Scanning-tunneling-microscopy observation of thermal oxide growth on Si(111)7×7 surfaces

The submonolayer regime of the thermal (600 °C) oxide growth process on Si(111)7×7 surfaces has been studied in comparison with room-temperature (RT) oxidation using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and STM simulation based on the empirical tight-binding method. STM measurements show randomly distributed dark or depressed areas on surfaces oxidized at room temperature and at 600 °C in the background of unreacted 7×7 adatoms. The area ratio of the total dark area to the entire surface is found to increase with oxygen exposure and to be comparable to the oxygen coverage measured by XPS, indicating that the dark areas represent oxidized regions. STM images reveal differences in the distribution and pattern of the dark areas between room-temperature and 600 °C oxidations, even with the same oxygen coverage. The dark areas of the 600 °C oxidation are highly segregated like islands, while at room temperature they are as small as single atoms and uniformly distributed over the entire surface. This result provides direct evidence of island formation during high-temperature oxide growth, as previously suggested in photoemission studies. Furthermore, STM images reveal that the islands of 600 °C oxidation are darker than the oxides of RT oxidation. Tight-binding calculations ascribe the darker areas to regions where a larger number of Si layers are oxidized. This result indicates that 600 °C oxidation proceeds in a vertical direction (normal to the surface) even in the submonolayer regime.