High-k ZrO2 thin films are grown on p-type silicon by metal-organic chemical vapor deposition based on zirconiumtetrakistrifluoroacetylacetonate as single-source precursor system. Annealing of the as-grown films is performed to investigate the impact of oxidative and reductive atmospheres On thin film properties. The composition of the ultrathin films is examined by Auger spectroscopy, whereas metal-oxide-semiconductor (MOS) structures are employed to extract electrical characteristics. Equivalent oxide thicknesses down to 2 nm and interface trap densities of 5 X 10(11)cm(-2) eV(-1) at midgap are obtained. MOS capacitors show extremely low leakage currents, promising to reduce gate leakage by more than a factor of 10(3) compared to SiO2. The correlation between compositional and electrical properties is discussed on the basis of postdeposition annealing procedures resulting in a consistent explanation of the observed effects. (C) 2003 American Vacuum Society.