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We identify and characterize the chemical nature of the oxide formed by the air oxidation of thin Al overlayers on Nb. Take-off angle experiments were performed to determine the sequential layer configuration. The thickness of the oxide did not exceed 20 Å, regardless of the thickness of the metallic Al originally deposited. Grain-boundary diffusion is suggested as a mechanism for the removal of most of the excess metallic Al. The observation of a small amount of residual metallic Al near the interface is in accord with Miedema’s theory of surface segregation which indicates that Al will segregate to the surface of a Nb-Al alloy. An Al overlayer as thin as 9.5 Å is sufficient to protect the underlying Nb film from oxidation and to form an effective tunnel-junction barrier.
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Kwo et al. (1982) studied this question.
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