Mg clusters on MgO surfaces: Characterization with metastable impact electron spectroscopy, ultraviolet photoelectron spectroscopy (HeI), and temperature programmed desorption

MgO films (2 nm thick) were grown on Mo and W substrates while metastable i mpact electron (MIES) and ultraviolet photoelectron spectroscopy (UPS) (HeI ) spectra were collected in situ. Apart from the valence band emission no a dditional spectral features have been detected with electron spectroscopies . After exposing the oxide surface to Mg (substrate temperature between 100 and 300 K) an additional peak not seen with UPS, located within the band g ap, shows up in MIES. This band, located at similar to 2 eV above the top o f the valence band with a full width at half maximum of similar to 1 eV at the lowest exposures, can be detected in MIES until its intensity falls bel ow a level of 10(-3) of that from the valence band. This additional emissio n is attributed to the formation of small, nonmetallic Mg clusters. The ene rgetic position of the cluster emission closely matches that of the expecte d ionization of surface F-S(+)/F-S centers. It is proposed that in the init ial phase of the Mg exposure, F center-type defects are produced close to e xtended defects, such as steps, corners, etc. These Mg-induced defects appe ar to play an important role as nucleation sites for cluster formation. At sufficiently large exposures a densely packed Mg film forms. (C) 1999 Ameri can Vacuum Society. [S0734-2101(99)20504-0].