High resolution X-ray diffraction measurements are performed on porous
silicon samples (p(+) type, 40% porosity) of various thicknesses (fro
m 0.03 to 20 mu m). In the thicker samples narrow Bragg peaks are obse
rved for as-formed samples, with profiles close to those of dynamical
diffraction theory. For thin samples (in the 0.1 mu m range) the Bragg
peak of the porous layer is not resolved, but appears as a shoulder o
n the substrate peak. Thickness fringes are observed, from which simul
ations give information on these thin PS layers, showing that the poro
us layer surfaces are well defined.