In this work, the manipulation of phosphorus diffusion as well as the reduction of the specific contact resistivity in Ge by carbon co-doping is explored. It is found that there is an optimum condition for carbon implantation to suppress the rapid P diffusion effectively. For such a condition, the implanted carbon sits at the half range of the pre-amorphized Ge layer. In this case, both the diffusion of P in the amorphous layer and in the Ge virtual substrate beyond the amorphous/crystal (a/c) i