National Institute Of Materials Physics - Romania

Photodetectors (PDs) based on three separate confinement-multiple quantum well (SCH-MQW) structures (with 2, 4 and 8 QWs) and sis graded-index (GRIN) SCH-MQW structures were studied for two types of devices: guided wave and non-guided wave devices. Non-guided wave devices were processed and the capacitance and the responsivity, in the wavelength range of 750-990 nm, were determined for various bias voltages. Guided wave PDs were simulated using the transfer matrix method for optical mode calculation and the minimum length of the detectors was calculated for absorbing 99% of the incident light. The junction capacitance of the guided wave PDs was calculated from the measured capacitance of non-guided wave devices and the minimum size of the devices, values of 215, 101, and 52 fF were obtained for 2, 4 and 8 QW SCH-MQW structures. Laser diodes were also fabricated from the three different SCH-MQW structures. Threshold current densities (J(th)) of 737. 755 and 1210 A cm(-2) were measured for 2, 4 and 8 QWs, respectively. Further improvement was achieved using GRINSCH-MQW structures and LDs with 4 QWs exhibited 33% decrease in J(th) and 22% increase in slope efficiency. (C) 2001 Elsevier Science B.V. All rights reserved.

We calculate the longitudinal conductivities of a two-dimensional noninteracting electron gas in a uniform magnetic field and a lateral electric or magnetic periodic modulation in one spatial direction, in the quantum regime. We consider the effects of the electron-impurity scattering anisotropy through the vertex corrections on the Kubo formula, which are calculated with the Bethe-Salpeter equation, in the self-consistent Born approximation. We find that due to the scattering anisotropy the band conductivity increases,:and the scattering conductivities decrease and become anisotropic. Our results are in qualitative agreement with recent experiments.

The SERS technique has been proved to be successful to study Single-Walled Carbon Nanotubes (SWNT's), with the observation of peculiar features. In this study, we present more specifically the Raman features which are strongly dependent on the morphology of the nanotube thin films, their thickness, the interfacial effects between films and support, the effect of solvent used to disperse the nanotubes onto the metallic surface.

The optical spectra of CdS nanocrystals embedded in polyaniline matrix have been studied. The presence of surface states is revealed both in absorption and emission spectra. The polymer matrix determines a shift of the absorption edge of CdS nanocrystals, towards higher energies.

Densification and piezoelectric performances of some fast fired lead zirconate-titanate ceramics were investigated in comparison with those made by conventionally sintering technique. Micro and nano PZT powders were used as initial powders for the compacts subjected to the Cast and conventionally sinterings. Density maxims of about 98 % of the theoretical density, were obtained for nanopowder compacts fast fired at 1350 degreesC for 20 minutes. The piezoelectric properties of the fast fired and conventionally sintered compacts were determined. Significant increase for d(33) piezoelectric charge constant was observed in the case of fast fired samples compared to conventionally sintered ones. The electromechanical planar coupling factor k(p) was also increased From 0.62 to 0.65 for fast fired samples. The densification mechanism was discussed in terms of a predominantly transient stresses induced during the initial stage of densification by the rather high temperature gradient of the fast fired samples.

Pyroelectric measurements were performed on pure and on L or D alanine doped TCS. Crystals were grown in paraelectric phase (similar to 52 degreesC) from solutions by slow evaporation of the solvent. Pyroelectric current was measured by, crossing the Curie point, both on heating and cooling. Spontaneous polarization P-S(T) was also recorded using the charge integration method. The pyroelectric current of pure TGS samples along with the spontaneous polarization were detected only on heating, using a poling procedure similar to those applied to ceramic ferroelectrics. The remarkable asymmetry of measured pyroelectric coefficient of D and I. alanine doped samples suggests a non-equivalent substitution of glycine in the host lattice.

The results of an in situ transmission electron microscopy study of the formation of Co-silicides on patterned (001) Si substrates are discussed. It is shown that the results of the in situ heating experiments agreed very well with the data based on standard rapid thermal annealing experiments. Fast heating rates resulted in better definition of the silicide lines. Also, better lines were obtained for samples that received already a low-temperature ex situ anneal. A Ti cap layer gave rise to a higher degree of epitaxy in the CoSi2 silicide.

It is shown that the figure of merit of a thermoelectric couple can be appreciably improved by using a pulsed current transferred from the thermoelectric circuit. In the frame of this theory un unusual drop of temperature in a thermoelectric circuit is possible. An ultrafast process of thermoelectric conduction is described, whose increased performance is realized by minimizing the effects of thermal dissipation.