Influences of hydrogen on initial oxidation processes of H-terminated Si(100) surfaces

The initial oxidation of H-terminated Si(100) surfaces by atomic oxygen at room temperature has been investigated by using high-resolution electron energy loss spectroscopy (HREELS). It has been found on H-terminated Si(100) surfaces with a 1 × 1 (dihydride) structure that there are two oxidation steps in the range of oxygen coverage, θ, less than unity. The adsorption of oxygen atoms on the SiSi back bond with a constant vibrational energy of about 128 meV for the SiOSi asymmetric stretching mode occurs in the first step of θ < 0.4, and an increase in the SiOSi vibrational energy can be observed in the second step θ > 0.4. Moreover, in the case of oxidation of clean surfaces, the SiOSi vibrational energy is monotonously increased with increasing oxygen coverage and is identical with that on H-terminated surfaces at θ > 0.4. It is considered that the structural relaxation of SiOSi bonds is promoted by a charge transfer from back bonds to surface SiH bonds.