Shugo Kubo

Synthesis of Y-Ba-Cu-O Thin Films on Sapphire Substrates by RF Magnetron Sputtering

Y-Ba-Cu-O thin films have been prepared on sapphire substrates by RF magnetron sputtering from sintered targets. The substrate temperature dependence of Ba and Cu concentrations against Y of as-sputtered films has been clarified. Film composition deviated from target composition. Cu concentration decreased significantly with increasing Ts, while Ba concentration was independent of the substrate temperature. Using the CuO-compensated Y1Ba6Cu10Ox target, assputtered high-Tc films (Tc onset=87 K, Tc endpoint=40 K) have been successfully obtained as a Ts of about 580°C.

Magnetic penetration depths in superconducting NbN films prepared by reactive dc magnetron sputtering

Penetration depth λ behavior in reactively sputtered NbN films has been examined. For NbN films deposited at 300u2009°C or higher by means of dc magnetron technique, λ values around 300 nm were obtained, very short compared with the previously reported values (500–700 nm). The measured λ dependences on deposition parameters are discussed in connection with film structure on the basis of the Ginzburg–Landau–Abrikosov–Gor’kov theory of type II superconductivity.

High-Tc Y-Ba-Cu-O Thin Films Prepared by Dual Magnetron Sputtering

Y-Ba-Cu-O thin films (200-300 nm thick) have been magnetron-sputtered on heated sapphire substrates. By means of dual sputtering configuration with a Cu and sintered Y-Ba-Cu-O target, films having compositions closer to Y1Ba2Cu3Oy are reproducibly obtained at a substrate temperature of about 600°C. They have a nearly single-phase perovskite-like structure with highly c-axis preferred orientation and show a Tc onset of 90 K. Remarkable improvement in the zero resistivity temperature (up to 73 K) is achieved by subsequent annealing of these films at 550-650°C, while annealing at higher temperatures leads to Tc degradation.

Examination of the Properties of Superconducting Nb–Ge Films Prepared by DC Magnetron Sputtering

The superconductivity and film structure of A15 Nb–Ge were extensively examined through film preparation utilizing the characteristic features of dc magnetron sputtering. Tco 22.9 K and ΔTc 0.5 K films of 2400 A thickness were obtained. Films of Tco>22 K and ΔTc<1 K could be prepared with good reproducibility. As a result of Auger analysis, it was concluded that impurities, especially oxygen, were unnecessary for metastable high-Tc Nb3Ge phase growth. Impurity oxygen was concentrated into a depositing film from surface impurity-rich layer and tended to make a hexagonal Nb5Ge3 phase formed. In order to control this and to attain high-Tc, film fabrication with a high deposition rate over short sputtering time was effective. A high-Tc Nb3Ge phase grew on a base layer 1000–1500 A thick. The layer was found to contain no impurities and rather to be rich in anti-site defects.

Fabrication and I-V Characteristics of High-Tc Nb3Ge Microbridges

High-Tc Nb3Ge microbridges are fabricated by electron-beam lithography combined with CF4 reactive sputter etching. The bridges are 0.48 µm wide and 0.36–0.6 µm long. The temperature dependence of Ic is found to be proportional to (1-t)5/2 at t>0.8 (t=T/Tc). This result means that the bridge region behaves like an inhomogeneous type-II superconductor film. 9.75 GHz microwave induced steps are observed throughout a wide temperature range. The steps are observed above 20 K in some bridges.

Electrical Resistivity of Internally Oxidized Cu–Al Alloys

The electrical resistivities at room temperature have been investigated precisely after partial and complete internal oxidation. The internal oxidation rates obtained through electrical resistivity measurements are always smaller than the real rates which are obtained metallo-graphically. This suggests that the resistivities of subscales after partial internal oxidation are greater than those after complete internal oxidation, and that it is wrong to estimate internal oxidation rates through resistivity measurements. After complete internal oxidation, deviations from Matthiessens rule (DMR) are observed in the internally oxidized Cu–Al alloys. The DMRs at 20°C have the same magnitudes as those of the residual resistivities and are nearly proportional to the volume fraction of dispersed Al2O3 particles. The DMR at 20°C in the Cu–1.0 vol%Al2O3, alloy is 0.037 µΩ-cm. The origin of the DMR is attributed to dispersed oxide particles.

Critical Currents of (103)- and (001)-Oriented Y-Ba-Cu-O Thin Films

Critical currents and flux pinning were examined in (103)- and (001)-oriented YBa2Cu3O7−x thin films at liquid nitrogen temperature (77.3K) under magnetic fields applied perpendicular to the film surface. Measurements were made in the [010] and [301] transport current directions for the (103)-oriented films. The field-dependent terms of the pinning force density F p were found to be described by the single Kramer scaling law irrespective of crystallographic orientation and transport current direction: F p/F p max ∝ b 1/2(1 − b)2, where F p max is the maximum pinning force density and b is the reduced field defined as b ≡ B/B i (B i: irreversibility field).

Flux Pinning Behavior in Amorphous MoSi Thin Films

Pinning force densities Fps for amorphous Mo0.75Si0.25 films (200~20 nm thick) for the use of memory devices utilizing Abrikosov vortices have been examined in the perpendicular and parallel fields. The maximum Fpmaxs in the perpendicular field decreased with decreasing film thickness. On the other hand, the Fpmaxx dependence on film thickness in the parallel field was found to have a peak at a thickness around 50 nm. Furthermore, Fp values for the thinner films in the parallel field became one order of magnitude larger than those in the perpendicular field. Pinning in these films, including the effect of a Si underlayer and overlayer for protection, was discussed.