Porous silicon nanocracking

We report on the collapse of porous silicon on the nanometer scale observed during a study of freeze dried low doped p-type samples. During the drying process highly porous layers, about 4 μm in depth, changed their colour from an initial bright brown to a well defined green at the end of the procedure. However, the mirror aspect of the sample was kept. This phenomenon which results from a change of the optical path (nd) cannot uniquely be attributed to a refractive index variation. Simulation of reflectivity spectra shows that compared to the expected values, the thickness is lower and the density of material higher. An analysis of the profile shows that the volume can be reduced by as much as a factor of six. X-ray and TEM observations have confirmed these data and have shown that these samples have lost their crystallinity and are completely amorphous. In situ observations shows that this compression occurs during the sublimation or evaporation phase. During this phase, if the sample is wetted again the initial colour of the sample can be recovered as an effect of the elasticity of the PS structure. We attributed the origin of this compaction to nanoscopic cracking due to capillary effects, as in the case of the well known macroscopic cracking.