Superconducting Ga-overdoped Ge layers capped with SiO2 – structural and transport investigations

Superconducting Ga-rich layers in Ge are fabricated by Ga implantation through a thin SiO₂ cover layer. After annealing in a certain temperature window, Ga accumulation at the SiO₂/Ge interface is observed. However, no Ga containing crystalline phases are identified. Thus it is suggested that the volatile Ga is stabilized in an amorphous mixture of all elements available at the interface. Electrical transport measurements reveal p-type metallic conductivity and superconducting transition. The superconducting properties of the samples with high Ga concentration at the interface change dramatically with etching the amorphous surface layer. A critical temperature of 6 K is measured before, whereas after etching it drops below 1 K. Therefore, one can conclude that the superconducting transport is based on two different layers: a Ga-rich amorphous phase at the interface and a heavily Ga-doped Ge layer. Finally, the comparison of the transport properties of Ga-rich Ge with those of Si demonstrates distinct differences between the interface layers and even the deeper lying doped regions.