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 mu m in depth, changed their colour
from an initial bright brown to a well defined green at the end of the proc
edure. However, the mirror aspect of the sample was kept. This phenomenon w
hich results from a change of the optical path (nd) cannot uniquely be attr
ibuted 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 volum
e 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 tha
t this compression occurs during the sublimation or evaporation phase. Duri
ng this phase, if the sample is wetted again the initial colour of the samp
le 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 capi
llary effects, as in the case of the well known macroscopic cracking. (C) 2
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