Publications

C-axis oriented and disordered aluminum nitride layers have been deposited by RF, respectively DC magnetron reactive sputtering mode. The same deposition conditions were used for gas flow and pressure. The differences between the two growing modes are explained by the influence of ion bombardment upon the growing layers, which is stronger in the RF mode in comparison with the DC one, at the same deposition rate.

We obtained a multilayer heteroepitaxial structure of In-0.53 Ga-0.47 As-type deposed on (100) oriented We selected deliberately different thickness and doping concentrations df each layer for their potential applications in very high frequency bipolar transistors. A sequence of six epilayers were realized by vapour phase epitaxy. The characterization of crystallographic and semiconducting properties in correlation with technological processes is performed by TSC-method and by double crystal InP substrate. We propose a kinematics model of X-ray diffraction in heteroepitaxial multilayer structures of InP/InGaAs. It is based on a discontinuous lattice misfit and elastic strain distribution. The model fits well the rocking curves obtained for both (004) and (224) reflecting planes. The values of the lattice misfit and the elastic strains in the plane of lamellae were calculated.

Magnetic materials in the CuZnTi ferrite system with the chemical composition Cu1-xZnxTiyFe2-yO4, with 0.5 less than or equal to x less than or equal to 0.62 and 0 less than or equal to y less than or equal to 0.05 were investigated as a function of sintering temperature and time and cooling speed in order to see the influence of these factors upon the Curie temperature and the permeability behaviour around the Curie point. The quality of the magnetic temperature transducers made from such materials is directly connected with the value of the slope of the mu(T) curve. The higher the slope the more sensitive will be the transducer. Slopes as high as 50%degrees C-1 were obtained by a proper cooling of the samples. this value bring the highest reported so far for such materials. The results are discussed in terms of the ionic distribution of the Cu, Zn and Ti ions over the tetrahedral and octahedral sites of the spinel lattice. The materials proved to be stable in time if the temperature does not exceed 200 degrees C, but higher temperatures may induce irreversible change in the material structure due to cation migration and electron transfer from copper ions to iron ions.

An analysis is given of the way in which thickness fluctuations in a multilayer can affect the grazing incidence X-ray reflectivity pattern. It is shown that peak shifts, broadening and height variations can result. They alter in a predictable way the parameter values derived from a fit with an ideal lattice. A principle for designed Bragg reflectors is also proposed.

The effect of intermediate grounding and sintering environment (air or sintered powder) on the superconducting characteristics of Bi(Pb)-Sr-Ca-Cu-O hulk ceramics obtained by spray-frozen freeze drying technique was investigated by X-ray diffraction, electric and magnetic measurements. Intermediate grounding leads to higher scattering coefficients of the superconducting characteristics. This result does not depend on sintering environment. Improved results are obtained when sintering is performed by immersing pellets "in powder" rather than placing them "on powder". The powder used as sintering environment was pyrolized precursor powder calcinated in air for 100 h. (C) 1998 Elsevier Science Ltd. All rights reserved.

Modeling of the I(V, T) dependence in thin p-n semiconductor structures shows that various behaviors in its shape factor can be correlated with and explained through the voltage dependence of the carrier concentration product np and its components, at the zero field injection-point, for various conditions controlled by thickness and doping. A ''second exponential'' high injection range exp (qV/nkT), with a shape factor n as high as four appears to be a consequence of the departure from quasineutrality due to significant minority carrier space charge, favored by low doping and thickness. In thin structures, the presence of a second exponential range is associated with the presence of a quadratic range (I similar to V-2) at very high current densities.

Modeling of the I(V,T) dependence in thin film semiconductor structures suggests that experimental data, on temperature and voltage dependent activation energy and shape factor of the current, give significant information on the recombination process within the structure. Grounds are thus laid for further investigation on quantum well effects within such structures. (C) 1996 American Institute of Physics.

An anomalous high signal was observed across lead sulphide-ferroelectric ceramic heterostructures illuminated at room temperature with light having wavelengths between 0.8 and 3.1 mu m. The frequency characteristic of this signal is similar to a high-pass filter and depends on the value of the load resistance in series with the lead sulphide thin film, This signal is only two to three times smaller than the usual photoconductive signal generated by the lead sulphide thin film. Its nature is not pyroelectric because of its frequency dependence. The interface charge released from the PbS-ferroelectric interface during polarization reversal was integrated and a hysteresis loop was obtained. Electrons and hole concentrations were evaluated from this curve. The values obtained are: 4.4 x 10(16) cm(-3) for holes and 3.4 x 10(16) cm(-3) for electrons.

The paraconductivity of Li-doped YBa2Cu3O7-x was measured. We have found that the character of the fluctuation changes as lithium content increases: instead of a crossover from 2D to 3D behavior (D is the dimensionality), as we have observed for a low doping level, a double crossover to the 2D percolative and 3D percolative regimes becomes manifest by increasing the lithium content.