Artificial barriers for Ba-K-Bi-O tunnel junctions

The authors have grown epitaxial bilayers and trilayers which utilize Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) base electrodes, native or artificial (MgO or SrTiO/sub 3/) insulating barriers, and Ag or BKBO counterelectrodes, respectively. The layers were deposited in situ by RF magnetron sputtering and characterized in situ. Electron diffraction showed the layers to be epitaxial with the symmetry of the desired structure maintained even in the first monolayer at the surface. X-ray photoelectron spectroscopy (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 approximately 25 mV to be present. In the range of thicknesses used for artificial barriers, 3-6 nm, the barriers apparently contained pin holes which caused the junctions to short. The BKBO films were K-rich with x approximately=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.<>