Publications

The luminescence spectra of fully oxidized porous silicon show two broad bands and one vibronic structure between 2 and 3.3 eV, all excited at 340 or 275 nm (3.65 and 4.51 eV). Two of these structures have not been described previously. Interestingly, these emissions are found in other forms of amorphous or disordered silica with high specific surfaces: frosted silica glass, silica gel, even various natural opals. These emissions disappear when the surface is passivated. They are, therefore, attributed to surface-related defects. The vibronic structure involves a nearly free SiO4 tetrahedron. Another vibronic structure found only in some opals is attributed to the uranyl group. (C) 2001 American Institute of Physics.

The relaxation of the irreversible magnetization of disordered YBa2Cu3O7-x crystals measured in the "flux-creep annealing" regime reveals that across the second magnetization peak (SMP) the barriers against flux motion remain finite at low current densities, which supports the existence of a crossover to a dissipation process involving the plastic deformation of the vortex system, In our experiments, the vortex creep process appears to be exclusively controlled by collective pinning barriers (diverging at low current densities) only below the onset of the SMP, where the vortex system is stable against dislocation formation. The (elastic) collective pinning barriers observed for magnetic field values close to the onset of the SMP (where the plastic barriers are high) could be related to the recently proposed collective pinning of individual dislocations. The proliferation of dislocations across the SMP leads to liquid-like behavior of the disordered vortex phase in the vicinity and above the peak field. (C) 2001 Elsevier Science B.V. All rights reserved.

In situ decomposition of silicon nitride films was observed by high-resolution electron microscopy. The films, which were produced by reactive pulsed laser deposition from Si wafer targets at 50-100 Pa ammonia pressure, had a prevalent content of hydrogen-doped, amorphous, non-stoichiometric silicon nitride. A layered morphology of the film, consisting of a density variation in the amorphous structure, developed under electron beam irradiation. This morphology became evident only in cross-sectional observation and was related to the stress-relaxation effect, based on a rearrangement of the bonds, We presume that the stress field anisotropy in the amorphous structure must be related to the presence of some bond texture in the as-grown, amorphous film. (C) 2001 Elsevier Science B.V. All rights reserved.

We investigated the superconducting critical temperature, transport critical-current density and the thermo-power properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiF)(y) samples. It was found that the midpoint critical temperature increases with increasing y, attaining approximate to 119.5 K for y = 0.15. The transition width shows a minimum and the critical temperature (zero electrical resistance) exhibits a maximum as a function of y. The observed behavior is correlated with the SEM analysis. Powder X-ray diffraction studies reveal that in our samples the amount of Bi2Sr2Ca2Cu3O10+delta high-temperature superconducting phase is maximal (70-78%) for y = 0.05-0.07.

Ce-doped BaTiO3 thin films prepared on silicon-platinium by r.f. sputtering has been investigated. BaTiO3 doped with 5.5 mol%CeO2 thin film was deposited at 550 degreesC substrate temperature in an Ar atmosfere. The crystal structure and shape were examined by X-ray diffraction and scanning electron microscopy with EDAX. Analysis by X-ray diffraction patterns show that the crystalline film with a cubic structure of BaTiO3, was obtained. The surface morphology (roughness, the grain size and the droplet size) of the thin film surface was examined by atomic force microscopy (AFM). The grain size is about 160 nm, the droplet size is about 0.675 mum and the roughness is 36.88 nm. EDAX analysis established a composition of the film to be identical with that of the target (BaTiO3 doped with 5.5 mol%CeO2). The broad peak in the capacitance versus temperature curve at the Curie point indicate that the r.f. sputtered Ce-doped BaTiO3 film is ferroelectric. The values of the capacitance of the thin film at 1 KHz were found to be 86 pF and the loss dielectric was tan delta = 0.0875. The film exibits a dielectric anomaly peak at 23 degreesC showing ferroelectric to paraelectric phase transition. (C) 2001 Kluwer Academic Publishers.

The concentration of primary radiation-induced defects has been previously estimated considering both the explicit mechanisms of the primary interaction between the incoming particle and the nuclei of the semiconductor lattice., and the recoil energy partition between ionisation and displacements, in the frame of the Lindhard theory. The primary displacement defects are vacancies and interstitials that are essentially unstable in silicon. They interact via migration, recombination, annihilation or produce other defects. In the present work, the time evolution of the concentration of defects induced by pions in medium and high resistivity silicon for detectors is modelled, after irradiation, In some approximations, the differential equations representing the time evolution processes could be decoupled. The theoretical equations so obtained are solved analytically in some particular cases. with one free parameter, for a wide range of particle fluences and/or for a wide energy range of incident particles, for different temperatures; the corresponding stationary solutions are also presented. (C) 2001 Elsevier Science B.V. All rights reserved.

We report on enhancements in the magnetization of two-dimensional electron systems in the quantum Hall regime. The observed discontinuities in the magnetization exceed the predictions for interacting electrons calculated in the Hartree-Fock approximation where only the exchange enhancement of the energy gaps is taken into account. We attribute the further enhancement to many-body correlation effects, i.e., filling factor dependent screening within the screened Hartree-Fock approximation. With this, the discontinuities in the magnetization no longer reflect the discontinuities of the chemical potential, but its behavior in the vicinity of integer filling factors.

An EPR study, of the paramagnetic centers formed by irradiation of polycrystalline K2Cr2O7 and BaCr2O7 was made, In the EPR spectra of the gamma irradiated powder samples, at room temperature, the presence of three types of paramagnetic centers iv as observed. According to their EPR parameters and thermal behaviour. these centers were attributed to the Cr(O)over dot(4)(3-), Cr-2(O)over dot(7)(3-) and Cr(O)over dot(3)(-) radicals respectively,. An adequate mechanism radiolysis has been established.

Trapping levels which govern the vertical transport in (nc-Si/CaF2)(n) multi-quantum wells were investigated using two different techniques: (a) the temperature dependence of the dark current and (b) the thermally stimulated depolarization current technique (TSDC). Measurements by the first technique showed that the conduction mechanism was thermally activated above 200 K with activation energies of 0.35-0.7 eV, These activation energies were found to increase with increasing electric field. TSDC measurements showed also the existence of at least one broad peak above 200 K with estimated activation energies in the range of 0.4-0.45 eV. Analysis of the peak by the fractional heating method showed a continuous distribution of defect states from 0.3 to 0.83 eV. (C) 2001 Elsevier Science B.V. All rights reserved.