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Thue-Morse: aperiodic multilayers made of porous silicon This section is aimed at the optimization of the fabrication process and the characterization of the Thue-Morse S0-S7 sequences by using the PSi technology.
phys. stat. sol. (c) 4, No. 6, 1966–1970 (2007) / DOI 10.1002/pssc.200674342 Optical properties of porous silicon Thue-Morse structures Luca De Stefano *, 1 , Ilaria Rea 1 , Lucia Rotiroti 1 , Luigi Moretti 2 , and Ivo Rendina 1 1 Institute for Microelectronics and Microsystems, National Council of Research, Via P. Castellino 111, 80131 Naples, Italy 2 DIMET “Mediterranea” University of Reggio Calabria, Località Feo di Vito, 89060 Reggio Calabria, Italy Received 17 March 2006, revised 15 September 2006, accepted 15 November 2006 Published online 9 May 2007 PACS 42.70.Qs, 78.66.Db, 78.67.–n, 81.05.Rm Dielectric aperiodic Thue-Morse structures up to 128 layers have been realized by the porous silicon technology. Normal incidence reflectivity measurements have been performed to investigate the photonic properties of the devices. A partial photonic band gap region, centered at 1100 nm and 70 nm wide has been observed for the S 6 and S 7 Thue-Morse structures. The S 6 multilayer has been studied as sensor device on exposure to several chemical substances. 1 Introduction © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim In recent years, dielectric photonic band gap (PBG) structures have attracted great attention for their property to forbid the propagation of the light at fixed wavelengths and the possibility to realize an alloptical integrated circuit which is the basic element of the optical computer [1, 2]. The simplest, one dimensional PBG structure is the Bragg mirror which is made of alternating layers of high (nH) and low (nL) refractive index, whose thicknesses satisfy the Bragg relation 2(nHdH+nLdL) = λBm where m is the order of the Bragg condition. Lot of theoretical and experimental studies on these periodic structures have shown that they are characterized by the presence of only one degenerate PBG in a period of the reciprocal space, even if it can be omnidirectional [3]. On the contrary, multiple PBGs are typical of aperiodic structures. The main example of 1D aperiodic multilayer is given by the Thue-Morse sequence which can be obtained alternating high (A) and low (B) refractive index layers following the substitution rules A→AB and B→BA [4]. The lower-orders of the Thue-Morse sequence are: S0=A, S1=AB, S2=ABBA, S3=ABBABAAB, S4=ABBABAABBAABABBA, and so on. Dielectric Thue-Morse multilayers have been already demonstrated using Si/SiO2 and TiO2/SiO2 as high/low refractive index materials [5, 6] and recently also in porous silicon (PSi) [7]. In this study we have fabricated and characterized the Thue-Morse S0-S7 sequences by using the PSi technology. The PSi is a very attractive material due to the possibility of realize in only one process multilayered structures that exploit a high quality optical response. PSi is obtained by the electrochemical etching of crystalline silicon in a hydrofluoridric acid (HF) based solution. The porosity and the thickness of a single layer are linear functions of the current density and the anodization time for a fixed doping level of the silicon wafer and HF concentration. The refractive index of the PSi film depends on its porosity, and can be calculated in the frame of several effective medium approximations, like the Bruggemann or Maxwell-Garnett models [8]. Moreover, the PSi is an ideal material for sensing applications because it is characterized by a very large specific surface area (200-500 m 2 cm -3 ), so that an effective interaction with several substances is assured. * Corresponding author: e-mail: luca.destefano@na.imm.cnr.it, Phone: +390816132375, Fax: +390816132598 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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cm -1 (Si-O-Si) and 1065, 880 cm -1
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Abstract Physica E 38 (2007) 188-19
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190 surface through Si-C bonds. We
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192 Acknowledgments The authors are
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Introduction: PSi hybrid systems Mu
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A polymer modified PSi optical devi
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5 10 15 20 25 Conclusions We have d
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Introduction to Lab-on-chip technol
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• Glass etching The technology fo
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Many lab-on-chip applications have
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Microsystems Based on Porous Silico
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PUBLICATIONS: J1. Luca De Stefano,
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P12. L. De Stefano, L. Rotiroti, I.
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C18. L. De Stefano, I. Rea, L. Roti
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Appendix
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White light source Gas channels Sea
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% Reflectance 110 100 90 80 70 60 5
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IOP PUBLISHING JOURNAL OF PHYSICS:
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J. Phys.: Condens. Matter 19 (2007)
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treatment in the range 600-800 nm,
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Sensors 2008 7 Figure 6. Resonance
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