Publications

Phase change materials are the most promising materials for innovation in the computer memory industry. Of great importance is the understanding of the processes that take place during the switching from the high resistivity state to the low resistivity state. In this paper we have tried to find out the correlations between the crystallo-chemical parameters and the switching properties of these materials. The crystallo-chemical parameters, average electronegativity, and glass forming ability (n) over bar(k) over bar/(Z) over bar have been calculated for a large number of tellurium chalcogenides. The switching parameters have been correlated with the physical properties of these materials: resistivity, activation energy, and average electronegativity. Change of the chemical bonding during switching would be responsible for the outstanding properties of the phase change materials.

Recent results have shown that size-effects in nanostructured materials may enable much more efficient thermoelectric devices. In this paper, the electrochemical preparation of Bi(1.95)Te(2.7)Se(0.35) and Bi(2.15)Te(2.55)Se(0.3) submicron wires arrays with nanosized grain structure is reported. X-ray diffraction (XRD) analysis has indicated the presence of nanosized crystallites in the structure of the wires; high resolution transmission electron microscopy (HRTEM) investigations have confirmed the formation of crystallites with geometrical dimensions of 5-14 nm and random orientations in the structure of these wires. These wires were further analyzed by field emission scanning electron microscopy (FESEM). The measurement of the transport properties showed that a highly degenerate semiconductor with a large thermopower (-125 mu V K(-1) at 300 K) and a low resistivity was obtained. (C) 2009 Elsevier Ltd. All rights reserved.

YVO4 nanocrystals doped with Eu (5 at.%) or Gd (1 at.%) were synthesized by precipitation method and annealed at various temperatures between 60 degrees C and 900 degrees C. The average size (coherence domain) of the YVO4 and nanocrystals, estimated from X-ray diffraction measurements, varies from similar to 7 nm to similar to 60 nm, function of the annealing temperature. The morphological transformations produced by thermal treatments in YVO4 nanocrystals were monitored using optical spectroscopy with Eu3+ as probe and EPR spectroscopy, the paramagnetic probe being Gd3+.

Technological process of interference lithography using two-layer chalcogenide photoresist were investigated. Top As(40)S(30)Se(30) layer is photoresist with a high selectivity and can be used for recording of interference pattern and formation of first lithographic mask. Second, more thick As(4)Ge(30)S(66) layer, almost is not sensitive to light, but dissolves in weak (0.05%) water solution of KOH. That, optimizing the etchant solutions for both layers, exposure and time of etching it is possible to carry out the technological process of formation of the lithographic mask with high modulation and with the groove form close to rectangular. This technology has been used for the fabrication of one- and two-dimensional periodic structures. Using two layer As(40)S(30)Se(30)-As(4)Ge(30)S(66) photoresist, we have fabricated the diffraction gratings and two-dimensional periodic structures with elements of submicron size. Relief parameters and diffractions properties of the obtained structures and their dependence on etching time are studied.

Matrix assisted photo-amorphization effect has been observed in thin chalcogenide films of composition As40S30Se30 deposited by thermal evaporation on silicon wafer substrate covered by silver. The initial films contain Ag2S and Ag4SeS crystallites embedded in an amorphous matrix. Illumination by a cold light halogen lamp induces the disappearance of the crystalline fraction and leads to significant changes in the diffraction pattern of the amorphous films.

Single-crystals of potassium hydrogen phthalate (KAP) doped with coumarin 6 (C6) were grown by solution evaporation technique. Powder X-ray diffraction. optical transmission and luminescence measurements were performed. The structure and morphology of the KAP crystals are not changed with the incorporation of the dye. Transparency of the dye-doped crystals is suited for non-linear optical (NLO) applications and UV cut-off is not changed when compared with the pure KAP crystals. The dye-doped crystals present an absorption band at 350 nm while the growth solution exhibits a peak at 400 nm. The doped crystals have a strong emission band at 450 nm that is excited at 350 nm and the second harmonic generating (SHG) properties are demonstrated using luminescence measurements. (C) 2009 Elsevier B.V. All rights reserved.

Electrochemical polymerization of indole in a LiClO4/CH3CN solution on a single-walled carbon nanotubes (SWNTs) film was studied by cyclic voltammetry, Raman scattering and FTIR spectroscopy. Comparing the cyclic voltammograms recorded on a blank Pt electrode with those obtained when carbon nanotubes films were previously deposited onto the Pt electrode, a down-shift of the indole reduction peak potential in the latter case was observed. Raman spectroscopy studies indicate that the electrochemical deposition of polyindole (PIN) onto the SWNT film results in a breaking of SWNT bundles into individual tubes. A covalent functionalization of SWNTs with PIN in the doped state is demonstrated by FTIR spectroscopy, when an increase in the intensity of the absorption band at 1045 cm(-1) is observed. Besides, Raman and FTIR studies performed on samples electrochemically prepared and thereafter post-chemically reacted with an NH4OH solution, indicate both a roping process of individual tubes with PIN as a binding agent and a strong steric hindrance effect. (C) 2009 Elsevier B.V. All rights reserved.

The knowledge of the effects of radiation in semiconductor devices, in particular in detectors, represents an important and active field of research. The influence of isovalent impurities, carbon and germanium, on the radiation damage of silicon for detectors is investigated in the frame of a quantitative phenomenological model for defect kinetics, developed previously by the authors. The concentrations of defects induced by irradiation in materials with different doping levels are calculated, as well as the leakage current and effective carrier concentrations in p-n junction detectors made from these materials. The beneficial effect of Ge on the radiation damage of silicon is deduced.

This work focuses on the investigation of radiation-induced defects responsible for the degradation of silicon detector performance. Comparative studies of the defects induced by irradiation with Co-60-gamma rays, 6 and 15 MeV electrons, 23 GeV protons and reactor neutrons revealed the existence of point defects and cluster-related centers having a strong impact on damage properties of Si diodes. The detailed relation between the "microscopic" reasons as based on defect analysis and their "macroscopic" consequences for detector performance is presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposure to high levels of Co-60-gamma doses can be completely understood by the microscopically investigated formation of two point defects, a deep acceptor and a shallow donor, both depending strongly on the oxygen concentration in the silicon bulk. Specific for hadron irradiation are the annealing effects which decrease (increase) the originally observed damage effects as seen by the changes of the depletion voltage and these effects are known as "beneficial" and "reverse" annealing, respectively. A group of three cluster-related defects, revealed as deep hole traps, proved to be responsible specifically for the reverse annealing. Their formation is not affected by the oxygen content or silicon growth procedure suggesting that they are complexes of multi-vacancies located inside extended disordered regions. (C) 2009 Elsevier B.V. All rights reserved.