Fabrication of Ramp-Edge Junctions With High $I_{\rm c}R_{\rm n}$ Products by Using Cu-Poor Precursor

We have fabricated ramp-edge Josephson junctions using a Cu-poor oxide layer as a precursor of the barrier. La_0.2 - Y_0.9Ba_1.9Cu₃O_x(La-YBCO) and La_0.2 - Yb_0.9Ba_1.9Cu₃O_x (La-YbBCO) were used for the base-electrode and the counter-electrode, respectively. A Cu-poor precursor was deposited on a pattered base-electrode at a substrate temperature (T_s) of approximately 660 degC by a pulsed laser deposition (PLD) method employing deposition conditions different from those for the counter-electrode layer. The fabricated junctions on a La-YBCO ground plane showed resistively and capacitively shunted junction-type current-voltage characteristics. They exhibited the products of the critical current (I_c) and the junction resistance (R_n) higher than 3 mV and excess current ratio less than 30% at 4.2 K. The I_cR_n products were nearly 1.5 times larger than those for junctions with an interface-modified barrier in a temperature range of 4-50 K. The junctions had a barrier region with the thickness of approximately 1 nm and a Cu-poor transition region narrower than that for the latter type of junctions.