Optical study of superconducting Ga-rich layers in silicon

We performed phase-sensitive terahertz (0.12–1.2 THz) transmission measurements of Ga-enriched layers in silicon. Below the superconducting transition T _c ^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 λ(T ) and the condensate density n_s = λ²(0)/λ²(T ) as functions of temperature demonstrate behavior typical for conventional superconductors with λ(0) = 1.8 μm. The terahertz spectra can be well described within the framework of Eliashberg theory with strong electron-phonon coupling: the zero-temperature energy gap is 2Δ(0) = 2.64 meV and 2Δ(0)/k_BT_c = 4.6, consistent with the amorphous state of Ga. At temperatures just above T_c, the optical spectra demonstrate Drude behavior.