The present disclosure provides a method of fabricating a semiconductor device. The method includes forming a high-k dielectric layer over a semiconductor substrate, forming a capping layer over the high-k dielectric layer, forming a metal layer over the capping layer, forming a semiconductor layer over the metal layer, performing an implantation process on the semiconductor layer, the implantation process using a species including F, and forming a gate structure from the plurality of layers including the high-k dielectric layer, capping layer, metal layer, and semiconductor layer.