Fabrication of all-epitaxial high-Tc SIS tunnel junctions requires an atomically flat superconducting thin film to be grown and a proper insulating material to be selected. First, we study the initial growth mode of YBCO thin films and show that reducing the growth rate results in a very smooth surface. Second, perovskite-related compound oxides, PrGaO3 and NdGaO3, which have a small lattice mismatch with YBCO and good wetability, are shown to be promising insulating materials for all-epitaxial SIS tunnel junctions. We believe that these concepts will be useful in the development of all-epitaxial high-Tc SIS tunnel junctions with good electrical properties.

We fabricated junctions with a porphyrin polyimide (PORPI) monolayer, and then investigated the electron transport properties of the junctions from the current-voltage (I-V) and d2V/dI2-V measurements. Polyimide LB films without porphyrin were used as tunneling barriers. One large peak was seen at a voltage around 1.9 V, due to the excitation of electron transitions in PORPI molecules, whereas a step structure was not observed in the I-V characteristic.