Oriented YBaCuO Films Research Articles

RHEED study of modified barrier for c-axis oriented YBaCuO trilayer junctions

We have studied a formation process of the modified barrier for YBaCuO trilayer junctions by using reflection high-energy electron diffraction (RHEED). c-axis oriented YBaCuO films were prepared on (1 0 0) oriented MgO substrates by using a co-evaporation method with pure O 3 vapor. The modified barriers were formed by an Ar plasma bombardment with an acceleration voltage of 1 kV and by a subsequent annealing with various gas conditions. The film surface exhibited a hallo pattern after Ar plasma bombardment at room temperature. As the film was annealed at 400, 600 and 710 °C under the vacuum condition, the RHEED pattern was drastically changed to weak spots, clear spots with a streak pattern, and a spotty pattern. The film annealed in the ozone vapor condition, on the other hand, showed a streak pattern.

Superconducting properties of YBa2Cu3O7 step-edge Josephsonjunctions prepared on CeO2-buffered sapphire substrates

Step-edge Josephson junctions in the c-axis oriented YBa2Cu3O7 films were fabricated on the CeO2-bufferedsapphire substrates. The step angles were controlled in the wide range of20-75° by an Ar ion milling technique. The I-V curves ofjunctions fabricated on the 35° step with the film thickness to thestep height ratio of about 0.8 exhibited a resistively shunted junction-likebehaviour with an ICRN product of about 250 µV and a criticalcurrent density of about 2×104 A cm-2 at 77 K. Thecritical currents of the step-edge junctions increased approximately linearlywith decreasing temperature but the normal resistance remained almostconstant. All the samples of the step-edge Josephson junctions satisfied ascaling behaviour of ICRN∝(JC)0.5, and a dc SQUID made out ofthose junctions exhibited a typical voltage modulation in an applied magneticfield.

Fabrication of YBaCuO wiring for YBaCuO trilayer junctions

We have investigated the advancement of the trilayer junction fabrication process toward the YBaCuO/PrBaCuO/YBaCuO trilayer junctions with the YBaCuO wiring layers. Run-to-run variation of J/sub c/ was obtained to be factor three for the YBaCuO junctions with Au wiring layer on the two substrates. We found that c-axis oriented YBaCuO films with T/sub c/ of 85 K were developed on an [110] SrTiO/sub 3/ substrates with CeO/sub 2//MgO buffer layers. From these results, we have attempted to fabricate YBaCuO trilayer junctions with the YBaCuO wiring layer.

Hall effect measurements on ion bombarded YBaCuO films

High quality c-axis oriented YBaCuO films, epitaxially grown on (100) SrTiO 3 substrates, were photolithographically patterned to allow Hall measurements at different temperatures 4K≦T≦300K in magnetic fields up to B≦5 T aligned parallel to the surface normal. In this way, the temperature and field dependence of the longitudinal ϱ xx(B,T) and transverse resistivity ϱ xy(B,T)] were determined before and after irradiation with 350 keV He + ions. The resulting changes of the temperature dependence of the longitudinal resistivity, Hall resistivity and inverse Hall angle are reported.

Model for reactive sputtering Optimisation results for YBaCuO thin films deposited on polycrystalline zirconia substrates

In order to study in situ elaboration of YBaCuO thin films sputtered on polycrystalline YSZ substrates, a model of reactive sputtering has been developed. c-axis oriented YBaCuO films were obtained with J c ≈3×10 4 A/cm 2 at 77 K measured on 55×55 μm 2 microbridges.

Direct high-resolution electron microscopy observations of sputtered a-axis oriented YBa2Cu3O7 films prepared by a self-template method on SrTiO3 substrates

A-axis oriented YBa2Cu3O7 films grown on a SrTiO3 substrate with and without the template layer have been studied by high-resolution electron microscopy. The self-template layer was found to be very effective to obtain highly crystalline a-axis oriented YBa2Cu3O7 film. A disordered cubic perovskite structure with a lattice parameter of 3.85 Å was found in the template layer which could be a good seed for further growth of a-axis film. In the film without the self-template layer, YBa4Cu3Ox cubic perovskite with lattice parameter 4.08 Å was observed to have grown at the interface as an island growth. Moreover, some faulted c-axis film at the very beginning of the template layer was observed even at the rather lower deposition temperature of 640 °C.

YBa2Cu3O7 direct current–superconducting quantum interference devices with artificial PrBa2Cu3O7 barriers above 77 K

Direct current–superconducting quantum interference devices (dc-SQUIDs) from YBa2Cu3O7 with PrBa2Cu3O7 barriers and insulating MgO layers that modulate up to a temperature of 85 K are prepared by laser deposition with a KrF excimer laser and an ex situ patterning process. In order to utilize the longer coherence length of c-axis oriented YBa2Cu3O7 films in the a-b plane, an edge geometry is used for the definition of the Josephson junctions. By the variation of the barrier thickness the critical current of the junctions can be controlled within close limits and can be chosen for optimum dc-SQUID performance at a given working temperature. At 80 K we find a flux-voltage modulation of 3.4 μV.

Anisotropic Josephson effect in all (103)-oriented YBa2Cu3O7−δ /PrBa2Cu3O7−δ′/YBa2Cu3O7−δ junctions

We report a demonstration of the anisotropic Josephson effect in all (103)-oriented junctions consisting of YBa2Cu3O7−δ /PrBa2Cu3O7−δ′/YBa2Cu3O7−δ film sandwiches. The junction shows a resistively shunted junction-like current-voltage characteristics with hysteresis and clear Shapiro steps under the microwave radiation. Depending on the direction of external magnetic field H, the junction exhibits anisotropic Fraunhofer interference patterns with different critical magnetic fields Hc0 and periods in the oscillation of critical current. The agreement between experimental and expected values by the Owen–Scalapino analysis for anisotropic Hc0 confirms that this anisotropic property comes from the anisotropy in the magnetic penetration depth of (103)-oriented YBaCuO films.

Contribution of nuclear microanalysis and of 18O tracer technique to study the oxygen sublattice in high Tc superconducting thin films

At first a short review of IBA contribution to the determination of composition and structure, including phonon properties of high Tc superconducting oxides is presented. In the second part, IBA contribution to the elucidation of the mechanisms of thin film growth is presented. The emphasis is on the complementarity of IBA, Raman spectroscopy and XRD techniques to characterize thin films of high Tc superconducting oxides. In the third part, some recent results related mainly to YBaCuO films grown on (100) oriented single crystalline bulk materials (MgO, LaAlO3, SrTiO3) is discussed. In these experiments, IBA, XRD and Raman spectroscopies were used to study the oxygen content in a series of YBaCuO films prepared in different conditions of pressure and temperature. In the case of c-axis oriented films a good agreement between these three methods was found for the films cooled down at high oxygen pressure and a significant disagreement for the films cooled down at low oxygen pressure, showing structures with anomalous c-axis parameter. In the case of a-axis oriented films grown on SrTiO3 substrates it was found that the low Tc values (∼ 70–80 K) are not correlated with the oxygen content but rather with a disorder in the oxygen sublattice. The disorder in the oxygen sublattice was studied using the 16O(α, α)16Oresonance in random and channeling geometry. These results are correlated with the data provided by Raman spectroscopy. The 18O tracer technique was used to estimate the diffusion coefficient in the a-axis oriented YBaCuO films showing a huge anisotropy of the 18O labelling. Combining Raman and IBA techniques, the selective 18O labelling of the CuO chain-planes was evidenced. The defects in the 18O enriched CuO chain-planes were studied using the 18O(p, α)15N nuclear resonant reaction in random and channeling geometries. Some preliminary results related to roughness of YBaCuO films are also discussed. The physical implications of these findings are analyzed.

Growth frustration of YBaCuO films at the transition from a- to c-axis orientation

Thin a-axis oriented YBaCuO films have been deposited in the self-template procedure with the deposition temperature in the second step, T s , as a parameter. At T s values around 790°C the conditions for a- and c-axis growth become similar leading to growth deterioration by a frustration effect. This effect kimits the maximum T s values applicable to ordering of a-axis films. In this series of experiments the a-axis films with negligible c-axis content had T c -values around 80 K.

Current-induced decoupling of superconducting planes in oriented YBaCuO thick films

The critical current of decoupling of superconducting CuO 2 planes is extracted from resistive data obtained on oriented YBaCuO films. Distinctive 3D—2D crossover in dissipation takes place at a temperature dependent current j c( T) that exhibits a clear tendency to vanish at T KT (Kosterlitz—Thouless transition temperature) of individual superconducting layers. That confirms the idea of full suppression of interlayer coupling due to 2D-vortex fluctuations close to T KT.

A-axis growth of YBa2Cu3O7 thin films on LaAlO3 induced by monolayer buffers of PrO2

The deposition of an intermediate buffer of praseodymium oxide (PrO2) of only monolayer thickness (about 2 Å) in a two-temperature process results in the growth of almost exclusive epitaxial a-axis oriented YBa2Cu3O7 (YBCO) on LaAlO3 (100). Under identical experimental conditions, but without the monolayer of PrO2, epitaxial c-axis oriented YBCO films with a critical temperature Tc of 86 K and a transition width of 1 K are obtained. Critical temperatures of these a-axis oriented films are typically 10 K lower than those of the c-axis oriented films. Our findings suggest that a single layer of PrBa2Cu3O7 is formed at the interface, inducing a-axis growth throughout the whole YBCO film.

Etching of screw dislocations in YBa2Cu3O7 films with a scanning tunneling microscope

We report on the systematic unrolling of screw dislocations with the scanning tunneling microscope at the surfaces of sputtered, c-axis oriented YBa2Cu3O7 films. The etching is dominated by field induced evaporation and not by mechanical milling of the surface. The process also enables the controlled drawing of nanometer scale grooves on the film surface.

Growth of C-axis oriented YBaCuO films on oxidized textured Ni sheets and on

This paper reports on YBaCuO films grown on oxidized textured Ni sheets and on (100) and (110) oriented NiO single crystals by magnetron sputter deposition from a stoichiometric tubular target. The properties of the YBaCuO films are investigated as function of various preparation parameters (oxidation procedure of the sheets, substrate temperature T{sub s}, HTSC film thickness). At T{sub s} = 780{degrees} C and for film thickness larger than 200 nm, the films have T{sub c} values around 80K. The textures of the Ni sheets, NiO layers and YBaCuO films are determined using X-ray diffraction in Bragg-Brentano geometry and scanning electron microscopy. It is remarkable, that on textured oxidized Ni the YBaCuO films grow c-axis oriented. For comparison the properties of films on differently oriented NiO single crystals are investigated. It is found that the film properties are better on (100) than on (110) surfaces.

YBaCuO films on silicon substrates: Fabrication, characterization, and use as phonon detectors.

Abstract Thermal coevaporation of Y, Ba, and Cu in an oxygen atmosphere produced c-axis oriented YBaCuO films with T c between 80 and 85 K on bare Si substrates. These films were laser patterned to produce microbridges which showed flux-flow resistance characteristics. We investigated the performance of a microbridge as a phonon detector, and measured its response to phonons of different frequency and polarization.

Temperature and field dependence of the magnetic penetration length of c-axis oriented YBa2Cu3O7 films

The penetration depth λ of c-axis oriented thin films of YBa2Cu3O7 in perpendicular magnetic fields up to 14T is determined from complex impedance measurements at 1.25kHz. Vortex core pinning dominates over the condensate term and determines the functional form of λ(T,H). The critical field Hcp(T) identified by λ → ∞ is associated with the vanishing of the curvature of harmonic pinning potentials. The critical temperature at which the pinning potentials vanish is field dependent, in contrast with assumptions elsewhere in the literature.

Scanning Auger microscopy as applied to the analysis of highly textured YBaCu 3O x thin films

Scanning Auger electron spectroscopy and scanning electron microscopy have been used to investigate the local composition and structure of highly textured c ̂ axis oriented YBaCuO films with thicknesses in the range 0.4–1 μm. The cuprate films were sputtered on MgO and sapphire (100)-oriented single-crystal substrates at room temperature followed by several anneal stages below or at 920°C in pure oxygen. The YBaCuO/sapphire sample was examined again after an additional 750°C air anneal for 24 h. By applying Auger line profiling on a freshly prepared cross-sectional surface of a thin cuprate film deposited on a sapphire substrate we have been able to show that barium aluminate segregation at grain boundaries is the main cause of the higher electrical resistance usually observed for cuprate films on Al 2O 3. The (drastic) reduction in T c can be attributed to the substitution of aluminium in the cuprate at copper sites. Severe interdiffusion has been observed for the epitaxial c axis oriented YBaCu oxide films grown on an MgO substrate, which leads to a deterioration in the superconductivity. The main reason for reduced T c and quality of cuprate films on MgO is the copper loss into the substrate, the depth of penetration of copper extending more than 400 nm below the YBaCuOMgO interface. From our experimental results it is evident that Auger line profiling is an important tool in the analysis of high T c superconducting thin films.

マグネトロンスパッタリングによるＹＢａＣｕＯ超電導テープの作製

The feasibility of preparing YBaCuO films on metallic substrates was discussed in an attempt to fabricate a superconducting tape. Deposition method employed was an rf magnetron sputtering. MgO buffer layers were found to be a favorable choice to prevent the adverse reaction between the superconducting film and the underlying substrate. Particular attention was given to the preferential resputtering effect due to the bombardment of negative ions during sputtering. The substrate-cathode configuration used to achieve stoichiometric film composition was described. Highly oriented YBaCuO films have been obtained on a Hastelloy-X tape with a MgO buffer layer. The highest zero resistance temperature obtained so far of the YBaCuO thin films on a metallic tape was 80.4K in its as grown state, and a critical current density was 200A/cm2 at 77.4K and 104A/cm2 at 60K. Preliminary results of YBaCuO films deposited on a bundle of Chromel fine wires were also reported.

Ion Beam Thinning of YBa2cu3o7 Films: Approach to a Limiting Thickness for Superconductivity of One Lattice Constant

AbstractGrazing angle ion-beam milling of c-axis oriented YBa2Cu3O7 films is shown to give smooth films which have metallic-like resistance transitions and superconduct at thicknesses approaching a lattice constant. The zero resistance transition temperature decreases uniformly with increased milling with most of the change occurring for thicknesses less than 100Å. The crossover to non-superconducting behavior occurs at “electrical” thicknesses near 10 Å with a sheet resistance above the onset of the superconducting transition having a value near h/4e2 = 6450Ω, a combination of fundamental constants predicted by theory and observed previously in low-Tc superconductors. Continued milling beyond this limit results in semiconducting behavior without a superconducting transition.