A method for forming an oxynitride gate dielectric in a semiconductor device and gate dielectric structure formed by the method are disclosed. In the method, an oxynitride layer is first formed on a silicon surface and then re-oxidized with a gas mixture containing oxygen and at least one halogenated species such that an oxynitride layer with a controlled nitrogen profile and a layer of substantially silicon dioxide formed underneath the oxynitride film is obtained. The oxynitride film layer can be formed by either contacting a surface of silicon with at least one gas that contains nitrogen and/or oxygen at a temperature of not less than 500° C. or by a chemical vapor deposition technique. The re-oxidation process may be carried out by a thermal process in an oxidizing halogenated atmosphere containing oxygen and a halogenated species such as HCl, CH2Cl2, C2H3Cl3, C2H2Cl2, CH3Cl and CHCl3.