ARTIFICIAL BARRIERS FOR Ba-K-Bi-O TUNNEL JUNCTIONS

We have grown epitaxial bilayers and trilayers which utilize Ba1-xKxBiO3 (BKBO) base electrodes, native or artificial (MgO or SrTiO3) insulating barriers, and Ag or BKBO counter electrodes, respectively. The layers were deposited in-situ by rf magnetron sputtering and characterized in-situ with RHEED, LEED, and XPS. Electron diffraction showed the layers to be epitaxial with the symmetry of the desired structure maintained even in the first monolayer at the surface. XPS showed no evidence of a chemical reaction at the BKBO/barrier interfaces. Tunneling measurements through a native insulating layer grown by exposing BKBO surfaces to air showed a gap of similar to 2.5 mV to be present. In the range of thicknesses used for artificial barriers, 3-6 nm, the bathers apparently contained pinholes which caused the junctions to short. The BKBO films were K-rich with x approximate to 0.5. Critical temperatures and lattice constants were consistent with reports for bulk superconductors with this composition. Normal-state resistivities of 70-100 mu Omega-cm at 30 K were the lowest ever reported in this system and significantly lower than for the highest-T-C composition of x = 0.4. Rocking curve widths of 0.7 degrees both in the growth direction and in the plane of the film were observed by x-ray diffraction for BKBO films on MgO and SrTiO3.