Microstructural control of porous silicon by electrochemical etching in mixed HCl/HF solutions

The role of hydrochloric acid in the fabrication of porous silicon was investigated with atomic force microscopy (AFM) and spectroscopic ellipsometry. The study was focused on surface morphology, in terms of roughness and power spectral density (PSD), and layer microstructure and quality in terms of thickness, porosity, interface sharpness and layer homogeneity. The results show that HCl can be used as an additional component in the standard hydrofluoric acid-based etching solution in order to affect different properties of porous silicon layers such as volume porosity, thickness and pore size. In addition, HCl/HF mixtures can be used to obtain samples with sharper porous silicon/silicon interfaces and less in depth inhomogeneities as compared with porous silicon samples having similar volume porosities and thicknesses and manufactured in conventional solutions containing HF only. Whereas large variations in both thickness and volume porosity can be obtained by conventional solutions, the HCl/HF interplay yield large variations in one of the mentioned parameters only.