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 x 1 (dihydride) structure that there are two oxidation steps in the range of oxygen coverage, theta, 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 theta < 0.4, and an increase in the Si-O-Si vibrational energy can be observed in the second step theta > 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 theta > 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.