HfxAl1-xOy thin films were deposited by pulsed-laser deposition (PLD) on p-type (001) Si substrates. Interfacial reaction at HfxAl1-xOy/Si interfaces was studied by scanning transmission electron microscopy (STEM) using high-angle annular dark field imaging technique and the energy dispersive X-ray (EDX) line scan analysis. The results showed that the interfacial reaction is due to Hf diffusion into the Si substrate forming Hf silicide. The presence of Al in the HfxAl1-xOy films was found to be responsible for the Hf silicide formation and it showed that increasing of Al content in the films reduced the interfacial reaction. Therefore, high-quality HfxAl1-xOy films possessing a reaction-ftee interface with Si may be obtained by optimizing the growth condition. (C) 2004 Elsevier B.V. All rights reserved.