Variations in the growth behavior, physical and electrical properties, and microstructure of the atomic layer deposited (ALD) HfO₂ gate dielectrics were examined with two types of oxygen sources: O₃ and H₂O for the given Hf-precursor of Hf[N(CH₃)(C₂H₅)]₄. The ALD temperature windows for O₃ and H₂O were 240–320 °C and 200–280 °C, respectively, with the growth rate of HfO₂ using O₃ being higher than that of the films using H₂O within the ALD window. While the film density of HfO₂ using O₃ decreased, the film density of HfO₂ using H₂O increased with the decreasing ALD temperature. As the deposition temperature decreased, the amount of impurity in the HfO₂ film with the O₃ oxidant increased due to the insufficient reaction, which led to the crystallization of the HfO₂ film into the tetragonal structure after the post-deposition annealing at 600 °C. The films with a lower density and a higher carbon-impurity concentration retained the portion of the tetragonal phase (∼30%) to the highest annealing temperature of 1000 °C. However, the HfO₂ films grown at 200 °C with H₂O showed the best electrical performance, which could be ascribed to the highest density, low impurity concentration, and negligible involvement of the interfacial low dielectric layer.