The improvements in the reliability of HfO2 thin films afforded by in situ NH3 injection during their atomic layer depositions were studied. The NH3 injection increased nitrogen content and reduced the residual carbon in the film, which resulted in the reduced leakage current density and improved reliabilities. A smaller flatband voltage shift, higher breakdown field, and the suppressed degradation of interface traps by constant voltage stress were obtained due to the solid SiNx interfacial layer formation, decreased carbon content, and reduction of electrical defects by oxygen vacancies. The solid interfacial SiNx layer improved the thermal stability of the films. Detailed analysis of the current-voltage characteristics revealed the electrical conduction behavior and nature of defects. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3098978] All rights reserved.