Growth of silicon nanowires on UV structurable glass using self organized nucleation centres

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Physica E 38 (2007) 40–43 Growth of silicon nanowires on UV-structurable glass using self-organized nucleation centres K. Tonisch a,Ã , F. Weise a , M. Stubenrauch a , V. Cimalla a , G. Ecke a , F. Will a , H. Romanus a , S. Mrotzek a , H. Hofmeister b , M. Hoffmann a ,D.Hu ¨ lsenberg a , O. Ambacher a a Institute of Micro and Nanotechnologies, Technical University of Ilmenau, P.O. Box 100565, 98684 Ilmenau, Germany b Max-Planck-Institut fu ¨ r Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany Available online 11 January 2007 Abstract We report on the growth of silicon nanowires on photostructurable glass by low-pressure chemical vapour deposition. Thereby, no additional catalyst was needed to stimulate the growth process. Instead, a self-organized crystallization process leads to the formation of metallic clusters and seed crystals within the glass, which are supposed to initialize the nanowire growth. The nanowires were contacted by direct deposition of Pt using a focussed ion beam system and characterized electrically. r 2007 Elsevier B.V. All rights reserved. PACS: 85.35.ÀP; 81.07.Àb; 61.43.Fs Keywords: Nanowires; UV-structurable glass; FIB 1. Introduction Due to its predominant role in semiconductor technol- ogy, the bulk properties of silicon are well understood. Therefore, low-dimensional silicon nanowires offer an ideal basis to study nano-size effects and their possible applica- tions. Low-pressure chemical vapour deposition (LPCVD) is an established method to grow silicon nanowires. Usually different catalyst materials, most common is gold, are used to stimulate the growth, which have to be deposited and patterned prior to the nanowire grow th. In this paper, a low-temperature growth of silicon nanowires was accomplished on UV-structurable glass without pre- deposition of any catalyst. These kinds of glasses already contain different ions like Ce, Ag, Sb, Sn (Li, Na, and K). By a heat pre-treatment of the UV-structurable glass, the Ag ions segregate into pure metallic clusters, which then serve as nucleation centres. A succeeding heating step leads to the formation of crystallized phases within the glass matrix, whereby the shape and density of the crystals is controlled by a self-organized process depending on the process parameters. As we describe in this communication, the growth of nanowires can be stimulated on the surface of such partially crystallized glasses without adding a metal catalyst. Various methods have been reported for contacting nanowires for electrical measurements. In the most common approach, the nanowire is removed from its growth substrate and placed on an additional sample by suspension in solvents and spin-on deposition [1]. Metal contacts can be defined by optical lithography [2], electron beam lithography [1] or by focussed ion beam (FIB) [3].We present the growth of nanowires without additional catalyst and their chemical and electrical characterization. For the latter, the nanowires were contacted by direct writing platinum leads using a FIB equipment. 2. Experimental The p hotostructurable glass fro m the LiO 2 –Al 2 O 3 –SiO 2 system wa s pretreated by a n exposure t o UV light and a crystallization ste p at 570 1C. The surface of th e glass was cleaned using only organic solvents. No sp ecific m odifications ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.01.001 Ã Corresponding author. E-mail address: katja.tonisch@tu-ilmenau.de (K. Tonisch). were necessary for the succeeding growth o f nanowires. Silicon nanowires were grown on by low-pressure CVD using silane as precursor. A small fraction of phosphine was added to supp ly the phosph or necessary for n-type doping. No additional catalyst was deposited prior to the growth process, but the glass c ontained s o-called micro-d opants su ch as A gO 2 which influences the crystallization behaviour of the glass, as will be shown i n the following section. An additional substrate of silicon covered with 800 nm thermal oxide has been used for electrical characterization. Large Ti/Au contact pads were patterned via optical lithography and lift-off technique. The nanowires were removed from the growth substrate and deposited on the thus prepared secondary substrate. A focussed ion beam system (FIB 200 ODP) with a liquid Ga source was used to connect the nanowires with the contact pads by writing Pt-connections. We used 30 k V Ga + at 350 p A t o decomp ose the metal-organic P t-precursor trimethylcyclopentadienyl-platinum [(CH 3 ) 3 CH 3 C 5 H 4 Pt] into predefined bridging leads. 3. Results The substrate was found to be covered homogeneously by a dense net of nanowires; ‘‘sea urchin’’-like bundles grow only where large inhomogeneities occurred at the surface (Fig. 1). The diameter of the nanowires varied from 50 to 300 nm, their length reached 10–20 mm. The photo- structurable glass was exposed to UV light with an energy flux of 60 J/cm 2 and tempered at 570 1C for several hours prior to the nanowire growth, which leads to the formation of Ag-clusters which init ialize the growth of lithium meta silicate (LMS) crystals within the glass [4]. As a result of this pretreatment, the glass partially crystallizes into dendrite like LMS crystals within the glass matrix. The Ag clusters and LMS crystals were analysed using transmission electron microscopy (Fig. 2). Whereas the Ag cluster itself is too small to initiate the vapour–liquid– solid growth (VLS-growth) of such large nanowires, the LMS crystals exhibit a size which seems to correspond to the diameter of the nanowires. Since the eutectic tempera- ture of the Ag–Si system of about 1100 1C is well above the nanowire growth temperature, the VLS growth mode can be excluded as growth model in this case. The dendrite LMS crystals lead to a mechanical distortion of the surrounding glass matrix [5], which might in turn change the surface free energy such that the growth of silicon nanowires is favoured. However, the init ialization of the nanowire growth requires additional investigations. The chemical composition of the nanowires was checked by energy dispersive X-ray analysis (EDX) in order to exclude the possibility of having grown silica or silicon nitride instead of pure silicon nanowires. However, the nanowires and the photostructurable glass were expected to contain mainly silicon and oxygen. Therefore, a direct measure- ment would not give a reliable composition for the nanowires. Additional nanowires were grown by the same LPCVD process on a substrate consisting of GaN on sapphire with the aid of an evap orated gold layer of 2 nm which served as catalyst. Though the growth of these nanowires follows the VLS growth regime, which is not the case on the photostructurable glass, the chemical composi- tion is expected to be the same since the gas composition, which supplies the material for the nanowire growth, did not change. As expected, the EDX analysis (Fig. 3) revealed high amounts of silicon and oxygen. Additional peaks for Ga, N and Au occurred due to the substrate and growth process. A carbon peak could be associated with a contamination of the surface due to the handling under ambient conditions. The quantitative evaluation of the EDX analysis gives a content of approximately 90.4 and 4.9 at% for silicon and oxygen, respectively. Though the growth process leads to a pure silicon nanowire, a thin layer of natural silicon oxide is formed soon after the removal of the substrate from the growth chamber. No ARTICLE IN PRESS Fig. 1. Scanning electron microscopy (SEM) images of homogeneously grown nanowires (right) and sea urchin-like bundles (left). K. Tonisch et al. / Physica E 38 (2007) 40–43 41 trace of the phosphor doping could be found due to the measurement range of EDX analysis. Fig. 4 shows a representative SEM image of a FIB-Pt connected nanowire in a two-probe geometry. The nominal Pt thickness was 500 nm, and the width of the leads was 4 mm. The FIB-deposition conditions must be caref ully chosen in order to avoid damage or even destruction of delicate structures by bombardment with massive Ga + ions. Therefore, the direct scanning of the nanowires with the ion beam prior to the contact writing was reduced as much as possible. We found no evidence for such damage. On the contrary, a light sputtering at the contact areas on the nanowires might even improve the contact resistance by removing the native oxide. The conductivity of the nanowires was calculated from their electrical resistance which was determined by current–voltage measurements. The linear I–V curves of several nanowires with a diameter of 200–300 nm is displayed in Fig. 5. The different slope is mainly caused by the different wire geometry resulting in a different value of the electrical resistance. Though the thickness of the SiO 2 -shell is not known, it was neglected, because the diameter of the nanowires was ARTICLE IN PRESS Fig. 2. High resolution transmission electron microscopy (HRTEM) image of LMS crystals (marked with solid arrows) and Ag cluster (marked with open arrows) in the glass matrix (left); the outline of one of the dendrite-shaped LMS crystals is roughly marked by a line. Detailed HRTEM image of a single Ag-cluster (right; inset: fast Fourier transformation of the image). Fig. 3. EDX analysis of silicon nanowires grown on a GaN/sapphire substrate with Au as catalyst. Fig. 4. SEM image of a two-probe Si-nanowire-based device with contacts patterned by FIB-Pt deposition. K. Tonisch et al. / Physica E 38 (2007) 40–4342 large compared to the thickness of a typical native oxide layer. The specific resistance was calculated to be about 3.5 Â 10 À3 to 9.4 Â 10 À3 O cm using the wire geometries which were estimated from SEM pictures. This is in the range of typical LPCVD-grown polycrystalline silicon which was grown with the same SiH 4 /PH 3 - ratio and had a specific resi stance of 9.6 Â 10 À4 to 8 Â 10 À3 O cm. 4. Discussion and conclusions Silicon nanowires were grown successfully on photo- structurable glass without the need of an additional catalyst and were analysed with regard to their chemical composition and their conductivity. Though the LMS crystals are a prominent candidate for initializing the nanowire growth, an extended study of the first stages of the growth process is necessary in order to gain a valid model. Further research will also focus on a more detailed characterization of the nanowires concerning their elec- trical and structural properties. The contamination of the substrate surface with carbon and gallium is expected to be high due to low background pressure of the FIB system and the necessity to use the Ga + -beam for visualizing, which might lead to an incorporation of gallium. This might result in an uncertainty of the conductivity measurements due to possible leakage currents across the substrate surface. Though the agreement of the specific resistance of the nanowires with that of polycrystalline silicon supports the correctness of the electrical measure- ments, additional analysis with Auger electron spectro- scopy or other surface-sensitive measurements are necessary. Another way to eliminate the uncertainty of the conductivity measurements is the undercutting of the nanowires. Additionally, a freestanding structure will enable a mechanical characterization concerning the resonant frequency of such nanowire-based NEMS and Young’s modulus of the nanowire itself. References [1] Y. Cui, Z. Zhong, D. Wang, W.U. Wang, C.M. Lieber, Nano Lett. 3 (2003) 149. [2] B. Lei, C. Li, D. Zhang, Q.F. Zhou, K.K. Shung, C. Zhou, Appl. Phys. Lett. 84 (2004) 4553. [3] C.Y. Nam, J.Y. Kim, J.E. Fischer, Appl. Phys. Lett. 86 (2005) 193112. [4] S. Mrotzek, A. Harnisch, D. Hu ¨ lsenberg, U. Brokmann, Glass Technol. 45 (2004) 97. [5] S. Mrotzek, Ph.D. Thesis, TU Ilmenau, Ilmenau, Germany, 2005. Further Reading [6] D.A. Dikin, X. Chen, W. Ding, G. Wagner, R.S. Ruoff, J. Appl. Phys. 93 (2003) 226. [7] E.C.M. Silva, L. Tong, S. Yip, K.J. van Vliet, Small 2 (2006) 239. ARTICLE IN PRESS Fig. 5. Two-probe I–V data from FIB-Pt contacted several Si nanowires with a diameter of 200–300 nm and a length of 1.5–3 mm. K. Tonisch et al. / Physica E 38 (2007) 40–43 43 . Physica E 38 (2007) 40–43 Growth of silicon nanowires on UV-structurable glass using self-organized nucleation centres K. Tonisch a,Ã , F. Weise a , M low-temperature growth of silicon nanowires was accomplished on UV-structurable glass without pre- deposition of any catalyst. These kinds of glasses already contain