Solution-growth of ultra-thin, insulating layers of zirconia for passivation and tunnel junction fabrication on YBCO thin films

Planar tunnel junctions fabricated with multilayers of solution-deposited zirconia as the insulating tunnel barrier on [001]- and [103]-oriented Y₂Ba₂Cu₃O_7-δ (YBCO) thin films display tunneling spectra with features as sharp, or sharper, than any planar tunneling spectra reported for YBCO to date. The multilayers of zirconia are deposited onto the thin film surfaces by repeated reaction with tetra-n-propyl orthozirconate in hydrocarbon solution followed by reaction with water in n-propanol solution at ambient temperature. Gap-like peaks are observed for all thin-film orientations, and well-defined zero-bias conductance dips and zero-bias peaks are observed for [001]- and [103]-YBCO surfaces, respectively. Junctions exhibiting the sharpest features do not show field-induced nor spontaneous splittings, consistent with what is predicted for a narrow tunneling cone. The zirconia multilayers appear to stabilize YBCO surfaces with respect to atmospheric degradation, and tunnel junctions fabricated from aged samples of zirconia on YBCO that had been stored for one year at ambient temperature display conductance characteristics comparable to those obtained from freshly prepared samples.