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

We have investigated the critical current, I
c
R, and oxide barrier shape in Nb/Ta/Ox/C.E. tunnel junctions. Here, layers of Ta in the thickness range OTa
<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
Ta
is increased, there is a more rapid decrease of I
c
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,
\bar{\phi}
, and width, s, of the oxide barriers which form on the Ta overlayers. It is observed empirically that
\bar{\phi} \sim 6/(s-s_{o})
where
s_{o} \sim10
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.