Subgap Leakage in $\hbox{Nb/Al}$– $\hbox{AlO}_{\rm x}\hbox{/Nb}$ Josephson Junctions and Run-to-Run Reproducibility: Effects of Oxidation Chamber and Film Stress

S K Tolpygo,R T Hunt,D J C Amparo,D T Yohannes,J A Vivalda

doi:10.1109/tasc.2012.2227438

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 c R 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 critically depend 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 to 300 MPa. In this range, V m depends neither on the stress in a Nb/Al base electrode nor in a 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 O 2 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 a turbomolecular pump results in high V m values and excellent run-to-run reproducibility.

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