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 degrees 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 similar to 4 nm with a void fraction of similar to 40%, obtained using an effective medium approximation; the AFM image for this sample indicates a roughened surface of similar to 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 (similar to 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.