NH4OH-treated Si(111) surfaces studied by spectroscopic ellipsometry and atomic force microscopy

Chemically treated Si(111) surfaces in NH4OH:H2O=1:5 alkaline etchant at 80°C were investigated using spectroscopic ellipsometry (SE) and ex situ atomic force microscopy (AFM). The SE data indicate that when a native oxide layer is partly etch-removed, the resulting surface is very rough. The roughened layer thickness in this case is approx.4 nm with a void fraction of approx.40%, obtained using an effective medium approximation; the AFM image for this sample indicates a roughened surface of approx.4.5 nm rms. Just after the oxide layer is etched away completely, the SE data yield the spectrum of a nearly flat Si surface. The AFM image confirms a microscopically flat surface (approx.0.2 nm rms) with the emergence of relatively large triangular hollows. With further etching, the Si surface becomes microscopically rougher, as revealed by the SE and AFM.