Published online by Cambridge University Press: 21 February 2011
The scanning tunneling microscopy (STM) images for samples treated by the NH4F (pH=8) solutions exhibited flat terraces and monolayer steps directed toward <112> directions. In addition, there exist two types of surface structures on the same surface: nearly flat parts consisting of terraces with 20-60 nm in width; step-bunched parts consisting of terraces with 1.0-1.5 nm in width which are close to (221) or (332) facets, respectively. The equilibrium width for the large terrace is determined by the competition of two etching processes with weakly-alkaline solutions (pH = 8 ): etching from the step edges and etching on the terrace. On the other hand, the narrow terrace with a few atomic distance is mainly etched from the step edges. In this case, the equilibrium shape is determined by the steric hindrance effect in addition to the anisotropic etching effect which makes the steps uniform.