Abstract The n-type doping of Ge is a self-limiting process due to the formation of vacancy-donor complexes (D _n V with n ⩽ 4) that deactivate the donors. This work unambiguously demonstrates that the dissolution of the dominating P₄V clusters in heavily phosphorus-doped Ge epilayers can be achieved by millisecond-flash lamp annealing at about 1050 K. The P₄V cluster dissolution increases the carrier concentration by more than three-fold together with a suppression of phosphorus diffusion. Electrochemical capacitance–voltage measurements in conjunction with secondary ion mass spectrometry, positron annihilation lifetime spectroscopy and theoretical calculations enabled us to address and understand a fundamental problem that has hindered so far the full integration of Ge with complementary-metal-oxide-semiconductor technology.