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Electron tunneling in tantalum surface layers on niobium

Physical Review B, ISSN: 0163-1829, Vol: 34, Issue: 1, Page: 217-225
1986
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  • Citations
    11
    • Citation Indexes
      11
  • Captures
    6

Article Description

We have performed electron tunneling measurements on tantalum surface layers on niobium. The tunnel junctions comprise 2000-A Nb base electrodes with 10100-A in situdeposited 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 IcR 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 IcR 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 r2=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 and width s in the form =6 eV/(s-10 A) 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. © 1986 The American Physical Society.

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