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ST322 - Subgap Leakage in Nb/Al-AlOx/Nb Josephson Junctions and Run-to-Run Reproducibility: Effects of Oxidation Chamber and Film Stress
Subgap Leakage in Nb/Al-AlOx/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-AlOx/Nb Josephson junctions (JJs) in superconducting electronics require high quality tunnel barriers with low subgap leakage that is usually characterized by figure of merit Vm=IcRsg, where Ic is the critical current and Rsg 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-AlOx/Nb trilayers, and the stress therefore should be carefully minimized and controlled. Contrary to this belief, we show that JJ quality (Vm) 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,Vm neither depends on the stress in Nb/Al base electrode nor in Nb counter electrode. We have found, however, that Vm 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 Vm and reduced Vm values, whereas dynamic oxidation at the same parameters but in a chamber with turbomolecular pump results in high Vm values and excellent run-to-run reproducibility.
Keywords - Nb/Al-AlOx/Nb Josephson junctions, AlOx 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).