Atom-resolved surface chemistry: The early steps of Si(111)-7×7 oxidation

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Abstract
The early stages of Si(111)-7×7 oxidation using scanning tunneling microscopy and scanning tunneling and photoemission spectroscopies have been studied. It has been found that there are at least two different oxygen-containing sites being formed. Their different surface site distributions and behavior as a function of O2- exposure show them to be two distinct early products. By correlating the spectroscopic results and the results of theoretical calculations one is able to identify one of these products as involving an O atom tying up an adatom dangling bond with a second O atom inserted in one of the adatom back bonds, while the other involves a single O atom inserted in an adatom back bond. The preference of these products for the faulted half of the 7×7 unit cell and for corner-adatom sites is explained in terms of a site-dependent sticking coefficient involving a process analogous to the gas-phase ‘‘harpooning’’ processes. It is shown that the majority of the resulting molecular precursors involve O2 interacting with a single dangling-bond site. Finally, mechanisms are proposed for the early steps of the oxidation process.

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