Single and multilayer porous silicon structures for photonic applications J. Anto Pradeep, Purabi Gogoi, Pratima Agarwal * Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781 039, India Available online 12 February 2008 Abstract Porous silicon (PS) apart from its diverse properties from bulk Si is also identified to be a photonic material. PS single and multilayer samples are prepared on a polished surface of (1 0 0) Si substrate. A peak fitting technique has been discussed for XRD analysis in the case of a dichromatic beam. Theoretical simulations of light transmission for some special structures are also presented. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Mb; 68.65.Ac; 42.25.Bs Keywords: Nanocrystals; X-ray diffraction; Photonic bandgap; Porosity 1. Introduction Electromagnetic waves in photonic crystals (PCs) behave analogously to electrons in semiconductors. This means, the concept of band structure, dispersion relation, Brillouin zones, and so on used more frequently with elec- trons can be applicable to photons as well [1,2]. Since pho- tons are non-interacting, band theory makes more sense for photons than for electrons. The predictions of the theory can be very well checked by experiments with PCs. The phenomenon like Bloch oscillations invited many debates [3] in its early days because of the difficulties with electrons can now be realized in PCs [4,5]. To observe any quantum phenomenon in solids, the coherence length of the electron should be greater than the system size. Since electrons are strongly interacting, the coherency is lost in a short time span. On the other hand, photon coherencies are greatly appreciated. The only difficulty in PCs is they are artificially made crystals unlike semiconductors. It also requires a high degree of sophisti- cation for fabrication. Fortunately, porous silicon (PS) apart from its many properties differed from bulk Si [6–8], is also identified to be a photonic material [9]. PS is widely prepared [10] by anodization of Si in an electrolyte of HF and C 2 H 5 OH with certain proportions. Constant and modulated current den- sities in time results in the formation of single and multi- layer PS, respectively. Refractive index in PS is related to the porosity [11] which in turn, controlled by the current density. The true refractive index of the material is that of Si, but in the region where the wavelength of light is much longer than the pore dimension, an effective refrac- tive index can be assigned. Since the pore dimensions are of few nanometers in certain PS [12], PCs can be realized in the range of visible or infrared wavelengths. Here, we report the preparation of single and multilayer PS, X-ray diffraction (XRD) studies for crystallite size estimation and photoluminescence (PL) characterizations on single layer PS (PSL), the achievability of refractive index modu- lation with depth in PS multilayers (PSML) and the theo- retical simulations of photon propagation for some special structures. 2. Experimental Porous silicon (PS) is prepared by electrochemical etch- ing of a monocrystalline, (1 0 0), p-type (Boron doped) sil- icon (c-Si) wafer in dark. Anodization is performed in an electrolyte of HF (48% aqueous) and ethanol with 1:1 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.092 * Corresponding author. Tel.: +91 361 2582702; fax: +91 361 2582749. E-mail address: pratima@iitg.ernet.in (P. Agarwal). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 354 (2008) 2544–2547