Porous silicon antireflection coatings have been obtained by stain etching in solutions of HF/HNO 3 for different etching times. The photoluminescence of the samples has been characterised. The morphology of the porous surface has been analysed by means of atomic force microscopy (AFM). For short etching times the porous surfaces reach reflectance values below 4%, and the photoluminescent intensity attains the maximum value. Also, a stabilization of the photoluminescent properties for short etching times is observed. This stabilization occurs for shorter etching times than those required for the stabilization of the porous layer width. The photoluminescence and reflectance results correlate well with the morphology data (i.e. the surface roughness and the lateral nanometer size of the imaged structures) obtained by AFM.
A comparison of the compositional and photoluminescent properties of stain etched (SE) and anodically etched (AE) porous silicon (PS) samples has been carried out. The silicon substrates used and the laboratory conditions are the same for both types of etching processes. The study is carried out varying the PS surface properties by means of different cleaning procedures and post-etching ambient conditions. The results demonstrate that the evolution of the photoluminescence and the composition are related for both types of PS (AE and SE). Thus, it seems highly likely that the photoluminescence mechanisms involved in both cases are similar.
We have studied the stress in porous silicon films as a function of depth and porosity using micro-Raman spectroscopy. Raman spectra were measured at different points along a cross section cleaved normal to the layer planes. Each spectrum was fitted using the phonon confinement model with the bulk phonon wavenumber as a free parameter. From the variation of this parameter we get the stress using the known dependence of phonon frequency on stress for bulk silicon. We observe a compressive stress at the interface with the substrate due to the lattice mismatch between porous and bulk silicon. The maximum value of the stress increases with porosity. The results obtained by Raman micro-spectroscopy agree well with the lattice mismatch measured by X-ray diffraction reported in the literature.
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