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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 [1,2]. 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 a maximum carrier concentration of 8 at.%, the state of the art, 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 of such disordered superconductor, increased by 30% in this optimized setup, in agreement with theory and opposite to previous results (Fig.1) [4].
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.% [5].
Mastering and understanding the materials properties has brought to the development of all-silicon devices, such as Josephson junctions and superconducting microwave resonators [6].
Indeed, SQUIDs and Josephson junctions were developed [7], thanks to the excellent, epitaxial, transparent interface between superconducting Si and semiconducting Si, that we have characterized both at room temperature and at sub-K temperatures as a function of the semiconductor doping.
[1] F. Chiodi, et al., Laser Annealing Processes in Semiconductor Technology (Elsevier), ch.9 (2021)
[2] Y. Baron et al., Appl. Phys. Lett. Materials 12 (12), (2024)
[3] G. Hallais, et al., Semicond. Sci.Tech. 38, 034003 (2023)
[4] L. Desvignes. PhD thesis, Université Paris Saclay (2023)
[5] S. Nath, et al., Phys. Status Solidi A, 221: 2400313 (2024)
[6] P. Bonnet, F. Chiodi, et al., Phys. Rev. Applied 17, 034057 (2022)
[7] F. Chiodi, et al., Phys. Rev. B 96, 024503 (2017)