National Institute Of Materials Physics - Romania

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of a coherent excitation and line shifts of the interband transitions due to carrier-carrier Coulomb interaction and carrier-phonon interaction are determined from a quantum kinetic treatment of correlation processes. We investigate the density dependence of both mechanisms and clarify the importance of various dephasing channels involving the localized and delocalized states of the system.

Investigations on compact devices developed for the second (2G) and the third (3G) mobile communications systems are presented in this paper. Four-pole and six-pole filters were designed by cross-coupling new compact planar resonators. The coupling constants were investigated as functions of coupling gaps. Several models of the pass-band filters were manufactured and characterized. They exhibit steeper responses due to the cross-couplings, which introduce transmission zeros on each side of the pass-band. Moreover, due to their shapes, the resonators exhibit a first,high order response at more than twice the fundamental frequency, therefore the filters show an enlarged rejection band. The measured filter responses are in a good agreement with the simulated responses. The developed filters are compact, cost-effective and suitable to be designed on high dielectric constant ceramic substrates, or for high temperature superconducting (HTS) circuits.

This paper presents a study of the nonhomogeneities in doped meta-dinitrobenzene (m-DNB) crystalline films. These growth defects lying in the film plane with dimensions smaller than the films thickness are correlated with the effect of the dopant on the crystallization regime in the dendritic growth conditions. To evaluate the dimension of the growth nonhomogeneities we have used a model based on the Kubelka-Munk theory, emphasising the general constraints imposed to the sample and the experimental set-up, and developing a more complex model through a step-by-step relaxation of these constraints, that ensures a better approximation for our experimental configuration. We have deduced higher scattering coefficients in m-DNB doped with 1,3 dihydroxybenzene (resorcinol) than in m-DNB doped with 8-hydroxyquinoline (oxine) films prepared by a rapid thermal solidification process. We also have emphasised that for the films of m-DNB doped with oxine the size of the scattering centers is larger than the wavelength of the radiation and in films of m-DNB doped with resorcinol the radiation scattering corresponds to a multiple scattering process leading to the superposition of different scattering mechanisms on centers with different dimensions.

Thin films of MgB2 on r-cut Al2O3 substrates have been grown by pulsed-laser deposition (PLD) using a Nd-YAG laser (fourth harmonic-266 nm) instead of the popular KrF excimer laser. The growth window to obtain superconducting films is laser energy 350-450 mJ and vacuum pressure with Ar-buffer gas of 1-8/10 Pa (initial background vacuum 0.5-1 x 10(-3) Pa). Films were deposited at room temperature and post-annealed in situ and ex situ at temperatures of 500-780 degrees C and up to 1 h. Films are randomly oriented with maximum critical temperature (offset of resistive transition) of 27 K. SEM/TEM/EDS investigations show that they are mainly composed of small sphere-like particles (<= 20 nm), and contain oxygen and some carbon, uniformly distributed in the flat matrix, but the amount of Mg and/or oxygen is higher in the aggregates-droplets (100-1000 nm) observed on the Surface of the film's matrix. Some aspects of the processing control and dependences on film characteristics are discussed. The technique is promising for future development of coated conductors.

We present some investigations on the electrical conductivity of two benzene substituted derivatives (m-DNB, benzil) emphasizing the correlation between the molecular structure, purity, structural defects and the particularities of the conduction mechanisms. The influence of inorganic/organic doping on the I-V plots of silicon/wide-gap organic semiconductor/silicon heterostructures has been analysed. The most significant increase in the conductance has been obtained for organic crystalline films of benzil doped with 3 wt.% m-DNB and m-DNB films doped with I wt.% oxine or 10 wt.% resorcinol. The influence of the silicon wafer's properties as resistivity, conduction type and surface processing on the carrier transport properties in these structures has been studied. We have remarked an increase in the conductance of the organic films of m-DNB doped with oxine or resorcinol in heterostructures realized with chemically polished, "n" type single crystal silicon wafers, compared to lapped ones. A special type of conduction mechanism given by a Poole-Frenkel dependence was evidenced in resorcinol doped m-DNB in the low voltages range. (c) 2005 Elsevier B.V. All rights reserved.

The properties of the electron paramagnetic resonance spectra of Gd3+ ions in PbWO4 single crystals have been investigated in the X-band microwave frequency region, in the 1.5 to 290 K temperature range. The observed S-4 symmetry of the local crystal field at the Gd3+ impurity ions strongly suggests that the Gd3+ ions substitute for the Pb2+ lattice cations, with charge compensation at a distance. The spin Hamiltonian parameters are comparable with those of the Gd3+ ions in other isomorphous tungstates. The temperature variation of the fine structure parameters B-2(0) and B-4(0) points to an energy transfer between the impurity ions and the host lattice, which takes place mainly through a local vibrational mode of frequency omega = 3.1 x 1013 rad s(-1).

Detectors processed on epitaxial silicon are a promising solution for the extreme radiation levels in the innermost tracking layers at future particle physics experiments as in the upgraded (S-LHC). In order to systematically investigate their radiation tolerance.. sets of 25 and 50-mu m-thick diodes had been irradiated with 24GeV/c protons up to fluences Of Phi(eq) = 10(16) cm(-2). The full depletion voltage was measured during full isothermal annealing cycles at both 60 and 80 degrees C. A voltage of only 290 V was measured for the 50 mu m diodes after Phi(eq) = 1016cm(-2). In contrast to likewise thin float zone (FZ) diodes, the depletion voltage in epi-devices decreases with annealing during foreseen operational and storage periods. Low-temperature storage during beam-off periods as inevitable for FZ devices could be avoided. The reverse annealing components reveal an overall similar functional time dependence as observed in FZ diodes. However, the most important stable component is drastically different. In contrast to the well-known negative space charge build up observed in FZ diodes, the behavior in epi-diodes can only be explained by an additional formation of positive space charge. The immense improvement of the radiation tolerance of epi-detectors is thus attributed to a compensation effect between acceptor generation and donor creation unique for the epi-material. Charge collection efficiency drops only to about 80% after Phi(eq) = 6 x 10(15)cm(-2) proving that epi-detectors are indeed adequate candidates for the tracking area at extremely large radiation levels. (c) 2005 Elsevier B.V. All rights reserved.

Four different kinds of silicon materials (standard float zone-FZ, oxygen enriched FZ-DOFZ, Czoehralski-Cz and thin epitaxial layers grown on Cz substrates-Epi/Cz) have been investigated in this work. They have been irradiated with (CO)-C-60-gamma- radiation and 24 GeV/c protons. The differences in the changes observed in the effective doping concentration (N-eff) after proton irradiation of Cz and Epi silicon can be explained by the balance between the formation of two types of defects-a deep acceptor (the I center, referred to as It, in this paper in order to avoid any confusion with interstitials) and a shallow donor (the bistable donor (BD) complex). While the formation of the I-P center depends on the concentration of interstitial oxygen, the BD is generated in materials with high concentration of oxygen dimers. Following the generation of the IO2i defect-detected after low irradiation fluences only in Epi and Cz material-the average dimer concentration in 50 put Epi/Cz was estimated to be of only 2.7 times lower than in Cz diodes. A value of similar to 2.7 x 10(-12) cm(2) for the electron capture cross-section of the Thermal Double Donors (TDDs), present in the Cz material, was also measured for the first time. The positive space charge introduced by ionization of the BD centers in Epi diodes was directly determined from TSC experiments for two irradiation fluences. The determined values show a linear fluence dependence for the formation of BD centers. By taking into account the BD and the I-p center generation as well as the donor removal, the change of the effective doping concentration N-eff at 20 degrees C in Epi/Cz and Cz diodes after an annealing time of 120 min at 60 degrees C can be explained up to a negative space charge introduction rate of similar to 2.2 x 10(-2) cm(-1) thought to be determined by negatively charged clusters. Long-time annealing experiments at 80 degrees C have shown that the generation of BDs represents a stable damage. (c) 2005 Elsevier B.V. All rights reserved.

We have studied a portion of H-T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements (chi(n)). Based on the behavior of the third harmonic modulus,vertical bar chi(3)vertical bar, the irreversibility line (IL) was obtained from the onset of vertical bar chi(3)vertical bar, and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being H-CR approximate to 0.1T. The obtained portion of B-irr(T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B=B-o(Tc/T - 1)" with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, T-m(H) = T-m(2D)[1 + b/(In B/B-cr )(1/v)] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as T-m(2D) = 36.7K (sample 1) and T-m(2D) = 27.3K (sample 2). (c) 2005 Elsevier B.V. All rights reserved.

A microscopic theory for the interaction of carriers with LO phonons is used to study the ultrafast carrier dynamics in nitride-based semiconductor quantum dots. It is shown that the efficiency of scattering processes is directly linked to quasi-particle renormalizations. The electronic states of the interacting system are strongly modified by the combined influence of quantum confinement and polar coupling. Inherent electrostaticfields, typical for InGaN/GaN quantum dots, do not limit the fast scattering channels.