Critical current density and microstructure of YBa2Cu3O7 thin films post-annealed at a low oxygen partial pressure

Post-annealing of thin films of YBa₂Cu₃O₇ (YBCO) has been performed at 29 Pa and 750°C. For films 0.6 m thick, a critical current density >1 MA cm^–2 is obtained at 77 K, with a sharp eddy current response at 25 MHz. Microstructural investigation of these films by crosssectional and planar transmission electron microscopy reveals that the YBCO film has thec-axis normal to the plane of the substrate in a continuous sheet of varying thickness, frequently covering the entire thickness of the film. Mutually perpendicular rods with thec-axis in the plane of the LaAlO₃ substrate are also seen. The microstructure and critical current density of these films are compared with those of previously reported films post-annealed in atmosphericpressure oxygen.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7– (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (31) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Superconducting behavior of high-Tc YBa2Cu3O7 thin films with BaO impurity produced by pulsed laser deposition

We have investigated the superconducting behavior of high-T _c YBa₂Cu₃O₇ (YBCO) thin films containing BaO impure phase produced by pulsed laser deposition. The thin films were characterized by the standard four-probe method, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD showed that all these thin films contained BaO impurity, with thec-axis normal to the surface of the substrates. The presence of impurity existed from substrate temperatureT _s of 727 to 796°C. When these thin films with BaO impurity were measured under the magnetic fields, it was found that the critical current densityJ _c increased slightly with increase in magnetic fieldB within the range ofB500 G, in the case ofB perpendicular to thec-axis of the film.     

Growth and Characterization of YBCO Thin Films by Sequential Deposition and Annealing

A novel thin film growth procedure, sequential deposition and annealing (SDA), which contains the advantages of both in situ and ex situ procedures, was proposed. Y₁Ba₂Cu₃O_{7 – x} (YBCO) high temperature superconducting thin films were grown and characterized by the SDA procedure. Purely c-axis-oriented YBCO thin films with no foreign phases and other oriented grains were successfully prepared. The superconducting transition properties of SDA-grown YBCO thin films were measured by measurement of inductance and resistance. The inductance measurements gave a T _c ^onset of 85 K and a T _c of 5 K. The resistance measurements gave a T _c ^onset of 90 K and a T _c of 5 K. Atomic force microscopy studies showed that SDA-grown YBCO thin films had micrometer-size grains surrounded by many nanometer-size grains. The nanometer-size grains in SDA-grown YBCO thin films are responsible for degradation of superconducting transition properties.     

Growth and Properties of Double-Sided YBCO Films Deposited by Off-Axis Sputtering for Microwave Applications

The double-sided YBa₂Cu₃O _y (YBCO) films were grown on LaAlO₃ (LAO) substrates by an off-axis magnetron sputtering system with two 2-inch sputtering guns. High-quality YBCO films within a 50-mm-diameter area were obtained. The homogeneous YBCO films revealed transition temperature T _c(R=0) of 86K and a critical current density J _c (zero field) of 1.5×10⁶ A/cm² at 77 K. Thesurface impedance of YBCO films was measured using a probe-coupling type microstripline resonator method. The surface resistance R _S of 1.8 was obtained at 77 K around 2180 MHz. Additionally, a HTS three pole bandpass filter was fabricated and revealed a good microwave characteristic. The influence of surface roughness on microwave properties of YBCO films was discussed.     

Estimation of the Surface-d to Bulk-s Crossover in the Macroscopic Superconducting Wavefunction in Cuprates

The concept of a surface d- and bulk s-symmetry in cuprate superconductors is applied to recent small-angle neutron-scattering results. These show a change of hexagonal to square vortex lattice as a function of the magnetic field along the c-axis. Identifying the hexagonal lattice with s- and the square with d-symmetry, the crossover distance from the surface d to the bulk s perpendicular to the c-axis is estimated to be 35 nm for LSCO and roughly 7 nm for YBCO, both at optimum doping. The crossover along the c-axis has to be of only a few layers distance to reconcile tunneling, photoemission, and pulsed femtosecond reflectivity experiments. These estimates are compatible with -rotation, NMR, and other experiments.     

Investigation of the Transport Properties of Coated Conductors and Comparison to Those of YBCO Films Deposited on Single-Crystal Substrates

The existence of non-superconducting zones in YBa₂Cu₃O_7??? (YBCO) films below the transition temperature to the non-resistive state can limit the performances of YBCO devices, especially of coated conductors for power transportation. In this work, we present detailed measurements of the temperature dependence of the surface critical current density of commercial YBCO-coated conductors. We compare our measurements to those carried out by other authors on the same types of samples and to measurements carried out on films deposited on SrTiO₃ single crystals. The existence of a T _c gradient along the YBCO films?? c-axis, probably linked to a gradient in the oxygen content of the films, seems established for all the investigated samples.     

Continuous Deposition of YBCO Films for Coated Conductor Using PLD Method

A reel-to-reel PLD system was set up for studying YBCO coated conductor. YBCO films could be continuously grown on the CeO₂/YSZ/Y₂O₃ buffered Ni-5W tape. Some deposition parameters were investigated. XRD θ–2θ scans were employed to characterize the c-axis orientation and in-plane texture of YBCO films deposited with different laser repetition rates and tape moving speeds. We investigated the dependence of critical current I _c on laser repetition rates and tape moving speeds for YBCO films. It had been found that a-axis oriented grains appeared as YBCO layer thickness increased, which prevented the values of I _c improved.     

Coupling behavior of Cu-O2 layers in oxygen-deficient YBCO films and ultrathin YBCO films

We have measured the resistivity as a function of temperature in a magnetic field at various field angles for oxygen-deficient YBCO films and ultrathin YBCO films; is the angle between the crystalc axis and the direction of the magnetic field. The angular dependence of the critical field,H _c2, shows a cusplike behaviour at =90° for the oxygen-deficient 60 K YBCO films indicating that the Cu-O₂ layers are two-dimensional in nature. BothT _c versus in a constant field andH _c2 as a function of show a plateau at =90° for 83 K (zero resistance) ultrathin YBCO films with a thickness of 20 units of the YBCO cell; these results indicate that there is a coupling between the Cu-O₂ layers. The dimensionality of the Cu-O₂ layers in oxygen-deficient YBCO films and ultrathin YBCO films is discussed in terms of the anisotropic Ginzburg-Landau model and the Tinkham thin-film model.     

Microstructure and electrical properties of YBCO thin films

Microstructures of c-axis oriented YBCO thin films made by high-pressure d.c. sputtering on LaAlO₃ and MgO substrates were examined by TEM. The a-axis oriented grains, second phases and micro-twins were frequently observed in the film. The a-axis oriented grains expanded along their c-axis directions during film growth. The a- and b-axis misorientations were observed in the film on MgO due to serious lattice mis-match between YBCO and MgO. The second phases were often accompanied with a-axis oriented grains suggesting they act as nuclei. These observed results were correlated with the measured T _c and J _c of the films.     

Special features in the synthesis and properties of thin Y-Ba-Cu-O high-temperature superconductor films free of secondary phases

Data on the structure and transport properties of thin Y-Ba-Cu-O (YBCO) high-temperature superconductor films obtained by magnetron sputtering of a stoichiometric target in a system with a 90° off-axis geometry are reported. It is shown that the films prepared under these conditions are free of copper-rich secondary phases and are characterized by the surface roughness height below 10 nm. The films possess a perfect structure and exhibit high superconducting properties: c-axis misorientation in microblocks FWHM(005)YBCO=0.4–0.5°; zero-resistance temperature T _co=89 K; critical (pinning) current density j _p=1.5–2 MA/cm² (77 K).     

Buffer layers for high-Tc thin films on sapphire

Buffer layers of various oxides including CeO₂ and yttrium-stabilized zirconia (YSZ) have been deposited onR-plane sapphire. The orientation and crystallinity of the layers were optimized to promote epitaxial growth of YBa₂Cu₃O_7– (YBCO) thin films. An ion beam channeling minimum yield of 3% was obtained in the CeO₂ layer on sapphire, indicating excellent crystallinity of the buffer layer. Among the buffer materials used, CeO₂ was found to be the best one for YBCO thin films onR-plane sapphire. HighT _c andJ _c were obtained in YBCO thin films on sapphire with buffer layers. Surface resistances of the YBCO films were 4 m at 77 K and 25 GHz.     

The effect of parallel electric fields on the orientation of polycrystalline zinc oxide thin films produced by reactive evaporation and oxidation of zinc

The effect of an electric field (detached electrodes) on the crystal orientation of zinc oxide thin film was investigated. Two methods, direct and indirect, were utilized to produce zinc oxide thin films. In the direct method (reactive evaporation) oxygen was introduced into the zinc vapour stream and ZnO films were deposited on cold substrates in a vacuum system. Parallel electric fields were applied during the reaction process. It was observed that the electric field had no effect on the preferred c-axis orientation of the ZnO crystals. There was no significant difference in the crystal size or surface texture of ZnO samples subjected to electric fields with respect to the sample that experienced no electric field. In the indirect method (oxidation) electric fields were applied during the evaporation of zinc then the samples were oxidized in air in a furnace at 600 C. Here the application of electric field improved the c-axis orientations. The crystal size remained unchanged, but the surface morphology was affected by the application of the electric field. Whenever the c-axis orientation improved, crystals on the surface became rounded and a more ordered microstructure was observed.     

Influence of deposition and annealing parameters on some properties of YBCO films prepared by spray pyrolysis

Superconducting YBCO films were prepared by chemical deposition of aerosol generated from nitrate solutions by pneumatic and ultrasonic atomizers. Single crystalline YSZ and MgO substrates were used. Two different substrate temperatures were applied, with the chemical composition of starting solutions being adjusted accordingly. Deposited precursor films were thermally processed under two basically different conditions, i.e., either in atmospheric O₂ atT900°C, or in atT600°C. The influence of these different deposition and annealing parameters onT _c andJ _c values, microstructure, and surface morphology is reported. By optimizing our technological procedure, a real possibility exists to prepare (by means of aerosol deposition) YBCO films withJ _c values of at least 10⁴–10⁵ A/cm², at processing temperatures (both deposition and annealing as well) not exceeding 600–700°C.     

Characterization of Superconductor Coplanar Resonators Deposited on Different Substrates by Thermal Coevaporation

This study is based on two commercially available YBCO thin films deposited by the thermal coevaporation method on different substrates (MgO and LaAlO₃). Those films should be optimized for microwave applications. The structure and microstructure of the film deposited on LaAlO₃ have been investigated, respectively by XRD and SEM. These characterizations showed the high quality of the films concerning the c-axis orientation and the smooth and homogenous morphology. The films have then been etched into two different coplanar line resonators by ionic method (YBCO/LaAlO₃) and chemical one (YBCO/MgO) and their microwave properties have been characterized in two different cryogenic experimental set-ups. Despite the differences between these coplanar resonators, we have obtained the same intrinsic parameters (λ₀ = 190 nm, T _c=87 K with γ = 3) corresponding to the data provided by THEVA and a very low surface resistance (R _s=0.4 m Ω at 31 K and 10 GHz).     

Thin and thick YBCO films grown by MOCVD processes. Study of the angular magnetic field dependence of current transport properties

Thin and thick YBCO Films have been grown by Aerosol Assisted as well as by thermal MOCVD. The Aerosol Assisted MOCVD technique allows the growth of YBCO films from a single liquid source at deposition rates of up to 10m/h.Transport measurements (I-V) on etched microbridges using a single pulse technique have been performed. The angular magnetic field dependence J_c() of the critical current density from T_c down to 50 K has been measured. Transport properties are reported and discussed with respect to the microstructural features as determined by X-ray diffraction and electron microscopy.     

Angular-Dependent Vortex Pinning Properties of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Quasi-Multilayers

A series of quasi-multilayers of YBa₂Cu₃O_7?δ (YBCO)/Y₂O₃ specifically 70 × (m YBCO/n Y₂O₃) were prepared on SrTiO₃ single crystal using pulsed-laser deposition (PLD) with a controlled deposition pulses of m = 40 and n = 2, 5, and 10 for YBCO and Y₂O₃, respectively. The x-ray diffraction patterns indicate that all the present quasi-multilayers exhibit good c-axis orientation. The angular dependence of critical current density (J _c ) on applied magnetic field directions are systemically measured to study the anisotropic vortex pinning performances for those quasi-multilayers. It is revealed that compared with the pure YBCO films, the quasi-multilayers with n = 2, i.e., a proper constituent pulse of Y₂O₃, exhibits the enhanced vortex pinning abilities in all angles between c-axis orientation and the applied magnetic field direction. As well, such a quasi-multilayer film (n = 2) shows the higher lift factor J _c (Θ)/ J _c (90°) and much better vortex pinning properties at high fields and high temperatures, showing promising potential for coated conductor application.     

Localization of carriers of Y-doped CaCuO2 films prepared by an organometallic-chemical-vapour-deposition method

Y-doped CaCuO₂ films have been formed on SrTiO₃(1 0 0) substrates in the range 730–790 °C. The lattice constants of c=0.3180 0.3183 nm were estimated from the X-ray diffraction (XRD) of c-axis-oriented Y-doped CaCuO₂ films. The values of the c-axis were shorter than those of the undoped CaCuO₂ film. Although the resistivity of a Y-doped CaCuO₂ film is significantly lower in comparison with that of an undoped CaCuO₂ film, no superconductivity was found in these Y-doped CaCu₂ films at temperatures higher than 12 K.     

Effects of Sm-doping on Structures and Properties of YBCO Coated Conductors Fabricated by TFA-MOD Process

The Sm-doping YBCO films, Y_1−x Sm _x Ba₂Cu₃O _y and Y₁Sm _x Ba_2−x Cu₃O _y (x=0.1, 0.3), were fabricated on LAO (00l) single-crystal substrates by TFA-MOD process in order to improve the superconducting properties of YBCO films. The XRD analyses indicated that these samples have a strong c-axis texture, but the secondary phase of BaCuO₂ are found from Y₁Sm _x Ba_2−x Cu₃O _y films with Sm³⁺ substitution on Ba²⁺ sites. High quality surface, better in-plane and out-of-plane texture of the Sm-doping films have been obtained compared with that of the undoped YBCO film. The Y_0.9Sm_0.1Ba₂Cu₃O _y films showed the highest value of T _c and J _c in self-field. Superconducting properties of all Sm-doping films are better than those of the pure film. These results strongly suggest that Sm-substitution on Y or Ba sites of YBCO phase is a powerful method to efficiently improve J _c and introduce artificial pinning centers in YBCO films.