PHOTODECOMPOSITION OF ADSORBED METHOXY SPECIES BY UV-LIGHT AND FORMALDEHYDE ADSORPTION ON SI(111) STUDIED BY XPS AND UPS

The adsorption of methanol and formaldehyde was studied on clean p-Si( 111) in an ultrahigh-vacuum chamber equipped with X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). M ethoxy species (-OCH3) were formed as evidenced by C 1s and O 1s core levels of 287.0 and 532.8 eV, respectively, and features in the UP spe ctra at 4.3 and 9.0 eV with a shoulder at around 10 eV below the Fermi level. The methoxy species changed gradually during UPS measurements at 298 K resulting in a new species characterized by a broad UPS spect ra centered at 7 eV. The C 1s binding energy (BE) value of the remaini ng species shifted to 284.6 eV, whereas the peak area of C 1s and O 1s did not change appreciably. These results suggest that methoxy specie s are decomposed by He-II (40.8eV) UV source on Si(111). It was found that the stability of methoxy species for photodehydrogenation as well as photodecomposition was dependent on the coverage of the species fo rmed after UV irradiation. Formaldehyde adsorption was carried out to elucidate the surface species obtained by thermal and photochemical de composition of methoxy species on p-Si(111). It was found that formald ehyde is adsorbed dissociatively on Si(111) at 298 K and the species r emaining at the surface resembles that obtained in photodecomposition of methoxy species.