Among the potential replacements for SiO@sub 2@ or SiO@sub x@N@sub y@ as gate dielectric in Si-based advanced MOSFET devices, hafnium oxide (HfO@sub 2@) and silicates (Hf@sub x@Si@sub y@O@sub z@) seem to be the most promising materials, combining high dielectric permittivity with low leakage current due to a reasonably high barrier height that limits electron tunnelling. In order to be effectively incorporated into ultra-large scale integration (ULSI) fabrication technology, the gate dielectric material must conserve its integrity in further processing steps. In particular, rapid thermal annealing of source and drain dopants, usually performed at and above 1000 °C, has been indicated as the most aggressive step. The main deleterious consequences of post gate dielectric deposition annealing reported so far include thickening of the SiO@sub 2@ interlayer and chemical reactions at both gate electrode/dielectric and dielectric/Si interfaces, with the consequent low! ering of the capacitance equivalent thickness, and diffusion of Hf into Si which reduces electron and hole mobilities in the transistor channel. Furthermore, these studies pointed out the need for controlling the effects of annealing in intentionally or unintentionally O@sub 2@-containing atmospheres (even at very low O@sub 2@ partial pressures), which renders post-deposition annealing even more aggressive to gate dielectric integrity, especially in the near dielectric-semiconductor interface region. The present work describes the results of composition, atomic transport and chemical reaction investigations following rapid thermal annealing in Ar, N@sub 2@ and O@sub 2@ of ultrathin HfO@sub 2@ and Hf@sub x@Si@sub y@O@sub z@ films deposited on Si in the temperature range 800 to 1000 °C. Film structures and compositions were established by high-resolution transmission electron microscopy, Rutherford backscattering spectrometry, nuclear reaction analysis, and X-ray photoelectron spectroscopy. Isotopic substitution and sub-nanometric depth resolution profiling using narrow nuclear resonant reaction profiling revealed that O migrates by means of a propagating front from the surface that reacts with the hafnium oxide or silicate networks as it advances, the main reaction channels being O-O and O-N exchanges. O penetration, incorporation in the bulk of the ultrathin film structures, and oxidation of the substrate forming SiO@sub 2@ here observed were significantly smalle! r than in previously studied Al, Zr and Gd oxides and silicates. Hf penetration into Si was observed, amounting between 10@super 12@ and 10@super 13@ Hf/cm@super 2@, which would degrade electron and hole mobilities in MOSFET transistor channels. Pre-annealing in non-reactive atmospheres like Ar and N@sub 2@ increase the stability and resistance to oxidation of the films.