Superconducting thick-films from a Y-Ba-Cu-O precursor

We have prepared scree-printed films of the Y-Ba-Cu-O composuperconductiund starting from a spray-pyrolysis precursor powder. BeO ceramic substrates are confirmed to be inert with respect to the film up to about 100 C. Electrical properties of oxigen annealed films are investigated,evidencing excellent superconducting behaviour,both in terms of Tc (=91K) and Jc (>100 Asquarecm at 77K)

Magnetic asymmetry in photoemission from Fe(100) with linearly polarized synchrotron radiation

We have measured partially angle-integrated photoemission with linearly polarized undulator radiation on the valence band and on the 3p and 3s core levels of iron in an Fe(100) single crystal. A large asymmetry is measured for 3p core levels when reversing the magnetization direction perpendicular to the photoemission plane. A smaller but clear asymmetry is also measured in the valence (3d) band spectrum, and no asymmetry is measured for the 3s core levels. The results are explained by the transverse spin polarization of the near-normal-emission photoelectrons which is induced by the light p polarization and by spin-orbit interaction in the initial state. From the line-shape change of the 3p core level as a function of magnetization reversal the fine structure of the 3p multiplet is derived. This effect provides a very efficient diagnostic of the surface magnetization direction allowing to measure magnetic ordering at surfaces similarly to the Kerr effect, but in a chemically specific and intrinsically surface sensitive way as shown by examples on ferromagnetically coupled and antiferromagnetically coupled Fe/Cr interfaces

Ag-based thick-film front metallization of silicon solar cells

The evolution of microstructure and electrical properties of silver-based thick-film metallizations of silicon solar cells prepared by infrared firing processes has been ivestigated. The performance of the cells are shown to be dependent on several dynamical and diffusive phenomena. In particular, the sintering of silver grains, silver diffusion in the glassand the flow of gas at the metal/silicon interface strongly affect important characteristics of the cells such as sheet and contatct resistivities and the adhesion of fingers and bus bars. The existance of an optimum value of the peak firing temperature is observed and explained in terms of competitive phenomena occuring at the metal/silicon interface. Moreover it is shown that IR firing treatments require aa careful consideration of the sequence of printing anf firing steps. The features of heat treatments performed in conveyor belt furnaces using Joule and infrared sources are compared

Effect of S segregation on the surface magnetism of Fe(100)

Bulk impurities of the ferromagnetic transition metals segregate at the surface at moderate temperature and severely modify the surface magnetic properties. S segregation on Fe(100) produces a stable c(2x2) reconstruction at 600 degrees C. We have measured the change of the magnetic properties of Fe(100) due to the formation of the c(2x2)S surface by measuring the exchange coupling along a path perpendicular to the surface via the spin-wave stiffness and the relative change of the iron surface magnetic moment by photoemission magnetic dichroism experiments and spin-polarization measurements of the secondary yield. A stronger coupling between the c(2x2)S surface and the bulk then for the clean Fe(100) free surface is found. Changes of the Fe 3p core hole magnetic splitting show that the surface magnetic moment of Fe is reduced by up to 20% upon S segregation with respect to the clean surface. Strong magnetic dichroism of the electron states just below the Fermi level indicates a filling of the surface minority spin band in the c(2x2)S/Fe(100) surface relative to the clean Fe(100) surface

Size effects in ruthenium-based thick-film resistors: rutile vs. pyrochlore-based resistors

The size effect, namely the change of sheet resistance, R, as a function of resistor length, has been investigated in layers whose conductive phase evoves from Pb-rich (Ru-deficient pyrochlores) to Pb2Ru2O6.5 and finally to RuO2 by increasing the firing temperature. It is found that Bi diffusion from the terminations is responsible for lower sheet resistance values in shorter resistors whatever the conductive phase is. On the contrary, Ag diffusion is responsible for lower sheet resistance values in shorte resistors only in the case of ruthenate conductive grains while the reverse is observed in RuO2-based layers. Size effect can be suppressed with Pt/Au-based terminations provided that Bi is contained and with Au-metallorganic-based contact provided that the peak firing temperature is not too high

Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure

In thin film ferroelectric capacitor the chemical and electronic structure of the electrode/FE interface can play a crucial role in determining the kinetics of polarization switching. We investigate the electronic structure of a Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy. The chemical, electronic and depth sensitivity of core level photoemission is used to probe the transient response of different parts of the upper electrode/ferroelectric interface to voltage pulse induced polarization reversal. The linear response of the electronic structure agrees quantitatively with a simple RC circuit model. The non-linear response due to the polarization switch is demonstrated by the time-resolved response of the characteristic core levels of the electrode and the ferroelectric. Adjustment of the RC circuit model allows a first estimation of the Pt/BTO interface capacitance. The experiment shows the interface capacitance is at least 100 times higher than the bulk capacitance of the BTO film, in qualitative agreement with theoretical predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.

Interface Electronic Structure in a Metal/Ferroelectric Heterostructure under Applied Bias

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is measured and is supported by transport measurements. Barrier heights depend on interface chemistry and on the FE polarization. A differential response of the core levels to applied bias as a function of the polarization state is observed, consistent with Callen charge variations near the interface.Comment: 9 pages, 8 figures. Submitted to Phys. Rev.

Surface vs. bulk magnetic moments from photoemission dichroism

The value of the surface spin magnetic moment of Fe(100) is obtained by measuring the energy spread of the photoemission dichroism spectra of Fe3p core levels from bulk and surface. The bulk and surface signals have been resolved exploiting the photoemission diffraction effects in a linear magnetic dichroism of the angular distribution (LMDAD) experiment with soft X-rays. The LMDAD lineshape and energy width of the bulk contribution has been independently obtained from angular and from photon energy dependent experiments: it provides a gauge for determining the magnetic moment of the surface Fe3p atoms in the hypothesis of a linear dependance between energy width of dichroism and local spin magnetic moment

Interface bonding of a ferromagnetic/semiconductor junction : a photoemission study of Fe/ZnSe(001)

We have probed the interface of a ferromagnetic/semiconductor (FM/SC) heterojunction by a combined high resolution photoemission spectroscopy and x-ray photoelectron diffraction study. Fe/ZnSe(001) is considered as an example of a very low reactivity interface system and it expected to constitute large Tunnel Magnetoresistance devices. We focus on the interface atomic environment, on the microscopic processes of the interface formation and on the iron valence-band. We show that the Fe contact with ZnSe induces a chemical conversion of the ZnSe outermost atomic layers. The main driving force that induces this rearrangement is the requirement for a stable Fe-Se bonding at the interface and a Se monolayer that floats at the Fe growth front. The released Zn atoms are incorporated in substitution in the Fe lattice position. This formation process is independent of the ZnSe surface termination (Zn or Se). The Fe valence-band evolution indicates that the d-states at the Fermi level show up even at submonolayer Fe coverage but that the Fe bulk character is only recovered above 10 monolayers. Indeed, the Fe 1-band states, theoretically predicted to dominate the tunneling conductance of Fe/ZnSe/Fe junctions, are strongly modified at the FM/SC interface.Comment: 23 pages, 5 figures, submitted to Physical review