X-ray-diffraction investigation of the anodic oxidation of porous silicon

A systematic study of porous silicon anodic oxidation is reported. We have studied the evolution of the lattice parameter versus oxidation levels for p- and p+-type materials by using high-resolution x-ray diffraction, which gives a determination of the strains of the porous layer. The analysis of an asymmetric reflection enables us to measure the lattice parameter parallel to the surface. Moreover a diffuse scattering, observed at the bottom of the Bragg peaks, comes from the porous structure. The study of this diffuse scattering with reciprocal space maps gives information about the pore size and shape. It is shown that the evolution of the lattice mismatch parameter is related to the surface stress variations. After a review of previous works on porous silicon strains and on surface stresses of silicon we discuss the origin of the strains of as-formed and oxidized porous silicon.