From active to passive oxidation:: O2 on Si(111)7X7

The interaction of oxygen with the Si(1 (1) over bar 1) 7 x 7 surface was studied by in situ real-time experiments in a high-temperature scanning tunneling microscope at temperatures between 350 and 600 degrees C for oxygen partial 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 occurrence of dark adatom slates. There are also small numbers of various other features 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 etching of step edges and holes and simultaneously homogeneous or heterogeneous oxide nucleation at surface defects with further lateral oxide growth affecting the next Si layer after terrace retraction. This competition leads to a rather rough surface morphology when the step edges an locally Dinned by the oxide. The oxidation reaction towards the passive regime is characterized by the formation of a homogeneous thin oxide (passivating film) on the entire surface without indications for an island growth mode from where further oxide growth proceeds slowly. From the temperature dependence of the step-flow mode etch rate in the active regime, we determine an activation energy of (1.6 +/- 0.2) eV which is comparable to the step edge detachment energy of atoms required for annealing of oxygen-induced vacancies on the terraces. (C) 1999 Elsevier Science B.V. All rights reserved.