A method for fabricating a semiconductor structure includes providing a substrate, forming an interface layer on the substrate, and then performing a first annealing process on the interface layer under a nitrogen-containing environment to form a nitrogen-containing layer from a top portion of the interface layer. The first annealing process also deactivates non-bonded silicon ions and oxygen ions in the interface layer. The method further includes forming a high-k dielectric layer on the nitrogen-containing layer, and performing a second annealing process on the high-k dielectric layer to allow nitrogen ions in the nitrogen-containing layer to diffuse into the high-k dielectric layer to reduce a density of active oxygen vacancies in the high-k dielectric layer. Finally, the method includes forming a gate electrode layer on the high-k dielectric layer.