Gallium-induced thin-film superconductivity in Ge and Si and its possible applications

We present two unconventional routes of embedding superconducting nanolayers in a semiconducting environment. On the one hand, ion implantation and subsequent annealing have been used to prepare intrinsic superconducting thin-films of Ga-doped germanium (Ge:Ga) with T_c of 0.5 to 1.2 K. On the other hand, the same technique has been applied to fabricate thin amorphous Ga-rich layers in silicon (Si:Ga) revealing a T_c of about 7 K. Extended structural investigations by means of XTEM, EDX, RBS/C, and SIMS have been performed in addition to low-temperature electrical transport and magnetization measurements. A narrow window of preparation parameters turns out to be necessary to obtain best sample properties in a reproducible way. While Ge:Ga films scale 60 - 100 nm and reveal a small critical current density J_c ≈ 3 A/cm², Si:Ga layers exhibit J_c > 1 kA/cm² and a thickness of 15 nm. Relative high and anisotropic critical fields have been found for both nanostructures proving their quasi-two-dimensionality. The implementation of prospective microstructuring may offer an on-chip combination of super- and semiconducting circuits that could be integrated in novel heterostructured devices.