STEP-EDGE JUNCTION OF YBCO THIN FILMS ON MgO SUBSTRATES

We have studied the microstructure of YBCO thin films epitaxially grown on MgO(100) and SrTiO3(100) substrates with step-edges using transmission electron microscopy (TEM). The step-edge junction is one of the simple artificial grain boundary junctions. The YBCO thin films were grown on the substrates with step-edges by means of laser deposition. Samples with steps of 200 and 300nm were prepared for the TEM cross section observation. The direction of a-b planes of YBCO thin films at the slope of the step were not parallel to the {100} plane of the MgO substrates but parallel to the ion-milled local surface of the MgO. Some grain boundaries were observed. While, the a-b planes of YBCO on the step-edge SrTiO3 substrate were parallel to the {100} plane of the SrTiO3 and were tilted at a right angle at the edges. The microstructure of the YBCO on SrTiO3 was different from that on MgO. It seems that this difference is due to the difference in the lattice matching. The DC SQUIDs with step edge junctions were fabricated and showed good modulation at 77K.