National Institute Of Materials Physics - Romania

In-plane (ab) and out-of-plane (c-axis) magnetoresistivity display different symmetry crossovers and/or transitions in 14 T magnetic field applied parallel to the CuO2 planes. The in-plane magnetoresistivity crosses over from four-fold symmetry below 6 K to two-fold symmetry at higher temperatures, which becomes dominant at temperatures higher than 40 K. The out-of-plane magnetoresistivity changes at 17 K from four fold symmetry to ordinary sin2 theta at higher temperatures. The behaviour of the c-axis magnetoresistivity can be ascribed to the antiferromagnetic ordering of the Pr spins whereas the symmetry change of the in-plane magnetoresistivity at 6 K might be attributed to commensurate to incommensurate crossovers of the spin subsystems. The antiferromagnetic order of the Cu(2) sublattice seems to have only a week effect on the magnetoresistivity.

The single-crystalline x-ray storage phosphor BaCl2:Ce3+ has been investigated by photo- and x-ray luminescence, and also by observing the afterglow and the photostimulated luminescence (PSL). A single active Ce3+ site has been observed with a characteristic emission doublet at 349 and 373 nm at room temperature. The PSL stimulation spectrum shows a clear resemblance to F-type absorption spectra in undoped BaCl2, indicating that the PSL-active electron centres are F-type centres. The results are compared with previous photoluminescence, PSL and new x-ray luminescence data on BaBr2:Ce3+. The integral PSL efficiency in the chloride is found to be somewhat smaller than in the bromide, which is still large enough for storage phosphor applications, while the efficiency in the bromide may be even larger or comparable to that of the commercial x-ray storage phosphor BaFBr2:Eu2+.

An asymmetric nanoscale cross junction is fabricated from a high-mobility Si/SiGe heterostructure. At T=4.2 K, the four-terminal current-voltage characteristics reveal a polarity-dependent breakdown of the negative bend resistance. The breakdown is accompanied by negative differential conductance found in the two-terminal current-voltage characteristics of the orthogonal current leads. We attribute this behavior to phonon emission by hot electrons. From gate-voltage-dependent measurements, we determine a phonon threshold of 19 meV. (c) 2005 American Institute of Physics.

A modified model of metal-semiconductor contacts is applied to analyze the capacitance-voltage and current-voltage characteristics of metal-ferroelectric-metal structures. The ferroelectric polarization is considered as a sheet of surface charge situated at a fixed distance from the interface. The presumable high concentration of structural defects acting as active electric traps is taken into account by introducing a deep acceptorlike level. The model is applied on a set of metal-Pb(Zr,Ti)O-3-metal samples with different Zr/Ti ratios, deposited by different methods, and having different thicknesses, electrode materials, and electrode areas. Values around 10(18) cm(-3) were estimated for the hole concentration from capacitance-voltage measurements. The space-charge density near the electrode, estimated from current-voltage measurements, is in the 10(20)-10(21) cm(-3) range. The total thickness of the interface layer ranges from 3 to 35 nm, depending on the Zr/Ti ratio, on the shape of the hysteresis loop, and on the electrode material. The simulated I-V characteristics is fitted to the experimental one using the potential barrier and Richardson's constant as parameters. The potential barrier is determined to be in the 1.09-1.37 eV range and Richardson's constant is 520 A cm(-2) K-2. (c) 2005 American Institute of Physics.

Surface-enhanced Raman scattering (SERS), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectroscopies were used to investigate composites based on single-walled carbon nanotubes (SWNTs) and different conducting polymers like polyaniline (PANI), polyparaphenylene vinylene (PPV) and poly 3-hexylthiophene (3-PHT). In the case of SWNTs/PANI, different composites are obtained by adding dispersed SWNTs powder to the polymer solutions and by chemical polymerization of aniline in presence of SWNTs. The difference originates in the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs in fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FT-IR spectra of the composites PANI/SWNTs and PANI/C-60, chemically synthesized. Electrochemical polymerization of aniline in an HCl solution on a SWNT film leads to a covalent functionalization of SWNTs with PANI. In this case, Raman scattering data suggest an additional nanombes roping with PANI as a binding agent. A post treatment with the NH4OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. In the case of PPV and 3-PHT, the effect of the concentration of SWNTs on the photoluminescence properties will be described and compared, as probes of interaction. (C) 2005 Elsevier B.V. All rights reserved.

A model for metal-ferroelectric-metal heterostructures with Schottky contacts is proposed. The model adapts the general theories of metal-semiconductor rectifying contacts for the particular case of metal-ferroelectric contact by introducing the ferroelectric polarization as a sheet of surface charge located at a finite distance from the electrode interface, a deep trapping level of high concentration, and the static and dynamic values of the dielectric constant. Consequences of the proposed model on relevant quantities of the Schottky contact such as builtin voltage, charge density, and depletion width, as well as on the interpretation of the current-voltage and capacitance-voltage characteristics are discussed in detail. (c) 2005 American Institute of Physics.

In this work, we investigate the electronic and optical properties of self-assembled InN/GaN quantum dots. The one-particle states of the low-dimensional heterostructures are provided by a tight-binding model that fully includes the wurtzite crystal structure on an atomistic level. Optical dipole and Coulomb matrix elements are calculated from these one-particle wave functions and serve as an input for full configuration interaction calculations. We present multiexciton emission spectra and discuss in detail how Coulomb correlations and oscillator strengths are changed by the piezoelectric fields present in the structure. Vanishing exciton and biexciton ground state emission for small lens-shaped dots is predicted. (c) 2005 American Institute of Physics.

Gallium Arsenide (GaAs) has increasingly become an important compound-semiconductor material suited for high speed digital circuits, microwaves and detectors for high energy physics. For the fabrication of ohmic contacts on GaAs (semi-insulating- SI) wafers with characteristics as: Cr doped, (100) oriented, and p similar to 10(6)similar to 10(7) Omega cm, has been used alloyed contact Au-Ge. The ohmic film was deposited in a high vacuum chamber (10(-8) Torr), on a plasma etched surface of GaAs, followed by a rapid thermal annealing (RTA) at 450 degrees C in low vacuum. For the fabrication of the rectifying contact-respectively the Schottky diode structure, we have used the same GaAs substrates. The wafers were degreased in a standard cleaning procedure, followed by a chemical etching. The Au-Ti contact has been deposited by thermal evaporation in vacuum (10(-6) torr) followed by a RFA procedure at 300-320 degrees C. The carrier transport mechanisms through M/S interface are strongly influenced by the doping concentration in the semiconductor and temperature. There are presented the experimental I-V characteristics for selected samples in dark and illumination conditions. Schottky barriers height on Au-Ti/GaAs was 0.7325 V in accord with mean values on n and p type GaAs (0.5-0.8 V). The aim of this work is to fabricate a competitive X-ray detector.

Barium acetylacetonate and titanium (IV) isopropoxide, dissolved in adequate solvents, were used to produce stoichiometric BaTiO3. The purity of the gel powder was > 99.98%, The particle size range was 80 nm for the dried gel powder and 1-1.5 mu m for the powder calcined at 950 degrees C. X-ray analyses indicates that the material calcined at 950 degrees C in air or, at 825 degrees C under vacuum, exhibits a tetragonal, poorly crystallized, BaTiO3 structure. The powder pressed and sintered at 1275 degrees C show high dielectric constants, i.e. 1300 at 25 degrees C and, 3020 at the Curie temperature (104 degrees C). The melting point for BaTiO3 prepared by sol-gel technique is 1290 degrees C.

A microstructural investigation on polycrystalline electroluminescent Alq(3) obtained by train sublimation method is reported in the paper. The morphology and crystalline structure of the obtained Alq3 phases have been analyzed by transmission electron microscopy performed on samples collected from 3 different regions along the sublimation tube.