National Institute Of Materials Physics - Romania

The three types of order in the non-crystalline solids are described: the short range order, the intermediate range order or medium-range order and the super-order. After pointing out how the diffraction patterns express the various types of ordering in the amorphous network, we discussed the structural models developed for some amorphous materials: tetrahedral semiconductors, diamond like-carbons and the triple coordinated pnictides (e.g. arsenic). We showed the performances of the computer-assisted modelling studies.

Superconducting samples of the Bi(Pb)-Sr-Ca-Cu-O system have been irradiated in transmission electron microscopy (TEM) to investigate the effect of a relative high fluence of electrons with 75 and 100 keV energy on the microstructure of the material. The diffraction pattern images show a dramatic change from very uniform lattice spots at ab crystalline planes to a circular pattern corresponding to damage and breaking of the materials in very small crystallites.

An amorphous phase has been demonstrated in porous silicon prepared by anodization of silicon wafers. The radial distribution function has been calculated on the basis of the X-ray diffraction pattern recorded from a typical sample. There was revealed an amorphous phase of composition SiO1.6.

Measurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p-type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72 eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time alpha(-1) is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.

High resistivity n-type silicon samples have been irradiated with similar to 1MeV neutrons at fluences between 10(12) and 10(14) n/cm(-2). The radiation induced changes in Hall coefficient and resistivity have been analysed by Hall Effect measurement during a storage time of approximately seven months at room temperature. The Hall coefficient measured for the most irradiated samples, exposed to fluences higher than 4x10(13) cm(-2), has switched from negative to positive values approximately 200 days after the irradiation. This experimental evidence explains the reverse annealing effect observed in neutron irradiated silicon detectors as being related to the creation of a deep acceptor level which causes the change in conductivity from n to p-type of the irradiated silicon bulk during self annealing. The behaviour of irradiated devices has been analyzed with a model taking into account donor removal and acceptor creation. Results are in agreement with others obtained with different experimental techniques.

The low-voltage-level current-voltage characteristics of Bi2Sr2Ca2Cu3O10+x tapes and bulk samples with preferential crystallite orientation were measured in applied magnetic fields both parallel and perpendicular to the samples' surfaces. The data were interpreted within the framework of the collective pinning model. The collective pinning exponent alpha changes from approximate to 1 to approximate to 0.5 (close to T-c), while in the case of unoriented samples it is constant (and equal to the theoretical value 1/2) as shown in an earlier paper. We suggest that this temperature dependence of alpha is due to the change of the dimensionality of the intragranular vortices.

Polycrystalline p-type InxGa1-xSb (x=0.20) for optoelectronic devices has been obtained by rapid direct synthesis from high-purity (99.9999%) In, Ga, and Sb. The uniformity of the composition has been checked by neutron activation analysis and also by x-ray diffractometry (Vegard-law approximation). From Hall-effect measurements an acceptor concentration of 3.2x10(17) cm(-3) and a mobility of 628 cm(2)A/ s for holes, at liquid nitrogen temperature, have been determined, and from absorption measurements, a quite diffuse edge for the optical absorption coefficient resulted. This paper describes the method of rapid direct synthesis of the ternary compound and the techniques used for the characterization of this material for special optoelectronic applications. (C) 1996 Society of Photo-Optical Instrumentation Engineers.

The crystallization kinetics of the amorphous alloy Fe77Gd3B20 were examined under isothermal flash annealing conditions both in air and in a vacuum. Magnetic measurements were used to determine the reaction order (n) and activation energy (E(a)), and Mossbauer spectroscopy (MS) was used to identify some resulting crystalline phases.

TIN, films are deposited on glass in a modified de planar three-magnetron geometry. The ellipsometric and reflectance measurements reveal the influence of the substrate bias and of the Ar:N-2 partial pressure ratio on the stoichiometry and optical properties of the films. The stoichiometric structure is obtained for only certain bias voltage and Ar:N, ratio values. The value of the plasma energy omega(p) confirmed the film stoichiometry, as determined from x-ray diffractometry. (C) 1996 Society of Photo-Optical instrumentation Engineers.