XPS and tunneling study of air-oxidized overlayer structures of Nb with thin Mg, Y and Er

Guided by the criteria of strong oxygen affinity and strong tendency toward surface segregation over Nb, artificial tunnel barriers of thin Mg(10-65 A), and Y(4-30 A) overlayers on Nb were investigated. Very high-quality tunnel junctions of the types Nb/Mg-oxide/Pb 0.9 Bi 0.1 and Nb/Y-oxide/Pb 0.9 Bi 0.1 were obtained, particularly in the latter case, with Δ Nb of 1.57 meV and excess conduction at 2 mV to be 1.4 × 10-3of that at 4 mV. As characterized by the XPS technique, Mg and Y overlayers as thin as 10 A are sufficient to protect the underlying Nb film from oxidation. The Mg-oxide and Y-oxide formed on the surface by air oxidation are mainly hydrated. The rapid loss of metallic Mg and Y near the surface can be accounted for by the grain boundary diffusion mechanism previously invoked to explain results on Nb/Al overlayers. The Nb spectrum of the Nb/30 A Y sample shows remarkably clean, metallic Nb feature, very comparable to that obtained on a Nb foil in-situ scrape-cleaned in ultrahigh vacuum. Artificial barriers formed by oxidation of thin rare-earth metal layers Er(5.1-20.4A) were also attempted. Good-quality tunnel junctions of the type Nb/Er-oxide/Pb 0.9 Bi 0.1 were made, but inferior to those made with Al, Mg, and Y overlayers. XPS shows clearly that the surface oxide consists of both Er-oxide and Nb-oxide.