Soft-x-ray-excited angle-resolved photoelectron spectroscopy studies on silicon dioxide films formed using oxygen radicals (OR) versus oxygen molecules (OM) are reported. Most of intermediate oxidation states of Si, so called suboxides, consisting of Si3+, Si2+, and Si1+ are localized at and near the SiO2/Si interfaces. The suboxides formed utilizing OM are located extremely close to the interfaces, while suboxides formed utilizing OR are located not only at the interface but also in the Si substrate. This implies the penetration of a part of OR into the Si substrate to form Si1+ and to relax the interfacial stress. Intermediate chemical bonding states of Si are formed in Si substrate closer to the interface as compared with those formed utilizing OM. This implies that the interfacial stress in the former is smaller than that in the latter.