Publications

The Optical Charging Spectroscopy (OCS) was applied for the first time to investigate the trapping levels in high resistivity silicon irradiated with neutrons. An analytical formula for the OCS discharging current has been obtained. The theoretical and experimental results proved that the OCS is more sensitive than the usual Thermally Stimulated Currents method.

High power pulsed laser diodes are of interest due to their potential use in medicine and peculiar applications. This paper presents failure analysis for catastrophic damage in GaAs/AlGaAs LOC pulsed laser devices. There is presented failure decrease of optical power as Ig (P/P-0) = f(t) where Po is initial power, P actual power, t-operating time. The damaged devices were investigated by optical and scanning electron microscopy (SEM) and there was performed a electron dispersion spectroscopy (EDS) analysis These experiments advanced the idea of dividing catastrophic degradation in three types, respectively: 1-catastrophic optical mirror damage (COMD), 2- catastrophic damage due to major mechanic defects (CDMMD), 3-catastrophic damage due to metal migration (e.g. Au) (CDMM). There was established an experimental criterion to characterise catastrophic damage.

The solution chemistry of oxalate coprecipitation of Bi, Si, Ca, Cu has frequently! been used in the synthesis of Bi-based ceramic systems, although there are different opinions referring to some major aspects, such as coprecipitation pH and precipitating agent. In the present work we established the precipitation conditions of Bi, Sr, Ca, Cu individual oxalates and of the quaternary! mixture. The initial and thermally treated oxalate powders were characterized bq! XRD, IR, BET methods and TEM observation. The decomposition conditions were established based on a DTA/TG study. The powders resulted from the decomposition of Bi and Cu oxalates contain the corresponding oxides and the Ca and Sr oxalates decomposition lead to the carbonates obtaining. In each case a high reactivity and a very small powder particle size were observed. A very good powder homogeneity has also been noticed in the final mixture of oxalates. Compared to superconducting ceramic materials realized by a classic route, the reactive oxalate powders allow the decrease of both thermal treatment periods and temperature, emphasizing a high microstructure homogeneity of the ceramic body. (C) 1998 Elsevier Science Limited. All rights reserved.

Amorphous multilayers based on Se and CdSe alternated sublayers were succesfully prepared by thermal vacuum evaporation and laser ablation with the periodicity of 22 nm. The multilayer structure is stable up to similar to 70 degrees C. In the samples prepared by laser ablation a large contraction of the multilayer stacking occurs by annealing. Amorphous SiOx/CdSe multilayers with the periodicity of similar to 15 nm were successfully prepared by thermal vacuum evaporation. The structure is stable up to 400 degrees C annealing temperature. Pulse excimer laser irradiation of the SiOx based multilayer produces an expansion of the interlayer distance with similar to 0.33%.

The ferroelectric/semiconductor heterostructures were fabricated by sol-gel deposition of lead titanate (PT) thin films on a single-crystalline p-type Si wafers. The ETT films were crystallyzed by a conventional thermal annealing for 30 min at temperatures ranging from 575 degrees C to 675 degrees C. Current-voltage and capacitance-voltage characteristics show a hysteresis which can be due to the spontaneous polarization of PbTiO3. The current-voltage characteristics exhibit a diode behaviour while the capacitance-voltage exhibits a large memory window, up to 3.5 V, for the films annealed at 600-650 degrees C. At the illumination with modulated light, a.c. photovoltage was detected on a broad range of wavelengths (0.35+4 mu m) for all samples. The spectral distribution of the photoelectric signal in the UV-Vis-IR domain shows two local maxima. A model is proposed to explain the observed experimental results.

Hardness, X-ray diffraction and transmission as well as infrared reflection measurements on virgin and ultraviolet (UV)-irradiated amorphous films of the GexAs40-xS60 system have been performed. The UV light induces film softening for x 19. The structure remains amorphous but the distance characteristic of medium range order decreases for x 19. Photo-contraction of the films (up to similar to 12%) has been revealed from IR spectra and X-ray transmission experiments. A main effect of UV light is the release of sulphur. (C) 1998 Elsevier Science B.V. All rights reserved.

A quantum confinement model is proposed to explain the carriers transport in a nanowire network of porous silicon. The electron Hamiltonian is written as the sum of the longitudinal and the transversal contributions. The last one corresponds to a two-dimensional infinite cylindrical quantum well, whose energy levels determine the activation energies observed in the temperature dependence of the dark current. The model is in excellent agreement with the experimental data.

The time-evolutions of both the electrical resistance in air, and of the response to reducing gases lead to the conclusion that the resistance of tin dioxide gas sensors is not governed by intergranular barriers. A conductance model is discussed, with a surface resistance and a bulk resistance connected in parallel. The long-termed evolution of the sensor resistance (ageing) is attributed to oxygen diffusion into the bulk.