The thermal stability and structural characteristics for gate stack structure of HfO 2 dielectrics grown by DC reactive magnetron sputtering were investigated. The structural characteristics and chemical states of the HfO 2 films in relation to the postannealing temperature were examined by X-ray diffraction and X-ray photoelectron spectroscopy. XRD patterns demonstrate that the as-grown HfO 2 films are amorphous and the structure of the annealed HfO 2 films undergoes a transformation of tetragonal to monoclinic phase with increasing annealing temperature and forms a polycrystalline structure at high annealing temperature. Analysis of XPS of Hf4f and O1s gives evidence that the as-grown HfO 2 film is nonstoichiometric. With increase of annealing temperature, both peaks of Hf4f and O1s keep on shifting towards lower binding energy, which indicates to be forming stoichiometric HfO 2 film. Annealing the as-grown thin film at low temperature leads to the formation of silicate at the interfacial regime resulting from the reaction between the nonstoichiometric thin film and parts of SiO x formed at the interfacial regime, and the Si–O–Hf and Si–O bonds in the silicate and SiO x attributes greatly to the XPS O1s peaks. High temperature annealing process, the O-deficient HfO 2 films will absorb oxygen atoms from the formed silicate interfacial layer and the oxygen ambient to form the fully oxidized HfO 2 thin films.