Time-resolved two-photon photoemission study of silicon surface at initial stage of oxidation

Time-resolved two-photon photoemission study has been performed on Si(1 1 1) surface at initial oxidation stage that is characterized by synchrotron-radiation photoemission spectroscopy. Transient 2PPE intensity from the conduction band minimum (CBM) shows a maximum at the delay time between 0.5 and 2 ps depending on the oxygen dosage. The temporal profile of 2PPE intensity from surface state within the bulk band-gap shows a more rapid decrease than that on Si(0 0 1) surface, indicating that the metallic surface state on Si(1 1 1) surface causes the shorter lifetime of unoccupied surface state. The prolonged lifetime of 2PPE intensity from CBM after a large amount of O2 exposure is caused by the disappearance of metallic surface state.