Superconducting Epitaxial Silicon – when you turn on the light

Résumé :

Since the discovery of BCS superconductivity in silicon by nanosecond laser ultra-doping with boron, theoretical and experimental works have endeavored to understand what triggers and controls the superconducting phase. Indeed, superconducting Si has great potential to develop a cryogenic electronics with the advantages of large scale integration and high reproducibility. Through the optimization of the nanosecond laser temporal profile, we achieved an excellent control of both the electrical and structural properties of ultra-doped Si thin layers, with state of the art maximum carrier concentration (8 at.%) in monocrystalline epilayers with few defects, 100% dopant activation up to and above the solubility limit, and a vertically homogeneous doping profile [1-3]. The control and improvement of the active doping is directly reflected in the control of the superconducting critical temperature Tc, increased by 30% up to 0.9 K in this optimized setup, and whose dependence with doping can now be modelled within BCS frame [3]. Furthermore, we demonstrated that superconductivity is not only controlled by doping, but also by the lattice deformation. Thus, it is possible to tune up to 50% Tc by modifying by 1% the lattice parameter, as shown through nanosecond laser incorporation of Ge up to 20 at.% [4].

The sensitivity of superconducting silicon allows for multiple modulation ‘knobs’: for instance Tc can be tuned by doping, allowing for the design of superconducting, metallic or semiconducting regions in a device, or by strain relaxation, e.g. through localized Ge ion implantation. In addition, the thin superconducting silicon layer can be affected by the absorption of photons either in the superconductor itself, or in the undoped silicon underneath. The effect of light irradiation was highlighted in superconducting coplanar waveguide resonators, whose resistivity easily matches the vacuum impedance. Due to the large disorder, a high kinetic inductance characterizes the resonators [5]. The quasiparticle dynamics was measured when submitting the resonators to either a light or a microwave pulse, these out-of-equilibrium measurements being also compared with the equilibrium generation-recombination noise spectral density. In addition, work is in progress to assess the effect of visible photons on the DC response of the superconducting silicon micrometer-size lines, where a single-photon counting mode with extremely low Dark Count Rates has been highlighted, and whose detection mechanism is currently investigated.

1. L. Desvignes, et al., arXiv:2603.06383 (2026) ; PhD thesis, Université Paris Saclay (2023)

2. G. Hallais, et al., Semicond. Sci.Tech. 38, 034003 (2023)

4. S. Nath, et al., Phys. Status Solidi A, 221: 2400313 (2024)

5. P. Bonnet, et al., Phys. Rev. Applied 17, 034057 (2022)

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Contact : florence.levy-bertrand@neel.cnrs.fr