On the potential-dependent etching of Si(111) in aqueous NH 4F solution

Abstract

The topography of p-Si(111) surfaces was investigated by STM after etching in concentrated NH 4F solution at cathodic, open circuit and anodic potentials for various durations. A significant influence of the etch potential as well as the etch rate on the resulting surface topography was found. The STM data were supplemented by I( V) curves and by Auger electron spectroscopy in dependence of the applied etch potential. The cathodic and open circuit etched samples show a smoothing of the surfaces due to anisotropic etching at step sites. For cathodically etched surfaces, the time taken to reach a smooth surface is approximately 10 2 times higher than for samples etched at the open circuit potential. In the case of anodic etching, the surfaces are roughened during etching and the formation of a pore structure is initiated on a rather short time scale. The results are discussed in terms of the potential-dependent competition of three different electrochemical processes. We identified a site-selective anisotropic etch process, being most significant 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) surfaces. A site-independent isotropic etch process has relative importance for anodic potentials. Finally, anodic oxidation dominates for the highest anodic potentials.