We investigated the effect a 1.2-nm-thick Al2O3 capping layer had on suppressing interfacial Si oxidation in a 2.6-nm-HfO2/0.35-nm-SiO2/Si(001) structure during postdeposition annealing in an oxygen ambient. An incubation period (IP) was initially observed during which the HfO2/Si interface exhibited remarkable stability without any interfacial SiO2 growth. This was then followed by very slow interface oxidation. Our detailed study suggested that low oxidant diffusion through the capping layer determined the effective IP. Furthermore, HfO2/Si interface oxidation, which proceeded through a two-step process that was similar to an uncapped structure, was severely constrained by the limited availability of oxygen at the Al2O3/HfO2 interface. © 2003 American Institute of Physics.