Atomic layer deposition (ALD) has received increasing attention in relation to the growth of high-permittivity (kappa) rare-earth oxides for advanced gate stack applications. Transistor reliability strongly depends on the oxide/semiconductor interface properties. In this study, we perform transmission electron microscopy measurements in the high-resolution mode coupled with electron energy loss spectroscopy experiments to probe at the nanometric scale interface layer (IL) issues for ALD-grown La2O3/Si stacks. Complementary results from electrical and X-ray diffraction measurements on selected samples are also discussed. We demonstrate that the La2O3 film reactivity with the Si surface can be controlled up to a certain extent by appropriately choosing the ALD precursor combination. In particular, we prove that the La(Cp)(3)+O-3 scheme is more attractive than the La(Cp)(3)+H2O one for depositing La2O3 films because it gives rise to a lower IL thickness and interface trap density and to a smaller critical sample thickness for the stabilization of the high-kappa hexagonal La2O3 phase.