Room temperature scanning tunneling microscopy images of Si(001)-(2 × 1) surfaces exposed to ${\mathrm{O}}_2$ at pressures $1\times {10}^8<P_{\mathrm{ox}}<4\mathrm{}\times {10}^{-6\mathrm{}}$ torr and temperatures $500<T<\mathrm{}700\mathrm{}$°C show dramatic surface roughening via formation of long step "fingers" and multilevel Si islands. This is caused by nucleation of oxide clusters, which pin step edges during oxidation-induced surface etching. The nucleation rate $J_{\mathrm{ox}}$ (estimated by counting pinning sites) was found to scale as $P_{\mathrm{ox}}^m$, with $m\ge 2$. This is consistent with a simple model in which two diffusing surface oxygen species are required to nucleate a stable oxide cluster.

• Received 26 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.999