Optical activation of Si nanowires by Er 3+ doped binary Si Al oxides films derived from sol gel solutions

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1 Optical Materials 30 (2007) Optical activation of Si nanowires by Er 3+ doped binary Si Al oxides films derived from sol gel solutions Lingling Ren a, Won-Young Jeung a, Hee-Chul Han b, Kiseok Suh c, Jung H. Shin c, Heon-Jin Choi b, * a Korea China Advanced Materials Cooperation Center, KIST, Seoul, Republic of Korea b School of Advanced Materials Science and Engineering, Yonsei University, Seoul , Republic of Korea c Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea Received 30 March 2006; accepted 15 December 2006 Available online 13 March 2007 Abstract The optical activation of Si nanowires (SiNWs) by the coating of Er 3+ doped binary silicon aluminum oxides (Si Al oxides) films derived from sol gel solutions is reported. Continuous and crack-free Si Al oxide film could be successfully coated onto an Si substrate where the SiNWs were grown. The strong Er 3+ luminescence of lm from the SiNWs was observed at a high Er concentration (5 at.%). These results suggest that the Al Si oxide film makes it possible to realize a strong Er 3+ luminescence by excluding concentration quenching while at the same time improving the quality of film. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Si nanowire; Sol gel; Luminescence; Er 3+ doped silicon and aluminum oxides; Concentration quenching 1. Introduction * Corresponding author. Tel.: ; fax: address: hjc@yonsei.ac.kr (H.-J. Choi). Erbium (Er) doped semiconductors have received considerable attention due to the luminescence wavelength of the Er 4f shell of approximately 1.54 lm, which is an important telecommunication wavelength [1,2]. Much progress has been made with Er-doping of silicon-rich silicon oxide (SRSO), which consists of nanocluster Si embedded inside a SiO 2 matrix [3 10]. However, these isolated nanocrystals cannot be efficiently addressed electrically. Using Si nanowires (SiNWs), however, may solve such a problem. These nanowires have diameters in the range of nm and lengths exceeding 1 lm, thus providing a very high areal density of Er 3+ ions for Er-doping. Moreover, the electrical doping, transport, and fabrication of active electronic devices based on SiNWs have been reported, demonstrating the ease of injection into, and transport along, the SiNWs [11,12]. The optical activation of Er coated SiNWs has already been achieved, and consisted of SiNWs embedded inside an Er-doped silicon oxide film in a previous study [12]. In that system, the Er cations that were introduced into a silicon oxide network tended to accumulate due to the absence of non-bridging oxygen [13]. The clustering of Er may induce concentration quenching. In fact, the quenching of the photoluminescence (PL) intensity at a high concentration of Er should be avoided in order to realize strong luminescence in Si-based optical devices. In order to prevent the clustering of erbium ions in the silica network, Al ions could be added into the silicon oxide structure. In this case, the optically harmful Er clustering could be prevented by the selective coordination of Al 3+ around the Er 3+ ions [14] allowing for a homogeneous dispersion of Er ions in the silicon oxide structure. The addition of Al ions could also change the refraction index of the silicon oxide due to the relatively high refraction index of /$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi: /j.optmat

2 498 L. Ren et al. / Optical Materials 30 (2007) Al 2 O 3 (Al 2 O 3, n = 1.64 is higher than SiO 2 (n = 1.45)) [15], thus improving the optical transfer efficiency. Furthermore, aluminum oxide possesses a low thermal expansion coefficient [16], high chemical durability, and good mechanical properties that are helpful in providing a high-quality, continuous film that is important in electron-injected Si optical devices [17 21]. Here, our preliminary study on the optical activation of SiNWs coated with Er 3+ -doped silicon and aluminum oxides derived from a sol gel solution is presented. The addition of Al ions into the silica film enables the production of a continuous film with little concentration quenching and strong Er 3+ luminescence at lm. 2. Experimental ASB, Al(O-sec-C 4 H 9 ) 3, and TEOS (tetraethylorthosilicate) were used as oxide precursors. The sol gel solution was prepared in two steps [20]. TEOS was initially dissolved in ethanol and prehydrolized by an addition of water and hydrochloridric acid (TEOS/EtOH/HCl/H 2 O = 1:10/0.01/ 2). The sol was aged at room temperature for 24 h, and an ethanolic solution of ASB peptized with hydrochloric acid (HCl) (ASB/EtOH/HCl/H 2 O = 1:50:0.1/2) was subsequently added to give a final Si/Al atomic ratio of 0:1, 1:0, 3:1 and it was aged for an additional 48 h at room temperature. For the preparation of the erbium-doped coatings, ErCl 3 Æ 6H 2 O was introduced in the aged TEOS and ASB solutions with an Er/Si and Er/Al atomic ratio of 0.06:1 and 0.17:1, corresponding to an Er concentration of 9 at.%, that is, approximately 30 times that in a previous study by the authors using an Er Si sol gel solution [12]. A transparent, homogeneous and pink sol gel solution was then obtained. SiNWs were grown on Si(100) wafers through a VLS (vapor liquid solid) mechanism in a quartz tube furnace using 2-nm-thick sputtered Au film as a catalyst [12]. To grow the SiNWs, H 2 and Ar gases were introduced at a flow rate of 100 sccm (standard cubic centimeter per minute), respectively, and SiCl 4 was then introduced using an H 2 carrier gas that was bubbled through the liquid SiCl 4 held at 5 C at a flow rate of 5 sccm. The growth temperature was C. Silicon wafers (5 10 1mm 3 ) growing with SiNWs were obtained. The Si wafers with SiNWs were dipped in the prepared sol gel solution. The dip-coating process was controlled, with each wafer dipped and removed three times at a rate of 100 mm/min. The samples were then heat-treated at 750 C for 1 h in air and at 950 C for 5 min in flowing Ar gas. They were then cooled to room temperature in order to achieve oxide-structure-coated SiNWs for the PL test. A thermogravimetric analysis (TGA, Perkin Elmer, TGA6) was carried out in order to determine the optimum heat-treatment conditions. During the measurements, the sample was heated at a rate of 10 C/min in temperature range of C in air. The Er 3+ photoluminescence (PL) spectra were measured using Ar laser, a grating monochrometer, and a thermo-electrically cooled InGaAs detector, using a standard lock-in technique. 3. Results and discussion 3.1. Characteristics of sol gel solution To prepare high-quality Er-doped oxide films on the SiNWs, the thermogravimetric behavior of Si Al sol gel solution was initially investigated. As shown in Fig. 1, significant weight loss below 150 C and a slight further weight loss between 150 C and C were observed. The weight loss below 150 C, at 55% of the total weight, was due to desorption and decomposition of the adsorbed/coordinated water and the organic solvent. The weight loss between 150 C and 600 C resulted from the polymerization of the hydroxyl groups and the formation of oxides at higher temperatures. It was reported that 900 C is the porous to non-porous glass conversion temperature for a Si Al oxide system [22]. Based on the thermogravimetric behavior of the sol gel film, 750 C was select as the heat treatment temperature of the sol gel solution capable of best disposing of the OH groups. At that temperature, the film remains continuous and the SiNWs are also clearly seen, as shown in Fig. 2. Fig. 3 shows the powder XRD patterns of the heat-treated sol gels with compositions of the Er Al (a), Er Si (b) and Er Si Al (c) solution heat-treated at 750 C. To address the effect of the addition of Al to Er Si solution, the Er Al and Er Si solutions were characterized and compared. For the Er Al solution, c-al 2 O 3 (PDFN ) and Er 2 O 3 phase (PDFN ) were observed, as shown in Fig. 3a, which is consistent with previous studies [16,18] where c-al 2 O 3 is achieved at a temperature lower than 900 C. A small number of new peaks were also observed, conceivably due to the new (Al,Er) 2 O 3 phase created due to the valence match between the rare-earth ions (Er 3+ ) and the substituted cation (Al 3+ ). Weight (%) Temperature ( o C) Fig. 1. TGA curve of Er 3+ doped Si Al sol gel.

3 L. Ren et al. / Optical Materials 30 (2007) does not change the structures. However, it is expected that a more homogeneous incorporation of Er ions into Al Si network compared to the Si network is likely by the selective coordination of Al 3+ around the Er 3+ ions [14] Characteristics of the coated SiNWs Fig. 4 shows a scanning electron microscope (SEM) image of the as-grown SiNWs on the substrate. As shown in Fig. 3, the dense arrays of straight SiNWs, which are approximately 100 nm in diameter and 10 lm in length, lie at an angle nearly 60 from the surface [23]. Fig. 2. SEM image of Er 3+ doped Si Al oxides coating on SiNWs film heated at 750 C (zoom image) derived from the sol gel solution (Si/ Al = 3:1). Fig. 3. Powder XRD patterns of Er 3+ doped Si and Al oxides sintered at 750 C. Triangle (n) represents Er 2 O 3 peaks; round (s) represents c-al 2 O 3 peaks. (a) Al 2 O 3 ; (b) SiO 2 ; (c) Al 2 O 3 SiO 2. Fig. 3b shows an XRD pattern of the Er Si oxides. This shows the amorphous structure due to the short range ordering of the Si network [23]. The Er 3+ -doped Si Al oxides also show an amorphous structure, as shown in Fig. 3c, indicating that the incorporation of Al into the Si network Fig. 4. SEM images of SiNWs. Fig. 5. SEM images of Er 3+ doped Si Al sol gel solutions coated on SiNWs film with different Si/Al ratio: (a) Si/Al = 0:1; (b) Si/Al = 1:0; (c) Si/Al = 3:1.

4 500 L. Ren et al. / Optical Materials 30 (2007) Fig. 5 shows SEM images of the films derived from the Er Al (a), Er Si (b) and Er Si Al (c) sol gel solutions. A small number of cracks in the film derived from the Er Al sol gel solution appeared (Fig. 5a), while numerous splits emerged in the film derived from the Er Si sol gel solution (Fig. 5b). However, after addition of Al ions into the Er Si solution, the film becomes continuous without any cracks, as shown in Fig. 5c. A zoomed image further shows that the sol gel solution has completely penetrated the SiNWs arrays, and had formed a crack-free, integrative film with tight SiNWs. The higher quality of film derived from Al Si solution is due to the comparable thermal expansion coefficient of the film over that of the Si substrate. The thermal expansion coefficient of SiO 2,Al 2 O 3 and Si is /K, /K and /K, respectively. Thus, the cracks in the Er Si oxide film are due to large differences in the thermal expansion coefficient between the SiO 2 and the Si binary system, whereas the cracks in the Er Al oxides are due to the crystallization of the film. In addition, an addition of Al to the Si-oxide while maintaining its amorphous structures matches the thermal expansion coefficient of the film ( /K, according to the rule of mixture) to the Si substrate and suppressed the formation of cracks during the heattreatment. Fig. 6 shows the room temperature PL spectra of the SiNWs coated with Er 3+ -doped Si Al oxide, pumped with the 477 nm line of an Ar laser. The 477 nm line was chosen as it is absorbed only by SiNWs and not directly by Er 3+ ions [12]. This ensures that Er 3+ excitation occurs via carriers only, and represents an accurate simulation of the situation under the current injection. A strong Er 3+ luminescence was observed lm from the SiNWs, indicating an energy transfer from carriers in the SiNWs to the Er 3+ ions. Moreover, the stronger PL intensity at the high Er concentration (9 at.%) was observed as compared to that at a low Er concentration (0.33 at.%) in a previous study [12]. This is due to the prevention of only optically harmful Er clustering by the addition of Al to PL Intensity Wavelength (μm) Fig. 6. Room temperature PL spectra of Er 3+ doped Si Al oxides coating SiNWs film heat-treated at 750 C, wave guide with 477 nm excitation from an Ar + laser. Si/Al = 3:1. the SiO 2 film [14], which prevents the concentration quenching. In addition, it can be seen that the FWHM (full width at half maximum) of the luminescence is approximately 56 nm. This emission band structure is especially typical, and may be attributed to the Stark splitting of Er 3+ embedded in the amorphous structure. It could also be attributed to the presence of many different environments for Er 3+ ions in the binary Si Al oxides. The broadening of the spectra suggests a wider and homogeneous distribution of Er 3+ sites in the matrix. 4. Conclusion Optical activation of Si nanowires (SiNWs) coated with Er 3+ doped binary Si Al oxides derived from a sol gel solution is achieved. The addition of Al ions into Er Si sol gels causes the coating film to be homogeneous and crack-free due to the comparable thermal expansion coefficient to the Si substrate. The addition of Al ions into Er Si sol gels also contributes to a strong Er 3+ luminescence at lm from the SiNWs observed at high Er concentrations due to the homogeneous dispersion of Er ions into the Si Al network, while excluding the concentration quenching. These two aspects have important implications for the fabrication of optical devices based on SiNWs. Acknowledgements This work was supported by Korea Research Foundation of the Korea Government (MOEHRD) (KRF C00024), and the Ministry of Information and Communications in Korea and the Basic Research Program in ETRI. References [1] A. Polman, J. Appl. Phys. 82 (1997) 1. [2] X. Orignec, D. Barbier, X.M. Du, R.M. Almeida, O. McCarthy, E. Yeatman, Opt. Mater. 12 (1999) 1. [3] N.-M. Park, T.-Y. Kim, S.H. Kim, G.Y. Sung, K.S. Cho, J.H. Shin, B.-H. Kim, S.-J. Park, J.-K. Lee, M. Nastasi, Thin Solid Films 475 (2005) 231. [4] F. Priolo, G. Franzo, D. Pacifici, V. Vinciguerra, F. Iacona, A. Irrera, J. Appl. Phys. 89 (2001) 264. [5] S.-Y. Seo, J.H. Shin, B.-S. Bae, N. Park, J.J. Penninkhof, A. Polman, Appl. Phys. Lett. 82 (2003) [6] T. Makimura, K. Kondo, H. Uematsu, C. Li, K. Murakami, Appl. Phys. Lett. 83 (2003) [7] P.G. Kik, A. Polman, J. Appl. Phys. 88 (2000) [8] M. Schmidt, J. Heitmann, R. Scholz, M. Zacharias, J. Non-Cryst. Solids (2002) 678. [9] P.G. Kik, A. Polman, Mater. Sci. Eng. B 81 (2001) 3. [10] L. Khriachtchev, M. Räsänen, Appl. Phys. Lett. 86 (141911) (2005) 1. [11] Z. Wang, J.L. Coffer, Nano Lett. 2 (2002) [12] K. Suh, J.H. Shin, O.-H. Park, B.-S. Bae, J.-C. Lee, H.-J. Choi, Appl. Phys. Lett. 86 (2005) [13] K. Aray, H. Namikawa, K. Kumata, H. Tatsutoku, Y. Ishii, T. Handa, J. Appl. Phys. 59 (1986) [14] Choon Kun Ryu, Kyekyoon Kim, Appl. Phys. Lett. 66 (1995) [15] X.J. Wang, M.K. Lei, Thin Solid Films 476 (2005) 41.

5 L. Ren et al. / Optical Materials 30 (2007) [16] I. Jaymes, A. Douy, D. Massiot, J.P. Coutres, J. Non-Cryst. Solids 204 (1996) 125. [17] G.N. van den Hoven, E. Snoek, A. Polman, Appl. Phys. Lett. 62 (1993) [18] M. Jimenez de Castro, R. Serna, J.A. Chaos, C.N. Afonso, E.R. Hodgson, Nucl. Instrum. Meth. B (2000) 793. [19] X.J. Wang, M.K. Lei, T. Yang, B.S. Cao, Opt. Mater. 26 (2004) 253. [20] L. Armelao, S. Gross, G. Obetti, E. Tondello, Surf. Coat. Tech. 109 (2005) 218. [21] X.J. Wang, M.K. Lei, T. Yang, H. Wang, Opt. Mater. 26 (2004) 247. [22] M. Nogami, J. Non-Cryst. Solids 178 (1994) 320. [23] A.I. Hochbaum, R. Fan, R. He, P. Yang, Nano Lett. 5 (2005) 457.