National Institute Of Materials Physics - Romania

Lead sulfide (PbS) thin films were deposited from a chemical bath onto SiO2/Si (n-type) substrates. Pseudo-metal-oxide-semiconductor devices were obtained by evaporating source and drain gold electrodes on a PbS surface and aluminum gate electrode on a Si substrate. Field-effect-assisted photoconductivity in the PbS layer was investigated at room temperature, in the 800-2700-nm-wavelength domain for different values and polarities of the drain and gate voltages. The best results were obtained for a positive gate, when both semiconductors are in depletion. An enhancement of about 25% of the photoconductive signal is obtained compared with the case when the gate electrode is absent or is not used. A simple model is proposed that explains the behavior of the dark current and photoconductive signal in PbS film with changing the gate voltage. (C) 2002 American Institute of Physics.

The low-temperature behavior of the Co-doped magnetite is studied. The Fe3-xCoxO4 powders (x = 0.01 - 1) have been prepared by the hydrothermal method. In order to investigate the cobalt influence on the structure of magnetite, transmission Mossbauer spectroscopy measurements were performed at temperatures close and below the Verwey transition temperature characteristic for pure magnetite (122 and 77 K, respectively). All spectra were fitted with five components and related to nonequivalent Fe2+ and Fe3+ sites. The hyperfine parameters of these sites as well as their relative fractions in the powder samples were determined. (C) 2002 Elsevier Science B.V. All rights reserved.

Creation of Ag and Ag-0 centers by irradiation with ionizing radiations has been investigated using NaCl and KCl crystals containing various concentrations of Ag+ ions and using Ca2+-codoped NaCl:Ag+ and KCI:Ag+ crystals. From the absorption spectra, it is observed that Ag- center perturbed by Ca2+ ion is also created besides the Ag- and Ag-0 centers. It is confirmed that Ag- and Ag-0 centers increase with increasing the concentration of doped Ag- ions. It is suggested that there are two kinds of Ag(c)(0)v(a) centers (i.e. Ag-0 accompanied by vacancy at the nearest neighbor) with different site symmetries depending on the location of vacancy. (C) 2002 Published by Elsevier Science B.V.

We studied by means of optical absorption and transmission electron microscopy the formation of Ag nanoclusters from Ag--ions in three different potassium halides (KCl, KI and KBr). By stepwise UV illumination at a suitable temperature, the electrons of Ag--ions were detached and the controlled formation and growth of Ag nanoclusters from Ago-atoms take place. At the same time, the detached electrons are captured by K+-ions, proved by the appearance of K-colloid bands. The kinetics of the conversion Ag- + hv + U --> Ag nanoclusters was followed by monitoring the Ag--ions and Ag nanoclusters absorption. The temperatures of illumination are different, depending on the matrix in which Ag nanoclusters are formed (220 degreesC for KCl, 160 degreesC for KBr and 140 degreesC for KI). By optical absorption and TEM we proved that the nanoclusters of Ag obtained by our method are separated, spherical and ovoidal, with average diameters between 7.5 and 50 nm. (C) 2002 Elsevier Science B.V. All rights reserved.

We found Oxygen-doped GaAs crystals to be suitable materials for CO2 laser optical component preparation, with application at 10.6 mum. An optical transmission of 55% in the IR spectrum range, between 2 and 15 mum has been reached for such a GaAs type material. The GaAs crystals that we have analysed were grown by two procedures: Horizontal Bridgman (HB) and Liquid Encapsulated Czochralski (LEC). The FIB method has been used for obtaining pure (undoped) crystals, while the oxygen-doped GaAs ingots were grown by LEC technique. The two types of samples processed in the same manner as regards mechanical polishing and chemical etching, which were investigated by Hall measurements, optical transmission spectrometry and elastic recoil detection analysis (ERDA) technique. The GaAs:O (LEC) has near semi-insulating properties as can be observed from the results of the electrical resistivity and Hall effect measurements. The ERDA spectrum shows an intense signal of oxygen in the bulk of GaAs:O (LEC) crystals, while the oxygen signal is not present in the ERDA spectrum of the undoped GaAs (HB). We consider that these results could recommend the ERDA technique as a possible qualitative and quantitative analysis in an ion-beam accelerator for oxygen content in oxygen-doped GaAs crystals. The analysis is not sensitive to the native oxide, as could be seen by measuring GaAs (HB) undoped crystals. (C) 2002 Elsevier Science B.V. All rights reserved.

In the case of AsSe:Sn and Ge27Sb13Se60 amorphous thin films deposited on Si(100) wafer, it was observed that the diffraction pattern is distorted by the escape-peak, which is very sensitive in intensity to the wafer's position in its own plane, This peak was found to be superimposed on the first sharp diffraction peak, which characterises the medium range order in the amorphous materials. A method for the minimisation of the escape-peak effect is presented. A different method is suggested to avoid this peak. (C) 2002 Elsevier Science B.V. All rights reserved.

Photodarkening relaxation under light exposure of a-As2Se3 amorphous films doped with 0.5 at.% of metals Sn, Mn, Sm or Dy and a-AsSe films doped up to 10.0 at.% Sn was studied to determine its dependence on the type and concentration of impurities and thermal treatments. Both factors reduce photodarkening with the degree of reduction dependent on the type of impurity. The relaxation process may be described by a stretched exponential with the dispersion parameter 0.4 less than or equal to alpha < 0.9 and the time constant increasing with Impurity concentration or thermal annealing. The results are discussed in the context of the recently formulated 'slip-motion' model of photodarkening in chalecgenide glasses. (C) 2002 Elsevier Science B.V. All rights reserved.

In-plane and out-of-plane magnetoresistivity (MR) measurements were performed on the same antiferromagnetic (AF)YBa2Cu3O6.25 single crystal using a six-lead configuration, in magnetic fields H applied along the ab plane. We identified three terms contributing to both MRs. The first term is anisotropic with respect to the in-plane field orientation, with a twofold symmetry for the in-plane MR and a fourfold symmetry for the out-of-plane MR. We show that these anisotropic features of the magnetoresistivity tensor can be understood in terms of the coupling between the spin and the elastic degrees of freedom. The second term in both MRs is negative and isotropic upon in-plane H rotation. We ascribe this term to charge-carrier scattering on the AF domain walls. Finally, the third term is positive, quadratic in H and seems to correlate with the AF phase transition.

Results of x-ray powder diffraction and magnetic measurements performed on RFe11.3W0.7 compounds, where R=Dy, Ho, Er, and Lu, are presented. The Curie temperature, saturation magnetization, and spin reorientation transitions are discussed in relation to several RFe12-xTx systems, where T=Ta, Nb, Mo, Ti, and the x-substituent concentration is within a narrow range x=0.5-1.0. In these systems, the dependence of the Curie temperature versus the unit cell volume, for a given R partner, suggests combined size and electronic effects produced by the T atom. The strengths of the rare-earth-iron and iron-iron exchange interactions are evaluated in the mean-field framework, and analyzed with reference to RFe12-xTx compounds, with T=Ta, Re, Nb, Mo, Ti, Cr, and V. Based on (i) the phenomenological mean-field model accounting for the sublattice of conduction band electrons, (ii) Brooks' reinterpretation of Campbell's formulation of the intersublattice exchange, and (iii) Wigner-Seitz cell analysis, we explore the role played by the conduction band electrons, particularly the nd-type electrons (n=3, 4, 5), on the intersublattice exchange coupling in the iron-rich RFe12-xTx class of compounds. It is proposed that the variation in the conduction-electron density, by varying the T element type and concentration, leads to a modification of the 5d band occupancy number and consequently, of the R-Fe intersublattice exchange coupling.