Production of YBa2Cu3O7−x superconducting thin films by pulsed pseudospark electron beam evaporation

Thin-film superconducting YBa₂Cu₃O_7–x layers have been produced in a single-step process by pulsed electron beam evaporation from a stoichiometric 1-2-3 target. The films were produced at the 100 surface of SrTiO₃ substrates heated to a temperature of approximately 1000 K in a pure oxygen atmosphere of about 10 Pa total pressure. After deposition the films were cooled in situ within 20 minutes to ambient temperature. At present, the films are polycrystalline and show a T_c,zero of 83 K with a transition width of 3–5 K. Critical current densities of 7·10⁴ A/cm² at 4.2 K and zero magnetic field have been achieved. The pulsed electron beams used in these experiments are produced by a pseudospark discharge; the estimated energy density deposited at the target surface by the electron beam is of the order of 4 J/cm².     

Growth of YBa2Cu3O7−x thin films on sapphire with a cerium-oxide sublayer

The possibility is demonstrated of preparing high-quality films of the high-temperature superconductor YBa₂Cu₃C_7-δ with thicknesses up to 2.6 μm by dc magnetron sputtering. It is found that inclusions consisting of CuO and YBa₂Cu₃O₈ coexist with the growing film and are “sinks” for defects, nonstoichiometric atoms, and mechanical stresses. Using x-ray diffraction and Rutherford backscattering, we find that the structural perfection of the films is improved by increasing the thickness when using the proposed fabrication technique. Zh. Tekh. Fiz. 69, 94–98 (January 1999)     

Preparation and properties of Y1−xHoxBa2Cu3O7−δ thin films by TFA-MOD method

Y_1?xHo_xBa₂Cu₃O_7?δ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) thin films were prepared on LaAlO₃ (0 0 1) substrates by trifluoroacetate metal organic deposition (TFA-MOD) without change of the processing parameters. The highest J_c was attributed to the sample of Y_0.6Ho_0.4Ba₂Cu₃O_7?δ thin film, whose critical current density is about 1.6 times as compared to that of YBa₂Cu₃O_7?δ thin film at 77 K and self field. The flux pinning type was not varied with Ho substitution and can be attributed to δl pinning model, which is attributed to the close ionic radius between the Y³⁺ and Ho³⁺ ions. The improvement of J_c by Ho substitution without change of the processing parameters will provide an effective route to enhance the J_c of YBCO-based thin films using TFA-MOD method.     

Effects of nanoscale defects on critical current density of (Y1−xEux)Ba2Cu3O7−δ thin films

In pulsed laser deposition of YBa₂Cu₃O_7?δ films, defect introduction into the films tends to anisotropically improve the pinning along the H||c direction due to the columnar growth mode of the process. In Eu-substituted samples, however, even though an increase in critical current density (J_c) in the H||c direction was observed for low fields (H = 0.2 T), the improvement was more notable for the H||ab-plane at both low and higher fields. Herein we present detailed TEM microstructural studies to understand these new trends in J_c(H), which are markedly different than flux pinning increases achieved with other methods, for example, with nanoparticle additions. Threading dislocations, observed in the Eu-substituted samples along the c-axis, account for J_c enhancement with H||c at low field. The enhanced ab-planar pinning in the Eu-substituted samples is attributed to the extensive bending of the {0 0 1} lattice planes throughout the film, and the crystal lattice defects with excess Cu–O planes, that were effective in increasing the J_c for H||ab at both low and high fields.     

Growth of non-polar Zn1−x Mg x O thin films with different Mg contents on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

We report the growth and characterization of a series of non-polar Zn_1?x Mg _x O thin films with different Mg contents, which have been prepared on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Structural properties are anisotropic and surfaces of films show stripes running along the c-axis direction. The films exhibit atomically smooth surface with the minimal root mean square surface roughness of 0.36 nm. Non-polar Zn_1?x Mg _x O thin film is much easier to obtain pure a-plane single crystal orientation when Mg content is high. The quality of the non-polar Zn_1?x Mg _x O thin films is evidenced by X-ray diffraction (XRD) rocking curves full-width at half-maximum of 1,350 arcsec for the ( \( 11\overline{2} 0 \) ) reflection and 1,760 arcsec for the ( \( 10\overline{1} 1 \) ) reflection, respectively. Room temperature photoluminescence peak shifts monotonously from 3.29 to 3.56 eV as Mg content increases from 0 to 0.13. Alloying with Mg is found to widen the bandgap energy of the ZnO.     

Preparation of high J c YBa2Cu3O7−δ superconducting thin films by ion beam sputtering deposition

Preparation of high T _c and high J _c YBa₂Cu₃O_7– superconducting thin films by ion beam sputtering deposition is reported. The main factors affecting the composition of the films and the orientation of the crystal grains have been examined. Experimental results show that the Y, Ba and Cu composition of as-deposited films can be conveniently and accurately adjusted by a combined sputtering target which consists of a large sintered target of YBa₂Cu₃O_7– and a small one that is Ba and Cu rich (YBa_2.5Cu_3.3O_x). Fabrication conditions of highly oriented superconducting thin films are described. YBa₂Cu₃O_7– superconducting films with zero resistance at 88–90.5K and critical current density J _c (at 77K) of 1.5×10⁵ A/cm² are obtained.     

Growth stages of YSZ-buffer layers and YBa2Cu3O7−x thin films on silicon substrates studied by scanning probe microscopy

Scanning probe microscopy has been applied to study various growth stages of YSZ (yttria-stabilized zirconia) buffer layers on silicon and of YBa₂Cu₃O_7–x thin films on YSZ/Si. YSZ buffer layers of 75 nm thickness exhibit a remarkable smooth surface with a rms roughness of about 0.5 nm for a surface area of 5 m×5 m. The subsequent growth of YBa₂Cu₃O_7–x thin films was investigated from nucleation to the formation of growth hills. Screw dislocations were found only in very rare cases.     

Thermal stability of YBa2Cu3O7−δ films deposited on Ni-W textured substrates by laser ablation

The degradation processes of YBa₂Cu₃O_6.77(3) films 600 nm thick deposited on substrates from a textured (100) foil of the Ni-W alloy with the YSZ (Y₂O₃ + ZrO₂) (100 nm) and CeO₂ (50 nm) buffer layers at 200°C in air have been investigated. It has been shown that the degradation proceeds in the same manner as in the bulk ceramic materials with the same oxygen content, namely, initially the single-phase material with the orthorhombic structure laminates into the fractions depleted (the main fraction) and enriched with oxygen. After holding for more than 20 h, the oxygen-depleted fraction is first subjected to the structural phase transformation with the transition of the orthorhombic structure into the cubic one, which is associated with the redistribution of barium and yttrium atoms over their sites in the presence of water vapors.     

Influence of deposition rate on the properties of ZrO_2 thin films prepared in electron beam evaporation method

ZrO2 thin films were prepared in electron beam thermal evaporation method. And the deposition rate changed from 1.3 to 6.3 nm/s in our study. X-ray diffractometer and spectrophotometer were employed to characterize the films. X-ray diffraction (XRD) spectra pattern shows that films structure changed from amorphous to polycrystalline with deposition rate increasing. The results indicate that internal stresses of the films are compressive in most case. Thin films deposited in our study are inhomogeneous, and the inhomogeneity is enhanced with the deposition rate increasing.     

Effect of process parameters on the properties of plasma-sprayed superconducting Y1Ba2Cu3O7−x coatings

A study on the effect of powder, spray and coating parameters on superconducting properties of plasma-sprayed Y₁Ba₂Cu₃O_7–x coatings has been carried out. Optimised spray parameters to yield strong, adherent and dense coatings on many substrates including AISI 304 SS and alumina have been obtained. Coatings show superconductivity after oxygen annealing at 950°C for an hour. Coatings thinner than 100 m do not show zero resistivity down to 77 K due to weak links between granular superconductors. Coatings prepared with fine powder have superior superconducting properties, viz. higherT _c andJ _c values than coarse powder coatings. The choice of substrate material strongly affects theJ _c values.     

YBa2Cu3O7−δ /CeO2 heterostructures on sapphire R-plane

(001)YBa₂Cu₃O_7−δ epitaxial films were prepared by laser ablation on surface. A thin (001)CeO₂ or (111)CeO₂ buffer layer was deposited between the substrate and the superconductor film to reduce their chemical interaction. In the initial stages of CeO₂ buffer formation, its orientation depended strongly on the intensity of cerium ion interaction with oxygen. Epitaxial growth of (001)YBa₂Cu₃O_7−δ films was achieved both on and . The T _c temperature of epitaxial (001)YBa₂Cu₃O_7−δ films was within 88–90 K, and the current J _c at 77 K was in excess of 10⁶ A/cm⁻². Fiz. Tverd. Tela (St. Petersburg) 40, 205–208 (February 1998)     

Photoluminescence studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Micro—structural analysis of YBa2Cu3O7—x thin films grown on different substrates by X—ray techniques

YBa2Cu3Ox(YBCO) thin films grown on different substrates with and/or without Eu2CuO4(ECO) buffer layer were investigated by X-ray wide angle diffraction,reflection,diffuse scattering and topography.Theresults show that for the yttria stabilized ZrO2(YSZ) substrate,the presence of an ECO buffer layer improves the crystalline quality of the YBCO film,while a negative effect is observed for the SrTiO3(STO) substrate.The lateral correlation length for a sample grown on a YSZ substrate with ECO buffer Layer is much greater than grown on an STO subetrate.The STO substrate used has mosaic structure.2001 Elsevier Science B.V.All rights reserved.     

Magnetic tri-axial orientation in (Y1−xErx)2Ba4Cu7O15−y superconductors

We report the tri-axial grain-orientation effects under a modulated rotation magnetic field for (Y_1?xEr_x)₂Ba₄Cu₇O_y [(Y, Er)247]. The magnetic easy axis at room temperature was drastically changed around x ～ 0.1; however, the Er-doping levels for the conversion of magnetic easy axes from the c-axis to the ab-direction and from the a- to b-axes were quite different. Tri-axial single-ion magnetic anisotropy of Er³⁺ was roughly 10 times greater than tri-axial magnetic anisotropy generated by both the superconducting CuO₂ plane and the blocking Cu–O chain layer. An appropriate choice of rare-earth (RE) ions in RE-based cuprate superconductors enables the reduction of the required magnetic field for the production of bulks and thick films based on the magnetic orientation technique.     

The microregion compositional variation in Y1Ba2Cu3O7−x material

We have investigated the microstructure and compositional distribution of the Y₁Ba₂Cu₃O_7−x superconductor with scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. It has been found that the microregion compositions are far from the ideal composition. This is especially due to the yttrium content which is higher than the expected amount in the interfacial region between crystalline grains.     

The hole concentration on oxygen sites in the highT c superconductor Y1−Ba2−Cu3−O7−x

From XPS core level spectroscopy the average copper charge on the Cu sites in the high temperature superconductor Y₁Ba₂Cu₃O_7–x is determined as function of the oxygen vacancy concentrationx. Analysis of these data leads to the suggestion that there are holes on the oxygen sites in the basal plane of the crystal structure. The probability for holes on these oxygen ions is rather constant for 0x0.3 with a value of 0.64 and decreases to zero forx=0.5. The dependence of the superconducting transition temperature on the hole concentration is discussed. An energy level diagram for Cu²⁺ and Cu³⁺ in YBa₂Cu₃O_7–x is constructed.     

Structural transition of secondary phase oxide nanorods in epitaxial YBa2Cu3O7−δ films on vicinal substrates

A theoretical study of a structural transition of secondary phase oxide nanorods in epitaxial YBa₂Cu₃O_7?δ films on vicinal SrTiO₃ substrates is presented. Two possible types of film/substrate interface are considered, with one assuming complete coherence, while the other is defective as manifested by the presence of antiphase grain boundaries. Only in the former case does the increase of the vicinal angle of the substrate lead to a substantial change of the strain field in the film, resulting in a transition of the nanorod orientation from the normal to the in-plane direction of the film. The calculated threshold vicinal angle for the onset of the transition and lattice deformation of the YBa₂Cu₃O_7?δ film due to the inclusion of the nanorods is in very good agreement with experimental observations. This result sheds lights on the understanding of the role of the film/substrate lattice mismatch in controlling self-assembly of dopant nanostructures in matrix films.     

Study of the effect of the substitution of O by F,S, and Cl on the superconductivity of Y1Ba2Cu3O7−δ

We have studied the superconducting transitions of Y₁Ba₂Cu₃D _x O_7–x– samples where O has been replaced by D=F, S, or Cl andx=0.2-2. No single phase compounds were obtained forx>0.2. No dramatic increase inT _c was registered. Generally the results can be interpreted as a dilution of the highT _c 1 2 3 phase by other phases as the dopant concentration increases, ultimately suppressingT _c completely. Possibly there is an increase ofT _c for an intermediate dopant range,x, in the Cl series.