Characterization of Porous Silicon Layers by Reflectance Spectroscopy
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CHARACTERIZATION OF POROUS SILICON LAYERS BY REFLECTANCE SPECTROSCOPY
W.TheiB, P.Grosse, I. Phys. Inst., Aachen Technical University, Germany H.Miinder, H.Lfith, Institut fi~r Schicht- und Ionentechnik (ISi), Forschungszentrwm Jfilich, Germany R.Herino, M.Ligeon, Laboratoire Spectom~trie Physique, Universit• Joseph-Fourier, Grenoble, France
Abstract The characterization of porous silicon layers by optical reflectance spectroscopy in the infrared, visible and UV is presented. A fit of simulated to measured spectra is used to interprete the experimental results. We stress that the microgeometry of the porous system determines the optical properties to a large extent and must be taken into account in a correct way in order to achieve reliable results.
Introduction Porous Silicon is - due to its quite strong photo- and electroluminescence in the visible - a very promising material for photonic devices. The origin of the efficient light emission is still under discussion but a relation of the extraordinary properties of porous silicon to the microgeometry with structures in the nanometer range seems to be very likely [1,2]. A nondestructive characterization of porous silicon by optical reflectance spectroscopy in the infrared, visible and near UV provides information about free charge carrier properties, impurities such as oxygen and hydrogen and electronic transitions - therefore, quantities closely related to the luminescence mechanism can be investigated by reflectance spectroscopy. However the inhomogeneous microstructure (fig. 1, left) must be taken into account whenever a characterization of a porous material by some analytical method has to be done. For the case of reflectance spectroscopy with light wavelengths much greater than 100 nm this can be done using effective medium theories. These average the dielectric functions of the individual microscopic phases to the macroscopically observable effective dielectric function that determines the optical properties of the system. In this averaging procedure the topology influence is treated more or less simplified by the various effective medium theories. A model based on the so-called Bergman theorem has turned out to be very successful and is used in this work. The characterization of the porous silicon layers is then done by a comparison of measured reflectance spectra to model calculations where microscopic parameters such as the carrier or impurity concentrations are varied until good agreement is achieved. Results for typical porous silicon layers are presented. Previously reflectance spectroscopy has been applied to characterize porous silicon layers [3,4] but the analysis was based on a quite simple effective medium theory which can be improved as shown in this work. Experimental methods In order to characterize the porous silicon layers the simplest experimental method has been chosen: reflectance spectroscopy. It is applicable to nontransparent samples (e.g. highly doped silicon substrates), no sample preparation (like cutting the sample in small pieces)
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