National Institute Of Materials Physics - Romania

A new class of thermotropic lanthanidomesogens has been designed and prepared. They are based on 4-pyridone ligands that possess mesogenic cyanobiphenyl groups attached to the 4-pyridone unit via a flexible long alkyl spacer and show a very high thermal stability (decomposition temperatures near 300 degrees C). Depending on the alkyl length spacer, these complexes exhibit a SmA phase with transition temperatures influenced by the number of mesogenic groups employed and the spacer length.

New type of dielectric resonator antennas (DRAs) with axial symmetry are proposed for frequencies around 5.8 GHz. Each element uses a high permittivity cylindrical resonator and a low permittivity resonator of a truncated cone shape. Two types of microstrip-fed DRAs were investigated. By coupling the resonance modes, the first type and the second type DRA exhibit fractional impedance bandwidth of 15.6% and 19.7%, respectively.

The paper presents a CDRA (Cylindrical Dielectric Resonator Antenna) designed for wireless military and commercial applications. The antenna has been designed to radiate at 6.7 GHz, with a 160 MHz bandwidth at -10 dB. The antenna is made from a dielectric material with high quality factor. The dielectric resonator is Ba1-xPbxNd2Ti5O14 (BNT) and its relative permittivity has a value of 55.2. The feed technique used is direct coupling to micro strip-line.

In this paper we report electro-optic (EO) sampling of THz pulses using ferroelectric crystal in the non-collinear Cherenkov phase-matching scheme, where an effective velocity matching is achieved in an EO crystal with a large refractive index in the THz frequency region. We demonstrate efficient THz EO sampling using LiNbO3 and BaTiO3 crystals in the Cherenkov-phase-matching scheme. It is shown that the efficiency of EO sampling with BaTiO3 is smaller than that with LiNbO3, even though the EO coefficient (r(33)) of BaTiO3 is three times higher than that of LiNbO3. The reason of the poor performance of BaTiO3 is attributed to the strong absorption in the THz frequency region.

The leakage current was studies in epitaxial ferroelectric Pb(Zr0.52Ti0.48)O-3 layer with common SrRuO3 bottom electrode and different metals as top contacts (SrRuO3, Pt, Ir, Ru). It was found that the dominant conduction mechanism in the 200-350 K temperature range and for voltages significantly larger than the coercive value is the thermionic emission governed by the Schottky-Simmons equation. The height of the potential barriers was estimated and was found that this is about the same for negative and positive voltage polarities. No correlation was found between the height of the potential barriers for different top contacts and the work function difference between the bottom and top electrodes. The results suggest that the potential barrier is controlled by the polarization charges in a similar way to the one reported for Pb(Zr0.2Ti0.8)O-3 and BaTiO3 epitaxial films with bottom SrRuO3 electrode and different metals as top contacts. It was also found that above 350 K the conduction mechanism changes to ohmic and/or space charge limited currents.

Single and double layer phase change memory structures based on GeTe and GaSb thin films were deposited by pulsed laser deposition (PLD). Their crystallization behavior was studied using in-situ synchrotron techniques. Electrical resistance vs. temperature investigations, using the four points probe method, showed transition temperatures of 138 degrees C and 198 degrees C for GeTe and GaSb single films, respectively. It was found that after GeTe crystallization in the stacked films, Ga atoms from the GaSb layer diffused in the vacancies of the GeTe crystalline structure. Therefore, the crystallization temperature of the Sb-rich GaSb layer is decreased by more than 30 degrees C. Furthermore, at 210 degrees C, the antimony excess from GaSb films crystallizes as a secondary phase. At higher annealing temperatures, the crystalline Sb phase increased on the expense of GaSb crystalline phase which was reduced. Extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges revealed changes in their local atomic environments as a function of the annealing temperature. Simulations unveil a tetrahedral configuration in the amorphous state and octahedral configuration in the crystalline state for Ge atoms, while Ga is four-fold coordinated in both as-deposited and annealed samples. (C) 2014 AIP Publishing LLC.

In this work we have systematically investigated the evolution of structure and stoichiometry in SrCuO2 films grown on TiO2-terminated SrTiO3 substrates as a function of the substrate temperature. Depending on the growth temperature SrCuO2/SrTiO3 films can exhibit either a pure tetragonal high-pressure phase, or a pure orthorhombic low-pressure phase, or a mixed phase. Our results indicate that at low substrate temperatures the non-equilibrium state of the growth process is responsible for the stabilization of the metastable tetragonal high-pressure structure in SrCuO2 thin films grown on ( 0 01) SrTiO3 substrates, whose lattice matches the metastable structure. In addition, at higher substrate temperatures thermodynamics become dominant over other factors and the SrCuO2 thin films are stabilized in the thermodynamically stable orthorhombic phase. (C) 2014 Elsevier B.V. All rights reserved.

Using forward current injection with densities in the range 15-30A/cm(2) we can effectively eliminate the radiation-induced boron-oxygen complex, which is the main compensating center in irradiated Si solar cells. It was found that for a given forward current density the elimination rate is decreasing with increasing irradiation dose. Additionally, some evidences have been obtained on the negative-U properties of the radiation-induced boron-oxygen complex.

It is shown experimentally that, in contrast to the stable configuration of (interstitial carbon)-(interstitial oxygen) complexes (CiOi), the corresponding metastable configuration (CiOi*) cannot be found in n-Si based structures by the method of capacitance spectroscopy. The rates of transformation CiOi* -> CiOi are practically the same for both n- and p-Si with a concentration of charge carriers of no higher than 10(13) cm(-3). It is established that the probabilities of the simultaneous formation of stable and metastable configurations of the complex under study in the case of the addition of an atom of interstitial carbon to an atom of interstitial oxygen is close to 50%. This is caused by the orientation dependence of the interaction potential of an atom of interstitial oxygen with an interstitial carbon atom, which diffuses to this oxygen atom.

In this paper, we report on the structural, optical, and electrical properties of a wide compositional range of AlxIn1-xN thin layers deposited on glass and polyethylene terephthalate substrates. AlxIn1-xN layers of controlled composition were obtained by a simple reactive magnetron co-sputtering protocol, using a single aluminium target with indium insets, by varying the Al/In target surface area ratio, and the composition of the deposition atmosphere. The relevant physical properties were investigated and discussed. It is shown that the texture of the thin films is dependent on the cation ratio, while the bowing parameters of lattice constants and band gap values are larger than those of epitaxial layers. (C) 2014 AIP Publishing LLC.