AN ION-SCATTERING STUDY OF THE INTERACTION OF OXYGEN WITH SI(111) - SURFACE ROUGHENING AND OXIDE-GROWTH

High-resolution medium energy ion scattering (MEIS) has been used to e xamine the initial oxidation of Si(111) at elevated temperatures (870- 1170 K) and low oxygen pressures (5 x 10(-8)-10(-5) Torr). These oxida tion parameters cover three different parts of the (p,T) phase space: the oxide growth regime (passive oxidation), the surface etching regim e (active oxidation), and a transition (''roughening'') regime. We sho w that in the passive oxidation regime, a non-stoichiometric oxide gro ws initially only in the surface layer. This stage is followed by ''bu lk'' (3D) oxidation. Our results demonstrate that under certain condit ions, the initial oxidation may be accompanied by roughening of the su rface. During active oxidation, Si leaves the surface as SiO. Vacancie s are formed stochastically, and the vacancies can form vacancy island s. At relatively low temperatures, the vacancy island formation occurs much faster than step flow can smooth the surface, leading to a rough ened surface. Finally, our experiments also support the existence of a transition regime between ''passive'' and ''active'' oxidation. We ob tain direct evidence for the presence of oxygen on the surface in this regime. In addition, both silicon and oxygen spectra show clear evide nce of roughening. Our interpretation, consistent with that of others, is that the surface is partially covered by passive surface oxide isl ands with the remaining area free of oxygen. Continuous etching of the bare silicon areas results in the surface developing a vertical rough ness as high as 20-30 Angstrom.