Ken'ichi Kuroda

Niobium-stress influence on Nb/Al-oxide/Nb Josephson junctions

Niobium‐stress influence on Nb/Al‐oxide/Nb trilayer Josephson‐junction characteristics is studied. Different junction fabrication techniques are examined such as conventional etching and new lift‐off processes. Nb‐film stress, relaxed during trilayer etching, can deteriorate junction quality in etching‐processed junctions smaller than a few micrometers square. Such deterioration does not occur in lift‐off processed junctions even when they are 1 μm square because stress relaxation occurs during trilayer deposition through lift‐off stencils. This new all‐Nb Josephson‐junction fabrication technique produces high‐quality junctions with Rsg/Rnn =24 (Rsg and Rnn are tunnel resistances at 2 and 4 mV, respectively) and a current uniformity of 1.5% for 100 1‐μm‐square junctions connected in series.

Preparation of YBa2Cu3O7-y High Tc Thin Films on NdGaO3 Substrate by Laser Ablation

As-grown thin films of YBa2Cu3O7-y with a smooth surface were obtained on lattice-matched NdGaO3 substrates by ArF laser ablation deposition. The as-grown thin film has a zero-resistance temperature (Tc) at 90 K. The full width at half-maximum (FWHM) of the X-ray diffraction peak (005) of the thin film was as narrow as 0.12 degrees. These results suggest that the as-grown YBa2Cu3O7-y films on NdGaO3 substrates were high-quality single crystals. NdGaO3 substrates are thought to be a promising substrate for epitaxial growth of YBa2Cu3O7-y high Tc thin films.

(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.

Preparation of new high-Tc superconducting oxide Bi-Sr-Ca-Cu-O thin film by electron beam deposition technique

Thin films of Bi-Sr-Ca-Cu-O superconducting oxide were prepared by electron beam deposition of Bi, SrF2, CaF2 and Cu on (100) MgO substrates followed by annealing in oxygen gas flow. Onset transition temperature is 110 K, and resistance drops to less than 5% of the value in the temperature range Tc+4 K, although zero resistance is observed at 60 K. Between 110 K and 60 K, there exist at least 2 resistance drops, indicating multiphase superconductors with different Tcs. X-ray diffraction analysis reveals that the films are highly oriented so that the c-axis is normal to the substrate and has a length of 30.605 A.

Small Nb/Al-Oxide/Nb Josephson Junction Fabrication Using Lift-Off Processes

A lift-off technique is applied to the fabrication of all-refractory Nb/Al-oxide/Nb Josephson tunneling junctions. It is found that the lift-off is very efficient for lowering the influence of stresses in refractory metal electrode on junction characteristics. Small junctions (1 µm) with low leakage current can be fabricated by lift-off technique, whereas an etching process increases leakage currents of small junctions. Current-voltage characteristics of Vm=25 mV at a current density of 10 kA/cm2 and a junction current standard deviation of 3.2% in 1 µm size junctions are obtained.

In-plane orientation and coincidence site lattice relation of (001) Bi2Sr2CaCu2Ox formed on (001) MgO

Abstract A (001) Bi 2 Sr 2 CaCu 2 O x thin film fabricated on a (001) MgO substrate has been found to consist of two kinds of domain with in-plane orientation of Bi 2 Sr 2 CaCu 2 O x [100]∥MgO [510] and Bi 2 Sr 2 CaCu 2 O x [100] ∥MgO [5 1 0]. It is shown that near coincidence site lattice theory supports the occurence of this in-plane orientation. The dependence of the in-plane orientation on the substrate temperature was also investigated.

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.

(01n)-Oriented BiSrCaCuO Thin Films Formed on CeO2 Buffer Layers

With the use of RF magnetron sputtering, (01n)-oriented BiSrCaCuO superconducting films were first formed using MgO(110) substrates with CeO2(110) buffer layers. The film obtained using the just MgO(110) substrate had (01n)- and (0n)-preferred orientations with the c-axis tilted 45° against the surface of the CeO2 buffer layer. Moreover, with the use of an off-oriented MgO(110) substrate, a uniquely oriented BiSrCaCuO film with the c-axis not perpendicular to the substrate surface was obtained without twin boundaries.

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.

BiSrCaCuO Thin Film Grown on SrTiO3 Substrate with Off-Oriented (110) Surface

A Bi2(Sr, Ca)3Cu2Ox thin film with the (117) orientation was formed on a slightly off-oriented (110) SrTiO3 substrate by single-target sputtering. The (110) substrate off-oriented by 5° is considered to have limited film growth in one direction. From RHEED and cross-sectional TEM observations, it was found that the c-axis of the film tilts against the substrate surface by 46-47° and the SrTiO3(110) plane by 41-42°. Also, the surface morphology observed by SEM has been much improved.