Interpretation of the dielectric function of porous silicon layers

The dielectric function of porous silicon layers depends strongly on the nanostructure of the silicon skeleton. In this paper we discuss the main effects, as determined from spectroscopic ellipsometric measurements of the dielectric functions of porous layers formed on p-doped material. Finite-size effects and the high inner surface area of the nanostructure lead to relaxation of k-momentum conservation as defined for infinite crystals and therefore to a broadening of the optical structure arising from interband critical points. In addition a small threshold energy shift is observed when the percolation of the structure is reduced. However, this shift is too small to explain red photoluminescence as a consequence of a pseudo-direct gap whose energy is blue-shifted by confinement.