Ellipsometric characterization of oxidized porous silicon layer structures

Electrochemically prepared porous silicon (PS) layers were oxidized thermally and investigated by spectroscopic ellipsometry (SE). The SE spectra were measured in the range of 270–850 nm with a rotating polarizer ellipsometer. The PS was modelled as a mixture of void and crystalline silicon or fine-grained polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions, i.e. the complex refractive index of the layer was calculated by Bruggeman effective medium approximation. The dielectric function of the fine-grained polycrystalline silicon was taken from the work published by G.E. Jellison, Jr., M.F. Chisholm, S.M. Gorbatkin, Appl. Phys. Lett. 62 (1993) 3348. The porosity, the layer thickness and the composition of the oxidized PS layers were determined. Oxidation at 900°C was performed after a stabilizing heat treatment at 320°C. The oxidation at 900°C for 10 min generated only a few nm silicon dioxide on single crystalline Si while in the case of PS with 57% porosity nearly complete oxidation was found. For PS with 68% porosity complete oxidation was observed.