Electron tunneling in tantalum surface layers on niobium.

Abstract

We have performed electron tunneling measurements on tantalum surface layers on niobium. The tunnel junctions comprise 2000-A\r{} Nb base electrodes with 10--100-A\r{} in situ--deposited Ta overlayers, an oxide barrier, and Ag, Pb, or Pb-Bi alloy counterelectrodes. The base electrodes were prepared by ion-beam sputter deposition. The characteristics of these junctions have been studied as a function of Ta-layer thickness. These include the critical current, bound-state energy, phonon structure, and oxide barrier shape. We have compared our results for the product ${I}_{c}$R versus tantalum-layer thickness with an extended version of the Gallagher theory which accounts for both the finite mean free path in the Ta overlayers and suppression of the ${I}_{c}$R product due to strong-coupling effects. Excellent fits to the data yield a value of the intrinsic scattering probability for electrons at the Ta/Nb interface of ${r}^{2}$=0.01. This is consistent with the value expected from simple scattering off the potential step created by the difference between the Fermi energies of Ta and Nb. We have found a universal empirical correlation in average barrier height \ensuremath{\varphi}\ifmmode\bar\else\textasciimacron\fi{} and width s in the form \ensuremath{\varphi}\ifmmode\bar\else\textasciimacron\fi{}=6 eV/(s-10 A\r{}) for measured junctions which holds both for our data and results for available data in the literature for oxide-barrier junctions. The latter are composed of a wide variety of base and counterelectrode materials. These results are discussed in the general context of oxide growth and compared with results for artificial tunnel barriers.