National Institute Of Materials Physics - Romania

Publications

5781 articles found

3881. Growth and characterization of high density stoichiometric SiO2 dot arrays on Si through an anodic porous alumina template

Authors: Kokonou, M; Nassiopoulou, AG; Giannakopoulos, KP; Travlos, A; Stoica, T; Kennou, S

Published: MAY 14 2006, NANOTECHNOLOGY, 17, 2151, DOI: 10.1088/0957-4484/17/9/011

In this work, we present the fabrication and full characterization of stoichiometric SiO2 nanoisland arrays ( dots) on silicon, grown through an anodic porous alumina template. Atomic force and transmission electron microscopy (AFM, TEM) were used to characterize the morphology, size, size distribution and density of the dots as a function of the anodization time used. It was found that dot density is lower for very short anodization times, and it stabilizes after a certain time. The dot height increases rapidly after nucleation, reaching values of 8 - 10 nm. With prolonged oxidation the dots continue to nucleate to fill the available area on the silicon surface underneath the porous alumina, while the well developed dots grow in height and width, reaching saturation values at 14 and 55 nm respectively. X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy ( EELS) were used to investigate the stoichiometry and surface coverage of the dots.

3882. Magnetic behaviour of UB4 at high temperatures

Authors: Galatanu, A; Yamamoto, E; Haga, Y; Onuki, Y

Published: MAY 1 2006, PHYSICA B-CONDENSED MATTER, 378-80, 1000, DOI: 10.1016/j.physb.2006.01.383

UB4 is a moderate heavy fermion compound which does not exhibit my magnetic order down to 0.3 K. In this work, the magnetic properties of UB4 are investigated in an extended temperature range, 2-800 K. on high-quality single crystals. A cross-over from a low temperature 5f itinerant behaviour to a high temperature 5f localized behaviour is observed, marked by broad peaks in the magnetic susceptibility measured along the main crystallographic axes. (c) 2006 Elsevier B.V. All rights reserved.

3883. Quantized conductance and bend resistance in an asymmetric Si/SiGe cross junction

Authors: Poenariu, SA; Wieser, U; Kunze, U; Hackbarth, T

Published: MAY 2006, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 32, 542, DOI: 10.1016/j.physe.2005.12.118

Ballistic electron transport, namely quantized conductance, bend resistance, and its polarity-dependent breakdown, is studied in an asymmetric nanoscale cross junction prepared from a high-mobility Si/SiGe hetero structure. The cross junction is composed of four orthogonal leads which merge into two wide and two narrow quantum point contacts (QPCs). For a wide-narrow combination of leads the two-terminal differential conductance as a function of finite DC bias voltage reveals oscillations and half-plateaus characteristic for quantized conductance. In four-terminal bend resistance configuration with current injection into orthogonal leads negative bend voltage develops between the probing leads. Above a given current threshold the negative bend resistance breaks down just in that bias current polarity where electrons enter the cross region from the narrow QPC. This breakdown is attributed to phonon emission by hot electrons. (c) 2006 Elsevier B.V. All rights reserved.

3884. New growth approach of high-quality oxide thin films for future device applications: independent control of supersaturation and migration

Authors: Endo, K; Badica, P; Sato, H; Akoh, H

Published: MAY 2006, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 19, S225, DOI: 10.1088/0953-2048/19/5/S12

In this paper we focus on supersaturation and migration of the atomic species involved in the growth of oxide films in order to search for new efficient methods for growth of flat, uniform and precipitate-free complex thin films with high quality. In this respect we have applied MOCVD to HTS thin films, 'interrupted growth' (consisting of on/off vapour deposition cycles, keeping the temperature and atmospheric conditions constant), similar to 'pulsed laser interval deposition' reported in the literature. We show that MOCVD can be easily and successfully applied to suppress island growth, contrary to the currently accepted idea that this approach is unique to PLD. This suggests that principles of supersaturation-migration-supersaturation cycles are universal for the films' growth. By this approach, high-quality Bi-2223 thin films with a uniform flat surface, having roughness less than half the c-axis unit cell, were grown by MOCVD. Using substrates with artificial steps of predefined width equal to 'twice the migration length' totally precipitate-free thin films were obtained: precipitates-segregates of impurity phases (Cu rich for HTS materials) are gathered at the step edges where the free energy is lowest. As-prepared films were used to fabricate intrinsic Bi-2223 and Bi-2212/Bi-2223 SIS-type Josephson junctions (IJJs).

3885. Electron-trapping centers and interstitials in chlorinated SrCl2 : Fe single crystals

Authors: Ghica, D; Nistor, SV; Goovaerts, E; Schoemaker, D; Vrielinck, H; Callens, F

Published: MAY 2006, PHYSICAL REVIEW B, 73, DOI: 10.1103/PhysRevB.73.174103

Electron-trapped Fe+-type centers, produced by x-ray irradiation at 80 K and further annealing at higher temperatures in iron-doped SrCl2 single crystals grown in chlorine gas, have been investigated by electron paramagnetic resonance. The Fe+(III) and Fe+(IIIa) centers, produced by annealing at temperatures higher than 200 K, exhibit monoclinic local symmetry with the two g down arrow- tensor principal axes situated in the (110) plane slightly tilted away from the [001] and [1-10] directions, respectively. The Fe+(IV) center, observed after several cycles of irradiation and annealing to 700 K, exhibits tetragonal local symmetry around < 100 > and a well-resolved four-component structure, attributed to the superhyperfine interaction with a neighboring monovalent impurity ion. The presence and properties of the low symmetry radiation-induced Fe+ paramagnetic centers are attributed to trapping and the thermally activated movement of chlorine interstitials. Both precursor Fe2+ and resulting Fe+ centers are perturbed by these interstitials, which are introduced in SrCl2:Fe crystals during growth under a chlorine atmosphere.

3886. Femtosecond pulse shaping for phase and morphology control in PLD: Synthesis of cubic SiC

Authors: Ristoscu, C; Socol, G; Ghica, C; Mihailescu, IN; Gray, D; Klini, A; Manousaki, A; Anglos, D; Fotakis, C

Published: APR 30 2006, APPLIED SURFACE SCIENCE, 252, 4862, DOI: 10.1016/j.apsusc.2005.07.099

Pulse shaping introduces the method that makes possible the production of tunable arbitrary shaped pulses. We extend this method to control the prevalent growth of cubic SiC films on Si (10 0) substrates by pulsed laser deposition at temperatures around 973 K from a SiC target in vacuum. We used a laser system generating 200 fs pulses duration at 800 nm with 600 mu J at 1 kHz. The obtained structures are investigated by electron microscopy, X-ray diffraction and profilometry. We observed grains embedded in an amorphous texture, characteristic in our opinion to the depositions obtained with very short pulses. We present a comparison of deposited films with and without pulse shaping. Pulse shaping promotes increased crystallization and results in the deposition of thin structures of cubic SiC with a strongly reduced density of particulates, under similar deposition conditions. (c) 2005 Elsevier B.V. All rights reserved.

3887. Growth and characterization of beta-SiC films obtained by fs laser ablation

Authors: Ghica, C; Ristoscu, C; Socol, G; Brodoceanu, D; Nistor, LC; Mihailescu, IN; Klini, A; Fotakis, C

Published: APR 30 2006, APPLIED SURFACE SCIENCE, 252, 4677, DOI: 10.1016/j.apsusc.2005.07.087

We achieved the growth of cubic silicon carbide (SiC) films on (100)Si substrates by pulsed laser deposition (PLD) at moderate temperatures such as 750 degrees C, from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), conventional and high-resolution transmission electron microscopy (TEM/HRTEM). The morphology of deposited films is dominated by columns nucleated from a thin nanostructured beta silicon carbide (beta-SiC) interface layer. The combined effects of columnar growth, tilted facets of the emerging columns and the presence of particulates on the film surface, lead to a rather rough surface of the films. (c) 2005 Elsevier B.V. All rights reserved.

3888. In situ grown epitaxial YBa2Cu3O7-x thin films by pulsed laser deposition under reduced oxygen pressure during cool-down time

Authors: Branescu, M; Vailionis, A; Ward, I; Huh, J; Socol, G

Published: APR 30 2006, APPLIED SURFACE SCIENCE, 252, 4577, DOI: 10.1016/j.apsusc.2005.07.137

We report novel pulsed laser deposition conditions that were used to obtain superconducting epitaxial YBCO thin films, grown in situ using an oxygen pressure lower than the usual one during the cool-down time. We studied the influence of the PLD conditions as substrate temperature, oxygen pressure, laser fluence, and number of laser pulses on the crystallographic and morphological features, and on the superconducting properties of the films. Good superconducting properties were obtained without a high temperature post-deposition annealing process. A maximum critical temperature of 88.6 K was obtained. (c) 2005 Elsevier B.V. All rights reserved.

3889. Thickness effect in Pb(Zr0.2Ti0.8)O-3 ferroelectric thin films grown by pulsed laser deposition

Authors: Lisca, M; Pintilie, L; Alexe, M; Teodorescu, CM

Published: APR 30 2006, APPLIED SURFACE SCIENCE, 252, 4552, DOI: 10.1016/j.apsusc.2005.07.149

Epitaxial Pb(Zr,Ti)O-3 (PZT) thin films with thicknesses in the range of 50-200 nin and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD). The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0)SrTiO3 (STON). SrRuO3 (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the ext,ended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT. (c) 2005 Elsevier B.V. All rights reserved.

3890. Detailed study of the CePd2-xNixAl3 magnetic phase diagram around its critical concentration

Authors: Sereni, JG; Pedrazzini, P; Bauer, E; Galatanu, A; Aoki, Y; Sato, H

Published: APR 19 2006, JOURNAL OF PHYSICS-CONDENSED MATTER, 18, 3802, DOI: 10.1088/0953-8984/18/15/022

Thermal, magnetic and transport measurements on CePd2-xNixAl3 alloys within the 0 <= x <= 1 range are reported, including applied pressure (p) and magnetic field on some selected samples. The low temperature results indicate that long range antiferromagnetic order is robust up to x = 0.2, whereas between 0.25 and 0.5 magnetic fluctuations give rise to non-Fermiliquid (NFL) behaviour. In this critical region, the low temperature specific heat can be described as due to two components, the major showing a NFL C-p/T = gamma(0) - gamma(1) root T dependence, while the minor one includes a decreasing fraction of short range order degrees of freedom. The latter is only observed close to the critical point, x(cr) approximate to 0.35. Electrical resistivity (rho) studies performed under pressure for x = 0.5 allow us to investigate the evolution of the NFL state around and beyond x(cr), where the exponent of rho proportional to T-n increases from n = 1 (for p = 0) up to n = 2 (for p = 12 kbar). This exponent is also observed at normal pressure on the x = 1.0 sample. indicating the onset of the Fermi liquid behaviour. Doping and pressure effects are compared by fitting high temperature resistivity data employing a unique function which allows us to describe the evolution of the characteristic energy of this series along a large range of concentration and pressure.