National Institute Of Materials Physics - Romania

The dependence of the carrier concentrations, of the resistivity and of the Hall coefficient of irradiated silicon on the neutron fluence has been investigated, starting from the supposition that the main phenomena induced by irradiation in the semiconductor bulk are shallow-donor removal and deep-centres creation. The free parameters of the model are the initial doping of the starting material, the permitted energy level values of the radiation-induced centres in the semiconductor band gap and their introduction rates. The influence of each parameter on the calculated dependences is studied in detail, for three cases: one deep acceptor-like centre, two deep accepters and one deep acceptor plus one deep donor-like centre. Each of the three cases is discussed in correspondence with different experimental results.

Diffusional interactions within the Ostwald Ripening process are analyzed in the present paper. An off-centre diffusion approach is performed to point out the direct correlation between the size of clusters. Herein the diffusion solution is derived as a function of both the growing and shriniking cluster sizes. Also, it is shown that the frequency transfer of particles between the shrinking cluster and the growing one may acquire high values due to the medium polarization. As a result, the temporal power law of the cluster growth derived in this theoretical model differs from that predicted by the LSW theory. Experimental data for Ag clusters embedded in a KCl matrix are analyzed both by the present theory and by the LSW theory.

Pb(Zr0.53Ti0.47)O-3 (PZT) thin films, prepared by sol-gel techniques and deposited on to Si/SiO2/Ti/Pt substrates, have been subjected to thermal annealing in a range of temperatures from 550-800 degrees C. The crystallization behaviour and phase coexistence (tetragonal and rhombohedral) were studied by X-ray diffraction. According to the values of the {110} peak intensity and {110} peak values, the crystallization full-width at half-maximum was more complete at higher temperatures (750, 800 degrees C). At a fixed Zr/Ti ratio close to the morphotropic phase boundary, the lattice parameters of the two phases changed with the annealing temperature. However, the tetragonality degree had relatively low values and the angular rhombohedral distortion was associated with a narrow angular range. The phase coexistence and the variation of the lattice parameters could be explained by the titanium diffusion through the platinum layer. Thus the formation of a {011} titanium rich layer at Pt-PZT interface will supply a titanium excess for the nucleation and growth of textured PZT grains.

The current-voltage characteristics of highly anisotropic Bi2Sr2Ca2Cu3Ox/Ag tapes (short samples) were investigated in a large temperature interval, down to 25 K below the critical temperature, in zero external magnetic field. For samples with a relatively low self-field critical-current density (of the order of 10(3) A/cm(2) at 77 K), the inter- and intragranular contributions to the dissipation process were clearly separated. The I-V curve shape indicates the existence of finite inter- and intragranular critical currents. The anisotropy of the intergranular critical current in weak applied magnetic fields revealed good agreement with the ''railway switch'' model for the preferred transport current path. It was shown that, in the case of a large anisotropy, the finite zero-field critical-current density at high temperatures originates from a size limitation of the two-dimensional vortex fluctuations. This size limitation is caused by the finite (a, b) plane extension of the microscopic current path.

Thin films of tungsten and titanium carbonitrides have been prepared by reactive laser ablation of tungsten and titanium targets in a methane (propane) atmosphere with small amounts of nitrogen, at low pressure. The films have been investigated by X-ray diffraction. It was shown that the films are mainly composed of tungsten carbides and titanium nitrides. The microhardness of the samples reaches 2600 GPa for tungsten based films and 2200 GPa for titanium based films.

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.