An ion scattering study of the interaction of oxygen with Si(111): surface roughening and oxide growth

Abstract

High-resolution medium energy ion scattering (MEIS) has been used to examine the initial oxidation of Si(111) at elevated temperatures (870–1170 K) and low oxygen pressures (5 × 10 −8–10 −5 Torr). These oxidation parameters cover three different parts of the ( p,T) phase space: the oxide growth regime (passive oxidation), the surface etching regime (active oxidation), and a transition (“roughening”) regime. We show that in the passive oxidation regime, a non-stoichiometric oxide grows initially only in the surface layer. This stage is followed by “bulk” (3D) oxidation. Our results demonstrate that under certain conditions, the initial oxidation may be accompanied by roughening of the surface. During active oxidation, Si leaves the surface as SiO. Vacancies are formed stochastically, and the vacancies can form vacancy islands. At relatively low temperatures, the vacancy island formation occurs much faster than step flow can smooth the surface, leading to a roughened surface. Finally, our experiments also support the existence of a transition regime between “passive” and “active” oxidation. We obtain direct evidence for the presence of oxygen on the surface in this regime. In addition, both silicon and oxygen spectra show clear evidence of roughening. Our interpretation, consistent with that of others, is that the surface is partially covered by passive surface oxide islands with the remaining area free of oxygen. Continuous etching of the bare silicon areas results in the surface developing a vertical roughness as high as 20–30 Å.