Electron orbital energies of oxygen adsorbed on silicon surfaces and of silicon dioxide

The electronic structures of oxygen adsorbed on silicon (111)2×1 cleaved, (111)7×7, and (100)2×1 surfaces and of oxidized silicon have been investigated by combining electron energy-loss spectroscopy (ELS) and ultraviolet photoemission spectroscopy (UPS). In addition the surfaces were characterized by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). The electronic spectra are discussed in terms of localized $s$- and $p$-like bonds. For adsorbed oxygen one $s$-like and four $p$-like electron states are found. The structure of the surface layer is amorphous. The energies of the electron states (UPS) as well as the transition energies (ELS) were found to be almost independent of face and surface structure. Silicon can be oxidized to form a Si${\mathrm{O}}_2$-type oxygen bond by either annealing or electron bombarding the oxygen-covered surface or by bombardment with oxygen ions. For Si${\mathrm{O}}_2$, three $2p$ levels and one $2s$ level are found as expected. The electron states are compared to those of the ${\mathrm{H}}_2$O molecule. Important final states for transitions observed in ELS seem to be excitons near the bottom of the silicon conduction-band minimum for adsorbed oxygen and ∼ 1.5 eV below the conduction band for Si${\mathrm{O}}_2$.