Thin Solid Films 460 (2004) 53–57 0040-6090/04/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.01.067 Determination of the refractive index of n - and p-type porous Si q samples S. Setzu *, R. Romestain , V. Chamard a,b, a a Laboratoire de Spectrometrie Physique, Universite Joseph Fourier – CNRS (UMR 5588), B.P. 87, 38402 St Martin d’Heres Cedex, France a ´ ´ ` INFM, Sezione di Cagliari – Dipartimento di Fisica, Citt. Universitaria, 09042 Monserrato (CA), Italy b Received 14 June 2003; received in revised form 26 November 2003; accepted 13 January 2004 Available Online 12 April 2004 Abstract Photochemical etching of porous Si layers has been shown to be able to create micrometer or submicrometer-scale lateral gratings very promising for photonic applications. However, the reduced size of this lateral periodicity hinders standard measurements of refractive index variations. Therefore accurate characterizations of such gratings are usually difficult. In this paper we address this problem by reproducing on a larger scale (millimeter) the micrometer scale light-induced refractive index variations associated to the lateral periodicity. Using this procedure we perform standard X-ray and optical reflectivity measurements on our samples. One can then proceed to the determination of light-induced variations of porosity and refractive index. We present results for p-type samples, where the photo-dissolution can only be realized after the formation of the porous layer, as well as for n -type samples, where light action can only be effective during the formation of the porous layer. q  2004 Elsevier B.V. All rights reserved. PACS: 81.05.Rm; 81.40.Tv; 81.70.Fy Keywords: Porous silicon; Refractive index; Photo-dissolution; Electro-photo-formation 1. Introduction Large refractive index variations with porosity, togeth- er with relatively simple fabrication techniques and low manufacturing costs, make porous silicon (henceforth PS) a very promising candidate for photonic applications w1–4x. PS formation is usually obtained by electrochem- ical dissolution of Si wafers in aqueous or ethanoic HF solutions. Porosity (and hence refractive index) control is usually achieved by regulating the formation’s current density along the dissolution direction. The presence of light w5,6x strongly affects the for- mation of PS layers through the participation of photo- generated holes in the dissolution process. It is thus possible to realize structures with a modulated lateral porosity w7x, simply by using spatially modulated illu- mination. Using a dark and clear fringe interference *Corresponding author. Tel.: q39-070-675-4934; fax: q39-070- 510-171. E-mail address: susanna.setzu@dsf.unica.it (S. Setzu). pattern, we generate a micrometer-scale lateral modula- tion of the porosity, similar to a lithographic mask at the sample’s surface. The clear fringes allow the acti- vation of the photodissolution reaction while in the regions under the dark fringes no activation of the photochemical process occurs, leading to unmodified porous layers. The effect of the photodissolution is of course an increase of the layer porosity by an amount that depends on the light and on the exposure time. To study lateral gratings we need to know the light- induced variations of the optical and structural parame- ters. However, direct determinations of these variations are difficult, due to the reduced size (micrometer or submicrometer-scale) of their lateral periodicity. There- fore we have used an alternative approach, based on the natural assumption that porosity and refractive index local variations are independent of the size of the illuminated area. Within this assumption, if we are able to reproduce photo-electro-chemical reactions on a larger scale, we can measure porosity and refractive index