From active to passive oxidation: O-2 on Si(111)7X7

The interaction of oxygen with the Si(1 (1) over bar 1) 7 x 7 surface was s tudied by in situ real-time experiments in a high-temperature scanning tunn eling microscope at temperatures between 350 and 600 degrees C for oxygen p artial pressures of 10(-8) to 10(-7) mbar during an exposure ranging from 0 to 200 L. The early adsorption stage (0-2 L) is dominated by the occurrenc e of dark adatom slates. There are also small numbers of various other feat ures such as bright and grey states which are partially reversible or show random walk behaviour. In the transition region between active and passive oxidation regimes (500-600 degrees C, 10(-8)-10(-7) mbar) we observe etchin g of step edges and holes and simultaneously homogeneous or heterogeneous o xide nucleation at surface defects with further lateral oxide growth affect ing the next Si layer after terrace retraction. This competition leads to a rather rough surface morphology when the step edges an locally Dinned by t he oxide. The oxidation reaction towards the passive regime is characterize d by the formation of a homogeneous thin oxide (passivating film) on the en tire surface without indications for an island growth mode from where furth er oxide growth proceeds slowly. From the temperature dependence of the ste p-flow mode etch rate in the active regime, we determine an activation ener gy of (1.6 +/- 0.2) eV which is comparable to the step edge detachment ener gy of atoms required for annealing of oxygen-induced vacancies on the terra ces. (C) 1999 Elsevier Science B.V. All rights reserved.