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 5×1011 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 103 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. © 2003 American Vacuum Society.