Relation between electroluminescence and photoluminescence in porous silicon

We present combined measurements of electroluminescence (EL) and photolumin escence (PL) in p-type porous silicon. The EL spectra were measured using a n electrolyte contact for electron injection into the porous face of the sa mple. Upon applying the current, the EL intensity first rises with time, re aches a maximum, and then decays to zero. (The whole process takes about ha lf an hour.) At the same time, the peak of the EL spectrum shifts from appr oximate to 850 nm in the beginning to approximate to 600 nm at the end of t he process. The FL, which was measured simultaneously, peaked at approximat e to 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 practic ally disappears. This shifts the PL peak towards the blue, to about the sam e wavelength as the EL peak (approximate to 600 nm) and the spectrum become s much narrower, comparable to the EL spectrum. The voltage across the samp le during the EL process shows a moderate increase up to the point where th e EL disappears, and then the voltage rises steeply. This behavior is assoc iated with the build-up of a thin oxide layer on the porous surface. The co mbined results of EL and FL, and especially the disappearance of the red pa rt in the photoluminescence spectrum at the end of the EL process, suggest that in addition to quantum confinement, localized surface states play an i mportant 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. (C) 2000 Elsevier Science S.A. All rights reserved.