Fabrication and optical characterization of nanoporous alumina films annealed at different temperatures L.F. Marsal * , L. Vojkuvka, P. Formentin, J. Pallarés, J. Ferré-Borrull Nano-electronic and Photonic Systems Group (NEPHOS), Departament d’Enginyeria Electrònica, Elèctrica i Automàtica, ETSE, Campus Sescelades, Universitat Rovira i Virgili, Avda. Països Catalans 26, 43007 Tarragona, Spain article info Article history: Received 28 July 2008 Received in revised form 12 September 2008 Accepted 24 September 2008 Available online 8 November 2008 PACS: 78.67.n 78.20.Ci 82.45.Yz 78.20.Bh Keywords: Nanoporous alumina Self-ordering process X-ray diffraction Transmission spectra abstract We report on the structural and optical properties of self-ordered nanoporous anodic alumina (NAA) films annealed at different temperatures. The morphology of NAA films is examined by scanning electron microscopy (SEM). The results show that the porous structure have hexagonally ordered arrays of nanopores with interpore distance in the range of 90–100 nm and pore size between 30 and 40 nm. The structural properties studied by X-ray diffraction (XRD) show out that the nanoporous alumina expe- riences a transition from the amorphous phase to gamma and alpha phase when annealed from 800 to 1200 °C. The optical transmission spectra of the annealed NAA films with different thicknesses (9, 24 and 45 lm) are measured in the wavelength range of 300–1000 nm. Numerical simulations based on an optical model of the films show a good agreement with the measurements. The refractive index (n) and the extinction coefficient (k) are determined for each annealed temperature. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction In recent years there has been a tremendous growth in the field of nanotechnology. Nanoporous anodic alumina (NAA) films have attracted a great scientific and technological interest as a material for nanotechnology applications. NAA is produced by electrochem- ical anodization of aluminium in acidic electrolyte. Masuda and Fukuda [1] proposed a two-step electrochemical procedure to pre- pare alumina with highly ordered nanopore arrays. The morphol- ogy of NAA can be effectively modified by adjusting the anodizing conditions such as the electrolyte, temperature, voltage or time, so that one can obtain a nanoporous structure with inter- pore distance in the range of 50 to 400 nm and pore sizes between 5 and 200 nm [2–6]. The unique geometrical properties of these NAA films provide a wide range of potential applications such as magnetic storage [7], sensors [8], light emitting diodes [9], solar cells [10] or photonic crystals [11,12]. Recently, NAA films have been used as template or host material for preparation of nano- wires [13,14], nanotubes [15,16] or metal nanohole arrays [17]. In order to extend the applicable fields of the NAA films a deep knowledge of the physical properties is required to understand the obtained results and to improve the derived devices. Specifically, the optical properties of NAA films are essential from the point of view of optoelectronic devices. Recent studies on the optical properties of NAA films have been focused on photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR) properties [18–20]. However, there are few de- tailed studies of wavelength dependence on refractive index and extinction coefficient and their dependence on structural properties. The principles of optical and structural properties, that are both dependent on each other, still need to be clarified. Therefore in this work, we focus on optical features of nanoporous alumina related to crystallization process. The structural properties of NAA films during crystallization were studied in-situ by increasing the tem- perature up to 1200 °C. The optical transmission spectra of amor- phous and crystallized NAA films with different thicknesses were measured in the ultraviolet, visible and near infrared range. From the optical transmission spectra, we determine the dependence of refractive index and extinction coefficient for each annealed temperature and crystalline phase. 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.09.008 * Corresponding author. Address: Dept. Electronic Engineering, ETSE, University Rovira i Virgili, Avda. Paisos Catalans 26, 43007 Tarragona, Spain. Tel.: +34 977559625; fax: +34 977559605. E-mail address: lluis.marsal@urv.cat (L.F. Marsal). Optical Materials 31 (2009) 860–864 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat