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FRANCESCA CHIODI - IEF Université Paris Sud

Superconducting Silicon and quantum devices

When ultra-doped, cubic silicon turns into a BCS superconductor even at ambient pressure [1]. However, the extreme boron doping concentration required to trigger superconductivity in this system is more than three times the boron solubility limit in silicon. Even though this concentration is impossible to reach using conventional micro-electronic techniques, epitaxial superconducting Si:B thin films can be realised using Laser Doping.
We have shown that the superconducting critical temperature Tc only depends of the boron dose (the number of boron atoms per unit surface), increasing above a threshold value up to a maximum of 0.72 K for a concentration of 2.5 1021 cm-3 in a 210 nm thick layer [2].
However, the initial linear increase of Tc vs the boron dose reaches a saturation for a boron dose corresponding to the strain-induced Si:B crystal relaxation. We are thus investigating the role of the strain in the onset of superconductivity. The tunable Tc and the possibility to take advantage from the silicon technology, make superconducting silicon devices potentially interesting, in particular in the frame of astrophysics detection. Since Si:B doesn’t transit directly from the semiconducting state to the
superconducting one, but first becomes metallic, it allows the fabrication of a large range of nanodevices, in which superconductors, metals and semiconductors can be coupled through extremely clean, epitaxially grown interfaces.
We have thus realised the first silicon superconducting devices : superconductor/normal metal/superconductor Josephson junctions [3], coplanar resonators, and superconducting quantum interference devices (dc-SQUID) [4]. The results obtained so far can be globally understood using the conventional models for metallic superconductor devices, but leave room even for a few surprises.

In collaboration with :
D. Débarre Institut d’Electronique Fondamentale,Orsay
J.-E. Duvauchelle, A. Francheteau, C. Marcenat, and F. Lefloch CEA, Grenoble
A. Grockowiak and T. Klein Institut Néel, CNRS & Université J. Fourier, Grenoble H. Lesueur CSNSM, Orsay
[1] E. Bustarret et al., Superconductivity in doped cubic silicon, Nature 444, 465 (2006)
[2] A. Grockowiak et al., Thickness dependence of the superconducting critical temperature in heavily doped SiB epilayers, Phys. Rev. B 88, 064508 (2013)
[3] F. Chiodi et al., Gas Immersion Laser Doping for superconducting nanodevices, Appl. Surf. Sci. 302, 209 (2014)
[4] J. E. Duvauchelle et al., Silicon superconducting quantum interference device, Appl. Phys. Lett. 107, 072601 (2015)


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