Highly n-type Ge attained by shallow As implantation and excimer laser annealing was studied with positron annihilation spectroscopy and theoretical calculations. We conclude that a high concentration of vacancy–arsenic complexes was introduced by the doping method, while no sign of vacancies was seen in the un-implanted laser-annealed samples. The arsenic bound to the complexes contributes substantially to the passivation of the dopants.