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Fabrication of Nb super-conducting SET

Niobium is the material of choice for most scientific and industrial applications involving super-conducting metals, due its a relatively large critical temperature (9.26K compared to 1.175K of aluminium). The technology for producing large (micrometer scale) structures is well established; however, the fabrication of small super-conducting nano-structures using Nb as a material is a well-known difficult issue. The problem is that unlike aluminium, niobium belongs to the class of refractory materials, thus it is significantly more difficult to evaporate and deposit in a reliable way.

We have developed a new technique for the nano-fabrication of small junctions; we have produced and measured structures such as single junctions and single-electron transistors. To prevent resist out-gassing, Nb is evaporated in a UHV chamber built in-house which has a large distance from the crucible to the sample holder; we have found the optimal evaporation and oxidation parameters for this configuration. For creating the junctions we use typically two-angle or three-angle shadow evaporation techniques. Using this method, we have fabricated single junctions, single electron transistors, and multiple junctions' structures with Nb only or with combinations of Nb and Al.

All these structures have good well-defined IV characteristics and, once the optimal recipe was found, the sample fabrication failure rate was low (10-20%). The measurements on these structures allow us to extract information about the sub-gap density of states. We have also studied in detail a single electron transistor with Al leads and Nb island, which displayed interesting transport phenomena such as Cooper pair resonances.

This technology is relevant for super-conducting nano-electronics in general. In the particular case of quantum computing, it could be used for the read-out systems of the qubits and for fabricating flux qubits. The main advantage of our technology with respect to other nano-fabrication techniques involving Nb is simplicity. In the case of charge qubits, the initial expectation that Nb islands connected to Al leads would reduce the quasi-particle contamination has not been fulfilled. At this moment, there exists no fabrication technique for overcoming this problem; therefore more material-science oriented studies will be necessary.

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University of Jyvaskyla
Seminaarinkatu 15; P.O. Box 35
40351 Jyvaskyla
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