National Institute Of Materials Physics - Romania

High density (above 93%) superconducting bulks of MgB2 with addition of hexagonal BN (h-BN) and cubic BN (c-BN) with compositions ((MgB2) + (BN)(x), x = 0.01, 0.03, 0.05) were obtained by ex situ spark plasma sintering. All the investigated samples have the critical temperature T-c = 38.8 K. The variation of the critical current density J(c) with the external magnetic field H for h-BN added sample is almost overlapping the J(c)(H) dependence for the pristine MgB2 sample. On the other hand, J(c) for the samples added with c-BN is larger at high magnetic fields, while the decrease of J(c) at low H is very small. At T = 20 K, a J(c) of 10(2) A cm(-2) is determined for the sample with x(c-BN) = 0.005 at H = 58 kOe, and for the sample with x(c-BN) = 0.01 at 54 kOe. Magnetic relaxation measurements indicate a significant flux pinning enhancement in MgB2 samples added with c-BN. It is proposed that the disorder at the interface caused by the convenient lattice matching relationship between the lateral plane of the MgB2 crystal prism and the face of the c-BN crystal cube is responsible for the observed vortex pinning increase.

Magnetic and electrical properties of the nanostructured ZnO films are affected by the nonrandom distribution of impurities in the film due to segregation at grain boundaries (GBs) or extended defects. However, mapping the nature and distribution of the impurities in the film is not trivial. Here we demonstrate a simple, statistically relevant, and nondestructive procedure of quantitative determination of the paramagnetic impurities segregated at the GBs in nanostructured semiconducting and insulating films. From correlated electron paramagnetic resonance and transmission electron microscopy investigations, we determined the localization of trace amounts of Mn2+ ions, present as native impurities in a ZnO film deposited by magnetron sputtering at room temperature. In the as-deposited ZnO film, the Mn2+ ions were all localized in nanosized pockets of highly disordered ZnO dispersed between nanocrystalline columns. After the samples had been annealed in air at >400 degrees C, the size of the intercrystalline region decreased and the diffusion in GBs was activated, resulting in the localization of a portion of the Mn2+ ions in the peripheral atomic layers of the ZnO columns neighboring the GBs. The proportion of Mn2+ ions still localized at the GBs after annealing at 600 degrees C was 37%. The proposed method for the assessment of the presence and nature of impurities and the quantitative evaluation of their distribution in semiconducting and insulating nanostructures is expected to find direct applications in nanotechnology, in the synthesis and quality assurance of thin films for spintronics and opto- and nanoelectronics.

Relaxor-to-ferroelectric transformations induced by varying electric fields and temperatures are studied experimentally in acube-on-cubetype epitaxial PbSc1/2Nb1/2O3 film grown on La1/2Sr1/2CoO3/MgO(001). Dielectric response, quasi-static and dynamic polarization, and dynamic current-voltage characteristics evidence the absence of spontaneous relaxor-to-ferroelectric transition. The electricfield-induced transformation from a glass-like relaxor state to a new dynamic polar state is detected at low temperatures below 100 K only. The frustration of ferroelectricity is discussed in relation to orientational anisotropy of the dipolar system in the epitaxial (001) film.

The GeTiO2 amorphous films were deposited by magnetron sputtering and subsequently annealed at 400, 550, 600 and 700 degrees C for nanostructuring. The structure of annealed films was investigated by X-ray diffraction and transmission electron microscopy. The transmittance spectra of all annealed GeTiO2 films were measured and simulated by using Bruggeman effective medium approximation considering components of TiO2 anatase, crystalline Ge, GeO2 and voids determined from the structure investigations. The electrical behavior of 400, 600 and 700 degrees C annealed films was studied by measuring current-voltage characteristics. We found that by increasing the annealing temperature the films thickness decreases from 330 nm (as-deposited films) to 290 nm (700 degrees C annealed films). The 400 degrees C annealed films are amorphous, while all the others annealed at higher temperatures are crystallized (X-ray diffraction and transmission electron microscopy). In the 550 and 600 degrees C annealed films we found the (TiGe)O-2 rutile structure which is formed by starting from the GeO2 tetragonal structure with high Ti content. Additionally, these films contain TiO2 anatase structure and cubic Ge nanocrystals. At 700 degrees C annealing temperature, a surface layer of GeO2 tetragonal nanocrystals is formed by Ge diffusion and a part of Ge is lost. The experimental transmittance spectra indicate a broadening of the transparency range by increasing the annealing temperature, and the simulated ones also indicate this behavior with the decrease of Ge content, the experimental and simulated spectra being in good agreement. Also, the increase of annealing temperature produces an increase of electrical conductivity. (C) 2014 Elsevier B.V. All rights reserved.

Voltage and frequency dependent capacitance measurements were performed on epitaxial BaTiO3 and Pb(Zr0.2Ti0.8)O-3 thin films deposited on single crystal SrTiO3 substrates with (001) and (111) orientations. The measured capacitors have common bottom SrRuO3 contact and different metals as top electrodes: SrRuO3, Pt, Cu, Al, and Au. The capacitance-voltage characteristics were used to extract information regarding the density of the free carriers and the linear contribution to the static dielectric constant. The frequency dependent impedance was used to develop a suitable equivalent circuit for the epitaxial ferroelectric capacitors. It was found that the frequency dependence of the imaginary part of the impedance can be well simulated, in all cases, using a circuit composed of Schottky-type capacitance related to electrode interfaces, contact resistance, and the R-C parallel connection related to the ferroelectric volume of the film. Values for the components of the equivalent circuit were obtained by fitting the experimental data with the simulated curves. These were then used to extract quantities such as dielectric constant in the ferroelectric volume, the width of the depletion layers, and the apparent built-in potential. It was found that, although the investigated capacitors are of different ferroelectric materials, grown on substrates with different orientations, and having different metals as top electrodes, the values for the capacitance associated with the Schottky contacts and the apparent built-in potential are not very different. The results suggest a strong influence of ferroelectric polarization on the electrode interface properties in the case of epitaxial ferroelectric films. (C) 2014 AIP Publishing LLC.

In this paper, we analyze the structural distortions observed by transmission electron microscopy in thin epitaxial SrRuO3 layers used as bottom electrodes in multiferroic coatings onto SrTiO3 substrates for future multiferroic devices. Regardless of the nature and architecture of the multilayer oxides deposited on the top of the SrRuO3 thin films, selected area electron diffraction patterns systematically revealed the presence of faint diffraction spots appearing in forbidden positions for the SrRuO3 orthorhombic structure. High-resolution transmission electron microscopy (HRTEM) combined with Geometric Phase Analysis (GPA) evidenced the origin of these forbidden diffraction spots in the presence of structurally disordered nanometric domains in the SrRuO3 bottom layers, resulting from a strain-driven phase transformation. The local high compressive strain (-4% divided by -5%) measured by GPA in the HRTEM images induces a local orthorhombic to monoclinic phase transition by a cooperative rotation of the RuO6 octahedra. A further confirmation of the origin of the forbidden diffraction spots comes from the simulated diffraction patterns obtained from a monoclinic disordered SrRuO3 structure. (C) 2014 AIP Publishing LLC.

Ge, GeO2 and Ge2C6H10O7 additions to MgB2 obtained by ex situ spark plasma sintering significantly enhance the critical current density J(c) in high magnetic fields. A J(c)(T = 20 K) of 102 A cm(-2) is obtained at 3.9 T in the pristine sample and at 5.8 T in the MgB2(Ge2C6H10O7)(0.0014) sample. The decrease in the critical temperature for added samples is less than 1 K and T-c(20 K, H = 0) shows a small decrease from 5.5 x 10(5) A cm(-2) in the pristine sample to 3.9 x 10(5) A cm(-2) in MgB2(Ge2C6H10O7)(0.0014) sample. Ge does not substitute into the MgB2 lattice. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Thin SiC films were grown on (1 0 0) Si substrates at temperatures from 400 to 1000 degrees C under various CH4 pressures by the pulsed laser deposition (PLD) technique using a KrF excimer laser. After deposition, films were in situ annealed at their deposition temperature under 500 mbar of CH4 for 1.0-1.5h. X-ray reflectivity investigations showed that films exhibited mass densities similar to SiC single crystal samples, while symmetrical and grazing incidence X-ray diffraction investigations found that films deposited at 800 degrees C or higher substrate temperatures were nanocrystalline. Modeling of spectroscopic ellipsometry measurements indicated that the refractive index values were similar to those reported for bulk SiC, while X-ray photoelectron spectroscopy investigations found that films contained in bulk a relatively low oxygen concentration of around 1.0 at.%. Nanoindentation results showed that the deposited SiC films were very hard, with hardness values above 40 GPa for films deposited at temperatures higher than 800 degrees C. (C) 2014 Elsevier B.V. All rights reserved.

GeSi nanostructured films were obtained by cosputtering from two Ge and Si targets and subsequent annealing in furnace in N-2 for 5 h at 700, 800 and 900 degrees C, with the aim to show the correlation between electrical properties and crystalline structure of the films. The as-deposited films are amorphous, have a Ge: Si composition of 55: 45 and 185 nm thickness. The film structure was investigated by XRD and TEM, and the electrical behaviour was studied by measuring current-voltage (I-V) and current-temperature (I-T) characteristics and by discussing them in correlation with the structure. Different electrical behaviours of GeSi films corresponding to different structures have been evidenced. The 700 degrees C annealed GeSi films are formed of nanocrystals (7-15 nm) separated by amorphous regions (1-2 nm). These films present a superlinear I-V characteristic typical of high field-assisted tunnelling through potential barriers (amorphous regions) between nanocrystals. The I-T characteristic at low temperature follows a T-1/2 law showing a thermally activated tunnelling of carriers between neighboring GeSi nanocrystals. The films annealed at 800 and 900 degrees C have a like behavior, they are completely crystallized and present linear I-V and Arrhenius I-T dependences reflecting the polycrystalline behaviour. [GRAPHICS] . (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The energy band alignment at interfaces between different materials is a key factor, which determines the function of electronic devices. While the energy band alignment of conventional semiconductors is quite well understood, systematic experimental studies on oxides are still missing. This work presents an extensive study on the intrinsic energy band alignment of a wide range of functional oxides using photoelectron spectroscopy with in-situ sample preparation. The studied materials have particular technological importance in diverse fields as solar cells, piezotronics, multiferroics, photoelectrochemistry and oxide electronics. Particular efforts have been made to verify the validity of transitivity, in order to confirm the intrinsic nature of the obtained band alignment and to understand the underlying principles. Valence band offsets up to 1.6 eV are observed. The large variation of valence band maximum energy can be explained by the different orbital contributions to the density of states in the valence band. The framework provided by this work enables the general understanding and prediction of energy band alignment at oxide interfaces, and furthermore the tailoring of energy level matching for charge transfer in functional oxides. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim