The oxidation state at tunnel junction interfaces

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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.