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The effect of point defects such as oxygen vacancy and carbon interstitial on structural characteristics of W/HfO2 was analyzed by a quantum chemical molecular dynamics method. Post-oxidation annealing is effective for eliminating the oxygen deficiency of gate dielectric films. However, the excess oxygen atoms may remain in the oxide film after the post-oxidation anneal. For HfO2, they produce acceptor states in band gap and thus, give rise to the shrinkage of the local band gap of the oxide. In addition, such excess oxygen interstitials have been found to form a tungsten oxide layer at W/HfO2 interface during the deposition process of a tungsten gate. It is very important, therefore, to minimize the concentration of point defects in the film to assure the reliability of the MOS structures.