Abstract
Many applications of Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junctions (JJs) in superconducting electronics require high-quality tunnel barriers with low subgap leakage that is usually characterized by figure of merit V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> = 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">sg</sub> , where I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is the critical current and R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sg</sub> is the subgap resistance at 2 mV and 4.2 K. It is widely believed, and there is considerable literature suggesting, that quality and reproducibility of JJs critically depend on the intrinsic stress in Nb/Al-AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb trilayers, and the stress therefore should be carefully minimized and controlled. Contrary to this belief, we show that JJ quality V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and reproducibility do not depend on the stress in the trilayer, at least in the studied range from -300 to 300 MPa. In this range, V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> depends neither on the stress in a Nb/Al base electrode nor in a Nb counter electrode. We have found, however, that V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> crucially depends on the way the tunnel barrier formation by thermal oxidation of Al is done. For instance, room-temperature dynamic oxidation (in O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> flow at low pressures) in a cryopumped chamber leads to poor run-to-run reproducibility of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> and reduced V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> values, whereas dynamic oxidation at the same parameters but in a chamber with a turbomolecular pump results in high V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> values and excellent run-to-run reproducibility.
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