ON THE POTENTIAL-DEPENDENT ETCHING OF SI(111) IN AQUEOUS NH4F SOLUTION

The topography of p-Si(111) surfaces was investigated by STM after etc hing in concentrated NH4F solution at cathodic, open circuit and anodi c potentials for various durations. A significant influence of the etc h potential as well as the etch rate on the resulting surface topograp hy was found. The STM data were supplemented by I(V) curves and by Aug er electron spectroscopy in dependence of the applied etch potential. The cathodic and open circuit etched samples show a smoothing of the s urfaces due to anisotropic etching at step sites. For cathodically etc hed surfaces, the time taken to reach a smooth surface is approximatel y 10(7)-times higher than for samples etched at the open circuit poten tial. In the case of anodic etching, tile surfaces are roughened durin g etching and the formation of a pore structure is initiated on a rath er short time scale. The results are discussed in terms of the potenti al-dependent competition of three different electrochemical processes. We identified a site-selective anisotropic etch process, being most s ignificant at cathodic potentials, which leads to preferred etching of surface sites of local (100) geometry as compared to sites with local (111) symmetry. This process results in a smoothing of the (111) surf aces. A site-dependent isotropic etch process has relative importance for anodic potentials. Finally, anodic oxidation dominates for the hig hest anodic potentials. (C) 1998 Elsevier Science B.V.