Oxygen Etching of a High-Index Si Surface

J.L. Skrobiszewski, A.A. Baski

Department of Physics, Virginia Commonwealth University, aabaski@vcu.edu

The interaction of oxygen with Si surfaces is of fundamental interest with regard to processes such as thin-film oxide growth and surface roughening during etching. Depending on substrate temperature and oxygen pressure, O₂ can either form an SiO₂ layer or etch the surface via the creation of volatile SiO. The etching process dominates at higher temperatures and lower pressures, but there is a transition regime where both processes can occur. In this study, we have used scanning tunneling microscopy to study oxygen etching in this transition regime for the high-index Si(5 5 12) surface. Si(5 5 12) is tilted 30.5° away from (001) towards (111) and forms a single-domain reconstruction composed of row-like structures. This anisotropic morphology produces surface structures during etching that are not observed on the nearby low-index surfaces.

The figure below shows STM images of oxygen–induced structures that occur in this transition regime. Surfaces were exposed at the indicated temperatures using O₂ pressures in the mid 10^–7 Torr range. At lower temperatures and higher exposures (see left image), a high density of pyramidal islands form on terraces and preferentially along step edges. These islands are ~1 nm in height and ~20 nm in size, where the pyramidal shape is due to the presence of a (113) facet [oriented 5º from (5 5 12)]. The creation of such islands is presumably due to oxide-induced pinning sites, which remain intact while the surrounding surface is etched away. At higher temperatures and lower exposures (see middle image), 1D islands form at steps and increase in length as the terraces are etched back. These 1D islands also incorporate (113) planes, as seen in the higher resolution image shown on the right. The presence of (113) facets indicates that this orientation is etched at a significantly lower rate with respect to the (5 5 12) surface. We have not observed such (113) faceting on the low index (001) and (111) surfaces, most likely because these surfaces are oriented significantly farther away from (113). Studies are now in progress to determine the O₂ etching behavior of the Si(113) surface.