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In this study, the degradation mechanism of the interface integrity between a hafnium dioxide thin film and a gate electrode thin film was investigated by using quantum chemical molecular dynamics. Effect of point defects such as excessive oxygen and carbon interstitials in the hafnium dioxide films on the formation of the interfacial layer between them was analyzed quantitatively. Though the defect-induced sites caused by oxygen vacancies and carbon interstitials were recovered by additional oxidation after the deposition of the hafnium oxide film, the excessive interstitial oxygen and carbon atoms remained in the film deteriorated the quality of the interface by forming new oxide or carbide of the deposited metal such as tungsten and aluminum. No interfacial layer was observed when a gold thin film was deposited on the hafnium oxide. The estimated changes of the interface structure were confirmed by experiments using synchrotron radiation photoemission spectroscopy.