Publications

Starting from the microscopic current associated with the single-electron wave functions and using Wannier functions suitably generalized for nonperiodic heterostructures we construct mesoscopic (either discrete or continuous) conserved currents associated with states having components only in the considered band. This provides a rigorous basis to the approximate envelope function approaches. As an application we analyze general one-dimensional heterojunctions, mapping explicitly the microscopic asymptotic states onto mesoscopic (one-band envelope) ones. This proves that, apart from the effective masses and the band offsets, the connection rules are characterized in general by three parameters, the values of which are unconstrained, confirming the results of phenomenological analyses.

PbS/PbTiO3/Si heterostructures were prepared by successive depositions of a PbTiO3 thin film using the sol-gel method and of a PbS thin film using the coprecipitation method on a bulk silicon wafer. The current-voltage characteristic of the heterostructure is of a diode type. The heterostructure is sensitive over a broad range of wavelengths, from 0.35 to 3 mu m, when illuminated with continuous light or with modulated Light. Three sensitivity peaks are observed, at 2.7, 0.95, and 0.38 mu m. The measured signals are attributed to a photovoltaic effect which appears in the studied heterostructure.

To model high injection phenomena in P-N junction devices it is usually necessary to use numerical analysis. This is because the standard procedure of dividing the structure into neutral zones and depletion layer (regional approximation) fails. In a recent paper, Yue et al. [J. Appl. Phys. 77, 1611 (1995)] proposed an extension of the Shockley theory, retaining the form of the conventional diffusion current-only model for the conduction mechanism. Their solution was based on the resolution of the ambipolar transport equation in the base. It is shown here by a numerical simulation of the complete structure, within the framework of the drift-diffusion model, that both the exponential current dependence: J alpha exp(eVa/2kT) as well as the Yue et al. approximation are valid only in the limiting case of a strongly extrinsic short base diode. (C) 1996 American Institute of Physics.

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.