Elie Track

Tunneling measurements have been performed on tantalum surface layers on niobium. The thickness of the tantalum layer ranges from 10 to 100 A. The critical current, bound-state energy, phonon structure, and oxide barrier shape are investigated. The results are compared 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$\sb{\rm c}$R product due to strong electron-phonon coupling effects. Excellent fits to the data yield a value of the intrinsic scattering probability for electrons at the Ta/Nb interface of r$\sp2$ = 0.01. In addition, a new fabrication technique–dual ion-beam sputtering–is used to deposit thin films of NbN. The properties of these films and of tunnel junctions formed with NbN as base electrode and native-oxide as well as artificial barriers are reported. A universal empirical correlation is found between the average barrier height $\phi$ and the effective barrier width d for measured junctions. This correlation, which holds both for our data and for available data in the literature for oxide-barrier junctions, is discussed in the general context of oxide growth and compared with results for artificial tunnel barriers. Finally, high quality Ta/PbBi tunnel junction of area $\leq$1 $\mu$m$\sp2$ and current density 10$\sp3$-10$\sp5$ A/cm$\sp2$ are produced using a window geometry. The electrical noise properties of these junctions are investigated. Discrete voltage switching events allow the identification of the effect of single localized states in the oxide barrier. The voltage and temperature dependence of the switching rates are consistent with a microscopic model based on the emission and capture of individual electrons at the localized sites within the barrier.