Investigation of nanostructured porous silicon by Raman spectroscopy and atomic force microscopy

A set of porous silicon layers was produced by stain etching using a solution of HF:HNO3 in a composition ratio of 500:1 with etching time varying from 1 to 10 min. The residual stress and the correlation length of the layers were obtained through the analysis of the micro-Raman spectra using a phonon confinement model including a term to account for the amorphous phase. The correlation length, which corresponds to the crystallite size, is found to decrease as the etching time increases, and agrees with the values obtained by atomic force microscopy analysis. The residual compressive stress tends to increase with etching time, as expected by the smaller crystallite size. However, an oscillatory behavior is detected for etching times higher than 4 min. This result correlates well with the photoluminescence measurements and can be understood by the porous silicon layer formation observed with the atomic force microscopy. (C) 2004 Elsevier B.V. All rights reserved.