Evaluation of free carrier losses to 1.54 lm emission in Si/Si:Er nanolayers on SOI substrate for optical gain observation N.N. Ha ⇑ , K. Dohnalová, T. Gregorkiewicz Van der Waals-Zeeman Institute, University of Amsterdam, 65 Valckenierstraat, NL-1018 XE Amsterdam, The Netherlands article info Article history: Received 25 June 2010 Received in revised form 5 August 2010 Accepted 25 August 2010 Available online 21 September 2010 Keywords: Er-doped silicon Optical gain Induced absorption abstract In this study we use a combination of variable stripe and shifting excitation spot methods to evaluate lin- ear low temperature (4.2 K) optical properties of Si/Si:Er multinanolayer grown on SOI substrate. In par- ticular, Er-1 luminescence at 1.54 lm under continuous wave excitation was examined. Absorption coefficients of 22.4 ± 4.2 and 45.1 ± 4.2 cm 1 at photon fluxes of 2.4  10 19 and 2.4  10 20 cm 2 s 1 , respectively, have been established. These are approximately three times higher than those found earlier for a similar structure grown on Si substrate. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Doping with transition metals [1,2] and rare-earth ions [3] is frequently used for tailoring of optical properties of semiconduct- ing hosts. Among different systems of Er-doped crystalline Si (Si:Er) are particularly well investigated [4]. Thanks to the indirect excitation mechanism via host, Er-doped crystalline silicon has a quite high excitation cross section of the 1.54 lm Er-related emission [5]. However, in spite of many ad- vances in Si photonics [6–8], no efficient optical gain has been demonstrated up to date [9]. We also note that present approaches to Si photonics require external resonant excitation or employ sophisticated fabrication procedures. Therefore, search for Si:Er la- ser continues. In our recent study of the Si/Si:Er multinanolayer structure on Si substrate [10], we showed that the free carrier losses have masked observation of any possible gain. Pulsed excitation has decreased the induced absorption by free carriers while one could expect its value to be independent of excitation regime. This was inter- preted in terms of a possible gain which was realized in the multi- nanolayer structure. On the way to engineer a more suitable design for optical gain realization, we now investigate multinanolayer structures grown on SOI substrate. With this structure, on one hand, we expect a higher gain coefficient due to isolating effect by the SOI layer. On the other hand, however, it could increase free carrier losses due to free carriers’ confinement under laser excitation. In this work, we have carried out the combination of variable stripe-length and shifting excitation spot experiments in the steady-state of excitation in order to evaluate these non-linear effects and the pos- sibility of net gain. 2. Sample preparation and experiments 2.1. Sample preparation The investigated samples were grown by sublimation molecu- lar-beam-epitaxy (MBE) method [11]. The total thickness of epitax- ial layer is about 2 lm, with 500 periods of 1.5 ± 2 nm Er-doped and 3 nm undoped Si layers on SOI substrates (NoSOI). The SOI substrate comprises a layer of 1.5 lm (100)-oriented, p-type, 18 X cm Si, 1 lm buried thermal oxide layer, and 500 lm (100)- oriented, n-type, 0.3–1.2 X cm substrate. For optical activation [9,10], annealing was carried out in a continuous flow of nitrogen (99.99% purity, 5  10 5 cubic feet per minute rate) at 800 °C for 30 min. The average concentration of Er +3 ions in the doped re- gions, determined by secondary ion mass spectroscopy, was [Er] = 3.5  10 18 cm 3 . A similar structure grown on Si substrate (NoSi) which was investigated in Ref. [10] is used as a reference for the current measurements. 2.2. Experiments Laser induced non-linear optical effects were studied by the variable stripe-length (VSL) method [13] combined with the shift- ing excitation spot (SES) method [14]. The first method is based on excitation of a stripe-like region on the sample (creating condition for single-pass optical amplification) while the second one involves 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.025 ⇑ Corresponding author. E-mail address: N.H.Ngo@uva.nl (N.N. Ha). Optical Materials 33 (2011) 1094–1096 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat