We have investigated annealing-time dependence in interfacial reactions between polycrystalline-Si (poly-Si) electrodes and HfO2/Si gate stacks using synchrotron radiation photoemission and x-ray absorption spectroscopy. From photoemission core-level spectra, we found that silicidation started at the upper interface between poly-Si electrodes and HfO2 gate dielectrics under the conditions of 700 °C and 3 min in ultrahigh vacuum. Before silicidation, we observed nonstoichiometric silicon oxide at the upper interface, suggesting the formation of oxygen vacancies which may cause Fermi level pinning. The interface layer between HfO2 gate dielectrics and Si substrates was changed into silicide by further annealing. In addition, from valence-band photoemission and O K-edge absorption spectra, we have found that the crystallization of HfO2 gate dielectrics is independent of silicidation.