Initial oxidation of Si(111)-7×7 surfaces studied by photoelectron spectroscopy combined with medium energy ion scattering

Atomically clean Si(111)-7×7 surfaces were oxidized at room temperature (RT), 500 and 600°C and the oxidized surfaces were analyzed in situ by synchrotron radiation light-induced photoelectron spectroscopy and medium energy ion scattering (MEIS). The absolute quantities of O and Si atoms forming oxide layers were determined precisely by MEIS as a function of oxygen exposure (1 L=1 Langmuir=1×10−6 Torr s). We also measured the Si 2p core level spectra, from which the coverage of each oxidation state (Si+, Si2+, Si3+ and Si4+) was determined. The present results showed that the oxidation at RT was almost saturated with O coverage of 1.7 ML (1 ML=0.783×1015 atoms/cm2) at O2 exposure of 5 L under O2 pressure of 1×10−8 Torr, and the backbonds of the adatoms and rest atoms were almost oxidized. At this stage, the 7×7 pattern was still observed by reflection high-energy electron diffraction. It is concluded that the atomic configuration dominating the oxidized surface is an ‘ins–ins–ins–ad’ structure [an O atom bonding on top of a Si adatom (rest atom) and three O atoms inserted into three backbonds of an adatom (rest atom)]. The oxidation at 500°C (1×10−7 Torr) is almost saturated at O2 exposure of 50 L with oxygen coverage of 2.9 ML, and the oxide layer basically consists of two types of domain with intrabilayer and interbilayer termination. The oxidation at 600°C (1×10−5 Torr) is saturated with O coverage of 4.1 ML, and the oxide/Si(111) interface is also terminated with the above two types of structure.