YBCO/PB TUNNEL-JUNCTIONS - REPRODUCIBILITY, CYCLABILITY AND ROLE OF THE OXYGEN-CONTENT AT THE YBCO SURFACE

We have realized planar tunnel junctions on liquid etched YBa2Cu3O7-delta (YBCO) single crystals with a native barrier and a Pb film as counterelectrode. Reproducibility of the tunneling characteristics has been tested on more than 150 junctions. Cyclability was good for 6 thermal cycles over the course of 6 months. The main features on the dV/dI vs V curves were gap-like structures at +/- 4-5 mV and +/-19 mV and well defined minima at +/- 36mV. The amplitudes of these structures show little change when the YBCO single crystals were exposed 24 hours to room atmosphere prior to the Pb evaporation. By heating the samples for 1h at 400-degrees-C in vacuum before the counterelectrode was applied to complete the junction, the structures broaden and shift towards higher energies. For all junctions, sub-gap leakage currents were less than 5% at 4.2K and the Pb gap and phonon structures were clearly shown on the tunneling characteristics indicating a good quality tunnel barrier.