A method for forming a semiconductor structure includes providing a semiconductor substrate; forming a gate dielectric layer on the semiconductor substrate; forming a metal-containing layer on the gate dielectric; and forming a composite layer over the metal-containing layer. The step of forming the composite layer includes forming an un-doped silicon layer substantially free from p-type and n-type impurities; and forming a silicon layer adjoining the un-doped silicon layer. The step of forming the silicon layer comprises in-situ doping a first impurity. (or need to be change to: forming a silicon layer first & then forming un-doped silicon layer) The method further includes performing an annealing to diffuse the first impurity in the silicon layer into the un-doped silicon layer.