Effect of the quantum confinement on the optical absorption of porous silicon, investigated by a new in-situ method

A new in-situ technique allowing accurate measurements of the porous silicon optical absorption coefficient α is described. This technique is based on the measurement of the photocurrent generated in the substrate under light excitation through the porous layer, and does not require to dry the porous layer or to detach it from the silicon substrate. The absorption spectra of p-type and p+-type porous layers have been measured from 1.5 to 3 eV for a wide range of porosities. The results are analysed by focusing on the possible effect of quantum confinement. It is concluded that confinement does not account for the α dependence upon porosity over the full investigated range, but that it is only responsible for the faster decrease of α towards the absorption edge that is observed for highly porous and luminescent layers. An absorption model is developed that takes into account the effect of the quantum confinement on the density of states of the silicon nanocrystallites and that complies with the observed spectral behavior.