We studied the effect of ambient O-3 concentration on the electrical properties of ultrathin HfO2 gate dielectric formed by atomic layer deposition (ALD) using Hf[N(CH3)(C2H5)](4) as a precursor. The HfO2 films were intended for use in replacement metal gate transistors and were fabricated on silicon substrates that had undergone a final clean using diluted hydrofluoric acid. From the results of capacitance-voltage measurements and transmission electron microscopy images, the interfacial layer for 25-50 g/N m(3) could be made about 0.3 nm thinner than that for 200 g/N m(3). The concentration of carbon impurities was at an acceptable level (1.2 x 10(20) cm(-3)) in the ALD HfO2 films by using the low O-3 concentration of 25 g/N m(3). The measured leakage current densities for 25-200 g/N m(3) of O-3 were reduced by about 5 orders of magnitude with respect to reference SiO2 films. From these results, it was judged that O-3 concentrations of 25-50 g/N m(3) were suitable for the fabrication of ultrathin HfO2 gate dielectrics and would improve their electrical properties. (c) 2006 The Electrochemical Society.