Masami Tanioku

(11n)-oriented BiSrCaCuO thin film formed on SrTiO3(110) substrate by RF magnetron sputtering

A superconducting BiSrCaCuO thin film was grown on a SrTiO3(110) substrate at 660°C by sputtering a single target. The RHEED observation showed that the c-axis of the film meets at an angle of about 42° with the substrate surface. The XRD measurement showed that the film has a main peak at 2 θ=31.2°. From these results, the film is considered to chiefly consist of a 2212 phase with (117) orientation and/or a 2212/2223 superlattice phase with (118) orientation. The film surface has a stripelike texture which probably corresponds to the film orientation. The zero-resistivity temperature of the film was lower by 20 K than that of the film formed on the SrTiO3(100) substrate.

Surface morphology and crystal structures of as-grown BiSrCaCuO thin films prepared by RF magnetron sputtering from three targets

As-grown superconducting BiSrCaCuO thin films were grown at 650°C on MgO(100) substrates by RF magnetron sputtering from three targets. The substrate was rotated over the three targets: Bi3Sr2Ca2Cu3Ox, Bi2Sr3Ca3Cu3Ox and Bi2Sr2Ca2Cu4.5Ox. By adjusting the staying time of the substrate over each target, it was possible to precisely control the chemical composition of the thin film. This paper describes the superconducting properties, surface morphology and crystal structures of sputtered films formed by varying the staying time. X-ray diffraction measurement showed that the crystal structure could be changed by varying the staying time. The film with the optimum composition showed zero resistivity at 75 K and had an exceedingly smooth surface. This film had the mixed phase which is considered to be a stacked structure consisting of a high-Tc phase and a low-Tc phase.

As-Grown Superconductivity of BiSrCaCuO Thin Films Prepared by Magnetron Sputtering with Three Targets: Bi2+a(SrCa)2Cu3Ox, Bi2(SrCa)2+bCu3Ox and Bi2(SrCa)2Cu3+cOx (a=b=1, c=1.5)

Superconducting BiSrCaCuO thin films were grown on MgO(100) substrates by magnetron sputtering with three targets. In order to precisely control the chemical composition of the metal elements, these three targets were simultaneously discharged; the heated (650°C) substrate was rotated above these targets. The superconducting transition temperature (zero resistivity) was 75 K in the as-grown state for the optimized sample. From the X-ray diffraction, The c-axis of the film was highly oriented normal to the substrate. The surface of the film was much smoother than that of the film grown with a single target.

Superstructure in Thin Films of Bi-Based Compounds on MgO

In situ epitaxial growth of BiSrCaCuO films by three target sputtering was performed on MgO substrates at a temperature of 650°C. From high resolution transmission electron microscopy, the film showed a superstructure consisting of the alternate stacking of the low-Tc and high-Tc phases with a c-axis of 34 A. The film in the [110] or [1 0] directions was parallel to MgO, and the transient layers between the substrate and the film were less than a few atomic layers thick. X-ray diffraction simulations indicated that the films contained random layer ordering of 34 A bi-layers (low-Tc+high-Tc and high-Tc+low-Tc) and random layer ordering of low-Tc and high-Tc phase layers.

Superconductivity of Bi system thin films formed by consecutively sputtering three Pb-doped targets of slightly different compositions

Superconducting Bi system thin films have been formed on MgO(100) substrates by RF magnetron sputtering from three lightly Pb-doped targets: Bi2.4Pb0.6Sr2Ca2Cu3Ox, Bi1.6Pb0.4Sr3Ca3Cu3Ox and Bi1.6Pb0.4Sr2Ca2Cu4.5Ox. To improve the superconductivity of the as-grown films, the films were kept in a sputtering chamber after deposition. The film, formed at 660°C and kept at the same substrate temperature and gas pressure as the sputtering conditions for 5 h after deposition, showed zero resistivity at 83 K and a resistivity drop at 115 K. The critical current density of the film was 4×105 A/cm2 at 77 K and 1×107 A/cm2 at 52 K.

Formation of As-grown BiSrCaCuO thin films by sputtering three targets heavily doped with Pb in atmospheres of O2 and N2O/O2 mixtures

As-grown BiSrCaCuO thin films have been formed on MgO(100) substrates at various temperatures by RF magnetron sputtering from three targets in atmospheres of neat O2 and N2O/O2 mixtures (1:1). The three targets had slightly different compositions heavily doped with Pb: Bi3Pb3Sr2Ca2Cu3Ox, Bi2Pb2Sr3Ca3Cu3Ox and Bi2Pb2Sr2Ca2Cu4.5Ox. Both films formed in O2 and N2O/O2 mixtures at 660°C showed zero resistivity at 77–78 K. A film obtained at 540°C by N2O/O2 mixtures showed superconductivity, while a film obtained at the same substrate temperature by neat O2 did not. Gas mixtures of N2O and O2 used instead of neat O2 as a sputtering gas were effective in lowering the growth temperature by 20–40 degrees.

Formation of As-Grown YBaCuO Thin Films by RF Magnetron Sputtering from Three Targets

As-grown YBaCuO thin films with 500 A thickness have been formed at 620°C on MgO(100) substrates by RF magnetron sputtering from three targets: Y1.5Ba2Cu3Ox, Y1Ba3Cu3Ox and Y1Ba2Cu4.5Ox. The chemical composition can be precisely controlled by this method. A smooth surface was obtained by adjusting the chemical composition. Zero resistivity was observed at 81 K for the best superconducting film. The films with a slightly Cu excessive composition showed somewhat higher Tc(zero) than the stoichiometric film. X-ray diffraction spectra of sputtered films did not change distinguishably upon varying of the chemical composition when the value of Ba/Y ranged from 1.5 to 2.3 and the value of Cu/Y ranged from 1.8 to 3.3. The c-axis of the film was highly oriented normal to the substrate surface, and the c-axis lattice constant was equal to that of a bulk sample.

Preparation of Ca-Doped YBaCuO Thin Films Containing 248 Phase by RF Magnetron Sputtering

Ca-doped YBaCuO thin films containing predominantly a 248 phase were prepared on SrTiO3(100) substrates by sputtering three Ca-doped targets at 500°C and annealing at 750°C. The as-deposited films formed at above 500°C predominantly had a 123 phase, and those at 300°C were amorphous. On the other hand, the as-deposited films obtained at 400–500°C had an unidentified major phase with a lattice spacing of 7 A, which changed into a 248 phase by annealing, whereas the phase of amorphous films and the 123 phase did not.

Formation of BiSrCaCuO Thin Films with (11n) Orientation on SrTiO3 (110) Substrates by RF Magnetron Sputtering

As-grown BiSrCaCuO thin films were grown on SrTiO3(110) substrates at 660 °C by consecutively sputtering. XRD measurement showed that the films with a low-T c phase composition and a high-T c phase composition had main peaks at 2Ө of 31.0° and 31.9°, respectively. The film with a low-T c phase composition is considered to mainly consist of a low-T c phase with (117) orientation.

As-grown Bi(Pb)SrCaCuO thin films formed by RF sputtering in atmosphere of N2O/O2 mixtures

As-grown Bi(Pb)SrCaCuO thin films have been formed on MgO(100) substrates at various temperatures by RF magnetron sputtering from three targets in atmospheres of O 2 and N 2 O/O 2 mixtures. The compositions of three targets were Bi 3 Pb 3 Sr 2 Ca 2 Cu 3 O x , Bi 2 Pb 2 Sr 3 Ca 3 Cu 3 O x and Bi 2 Pb 2 Sr 2 Ca 2 Cu 4.5 O x . Both films formed at 660°C in O 2 and N 2 O/O 2 mixtures showed zero resistivity at 77-78K. A film obtained at 540°C in N 2 O/O 2 mixtures showed superconductivity, while a film obtained at the same substrate temperature in neat O 2 did not. Gas mixtures of N 2 O and O 2 used instead of neat O 2 as a sputtering gas were effective in lowering the growth temperature for superconductivity by 20-40 degrees.