Porous silicon characterization by x-ray reflectivity : problems arising from using a vacuum environment with synchrotron beam

Thin (300 nm) porous silicon layers have been formed on p-doped silicon substrate to be characterized by x-ray reflectivity. The objective was to determine the layer thickness, the depth profile of the porosity and the interface roughness as functions of the anodization conditions. A very intense synchrotron beam was used (≈1019 photons m-2 s-1) and under vacuum (≈0.13 Pa) was chosen for the measurements to limit as much as possible the well know phenomenon of oxide film growth on the pore walls during sample exposure to air. Results are reported for two silicon substrates anodized with different current densities and electrolyte compositions. They show that, despite the precaution of making measurements under vacuum, an unexpectedly fast sample evolution took place during irradiation, leading to a dramatic reduction in porosity. A plausible explanation of this phenomenon is reported, and the severe implication in comparing structural data obtained by different characterization techniques is underlined.