Influence of the doping level on the microstructure of P-type porous silicon studied by small-angle X-ray scattering

Small-angle scattering of X-rays (SAXS) can lead to a better understanding of the microstructure of porous silicon because the pore radii range (2-10 nm) corresponds to the small-angle scattering investigation range (1-100 nm). A recent SAXS study performed by some of the present authors on porous layers prepared on lightly doped and heavily doped silicon wafers revealed strong differences according to the dopant type. Here the dependence of the microstructure of P-type porous silicon on its doping level is focused upon. In nondegenerate P-type silicon, the porous structure is an apparently random distribution of voids. The scattering pattern obtained in that case is isotropic. In degenerately doped P-type silicon (P⁺), the porous film structure consists of many long voids running perpendicular to the surface, with small `buds' on their sides. SAXS experiments carried out with a two-dimensional detector revealed the anisotropic character of the P⁺ microstructure.