HfO2 layers, 25-Å thick, were grown by cyclic Hf sputter deposition and room temperature oxidation steps on chemically oxidized Si(001). Subsequent in situ annealing and TiN deposition yield a high-κ gate-stack for which the original 8-Å-thick SiO2 layer is eliminated, as confirmed by transmission electron microscopy. Transistors fabricated with this gate-stack achieve an equivalent oxide thickness in inversion Tinv =9.7Å, with a gate leakage Jg =0.8 A/cm2. Devices fabricated without in situ annealing of the HfO2 layer yield a Tinv which increases from 10.8 to 11.2 Å as the oxidation time during each HfO2 growth cycle increases from 10 to 120 s, also causing a decrease in Jg from 0.95 to 0.60 A/cm2, and an increase in the transistor threshold voltage from 272 to 294 mV. The annealing step reduces Tinv by 1.5 Å (10%) but also increases the gate leakage by 0.1 A/cm2 (30%), and causes a 61 mV reduction in Vt. These effects are primarily attributed to the oxygen-deficiency of the as-deposited HfO2, which facilitates both the reduction of an interfacial SiO2 layer and a partial phase transition to a high-κ cubic or tetragonal HfO2 phase.