Depending on the dopant concentration, two distinct types of porous silicon can be formed during the anodization of silicon in hydrofluoric acid. A range of samples of both types of porous silicon has been investigated using x-ray double crystal diffraction techniques. The crystal lattice of porous silicon is found to be tetragonally distorted. In the plane of the substrate, the interplanar spacing of the porous film is identical to that of the substrate but is increased in the direction normal to it. The increase is typically 700 ppm in the type of film formed on heavily doped silicon and 6000 ppm in that on lightly doped silicon. We propose that stresses, generated by the growth of a native oxide on the surface of the pores, are responsible for the observed increase in lattice parameter. The different interplanar spacings of the two types of film are related to the observed differences in their oxygen contents which are a consequence of their different surface area to volume ratios.

Young, I. M., Beale, M. I. J., & Benjamin, J. D. (1985). X-ray double crystal diffraction study of porous silicon. Applied Physics Letters, 46(12), 1133–1135.