New Phenomenon in the Absorption of Oxygen on Silicon

An O₂ adsorption state on the Si(111) 2×{}1 surface has been found which will remove surface states without producing a chemical shift of the Si $2p$ core level, suggesting a strongly covalent bond. This is accomplished by using an initial exposure pressure of 10^-8 Torr. Exposures using an initial exposure pressure of 10^-6 Torr also remove the surface states but produce a 2.0 eV Si $2p$ chemical shift indicating an ionic bond different from that of SiO₂ (shift of 3.8 eV).