Oxygen Etching of Si surfaces
(2-pg summary in PDF format)
The interaction of oxygen with Si surfaces is of fundamental importance in Si-based semiconductor research and technology. The exposure of a Si substrate to molecular O₂ produces oxide growth and/or etching, depending on the substrate temperature and oxygen pressure. Lower temperatures and higher exposures promote surface oxidation, whereas higher temperatures and lower exposures result in etching of the Si surface. There is a transition regime, however, where both mechanisms are in competition with each other. We have used STM to study this transition regime for the low-index Si(001) and Si(111) surfaces, and the high-index Si(113) and Si(5 5 12) surfaces.

Si(5 5 12) Surface

The clean Si(5 5 12)2×1 surface forms a single domain of row-like structures oriented along the [110] direction (0.77 × 5.35 nm² unit cell). Although this surface reconstruction is relatively complicated, two types of Si row structures are most prevalent, tetramer rows and p-chains. The most commonly observed surface defects are disruptions in the periodicity of the Si(5 5 12) unit cell, and the presence of adsorbed dimers.

Au "Nanowires" on Si(5 5 12)

When low coverages of Au or Ag (q < 0.25 ML) are deposited onto Si(5 5 12) and moderately annealed (~ 450°C), long overlayer rows are formed. This "nanowire" phase of growth is shown to the right, where the brightest rows correspond to Ag . These rows are ~ 2 nm wide and have a minimum inter-row spacing equal to the underlying Si periodicity (5.4  nm).  Noble metals optimally interact with the Si substrate, preferentially bonding to the surface without significantly disrupting the underlying periodic structure.