Subgap Leakage in Nb/Al-AlO_x/Nb Josephson Junctions and Run-to-Run Reproducibility: Effects of Oxidation Chamber and Film Stress

Sergey K. Tolpygo, D.J.C. Amparo, R.T. Hunt, J.A. Vivalda, and D.T. Yohannes

Abstract - Many applications of Nb/Al-AlO_x/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_m=I_cR_sg, where I_c is the critical current and R_sg 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 depends critically on the intrinsic stress in Nb/Al-AlO_x/Nb trilayers, and the stress therefore should be carefully minimized and controlled. Contrary to this belief, we show that JJ quality (V_m) and reproducibility do not depend on the stress in the trilayer, at least in the studied range from -300 MPa to 300 MPa. In this range,V_m neither depends on the stress in Nb/Al base electrode nor in Nb counter electrode. We have found, however, that V_m crucially depends on the way the tunnel barrier formation by thermal oxidation of Al is done. For instance, room-temperature dynamic oxidation (in O₂ flow at low pressures) in a cryopumped chamber leads to poor run-to-run reproducibility of V_m and reduced V_m values, whereas dynamic oxidation at the same parameters but in a chamber with turbomolecular pump results in high V_m values and excellent run-to-run reproducibility.

Keywords - Nb/Al-AlO_x/Nb Josephson junctions, AlO_x tunnel barrier, subgap leakage, intrinsic stress, hydrogen in Nb, hydrogen chemisorption, superconducting digital circuits.

IEEE/CSC & ESAS European Superconductivity News Forum (ESNF) No. 22 October/November 2012.  Reference No. ST322; Category 4.
The published version of this preprint appeared in IEEE Transactions on Applied Superconductivity 23, 1100305 (June 2013).