Depth inhomogeneity of porous silicon layers

The two possible causes of depth inhomogeneities of the microstructure of porous silicon are changes in the HF concentration with depth and a varying chemical etching rate of the porous silicon layer. During anodization chemical etching will become important for microporous silicon — e.g. p-porous silicon — due to the large internal surface area, especially at long etching times. On the other hand, a considerable decrease of the HF concentration will occur during etching with high current densities to produce p+-porous silicon with high porosities. We have investigated the depth inhomogeneity of porous silicon layers by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence measurements. From a line shape analysis of the Raman signal a size distribution of nanocrystals is deduced. For p-porous silicon smaller nanocrystals are found near the surface of the layer; for p+-porous silicon etched with high current densities smaller nanocrystals are found near the porous silicon/substrate interface.