Publications

The paper presents an analysis of the electrical transport, phototransport and photoluminescence measurements performed on silicon-based nanocrystalline systems. The experimental results are discussed within the frame of a quantum confinement model. It is proved that the differences between the energy levels identified in the measurements are related to the quantum selection rules specific to each process. It is also proved that, at nanometric scale, the nature of the atoms composing the nanocrystal represents a first order correction, while the size represents a zero order factor.

This paper presents the manufacturing and the characterization of AlN membrane supported F-BAR structures. The 2 mu m thin AlN layer was grown using sputtering techniques, on a high resistivity (111) oriented silicon substrate. Conventional contact lithography, e-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachinig techniques were used for the release of the AlN membrane. The bottom side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a resonance around 1.6 GHz. The extracted value of acoustic velocity is in good agreement with that reported by other authors on materials fabricated by other methods.

The mechanism of charge storage in p Si/SiO2 /nano-crystalline PbS heterostructures is investigated using capacitance - voltage (C-V) and conductance-voltage (G-V) measurements at different frequencies. The nano-crystalline thin PbS films were obtained using the Chemical Bath Deposition method with a shorter reaction time and without doping element in the reducing bath. The C-V curves show hysterezis indicating charge storage with a maximum charge density of 3x10(11) cm(-2) for the highest measurement frequency of 100 kHz. The charge storage capacity is enhanced in the rase of using a nano-crystalline PbS thin film instead of a standard macro-crystalline film.

The combined investigation techniques (EMPA, ME, NGR) evidenced the third level of the inclusions at nano-size scale for different metamorphic deposits. So, NGR technique reveals the contribution of the (Mn, Fe) oxides - small particles at standard (RT) at very low temperature (LHeT) in the Fe-57 gamma- ray signal. On the other hand the ME techniques confirmed the presence of the nanometric Fe and Mn oxides inclusions in manganoan fayalites (Razoare deposit) and "coral like" aggregates of nanometric Au -"solid-bubbles" in the gold ores of the Southern Carpathians.

The spectroscopic investigation of the highly transparent rare earth (Pr3+, Nd3+, Ho3+, Er3+ Tm3+, Yb3+) doped Sc2O3 ceramics produced by the solid-state synthesis technique indicate that these materials could substitute the single crystals in construction of solid-state lasers. These studies indicate also that the rare earth doped transparent ceramics could extend considerably the variety and performances (new active systems, wavelength ranges or emission schemes) of these lasers.

The paper presents a quantum confinement model which describes the phototransport processes in nanocrystalline porous silicon. The model supposes that the nanocrystallites surface/interface acts as the walls of an infinite rectangular quantum well, introducing discrete confinement energy levels. This model is proved able to also describe the photoluminescence and the temperature dependence of the dark current in the same samples. Its results are in agreement with the Microstructure investigations.

The temperature behavior of the main piezoelectric parameters, such as the electromechanical coupling factor k(p), the mechanical quality factor Qm, the dielectric permittivity epsilon (real and imaginary part), the loss tangent tan delta, the piezoelectric charge constants d(33) and d(31) and the voltage constants 933 and 931 of a soft type piezoelectric material was investigated. The results showed that temperatures under 150 degrees C do not influence these parameters, which indicate that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect and above that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

We investigated the behaviour of YBa(2)CU(3)o(7-delta) ceramics by multiharmonic ac-susceptibility. We have found an unexpected improvement of the intragranular response at the highest fluence 9.98 10(17) neutrons/cm(2). However, this fact is accompanied with a continuous suppression of the intergranular properties. We assume that this behavior is the result of the change of the nature of defect distribution at high fluence, when the uniform distribution of defects starts to develop space instabilities as a result of the subtle interplay of the in-cascade rate of interstitial loops production, cascade collapse of the vacancy loops, and the excess network bias.

The morphological changes induced by the thermal treatments of YAG: Eu crystals synthesized by a sol-gel method have investigated using the Mossbauer and optical techniques. Both methods exhibit a drastic change of the spectral parameters at amorphous - garnet phase transition, followed by a slow evolution of these parameters with the increase of the annealing temperature. This slow evolution is due to the increase in size of the nanocrystals.

Accelerate ageing of lead zirconate-titanate ferroceramics (PZT) by fast neutron irradiation is investigated. The ferroelectric behavior of Pb1-wMw(Zr,Ti)O-3, where w < 0.5 wt%, M = Li, Nb, Cr, Bi, Mn, Sb, La for Zr/Ti ratio = 51.5/48.5, prepared by solid state reaction, is strongly affected by fast neutron irradiation at Phi(i) = (10(13) divided by 10(18)) n/cm(2) integrated fluxes. Structural investigations (XRD and SEM) reveal modifications of the microstructure, porosity and elementary cell parameters. High irradiation levels induce tetragonal-cubic at the same time with a ferro-paraelectric transition. The micrographs show the evolution of 90 degrees and 180 degrees walls as well as herringbone structure typical for tetragonal distortions with irradiation level. A relation between macroscopic characteristics (piezoelectric and dielectric) and microscopic properties (crystallographic parameters, porous structure and grain morphology) as function of irradiation doses can be established using a model based on microstructural parameters. These results offer opportunities to understand the fundamentals of radiation effects upon piezoelectric ceramics. (c) 2007 Elsevier Ltd. All rights reserved.