Relation between electroluminescence and photoluminescence in porous silicon

We present combined measurements of electroluminescence (EL) and photoluminescence (PL) in p-type porous silicon. The EL spectra were measured using an electrolyte contact for electron injection into the porous face of the sample. Upon applying the current, the EL intensity first rises with time, reaches a maximum, and then decays to zero. (The whole process takes about half an hour.) At the same time, the peak of the EL spectrum shifts from ≈850 nm in the beginning to ≈600 nm at the end of the process. The PL, which was measured simultaneously, peaked at ≈750 nm in the beginning and was much wider than all of the EL spectra. Towards the end of the EL process, the red part of the PL spectrum practically disappears. This shifts the PL peak towards the blue, to about the same wavelength as the EL peak (≈600 nm) and the spectrum becomes much narrower, comparable to the EL spectrum. The voltage across the sample during the EL process shows a moderate increase up to the point where the EL disappears, and then the voltage rises steeply. This behavior is associated with the build-up of a thin oxide layer on the porous surface. The combined results of EL and PL, and especially the disappearance of the red part in the photoluminescence spectrum at the end of the EL process, suggest that in addition to quantum confinement, localized surface states play an important role in the luminescence process, at least in the red part of the spectrum. Such states may be associated with adsorbed species and disappear upon oxidation.