Fabrication of Ramp-Edge Junctions With High <i>I_CR_N</i> Products by Using Cu-Poor Precursor

Abstract

We have fabricated ramp-edge Josephson junctions using a Cu-poor oxide layer as a precursor of the barrier. La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> – Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (La-YBCO) and La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub> – Yb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.9</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&#x01d4cd;</sub> (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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) of approximately 660°C 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) and the junction resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> ) higher than 3 mV and excess current ratio less than 30% at 4.2 K. The I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> 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.