Subgap Leakage in $\hbox {Nb/Al}$ - $\hbox {AlO}_{\rm x}\hbox {/Nb}$ Josephson Junctions an

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 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 a turbomolecular pump results in high V_m values and excellent run-to-run reproducibility.