phys. stat. sol. a) 182, 291 2000) Subject classification: 78.20.Bh; 78.20.Ci; 78.55.Mb; S5.11 Effective Medium Correlations for Experimental Absorption Data J. Tagu Èen Äa-Marti Ânez 1 ), J.A. del Ri Âo, and J.E. Lugo Centro de Investigacio Ân en Energõ Âa UNAM, A.P. 34, 62580 Temixco Mor., Me Âxico Received March 12, 2000) The goal of this work is to model experimental optical data in porous silicon within the framework of the effective medium theories and to analyze the role of including the effect of charges in the nanostructures. The presence of charge produces a blueshift in the optical response similarly to quantum confinement. Also, the intensity of the excitation luminescence spectra decreases when the charge is present in the nanostructures. We explore these effects experimentally and we con- clude that charge may give a significant change in the optical properties of porous silicon. Introduction The observed photoluminescence in electrochemically edged porous sili- con PSi) [1] is the reason behind the great interest in the possible applications of this material. Although there has been a big research effort, there are still some effects which remain to be studied. In particular, it has been pointed out that the electrochemi- cal etching includes interactions with induced charges that in turn induce electrostatic polarization [2]. Furthermore, when the electrochemical reaction ends, PSi contains a lot of dangling bonds [3], where some charge could to be trapped altering the opto- electrical properties of PSi [4]. Certainly, the presence of any charge modifies the di- electric function regardless of its origin. Different approaches to study the dielectric function in quantum confined systems show that an appreciable reduction takes place when the size of the structures is of the order of 2 nm and below [2, 5, 6]. However, the reduction in the dielectric function in the limit of long wavelengths is not only due to changes in the electronic spectrum. For example, these changes represent only a small role in the increment of the binding energy of impurities for small nanoscale silicon particles, while most of the increase is due to the induced polarization charges at the boundary of the dielectric discontinuity [2]. Lannoo et al. [4] have calculated the binding energy of hydrogenic impurities in PSi. They considered Coulomb charging effects and the so-called image corrections to analyze the dielectric properties for charged semiconductor crystallites and they found a shift of the conduction band edge. Tsu and Babic [2, 7] have also studied the effect on the binding energy of a charged inclusion and they have explained the origin of the red±orange luminescence in PSi by using polarization phenomena. The effective dielectric function e eff represents the macroscopic response of an inho- mogeneous system to the electromagnetic field. This function depends on the dielectric functions and the volume fractions of each component of the system. In previous works [8±10] we have used effective medium approximations to describe the optical and elec- J. Tagu Èen Äa-Marti Ânez et al.: Effective Medium Correlations for Absorption Data 291 1 ) Corresponding author; Fax: 52 73220014; e-mail: jtag@servidor.unam.mx