Tunnelling Measurements on Thin Films of YBa sub 2 Cu sub 3 O sub (7-delta)

We have fabricated tunnel junctions of the type YBa sub 2 Cu sub 3 O sub (7-delta) /barrier/Pb that show reproducible dV/dI characteristics. The YBa sub 2 Cu sub 3 O sub (7-delta) films were chemically etched and then exposed to the ambient in order to form a natural tunnel barrier. Below the T sub c of the YBa sub 2 Cu sub 3 O sub (7-delta) films a structure associated with superconductivity appears in the conductance as a function of voltage. We discuss differences and similarities of these features and those obtained on tunnel junctions on chemically etched YBa sub 2 Cu sub 3 O sub (7-delta) single crystals.