A novel process for fabricating refractory sperconducting tunnel junctions is described, which is useful with both deposited and native oxide barriers. The distinguishing feature of the method is that the entire superconductor-barrier-superconductor sandwich is formed before the patterning of any layer. Isolated Josephson junctions are then formed by anodizing through the upper electrode, while the devices themselves are protected by a photoresist mask. Using this process, Nb-Si:H-Nb junctions have been fabricated, whose product of critical current and subgap resistance exceeds 10 mV and whose critical current density varies by about 50% over a 2-in. diameter wafer.

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Using a generalized two-fluid picture to describe a quasi-one-dimensional superconductor near To we provide a heuristic derivation for a set of equations governing the temporal and spatial evolution of the charge imbalance (or branch imbalance) in the quasiparticles. We show that these equations are isomorphic to those that describe a simple electrical transmission line, so that charge imbalance waves may propagate in the superconductor in analogy with electrical signals that propagate down the transmission line. We propose as a model for a phase-slip center in a superconducting filament a localized Josephson oscillator coupled to the transmission line. Applying standard transmission-line theory to solve the problem, we show that the Josephson oscillations in the center generate charge imbalance waves that propagate out to a frequency-dependent distance of the order of the quasiparticle diffusion length A(~. = (Dro.)l/2before they damp out. The time-averaged behavior of the model reduces to the earlier model of Skocpol, Beasley, and Tinkham. A novel consequence of the model is a prediction of intrinsic hysteresis in the dc current-voltage relation. The model also provides a convenient framework for dealing with ac effects in phase-slip centers, including resonance and synchronization in systems of closely spaced phase-slip centers and microbridges.

We have used telegraph noise spectroscopy to study the properties of individual localized electron traps of a particular type in thin layers (t<∼6 nm) of rf sputtered amorphous Si and amorphous Si:H. The results indicate that these traps have bistable ionic configurations: The trapping kinetics are dominated by transitions between two different ionic configurations with an associated change in trapped charge. Above ∼20 K, configurational transitions are by thermally activated hopping; below 20 K, transitions appear to be induced by zero-point oscillations.

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