Influence of the electrolyte viscosity on the structural features of porous silicon

The influence of an increase in the electrolyte viscosity, obtained by reducing the chemical bath temperature, replacing a volume fraction of ethanol with the heavier glycerol and increasing the anodization current density, was studied in porous silicon layers formed on p-type silicon. Synchrotron X-ray reflectivity and cross-sectional transmission electron microscopy techniques were used for 300-nm-thick porous silicon layers. For greater thicknesses (10 μm), the temperature effect was investigated by multicrystal X-ray diffraction with a laboratory source. It was found that higher average porosities and lower roughnesses at the interface between porous silicon and substrate are obtained when the electrolyte viscosity increases. Moreover, the formation of flatter porosity depth-profiles is favored by lowering the electrolyte temperature. The importance of these effects on the performances of optical devices based on porous silicon is underlined.