Superconductivity in Ga-doped Germanium

We report the first observation of superconductivity in heavily p-type doped germanium at ambient pressure conditions. Using Ga as dopant, we have produced a series of Ge:Ga samples by ion-beam implantation and subsequent short-term (msec) flash-lamp annealing. The combination of these techniques allows for Ga concentrations up to 6 %, i.e., a doping level which is clearly larger than the solubility limit and not accessible to any other method so far. Transport measurements reveal superconducting transitions with T_c up to 0.5 K. In more detail, we observe a strong dependence of the superconducting critical parameters on the annealing conditions. Further, we find a strong anisotropy of the superconducting critical field reflecting the two-dimensional character of the superconducting state in the ~ 60 nm thin Ge:Ga layer. We find critical magnetic in-plane fields following a linear temperature dependence up to a maximum field which is even larger than the Pauli-Clogston limit. Ge:Ga appears to be a superconductor in the extreme type-II limit with a very small Cooper-pair density. After its finding in Si [1] and diamond [2], our work reports another unexpected observation of superconductivity in doped elemental semiconductors.