Publications

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation, The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-mu m-wide stripes was as high as 6.4 W before catastrophical optical degradation, If scaled to continuous-wave (CW) conditions, this value would be 800-1100 mW, which would mean a factor of 2-2.7 times more than reported for the best devices with normal design for threshold minimization, The absorption coefficient for the symmetrical structure is as low as 1.1 cm(-1), in spite of the low trapping efficiency of carriers in the quantum well (QW), The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions), Threshold current densities were 800 A/cm(2) for 5-mm-long devices in the symmetrical case and 2200 A/cm(2) in the asymmetrical one, The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed.

The active current and brightness pulses have been measured and recorded for good thin film electroluminescent devices. The transferred charge and the total number of emitted photons per pulse have been evaluated by direct numerical integration. Both these quantities depend almost linearly on the amplitude of the applied voltage pulses. In the investigated range of low frequencies (1 to 100 Hz) it was found that the measured efficiency per pulse is independent of voltage, brightness and frequency.

The correlation between the structure, chemical composition, electrical and magnetic properties of Bi2Sr2CaCu2O8+delta single crystals is presented. As revealed by SEM measurements, the small samples (approximately 1 mm x 1 mm x 0.2 mm) are stacks of thin plates (1-2 mu m). Microcompositional measurements (energy dispersive spectroscopy) show the Bi2Sr2CaCu2O8+delta (2212)-phase and a small quantity of residual phases enriched in bismuth and strontium and calcium-deficient. Magnetically modulated microwave absorption measurements (MAMMA) lead to a value for the anisotropy of the upper critical field of about 7, which is more than twice the reported value for YBa2Cu3O7-delta single crystals (also determined from MAMMA responses). The plot of the resistance versus temperature suggests that only the first 10-20 plates from the surface of the sample have a contribution to the sample conductivity. The imaginary part, chi'', of the magnetic susceptibility of our samples shows three peaks.

The temperature and frequency dependence of tg delta and epsilon' for Ni0.8Zn0.2CoxFe2-x ferrites was studied in the 10(2)-10(6) Hz frequency range and 20-200 degrees C temperature range. The experimental curves exhibit aspects very dependent of composition. To verify that interfacial losses, expected to appear in this type of system are compatible with the experimental results a calculation of the dielectric permittivity and dielectric loss using the Koops' model was made. A good correlation between calculated and experimental curves was found.

Mn-Zn ferrites substituted with Ti4+ ions are now becoming commercially important. The substitution with titanium ions in Mn-Zn ferrite is recommended for ferrites with low losses and low temperature factor of initial permeability, T-k/mu(i) because of the shift of the secondary maximum of permeability, SMP, towards lower temperatures [1] and the increase of the d.c.resistivity [3,4,5]. The present paper reports the results obtained by substitution of a small amount of titamium ions to high permeability ferrite Mn0.51Zn0.43Fe0.062+Fe2O4, by adequate reduction of Zn2+ content. The magnetic and electric properties improve: d.c. electrical resistivity increases while the loss factor tg delta/mu(i) and disaccommodation decrease. By addition of 0.35% by weight TiO2 the initial permeability, at 20 degrees C, improves.. We remark the flattening of mu(i)(T) curves with the increase of content of Ti4+ ions and the diminution of the peak at the Curie point, facts that can be associated with two points of anisotropy compensation instead of one, the second minimum of K-1 being in the neighborhood of T-c. Structural and magnetic properties of high permeability ferrite with substitution of titanium ions confirm thar Ti(4+)ions make pairs with Fe2+ ions.[4,6].

The present paper reports the magnetic and structural properties of manganese -zinc ferrites obtained by the addition of small amount of Nb2O5 (0.03-0.1%g). For power applications of Mn-Zn ferrite it is necessary to have the Secundary Maximum of Permeability (SMP) and correspondingly low losses, in the domain of positive temperature, 50-100 degrees C. The magnetic properties: initial permeability, power losses, temperature factor of permeability and T-c improve by the addition of 0.03-0.07%g Nb2O5 to the Mn-Zn ferrite.Magnetic and structural results: XRD, SEM2+EDX analysis, suggest that Nb5+ ions partially dissolve in the ferrite lattice, substituting Fe3+ octahedral and loosely bounding Fe2+ ions.

Binary rare-earth ceria-ytterbia and ceria-dysprosia and pure CeO2 have been subjected to CO2-laser irradiation. All the samples have been prepared by precipitation and controlled drying. Irradiated samples as well as the samples calcinated at 1273 K have been studied using TG-DTA, TEM and SAED, WAXS and FT-IR. Introduction of a second rare-earth oxide, like ytterbia or dysprosia led to an important increase of 'stability' towards the CO2-laser irradiation. Thermal investigation of these samples revealed in fact a 'metastability' because, for the temperatures of about 500 K, a release of oxygen has been observed. This behaviour could recommend CO2-laser irradiation as a possible alternative for activation of the oxidation catalysts.

The average density of zeros for monic generalized polynomials, P-n(z)=phi(z)+Sigma(k=1)(n)c(k)f(k)(z), With real holomorphic phi, f(k) and real Gaussian coefficients is expressed in terms of correlation functions of the values of the polynomial and its derivative. We obtain compact expressions For both the regular component (generated by the complex roots) and the singular one (real roots) of the average density of roots. The density of the regular component goes to zero in the vicinity of the real axis like \Im z\. We present the low- and high-disorder asymptotic behaviors. Then we particularize to the large-n limit of the average density of complex roots of monic algebraic polynomials of the form P-n(z)=z(n)+Sigma(k=1)c(k)z(n-k) with real independent, identically distributed Gaussian coefficients having zero mean and dispersion delta=1/root n lambda. The average density tends to a simple, universal function of xi=2n log \z\ and lambda in the domain xi coth(xi/2) much less than n \sin arg(z)\, where nearly all the roots are located for large n.

PbS films were chemically deposited on poled and unpoled ferroelectric ceramic substrates and on glass. The sensitivity of the PbS thin films and the concentration of trapping centers inside these films depend on the nature of the substrate and, in the case of poled ferroelectric substrates, depend on the polarity of the ferroelectric polarization as well. A considerable enhancement of the photoconductive signal at liquid nitrogen temperature was found in the case of PbS films deposited on the positive face of a poled ferroelectric substrate. (C) 1997 Elsevier Science S.A.

Infrared photoconductive detectors were prepared from three undoped n-type InSb single crystals with different electron concentration and mobility values. The single crystals were grown by the Czochralski method. The spectral responsivity and spectral detectivity of the detectors were determined as functions of the power dissipation density of the device. A remarkable improvement in performance was observed for the purest specimen in comparison with earlier reported data. (C) 1997 Elsevier Science S.A.