Yoshihiro Ishimaru

Leveling Technique for High Tc Superconducting Films and Circuits

We examined a leveling technique for high Tc superconducting (HTS) films and circuits. YBa2Cu3O7-x (YBCO) film with a smooth surface and a high critical temperature was fabricated by using leaving techniques. The method is a polishing technique using polymer fixing large particles. The 100-nm-height particles of a 200-nm-thick film were reduced to 3-nm roughness with a little over-polishing. Through this process, the Tc of 85 K did not decrease. The polishing technique is also useful for removing the side wall produced in the milling process and digging out an electrode from the insulating layer. By controlling the polishing pressure, the polishing stopped at the flat surface automatically.

Edge-Type Junction with Ito Barrier

We fabricated edge-type Josephson junctions with 17 nm thick indium-tin-oxide (ITO) (1wt% Sn) barriers. We examined 8 of these junctions with widths from 6 μm to 10 μm at temperatures from 20 K to 80 K. All junctions showed resistively shunted junction (RSJ) type I-V characteristics from 20 K to 77 K. At 20 K, the average values of the critical current density Jc and of the critical current normal resistance products (IcRn) were 2.7x104A/cm2and 1.7 mV, respectively. The dispersion values of Ic and IcRn were σ I= 33% and σ IcRn=21%, respectively. Ic was suppressed well by a magnetic field.

Fabrication of smooth surface on liquid phase epitaxially grown Y1Ba2Cu3O7-δ Films by mechanical polishing

We successfully created a smooth surface on liquid phase epitaxially (LPE) grown YiBa2Cu3O7-δ (YBCO) films by employing a mechanical polishing process. We used an alumina powder with an average grain diameter of 0.03µm which was dispersed in oil for mechanical polishing, which made it possible to avoid a degradation of the Tc value for YBCO. While the as-grown surface of LPE-YBCO films contains various pyramidal structures with heights of about 400nm, the polished surface has only a roughness of 2 to 3nm as measured by a step profiler. A χmin value of 2.6% according to Rutherford backscattering spectrometry suggests that the damage after mechanical polishing is negligible. After annealing in O2 gas at 1 atm, the polished YBCO films transformed from tetragonal to orthorhombic phase and had a Tc value of about 91K.