Deep submicron Nb/AlO x /Nb tunnel junctions and single electron transistors were fabricated by electron beam gun shadow evaporation, using stencil masks consisting of the thermostable polymer polyethersulfone and germanium. The I(U) characteristics of the single electron transistors show special features due to the tunneling of single Cooper pairs and quasiparticles. Significant e-periodic gate modulation is observed in both the superconducting and the normal state and a gap energy ⌬ eff of up to Ϸ1 meV has been achieved. The special features of using the refractory metal Nb in combination with the shadow evaporation technique are discussed.

We have developed a reliable method for the direct measurement of dielectric losses in thin films using a special design of lumped element superconducting resonators. The method presented allows obtaining quantitative values for the losses in the film volume as well as the metal/dielectric interfaces. Numerous different resonator geometries were designed and measured in order to study the losses in dielectric thin films usually used for Josephson junction fabrication, such as Nb 2 O 5 , SiO and SiN x , at low temperatures and low gigahertz frequencies. The results show that for such amorphous materials, the bulk losses clearly exceed the interface losses. Furthermore, the frequency dependence of the losses in this working regime was studied for the first time. Our results are in good agreement with the universal law describing the frequency dependence of dielectric losses and suggest that the losses are due to many-body interacting dipoles, which is an important fact for the theoretical modelling of two-level fluctuators. We also investigated the losses in multi-layer films and find that the results are in good agreement with the theoretical expectations, which allows optimization of the dielectric multi-layers used in Josephson junction and qubit fabrication.

No abstract

A reliable process has been developed for the fabrication of all-Nb single-electron circuits, based on spin-on glass planarization. The process steps are the in situ growth of Nb/AlO x /Nb sandwich, definition of the patterns of junctions, base electrodes and wiring by use of reactive ion etching and the planarization of a spin-on glass insulation between base electrode and wiring. A single electron transistor made of 0.3×0.3 µm 2 area junctions clearly shows the e-periodic Coulomb blockade modulation by a voltage applied to a gate.Up to now, Nb tunnel junctions have remained the most popular elements of superconductive electronics. Since the invention of the Nb/AlO x /Nb trilayer by Gurvich et al.1 , the fabrication of complex circuits of micrometerscale junctions has been well established. In the past years, there was a strong motivation to develop a technique for the reliable fabrication of Nb Josephson junctions of smaller sizes, characterized by smaller selfcapacitances, which are advantageous for application in both Josephson devices 2,3 and single electron circuits. Moreover, due to rather large values of the superconductor energy gap of Nb (∆ ≈ 1.4 meV) and, hence, larger Josephson-junction coupling, the small-capacitance Nb junctions are very promising for the single electron tunneling (SET) applications in which the interplay between the Coulomb charging energy E C and the Josephson coupling energy E J is realized 4 . A number of attempts were made to fabricate submicron-scale Nb/AlO x /Nb tunnel junctions. These approaches can be divided into two groups: first, the techniques in which the tunnel barrier was formed ex situ 5,6 , and, secondly, those in which it was grown in situ [7][8][9]

No abstract