Materials Science and Engineering B69 – 70 (2000) 182 – 193 Ellipsometric characterization of oxidized porous silicon layer structures T. Lohner a, *, M. Fried a , P. Petrik a , O. Polga ´r a , J. Gyulai a , W. Lehnert b a Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary b Fraunhofer -Institut fu ¨r Integrierte Schaltungen, Schottkystrasse 10, D-91058 Erlangen, Germany Abstract Electrochemically prepared porous silicon (PS) layers were oxidized thermally and investigated by spectroscopic ellipsometry (SE). The SE spectra were measured in the range of 270–850 nm with a rotating polarizer ellipsometer. The PS was modelled as a mixture of void and crystalline silicon or fine-grained polycrystalline silicon with enhanced absorption due to extensive grain-boundary regions, i.e. the complex refractive index of the layer was calculated by Bruggeman effective medium approxima- tion. The dielectric function of the fine-grained polycrystalline silicon was taken from the work published by G.E. Jellison, Jr., M.F. Chisholm, S.M. Gorbatkin, Appl. Phys. Lett. 62 (1993) 3348. The porosity, the layer thickness and the composition of the oxidized PS layers were determined. Oxidation at 900°C was performed after a stabilizing heat treatment at 320°C. The oxidation at 900°C for 10 min generated only a few nm silicon dioxide on single crystalline Si while in the case of PS with 57% porosity nearly complete oxidation was found. For PS with 68% porosity complete oxidation was observed. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Oxidation; Spectroscopic ellipsometry; Optical modelling; Effective medium approximation www.elsevier.com/locate/mseb 1. Introduction The application of porous silicon (PS) requires ap- propriate control of the electrical and optical proper- ties, which depend essentially on the morphology, the porosity and the thickness. Due to the enhanced surface area PS is a highly reactive material and sensitive to ageing effects leading to modification of its properties. The specific surface area of PS is very large: a few hundred m 2 /cm 3 according to the morphologies (‘columnar’ vs. ‘spongy’ like) [1]. Spectroscopic ellipsometry (SE) is a widely used non- destructive method for characterization of PS layers [2–16]. The depth inhomogeneity of PS layers was investigated by Tho ¨ nissen et al. by SE, Raman spec- troscopy and photoluminescence measurements [4]. They established that the thinning of the structure at the surface is related to the effects of chemical etching because of the increasing storage time of the topmost layers in the electrolyte. A porosity graded layer model was applied for porosity depth profiling of thin PS layers using variable-angle SE by Pettersson et al. [13]. Krzyzanowska et al. investigated refractive index an- isotropy of PS layers with columnar structure using multiple angle of incidence ellipsometry [17]. The illu- mination assisted formation of PS layers was studied by Tho ¨ nissen et al. [18]. Molna ´r et al. applied SE for characterization of PS layer fabricated using illumina- tion for LED structures [12]. Quantitative adsorption studies of protein molecules (human serum albumin and fibrinogen) were performed in thermally oxidized PS layers using SE [16]. Concerning the morphology and pore structure of PS ion beam analytical methods proved to be powerful techniques, for example one can determine morphological details, as porosity, average pore diameter and anisotropy of the pore directions by investigation of the widening of the 16 O(,) 16 O nuclear resonance peak [19,20]. Surface passivation of PS is an essential issue for many device application. In PS structures designed to operate as optical elements the atmospheric oxidation of the inner surfaces modifies the dielectric function of the layers. One approach to this is a passivation of PS layers in rapid thermal oxidation process [6,21]. An- other approach to this problem is thermal nitridation * Corresponding author. Fax: +36-1-3959284. E-mail address: lohner@mfa.kfki.hu (T. Lohner) 0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0921-5107(99)00257-3