Two-dimensional photonic crystals with pure germanium-on-insulator M. El Kurdi a , S. David a , X. Checoury a , G. Fishman a , P. Boucaud a, * , O. Kermarrec b , D. Bensahel b , B. Ghyselen c a Institut d’Electronique Fondamentale, CNRS Univ Paris Sud, Ba ˆtiment 220, F-91405 Orsay Cedex, France b STMicroelectronics, 850 rue Jean Monnet, 38920 Crolles, France c Soitec, Parc technologique des fontaines, 38190 Bernin, France Received 19 July 2007; received in revised form 26 September 2007; accepted 3 October 2007 Abstract We have investigated pure germanium two-dimensional photonic crystals. The photonic crystals which exhibit resonances in the near infrared spectral range were fabricated on germanium-on-insulator substrates using standard silicon-based processing. The germanium- on-insulator substrate consists of a thin layer of pure germanium-on-oxide deposited on a silicon substrate. The optical properties are probed by the direct band gap optical recombination of pure germanium at room temperature. Resonant optical modes are evidenced between 1.68 and 1.53 lm in different type of hexagonal cavities (H1–H5). The spectral position of the modes is controlled by the lattice periodicity and air filling factor of the photonic crystals. Close to the Ge band edge, the quality factors are limited by the bulk material absorption. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.70 Qs; 78.55. –m; 78.66.Db Keywords: Photonic crystal; Germanium; Photoluminescence 1. Introduction Very strong progress has been achieved over the past years in the field of silicon photonics either for passive or active devices. Cavities with record quality factors have been demonstrated by using photonic heterostructures fab- ricated in silicon membranes [1]. On-chip optical buffers with very long optical delays lines have been fabricated with microring resonators and group delays up to 500 ps with an ultracompact design have been reported [2]. Pulsed and continuous wave Raman lasers with silicon as the active medium have also been demonstrated [3]. These advances are not limited to pure silicon. A strong interest is also devoted to structures containing Ge since this mate- rial is compatible with the standard CMOS processing of the microelectronics industry. Ge/Si self-assembled islands can be used as internal sources to probe the optical proper- ties of two-dimensional photonic crystals [4,5]. Pure Ger- manium is characterized by a very large refractive index (4.1–4.2 in the near infrared as compared to 3.5 for sili- con). It also exhibits a direct band gap around 0.8 eV at room temperature not far in energy from the indirect band gap (0.66 eV). This direct band gap results in strong absorption coefficients at 1.5 and 1.3 lm. Pure germanium photodetectors on silicon with very high cut-off frequencies and large responsivities have been demonstrated with this system [6,7]. Quantum confined Stark effects based on exci- tonic transitions with pure germanium quantum wells grown on relaxed buffer layers on silicon have also been 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.10.008 * Corresponding author. Tel.: +33 1 69 15 40 92; fax: +33 1 69 15 40 90. E-mail address: philippe.boucaud@ief.u-psud.fr (P. Boucaud). www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 846–850