New Phenomenon in the Absorption of Oxygen on Silicon

An ${\mathrm{O}}_2$ 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 ${10}^{-8\mathrm{}}$ Torr. Exposures using an initial exposure pressure of ${10}^{-6\mathrm{}}$ Torr also remove the surface states but produce a 2.0 eV Si $2p$ chemical shift indicating an ionic bond different from that of Si${\mathrm{O}}_2$ (shift of 3.8 eV).