Ultrathin HfxSiyOz and HfxSiyOz/Al2O3, grown on Si surfaces by atomic layer chemical vapor deposition, were characterized in terms of their interface properties using X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy. The formation of Hf-silicide at HfxSiyOz/Si interfaces was induced by the reaction of metallic Hf atoms with Si substrate atoms; this formation was suppressed by the presence of Al2O3 interlayers between HfxSiyOz and Si. As the Al2O3 interlayer thickness increased, the Hf 4f XPS peak intensity from Hf-silicide decreased and completely disappeared for Al2O3 thickness greater than 10 nm. Effects of Al2O3 interlayers on the atomic structure of the films were also discussed. (c) 2005 The Electrochemical Society.