Oxygen adsorption on a Si(1 0 0) substrate: effects on secondary emission properties

Secondary anion and electron yields resulting from low-energy (50–500 eV) Na+ bombardment of an oxygen-adsorbed Si(1 0 0) substrate have been measured as a function of oxygen exposure and of Na+ impact energy. Adsorbate coverage ranges from none to over half a monolayer. The dominant sputtered anion was found to be O− with SiO2− being a minor constituent. Kinetic energy distributions of the secondary anions and electrons were also measured. The presence of an adsorbate was observed to enhance secondary anion emission to a significant degree whereas secondary electron emission was minor, in sharp contrast to what has been observed for metallic substrates. The mechanism for secondary emission appears to involve electronic excitation of SixO−; it is suggested that electron emission is governed by a process similar to Penning ionization, in which a vacancy created by the excitation of SixO− may be filled by an electron from the valence band. The variation in the work function as oxygen accumulated on the surface was observed to be small.