Transition during the growth of nanoporous columns in p-type silicon: the origin of macropores

This work studies the evolution of the porous silicon layer growth at two current densities for p-type wafer. It shows that the porous layer is produced by the nucleation and growth of nanoporous columns. Below a threshold current density, the columns become separated by silicon walls i.e. a transition from a homogeneous nanoporous layer to a filled macropore layer takes place (p-Si, 10-15 Omega.cm). The nanoporous filling into the macropores shows an anisotropic structure parallel to the current direction. When the nanoporous material of such sample is dissolved, a macroporous structure appears. The transition is accompanied by a decrease in valence of dissolved silicon while the porosity and the porous silicon growth rate are not affected. Finally, taking the benefit of this morphological change, various patterned substrates are produced. They are characterised by a faceted motif and a narrow size distribution.