Optical study of superconducting Ga-rich layers in silicon

We performed phase-sensitive terahertz (0.12\char21{}1.2 THz) transmission measurements of Ga-enriched layers in silicon. Below the superconducting transition ${T}_{c}^{\mathrm{middle}}=6.7$ K we find clear signatures of the formation of a superconducting condensate and of the opening of an energy gap in the optical spectra. The London penetration depth $\ensuremath{\lambda}(T)$ and the condensate density ${n}_{s}={\ensuremath{\lambda}}^{2}(0)/{\ensuremath{\lambda}}^{2}(T)$ as functions of temperature demonstrate behavior typical for conventional superconductors with $\ensuremath{\lambda}(0)=1.8\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}$m. The terahertz spectra can be well described within the framework of Eliashberg theory with strong electron-phonon coupling: the zero-temperature energy gap is $2\ensuremath{\Delta}(0)=2.64$ meV and $2\ensuremath{\Delta}(0)/{k}_{B}{T}_{c}=4.6$, consistent with the amorphous state of Ga. At temperatures just above ${T}_{c}$, the optical spectra demonstrate Drude behavior.