Critical current behavior and oxide barrier properties of Ta surface layers on Nb

Abstract

We have investigated the critical current, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R, and oxide barrier shape in Nb/Ta/Ox/C.E. tunnel junctions. Here, layers of Ta in the thickness range O<D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ta</inf> <1000Å were deposited, in situ, on 2000Å thick Nb underlayers. Junctions were completed with Pb, PbBi, and Ag counter-electrodes. We find that as D <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ta</inf> is increased, there is a more rapid decrease of I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R compared with the effective energy gap, in accord with an extended version of the Gallagher theory including strong-coupling and electron-scattering effects. In addition, we have investigated the average barrier height, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{\phi}</tex> , and width, s, of the oxide barriers which form on the Ta overlayers. It is observed empirically that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\bar{\phi} \sim 6/(s-s_{o})</tex> where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s_{o} \sim10</tex> Å and φ is measured in eV. This relationship is also found to hold for barrier formation on a wide variety of pure and composite metallic systems. These results are discussed in conjunction with the Fromhold-Mott-Cabrera theory for self-limiting oxide growth on metal surfaces.