While the debate about the exact cause of Fermi level pinning in metal-high-k dielectric gate stacks is ongoing, several solutions for engineering the threshold voltage Vt of the gate stacks have been proposed. Engineering the flat-band voltage Vfb translates into an effective control of the threshold voltage. This study uses ion implantation as a tool to adjust Vfb by doping the gate stack. It is shown that lanthanide implantation can modulate the effective work function for n-type gate electrodes. Ion implantation of dysprosium (Dy) and lanthanum (La) into the gate stack achieves significant flat-band voltage shifts of about -1 and -3 V, respectively, for a dose of 1×1014 cm-2. By increasing the implantation dose and energy, larger shifts in the flat-band voltage are obtainable.