it does not and coincides with the first term in the expansion of the Landauer formula w.r.t. tau.

The smart-cut (TM) process is based on inducing and processing structural defects below the free surface of semiconductor wafers. The necessary defects are currently induced by implantation of light elements such as hydrogen or helium. An alternative softer way to induce shallow subsurface defects is by RF-plasma hydrogenation. To facilitate the smart-cut process, the wafers containing the induced defects need to be subjected to an appropriate thermal treatment. In our experiments, (0 0 1) Si wafers are submitted to 200 and 50 W hydrogen RF-plasma and are subsequently annealed. The samples are studied by transmission electron microscopy (TEM), before and after annealing. The plasma-introduced defects are {1 1 1} and {1 0 0} planar-like defects and nanocavities, all of them involving hydrogen. Many nanocavities are aligned into strings almost parallel to the wafer surface. The annealing is performed either by furnace thermal treatment at 550 degrees C, or by in situ heating in the electron microscope at 450, 650 and 800 degrees C during the TEM observations. The TEM microstructural studies indicate a partial healing of the planar defects and a size increase of the nanometric cavities by a coalescence process of the small neighbouring nanocavities. By annealing, the lined up nanometric voids forming chains in the as-hydrogenated sample coalesced into well-defined cracks, mostly parallel to the wafer surface.

The paper focuses on the utilization of radiofrequency discharges at intermediate pressures for plasma removal of carbon residuals or layers from narrow gaps and channels. It is shown that by proper handling of the discharge power and pressure it is possible to control the development of plasma inside narrow gaps, in the proximity of surfaces to be cleaned. The discharge operation inside gaps having the width in the range 0.6-2 mm was demonstrated. The cleaning effectiveness is exemplified for castellated surfaces having rectangular gaps coated with hydrogenated amorphous carbon layers. Cleaning of large surfaces can be approached with a movable "plasma shower" type device. (C) 2011 Elsevier B.V. All rights reserved.

The combined processes of plasma nitriding and cathodic arc deposition of (Ti,Cr,Nb)CN coatings were applied to HSS substrates. The nitrided layers, obtained in a mixture of H-2 (70%) and N-2 (30%) at two different temperatures (480 degrees C and 510 degrees C), were examined for the microhardness depth profiles. Characterization of the duplex coatings was performed by investigating elemental and phase composition, texture, hardness, friction and wear. XRD and XPS analyses revealed the formation of a mixture of a carbonitride fcc solid solution, in a dominant proportion, and metallic chromium. The film hardness was measured to be similar to 34 GPa. The duplex (Ti,Cr,Nb)CN coatings exhibited superior tribological behavior as compared to both nitrided layers and non-duplex coatings. (C) 2011 Elsevier B.V. All rights reserved.

A large model of amorphous silicon (2052 atoms) with 0.5% dangling bonds has been built and investigated. The refinement of the coordinates evidenced the presence of small domains with advanced ordering. These domains preclude the formation of crystallization nuclei and play an essential role in the redistribution of the defects in the material with homogenization of the free energy and stabilization against aging. An effect of amorphization of the ordered nuclei due to free energy redistribution is assessed. The glass relaxation of a-Si (a-Ge) occurring during heating below T(g) receives a natural explanation as a structural change from local quasi-ordering to homogeneous disordering. (C) 2011 Elsevier B.V. All rights reserved.

The switching mechanism in phase change memories was described on the basis of minimum switching unit: the commuton. A commuton is a minimum cluster of atoms that supports a reversible phase change from high to low electrical conduction state and back under the influence of an external signal. The switching process in a phase change chalcogenide film was modeled using two dimensional cellular automata approach. A system of 50 x 50 cells, each cell containing a commuton, was simulated. In the particular case of Ge(2)Sb(2)Te(5) (investigated here) this system corresponds to a 30 x 30 nm area. The formation of the percolation path as a function of phase change induced in commutons explains the switching phenomenon. The influence of the percent of defects in the material on the percolation threshold has been studied. (C) 2011 Elsevier B.V. All rights reserved.

A new method, that involves Evanescent-Wave Cavity Ring-Down Spectroscopy (EW-CRDS), is presented. This method is useful for the study of thin films on optical solid surfaces. The new design of experimental set-up incorporates a semicylindrical prism with a plane surface where the Total Internal Reflection (TIR) effect appears. The antireflex (AR) coated cylindrical surface of the prism induces the stability of resonant cavity. The evanescent wave generated at the plane surface of the prism probes the absorption by matter in the vicinity of the prism. A general discussion of design criteria is presented to quantify intrinsic losses, and then absorption spectra for Rhodamine chloride R590 from 555 to 560 nm are presented to demonstrate the sensitivity of the system. The layer of R590 on BK7 surfaces is deposited from a solution of R590 in ethanol (92% purity) with 51 mg/l concentration. After vaporization of the ethanol the dye layer on surfaces is supposed to be uniform. This implementation of TIR surface in a resonant cavity provides a powerful new spectroscopic tool especially to angle-resolved diagnostic for interfaces and thin-films. The loss spectrum can be obtained for an angular range from the TIR critical angle to a maximum angle when the resonance of system is lost.

Noble solid gases are promising detector materials to be used in the search for dark matter. In the present paper a systematic analysis of the transient phenomena associated with the stopping of recoils in noble gases in the solid phase is performed for the first time. The investigated energy range of the recoils corresponds to the elastic scattering of WIMPs from the galactic halo in these materials. A thermal spike model, previously developed by the authors, is extended and applied to solid noble gases. Ionization, scintillation and nuclear. energy loss processes are considered and included in the model, as well as the coupling between the subsystems. The development of the temperature pulse in space and time in solid Ar, Kr and Xe is analysed for different energies of the WIMP, and for different initial temperatures of the material. Phase transitions are possible in particular cases. The results of the model could be used as supplementary information in respect to ionization and scintillation, for detection and particle identification.

Europium doped hydroxyapatite (Eu:HAp) nanocristalline powders was synthesized by co-precipitation method by setting the atomic ratio of Eu/[Eu + Ca] at 0% and 2% and [Ca+Eu] /P at 1.67. The structural properties were characterized by X-ray diffraction (XRD). Nitrogen adsorption-desorption isotherms were obtained on different samples using BET method. The X-ray diffraction analysis revealed that hydroxyapatite is the unique crystalline constituent of all the samples, indicating that Eu has been successfully inserted into the HAp lattice. Based on N-2 adsorption-desorption isotherms investigation, the pore size, surface area and pore volume of europium doped hydroxyapatite are 16.57 nm, 115.06 m(2)/g and 0.48 cm(3)/g.

Indium oxide-doped hematite xIn(2)O(3*) (1 - x) alpha-Fe2O3 (molar concentration x = 0.1-0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In3+ substitution of Fe3+ in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In3+ and Fe3+ into hematite and respectively, In2O3 lattices occur simultaneously. The lattice parameters of alpha-Fe2O3 (a and c) and In2O3 (a) vary with milling time. For x = 0.1, Mossbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In3+ for Fe3+ in hematite lattice. For x = 0.3, Mossbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In3+ substitution of Fe3+ in hematite lattice and Fe3+ substitution of In3+ in two different sites of In2O3 lattice. For x = 0.5 and 0.7, Mossbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In3+ substitution of Fe3+ in hematite lattice and Fe3+ substitution of In3+ in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn(2)O(3*)(1 - x)alpha-Fe2O3 solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.