Model for the emission ofSi+ions during oxygen bombardment of Si(100) surfaces

The variation in the emission of ${\mathrm{Si}}^{+}$ ions from ion-beam-induced oxidized silicon surfaces has been studied. The stoichiometry and the electronic structure of the altered layer has been characterized using x-ray photoelectron spectroscopy (XPS). The XPS analysis of the Si 2p core level indicates the strong presence of suboxide chemical states when bombarding at angles of incidence larger than 30 °. Since the surface stoichiometry or degree of oxidation varies with the angle of incidence, the corresponding valence-band structures also differ among each other. A comparison between experimental measurements and theoretically calculated Si and ${\mathrm{SiO}}_2$ valence bands indicates that the valence bands for the altered layers are formed by a combination of those two. Since Si-Si bonds are present in the suboxide molecules, the top of the respective new valence bands are formed by the corresponding 3p-3p Si-like subbands, which extend up to the Si Fermi level. The changes in stoichiometry and electronic structure have been correlated with the emission of ${\mathrm{Si}}^{+}$ ions from these surfaces. From the results a general model for the ${\mathrm{Si}}^{+}$ ion emission is proposed combining the resonant tunneling and local-bond-breaking models.