National Institute Of Materials Physics - Romania

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.

Ba0.5Sr0.5TiO3 ferroelectric ceramics (BST) were prepared by solid-state reaction. The effect of sintering temperature T, on microwave dielectric properties was studied. Also, the influence of 1 wt % MgO and 1 wt % MnO2 doping on the dielectric parameters was investigated. The morphological aspects of samples were studied by scanning electron microscopy (SEM). The microwave measurements at room temperature revealed dielectric constant around 1.000 and loss smaller than 1 % at 1.1 GHz. Also, the dielectric parameters at low frequency were determined. The results indicate that the BST dielectric ceramics are suitable for manufacturing electric field controlled components such as tunable resonators, phase shifters, steerable antennas etc.

We report experimental data that indicate an anti-Stokes Raman emission, reminiscent of coherent anti-Stokes raman scattering. It originates from a wave mixing process between the incident laser light (omega(l)) and Stokes Raman light (omega(s)) generated by a surface enhanced Raman scattering (SERS) mechanism. The variation of anti-Stokes SERS spectra of single-walled carbon nanotubes, copper phthalocyanine, and poly(bithiophene) as a function of the film thickness, the laser excitation intensity, and the numerical aperture of the microscopic objective used for collecting the scattered light demonstrate the described phenomenon.

A scanning Hall probe microscope (SHPM) with an effective spatial resolution of 1 similar to mu m has been used to study the local induction in high quality superconducting Bi2Sr2CaCu2O8+delta single crystals at high temperatures and low magnetic fields. We observed, for the first time to our knowledge, an anomalous splitting of the peak of first full penetration of magnetic field, We discuss the observed splitting, which is connected to the effects of surface and geometrical barriers on the vortex lattice.

Scanning Hall probe microscopy and magnetometry have been used to study pancake vortices (PV's) in highly anisotropic Bi2Sr2CaCu2O8+delta (BSCCO) thin films. For small perpendicular applied fields we observe well-defined PV stacks which are randomly pinned due to a high density of relatively strong pinning centers in the films. When a strong in-plane magnetic field is added images reveal stripelike flux structures which are quite closely aligned to the in-plane field direction. We conclude that this is a manifestation of Josephson vortex (JV) "decoration" by PV's, which was recently demonstrated in highly ordered BSCCO single crystals in the crossing lattices regime, allowing independent estimates of the anisotropy parameter to be made. This interpretation is supported by the observation that the presence of an in-plane field suppresses the irreversible magnetization of thicker films. Estimates of the JV-PV crossing force and the PV pinning force are found to be comparable in magnitude and fully consistent with this scenario. We believe that this is the first confirmation of the existence of interacting crossing vortex lattices in strongly pinning BSCCO thin films and could find potential applications in superconducting devices.

The effects produced by heavy ion irradiation in KBr:Ag and KCl:Ag crystals are investigated using optical spectroscopy. For KCl, the results are compared with the effects obtained in the pure crystals. After irradiation with heavy ions up to 11.2 MeV/u specific energy, the absorption bands suffer a series of changes, related to the structure of defects and also to their dimensions. In the high Ag concentrated samples, the Ag+ ions are changing their valence to Ag- while in the samples containing Ag nanoclusters the irradiation induces changes in the nanocluster size.