Selective plasma oxidation of oxygen-deficient YBa₂Cu ₃O_7-x

A selective plasma oxidation method was developed to make superconducting patterns in nonsuperconducting deoxygenated YBa₂Cu₃O _7-x (YBCO) films. Superconductor- nonsuperconductor-superconductor junctions were fabricated in oxygen-deficient c-axis-oriented polycrystalline YBCO films by selective plasma oxidation using Au/BaF₂ oxidation masks. These devices displayed SNS-like or SIN-like nonlinear current-voltage relationships, depending on the device size and oxidation time. The oxygen penetration depth beneath the oxidation mask was estimated to be about 2 μm for the polycrystalline films. Oxygen supplied to 300-nm-thick films for 5 min changes its phase, determined from x-ray diffraction patterns, from tetragonal to orthorhombic I having a critical temperature of about 85 K. X-ray diffraction and magnetic susceptibility measurement showed that the plasma oxidation can supply oxygen to a deoxygenated 0.7×0.5×0.05 mm YBCO single crystal, changing the entire crystal from tetragonal to the orthorhombic II phase whose critical temperature is about 50 K. Four-probe resistance measurement of the plasma oxidized single crystal showed that the phase of the surface of the sample became orthorhombic I with the critical temperature of 84 K by plasma oxidation.