Metastable moleculary chemisorbed oxygen adsorbate on Si(111)-77 surface observed by synchrotron radiation real-time photoelectron spectroscopy

Yoshigoe, Akitaka ; Teraoka, Yuden

Recent experimental works regarding the metastable oxygen adsorbate on Si(111)-77 at room temperature report that the oxygen molecule chemisorbs on the Si adatom inherently having some oxygen atoms at the backbond sites. Since the numbers of oxygen atoms at the backbond sites of the Si adatom have been ambiguous only from O1s XPS, the mechanism of initial oxidation processes has been under debate. In this talk, we report the initial oxygen adsorption processes investigated by real-time Si2p XPS using high resolution synchrotron radiation in addition to O1s one. All experiments were performed at SUREAC2000 at BL23SU in SPring-8. The oxygen gas at 5.310Pa was exposed to the clean Si(111)-77 surface at room temperature and the oxidation process was in-situ monitored by real-time XPS for O1s and Si2p alternate measurement using synchrotron radiation. Comparing the chemical shifts of O1s with those of Si2p XPS, we found that the oxidation states corresponding to Si and Si were formed at the observable dosage for the metastable O adsorbate. Thus, it is found that the metastable O species adsorb at the on-top site of Si adatom bonding to an oxygen atom at that backbond, i.e. ins-paul structure. Moreover, the oxidation states with higher numbers, such as Si and Si, appeared at the oxidation time when the metastable oxygen adsorbate almost vanished. This result suggests that the oxygen molecule of an ins-paul dissociates to create two oxygen atoms. One oxygen atom remains at the on-top site of the Si adatom and it is assigned to be an ad-oxygen. The other moves to the Si adatom backbond sites to become an ins-oxygen. We concluded that the Si oxidation structure is the insx2-ad structure just after the disappearance of metastable oxygen adsorbate.