Tuning superconductivity in Ge:Ga using Ga+ implantation energy

Abstract

High-fluence gallium $({\mathrm{Ga}}^{+})$ implantation at medium energies is proven to be an effective tool in forming superconducting (SC) thin films in germanium (Ge). By changing the post-implantation annealing conditions nanocrystalline to single-crystalline Ge matrices have been produced. Irrespective of crystallinity, such processes have mostly led to supersaturated Ge:Ga films where superconductivity is controlled by the extent of coherent coupling between Ga precipitates. Here we use ${\mathrm{Ga}}^{+}$ implantation energy as a means to tailor the spatial distribution and the coupling energy of the Ga precipitates. By systematic structural and magneto-transport studies, we unravel the complex connection between the internal structure of Ge:Ga films and their global SC parameters. At the shallowest implantation depth, we observe the strongest coupling leading to a robust superconductivity that sustains parallel magnetic fields as high as 9.95 T, above the conventional Pauli paramagnetic limit and consistent with a quasi-2D geometry. Further measurements at mK temperatures revealed an anomalous upturn in perpendicular critical field ${\mathrm{B}}_{\ensuremath{\perp}}$ vs temperature whose curvature and thus origin may be tuned between weakly coupled SC arrays and vortex glass states with quenched disorder. This warrants future investigations into Ge:Ga films for applications where tunable disorder is favorable, including test-beds for quantum phase transitions and superinductors in quantum circuits.