Frame assisted H2O electrolysis induced H2 bubbling transfer of large area graphene grown by chemical vapor deposition on Cu

An improved technique for transferring large area graphene grown by chemical vapor deposition on copper is presented. It is based on mechanical separation of the graphene/copper by H2 bubbles during H2O electrolysis, which only takes a few tens of seconds while leaving the copper cathode intact. A semi-rigid plastic frame in combination with thin polymer layer span on graphene gives a convenient way of handling- and avoiding wrinkles and holes in graphene. Optical and electrical characterizations prove the graphene quality is better than that obtained by traditional wet etching transfer. This technique appears to be highly reproducible and cost efficient. © 2013 American Institute of Physics.

Light absorption and electrical transport in Si:O alloys for photovoltaics

Thin films (100-500 nm) of the Si:O alloy have been systematically characterized in the optical absorption and electrical transport behavior, by varying the Si content from 43 up to 100 at. %. Magnetron sputtering or plasma enhanced chemical vapor deposition have been used for the Si:O alloy deposition, followed by annealing up to 1250 °C. Boron implantation (30 keV, 3-30× 1014 B/cm2) on selected samples was performed to vary the electrical sheet resistance measured by the four-point collinear probe method. Transmittance and reflectance spectra have been extracted and combined to estimate the absorption spectra and the optical band gap, by means of the Tauc analysis. Raman spectroscopy was also employed to follow the amorphous-crystalline (a-c) transition of the Si domains contained in the Si:O films. The optical absorption and the electrical transport of Si:O films can be continuously and independently modulated by acting on different parameters. The light absorption increases (by one decade) with the Si content in the 43-100 at. % range, determining an optical band gap which can be continuously modulated into the 2.6-1.6 eV range, respectively. The a-c phase transition in Si:O films, causing a significant reduction in the absorption coefficient, occurs at increasing temperatures (from 600 to 1100 °C) as the Si content decreases. The electrical resistivity of Si:O films can be varied among five decades, being essentially dominated by the number of Si grains and by the doping. Si:O alloys with Si content in the 60-90 at. % range (named oxygen rich silicon films), are proved to join an appealing optical gap with a viable conductivity, being a good candidate for increasing the conversion efficiency of thin-film photovoltaic cell. © 2010 American Institute of Physics.

Influence of the matrix properties on the performances of Er-doped Si nanoclusters light emitting devices

We investigated the properties of light emitting devices whose active layer consists of Er-doped Si nanoclusters (nc) generated by thermal annealing of Er-doped SiOx layers prepared by magnetron cosputtering. Differently from a widely used technique such as plasma enhanced chemical vapor deposition, sputtering allows to synthesize Er-doped Si nc embedded in an almost stoichiometric oxide matrix, so as to deeply influence the electroluminescence properties of the devices. Relevant results include the need for an unexpected low Si excess for optimizing the device efficiency and, above all, the strong reduction of the influence of Auger de-excitation, which represents the main nonradiative path which limits the performances of such devices and their application in silicon nanophotonics. © 2010 American Institute of Physics.

Microstructural evolution of SiOx films and its effect on the luminescence of Si nanoclusters

In this paper we demonstrate that the structural and optical properties of Si nanoclusters (Si ncs) formed by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering are very different. In fact, at a fixed Si excess and annealing temperature, photoluminescence (PL) spectra of sputtered samples are redshifted with respect to PECVD samples, denoting a larger Si ncs size. In addition, PL intensity reaches a maximum in sputtered films at annealing temperatures much lower than those needed in PECVD films. These data are correlated with structural properties obtained by energy filtered transmission electron microscopy and electron energy loss spectroscopy. It is shown that in PECVD films only around 30% of the Si excess agglomerates in clusters while an almost complete agglomeration occurs in sputtered films. These data are explained on the basis of the different initial structural properties of the as-deposited films that become crucial for the subsequent evolution. © 2008 American Institute of Physics.

Thermal evolution of Er silicate thin films grown by rf magnetron sputtering

Stoichiometric Er silicate thin films, monosilicate (Er2SiO 5) and disilicate (Er2Si2O7), have been grown on c-Si substrates by rf magnetron sputtering. The influence of annealing temperature in the range 1000-1200 °C in oxidizing ambient (O 2) on the structural and optical properties has been studied. In spite of the known reactivity of rare earth silicates towards silicon, Rutherford backscattering spectrometry shows that undesired chemical reactions between the film and the substrate can be strongly limited by using rapid thermal treatments. Monosilicate and disilicate films crystallize at 1100 and 1200 °C, respectively, as shown by x-ray diffraction analysis; the crystalline structures have been identified in both cases. Moreover, photoluminescence (PL) measurements have demonstrated that the highest PL intensity is obtained for Er2Si2O7 film annealed at 1200 °C. In fact, this treatment allows us to reduce the defect density in the film, in particular by saturating oxygen vacancies, as also confirmed by the increase of the lifetime of the PL signal. © 2008 IOP Publishing Ltd.

The influence of substrate on the properties of Er2O3 films grown by magnetron sputtering

The structural properties and the room temperature luminescence of Er2O3 thin films deposited by RF magnetron sputtering have been studied. Films characterized by good morphological properties have been obtained by using a SiO2 interlayer between the film and the Si substrate. The evolution of the properties of the Er2O3 films due to rapid thermal annealing processes in O2 ambient performed at temperatures in the range 800-1200 °C has been investigated in details. The existence of well-defined annealing conditions (temperature of 1100 °C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. The same annealing processes are less effective when Er2O3 is deposited on Si. In this latter case interfacial reactions and pit formation occur, leading to a material characterized by stronger non-radiative phenomena that limit the PL efficiency. © 2006 Elsevier B.V. All rights reserved.

Optical and structural properties of Er2O3 films grown by magnetron sputtering

The structural properties and the room temperature luminescence of Er 2O3 thin films deposited by magnetron sputtering have been studied. In spite of the well-known high reactivity of rare earth oxides towards silicon, films characterized by good morphological properties have been obtained by using a SiO2 interlayer between the film and the silicon substrate. The evolution of the properties of the Er2O3 films due to thermal annealing processes in oxygen ambient performed at temperatures in the range of 800-1200°C has been investigated in detail. The existence of well defined annealing conditions (rapid treatments at a temperature of 1100°C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated; under these conditions, the thermal process has a beneficial effect on both structural and optical properties of the film, and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as-deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. Finally, the conditions leading to a reaction of Er2O3 with the substrate have been also identified, and evidences about the formation of silicate-like phases have been collected. © 2006 American Institute of Physics.

Carbon nanotube growth for through silicon via application.

Through silicon via (TSV) technology is key for next generation three-dimensional integrated circuits, and carbon nanotubes (CNT) provide a promising alternative to metal for filling the TSV. Three catalyst preparation methods for achieving CNT growth from the bottom of the TSV are investigated. Compared with sputtering and evaporation, catalyst deposition using dip-coating in a FeCl2 solution is found to be a more efficient method for realizing a bottom-up filling of the TSV (aspect ratio 5 or 10) with CNT. The CNT bundles grown in 5 min exceed the 50 μm length of the TSV and are multi-wall CNT with three to eight walls. The CNT bundles inside the TSV were electrically characterized by creating a direct contact using a four-point nanoprober setup. A low resistance of the CNT bundle of 69.7 Ω (297 Ω) was measured when the CNT bundle was contacted midway along (over the full length of) the 25 μm deep TSV. The electrical characterization in combination with the good filling of the TSV demonstrates the potential use of CNT in fully integrated TSV applications.

Tantalum-oxide catalysed chemical vapour deposition of single- and multi-walled carbon nanotubes

Tantalum-oxide thin films are shown to catalyse single- and multi-walled carbon nanotube growth by chemical vapour deposition. A low film thickness, the nature of the support material (best results with SiO2) and an atmospheric process gas pressure are of key importance for successful nanotube nucleation. Strong material interactions, such as silicide formation, inhibit nanotube growth. In situ X-ray photoelectron spectroscopy indicates that no catalyst reduction to Ta-metal or Ta-carbide occurs during our nanotube growth conditions and that the catalytically active phase is the Ta-oxide phase. Such a reduction-free oxide catalyst can be technologically advantageous. © 2013 The Royal Society of Chemistry.

Use of 2G coated conductors for efficient shielding of DC magnetic fields

This paper reports the results of an experimental investigation of the performance of two types of magnetic screens assembled from YBa2Cu3O7-d (YBCO) coated conductors. Since effective screening of the axial DC magnetic field requires the unimpeded flow of an azimuthal persistent current, we demonstrate a configuration of a screening shell made out of standard YBCO coated conductor capable to accomplish that. The screen allows the persistent current to flow in the predominantly azimuthal direction at a temperature of 77 K. The persistent screen, incorporating a single layer of superconducting film, can attenuate an external magnetic field of up to 5 mT by more than an order of magnitude. For comparison purposes, another type of screen which incorporates low critical temperature quasi-persistent joints was also built. The shielding technique we describe here appears to be especially promising for the realization of large scale high-Tc superconducting screens.

DC and AC Shielding properties of bulk high-tc

We have studied numerically and experimentally the magnetic flux penetration in high-Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of high-Tc superconducting ceramics. We have measured the field attenuation for applied magnetic fields in the frequency range 5 mHz-0.1 Hz by Hall probe measurements and at audio frequencies using a sensing coil. A simple 1D analysis using the Kim critical state model was found to be able to reproduce the experimental data satisfactorily. We have also determined the phase shift between the internal and the applied field both experimentally and numerically. Finally, we have studied the sweep rate dependence of the magnetic shielding properties, using data recorded either at several constant sweep rates dB /dt or at several AC fields of various amplitudes and frequencies. Both methods agree with each other and lead to a n-value of the E ∼ Jn law equal to ∼40 at 77 K. © 2009 IEEE.

YBa 2Cu 3O 7-δ thick films on Ag prepared by the Electrophoretic Deposition technique

YBa 2Cu 3O 7-δ thick films have been deposited onto Ag substrates by the Electrophoretic Deposition (EPD) technique. Different microstructures and electrical behaviours were observed depending on the starting powder. Coatings prepared from commercial powder displayed significant porosity and the superconducting transition width was found to be magnetic-field dependent. Films produced from home-made coprecipitated powder are denser but contain some secondary phases. No dependence of the resistive transition as a function of magnetic field (H 20 Oe) was observed in that case. © 2006 IOP Publishing Ltd.

Anisotropic behaviour in the magnetic field dependence of the low temperature electrical resistance of calcium-doped lanthanum manganate thin films grown by RF magnetron sputtering

We report about the magnetoresistive properties of calcium-doped lanthanum manganate thin films grown by RF magnetron sputtering on single crystalline LaAlO3 and MgO substrates. Two orientations of the magnetic field with respect to the electrical current have been studied: (i) magnetic field in the plane of the film and parallel to the electrical current, and (ii) magnetic field perpendicular to the plane of the film. The film grown on LaAlO 3 is characterised by an unusual magnetoresistive behaviour when the magnetic field is applied perpendicular to the film plane: the appearance of two bumps in the field dependence of the resistance is shown to be related to the occurrence of anisotropic magnetoresistive effects in manganate films. © 2004 Elsevier B.V. All rights reserved.

YBa2Cu3O7 tapes prepared by sol-gel deposition techniques: Microstructure and structural characterizations

We report on the preparation conditions of YBa2Cu3O7 polycrystalline superconducting tapes by a sol-gel deposition technique. We present some discussion on the compatibility between the nature of the substrate, the use of a buffer layer, and the conditions used to prepare appropriate superconducting YBa2Cu3O7 materials. We report also on the microstructural characterizations performed in order to evaluate the crystallites size, degree of orientation and connectivity. © 2002 Elsevier Science B.V. All rights reserved.