National Institute Of Materials Physics - Romania

Electrical conduction mechanisms in Si-SiO2 nanocomposite films were experimentally investigated and theoretically modelled. The films with silicon volume concentration varying from 0% to 100% were deposited on rectangular quartz substrates. They were prepared by co-sputtering CVD from two targets, one of silicon and one of silicon dioxide. Their middle part is formed by Si nanodots embedded in SiO2. 50 parallel aluminium electrodes were deposited, forming 49 coplanar samples, centred around x = 45% Si concentration. The microstructure investigations proved that the silicon nanodots have diameters between 4 nm (x = 23%) and 36 nm (x = 81%). The current-voltage characteristics were measured at room temperature, in the -25 divided by +25 V interval. The obtained curves are symmetric and superlinear. As the current - voltage characteristic is determined by the maximum resistance met by the carriers in their path, the experimental curve is modelled by the field-assisted tunnelling under Coulomb blockade. The used parameters are correlated with previous results obtained on nanocrystalline porous silicon. The correlation coefficient between the theoretical formula and the experimental data is greater than 99.95%.

Thallium related paramagnetic centers of Tl2+ (6s(1)) and Tl-0 (6p(1)) type were produced by low temperature x-ray irradiation in ferroelectric Rb2ZnCl4:Tl single crystals. Extensive EPR investigations have been performed in order to elucidate their intrinsic properties and sensitivity as paramagnetic probes for structural phase transitions studies. Compared to the s-type Tl2+ centers, already used in many such studies, it was found that the p-type Tl-0 centers are much more sensitive to the small variations in the local crystal field associated with these transformations. Their EPR spectra provided information about the unit cell tripling in the P2(1)cn phase, as well as the symmetry lowering and lattice dynamics in the C1c1 phase. The temperature induced, continuous changes observed in the EPR spectra of both Tl-0 and Tl2+ centers were explained by the influence of the soft modes responsible for the 74-K structural phase transition.

The electron paramagnetic resonance (EPR) properties of the Mn2+ ions in PbWO4 single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T = 20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn2+ impurity ions is tetragonal, strongly suggesting that the Mn2+ ions substitute for the Pb2+ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn2+ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains. (C) 2003 Elsevier Ltd. All rights reserved.

Ceramics in the ternary system (Pb,Sr)TiO3-PbZrO3-Pb(Mg1/3Nb2/3)O-3, in the ratio 85.976/5.817/8.207 mol%, was prepared by mixed oxide route. With about 13.98 at % Sr+2, as isovalent substitution in A-position of the lead titanate, high-density samples were obtained for the sintering temperatures of about 1220 and 1230 degreesC. Tetragonal phases were identified both by XRD method and selected area electron diffraction pattern (SAED). The lattice parameter ratio, c/a, of about 1.017 indicated a decrease of tetragonal distortion. The typical TEM micrographs showed domains of about 50 nm wide. The reflection spectra were recorded to prove the presence of the transition Ti+4 to Ti+3. The principal mechanical, electrical and electromechanical properties were discussed as a function of the sintering temperature. The variation of the relative constant with the temperature and frequency evidenced a relaxation-like behavior of this ternary system. By combining the ferroelectric and relaxor end member we tried to improve the piezoelectric properties of a ceramic with a high content of lead titanate, and to achieve a material with both piezoelectric and relaxor-like properties. (C) 2003 Elsevier Ltd. All rights reserved.

The paper presents the inicrostructure of the Si/SiO2 nanocomposite films prepared by co-sputtering from Si and SiO2 targets. High resolution transmission electron microscopy and electron diffraction were used to reveal and compare the structures with different silicon/oxygen (SiOx, 0 < x < 2) coinposition. Si nanocrystallites have been evidenced clearly in the SiO1.6 areas. Percolation limits between the Si crystallites was observed in the x=1 areas. The morphology of the Si nanocrystallites and the amorphous SiO2 zones were compared in regions with different composition.

Silicon detectors represent an important option for the Large Hadron Collider and for its upgrades in luminosity and energy, especially for the tracking system. The main limitation in their utilisation comes from the degradation in the hostile radiation environment where they, will work for long time, without the possibility to be changed. The main goal of this paper is to study, the role of growth technology on stable defect concentrations produced by long-time continuous irradiation in the radiation field estimated for LHC and SLHC.

Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P2(1)cn <----> ClcI structural phase transition that takes place in this compound at 74.6 K. From the two types Of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition. (C) 2003 Elsevier Ltd. All rights reserved.

The alkoxidic route and the spinning deposition were used to prepare monolayer sol-gel indium tin oxide (ITO) films. The morphology and crystalline structure were investigated by cross-section transmission electron microscopy (XTEM) and atomic force microscopy (AFM). The ITO sol-gel mono-layer contains three regions of different porosities. The basic crystalline structure is that of the In2O3 lattice. The optical properties have been studied by optical transmission and spectroscopic ellipsometry. (C) 2003 Elsevier Science B.V. All rights reserved.

A model for trapping phenomena in nanocrystalline silicon investigated by the optical charging spectroscopy method is proposed. The model takes into account all the possible contributions to the discharge current. The results previously obtained on fresh and passivated porous silicon samples are interpreted within the framework of the model, which provides a good fit to the experimental data. The role played by the different kinds of trapping centers (donors and acceptors, surface and bulk) and the different trap parameters is analyzed. (C) 2003 American Institute of Physics.

In this paper we report correlations between the structure, the photoluminescence and the transport properties of luminescent porous silicon. These correlations combined with the observed temperature dependence of tunneling characteristics yield quite a wholesome (pea-pod-like) model for this system. (C) 2002 Elsevier Science B.V. All rights reserved.