Fabrication of Nb-based superconducting single electron transistor

J. Jussi Toppari, G.-S. Paranoanu, Ari Halvari +2 more

Giampiero Amato, Emanuele Enrico

Current Status and Technological Limitations of Hybrid Superconducting-Normal Single Electron Transistors

Y. Harada, David B. Haviland, Per Delsing +2 more

A new four-layer resist system was developed to fabricate small capacitance niobium tunnel junctions. Nb/AlOx/Al junctions were used to make a superconducting single electron transistor composed of two dc SQUIDs in series. The current-voltage characteristics were measured and two types of resonant tunneling of Cooper pairs were observed at different magnetic fields.

J. Jussi Toppari, Thomas Kühn, Ari Halvari +3 more

We have fabricated and measured superconducting single-electron transistors with Al leads and Nb islands. At bias voltages below the gap of Nb we observe clear signatures of resonant tunneling of Cooper pairs, and of Coulomb blockade of the subgap currents due to linewidth broadening of the energy levels in the superconducting density of states of Nb. The experimental results are in good agreement with numerical simulations.

S. L. Pohlen

R. Dolata, Hansjorg Scherer, Alexander B Zorin +1 more

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.