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Trapping levels in fresh (one month) and naturally aged (one year) nanocrystalline porous silicon have been investigated using the optical charging spectroscopy method. Four significant maxima and/or shoulders were observed for fresh samples and five for aged ones. They have been attributed to five and six trapping levels, respectively. The trapping centers corresponding to the most shallow four levels are situated at or nearby the internal surface of the porous silicon films.

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The trap parameters of defects in Si/CaF 2 multilayered structures were determined from the analysis of optical charging spectroscopy measurements. Two kinds of maxima were observed. Some of them were rather broad, corresponding to "normal" traps, while the others, very sharp, were attributed to stress-induced traps. A procedure of optimal linear smoothing the noisy experimental data has been developed and applied. This procedure is a Electronic mail: ciurea@infim.ro 2 based on finding the minimal value of the relative error with respect to the value of the smoothing window. In order to obtain a better accuracy for the description of the trapping-detrapping process, a Gaussian temperature dependence of the capture crosssections characterizing the stress-induced traps was introduced. Both the normal and the stress-induced traps have been characterized, including some previously considered as only noise features.