Thermal stability and electronic structure of xHfO2∙(100-x)SiO2 (HfSiO) (x=25%, 50%, and 75%) grown by atomic layer deposition were investigated by various measurement tools. The quantity of incorporated SiO2 content changes the binding energy of HfO2 as the charging effect in the silicate is enhanced with the quantity SiO2. When the annealing temperature is increased over 800 °C, phase separation between SiO2 and HfO2 is observed in the films with 50% and 75% HfO2, while it does not occur in a Hf-silicate film with a high mole fraction of SiO2. The phase separation begins in the surface region via the segregation of SiO2. After the annealing treatment, the quantity of SiO2 supplied to the film surface due to interfacial interactions influences the phase separation process, resulting in no phase separation being observed, even at a high annealing treatment of 900 °C.