Oxidation of Si(111)7 × 7 using alkali metal atoms: evidence for local promotion mechanisms

The oxidation of Si(111)7 × 7 covered with potassium atoms has been investigated with high resolution electron energy loss (HREEL) spectroscopy and temperature programmed desorption (TPD). The amount of SiO₂ produced (monitored using the 140 meV HREEL excitation) increased in proportion to the K coverage up to 25 ML. This result indicates that the promotion effect of adding potassium atoms is strongly local, yet the HREEL and TPD spectra clearly demonstrate that the details of the promotion mechanism depend on the K coverage. The HREEL spectrum recorded after a 0.8 ML potassium layer was exposed to oxygen shows evidence only of O-Si bonding, even though the K TPD spectrum exhibits a shift toward higher temperature and a change in peak shape when oxygen was present. Oxygen interacts with the K atoms in a submonolayer, but indirectly via the Si atoms to which they are bound. In contrast, oxygen interacts with the K atoms in a multilayer directly since the HREEL spectrum shows that stable potassium oxides K_xO_y form. Heating these compounds above 300 K causes them to decompose yielding K atoms which desorb and reactive O species which diffuse to make SiO₂.