  We demonstrate that at the usual 1E-7 torr range of base pressures in the
sputtering chamber, X-ray photoelectron spectroscopy shows the existence of a
thin AlOx layer at the Nb/Al interface in both Nb/Al-AlOx/Pb tunnel junctions
and Nb/Al bilayers. This is due to the time elapsed between the deposition of
the Nb and Al bottom layers, even at times as short as 100 s. We also give some
direct evidence of the oxidation of the top Pb electrode on the Nb electrode
surface. Such oxidation probably occurs at the pinholes of the intermediate
Al-AlOx layer of the junctions, as a consequence of the oxidation state at the
Nb/Al interface. We therefore suggest that both the base pressure and the time
lapse between layer depositions should be carefully controlled in magnetic
tunnel junctions.
