- Email: [email protected]
Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates
ARTICLE IN PRESS
Physica E 37 (2007) 11–15 www.elsevier.com/locate/physe
Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates Renato Angeluccia,, Rita Rizzolia, Vincenzo Vinciguerrab, Maria Fortuna Bevilacquac, Sergio Guerria, Franco Corticellia, Mara Passinia a CNR-IMM Bologna, via Gobetti 101, Bologna 40129, Italy STMicroelectronics, 10 Science Park Road, #02-08, 117684, Singapore c STMicroelectronics, Piazzale Enrico Fermi 1, Porto del Granatello—Portici, Napoli 80055, Italy b
Available online 6 October 2006
Abstract In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly microfabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide ‘‘pillars’’, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De; 61.46.Fg; 73.63.Fg Keywords: Carbon nanotubes; Catalytic chemical vapor deposition; Iron
1. Introduction Carbon nanotubes (CNTs) [1,2] are novel allotropic 1D (one dimensional) forms of carbon consisting of hollow cylindrical-shaped molecules with graphitized walls that can be considered as molecular nanowires. CNTs have emerged in the field of molecular electronics because of their excellent conducting properties allowing the manufacturing of devices such as FETs (field effect transistors) [3–5] and SETs (single electron transistors) [6–8]. A strong key requirement for a successful CNTs integration with silicon is the development of a growth technology compatible with the standard manufacturing processes of the microelectronic industry. Moreover, the ability to grow CNTs with predictable and reproducible properties, i.e. controlling their semiconducting or metallic characteristics, on suitable substrates, at a fixed location Corresponding author. Tel.: +39 0516399120; fax: +39 0516399216.
E-mail address: [email protected] (R. Angelucci). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.07.043
and with a desired orientation on surfaces, is still a great challenge for the fabrication of hybrid carbon micro and nanocircuits. This ability, including the control over CNTs diameter and chirality, would allow the exploitation of their unique electrical, thermal and mechanical properties, in a wide range of electronic devices as emitters in flat panel displays, bio and DNA sensors and in the IC thermal management. Considering this scenario, we have developed a site-selective catalytic chemical vapor deposition (CCVD) synthesis of CNTs on silicon-based substrates, which is based on a standard process of the silicon technology, the CVD technique. In this paper, a modified CCVD process, exploiting iron nanoparticles as catalysts, methane as carbon gas feedstock and hydrogen as carrier gas at 900 1C, has been developed in order to get reproducible horizontally oriented nanotubes for FETs and other electronic devices. Modifications of the CCVD process are necessary in order to switch from multi-walled nanotubes (MWNTs) to single-walled nanotubes (SWNTs) and from a vertical to a horizontal growth of isolated SWNTs.
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Single horizontal CNTs satisfy both the requirements of an easier process to manufacture a CNT–FET and an unequivocal structural and electrical characterization of the single nanotube. 2. Experimental In this study, we investigate methods to control the directionality and location of nanotubes by means of patterned substrates and proper dispersion of iron nanoparticles. In the specific, substrates of silicon oxide and polysilicon have been patterned with trenched structures, about 500 nm deep, and their embossed surface has been used to pattern the iron-catalyst. The width of the trenches ranged between 1.4 and 5 mm. In the case of thermal SiO2, the trench patterns have been obtained by a dry etching process. In order to smooth the finishing SiO2 surface a wet etching process in NH4F and HF mixture and a soft cleaning process in ozone, have been performed. The polysilicon layer has been deposited by CVD of silane on a thermal silicon oxide substrate. The polysilicon mesa structures have been defined using an anisotropic dry etching process, avoiding the rounding effects by means of sacrificial oxide layers. Iron nanoparticles have been obtained both from an evaporated Fe film, deposited by e-gun at a rate of 5 A˚/s for 2 s, to get a nominal layer thickness of 1 nm and from iron nitrate solutions accurately dispersed on the substrates. In the case of catalyst delivered by iron solution, a liquid-phase catalyst, 0.4 mM of iron nitrate Fe(NO3)3 9H2O in isopropilic alcohol (IPA), was dispersed on the substrate both by spin coating and dip coating techniques. Hence, for spin coating, 100 ml of the solution was transferred onto the sample, spun at 500 rpm for 5 s and calcined in oven at 80 1C for 10 min; for dip coating the samples were immersed in a beaker with 200 ml of IPA iron solution for 4 h and then dried in air. The CNTs were grown by CCVD in a hot-wall quartz
furnace at 750 Torr, 900 1C and with a methane flow rate of 140 sccm. The catalyst nanoparticles were formed in H2 atmosphere during the heating up to the deposition temperature, (60 min in H2 ¼ 200 sccm) and the following annealing step (900 1C, 20 min, H2 ¼ 16 sccm). All the samples have been characterized by using the field emission scanning electron microscopy, FE-SEM, (Leo 1550, with 1 nm/4 nm lateral resolution) as the customary characterization technique for the morphology. Deeper and more accurate information on the CNTs structure have been carried out by means of the micro-Raman spectroscopy (Renishaw 1000 spectrometer with a HeNe laser source, at 633 nm and maximum output power of 15 mW) and AFM (dimension 3100, nanoscope III, Digital Instruments) technique. The Raman D-band, G-bands and radial breathing modes (RBM) features have been fitted by Lorentzian curves. The SWNTs diameters have been calculated by the RBM peak positions using the relation oRBM ¼
C1 dk
þ C2
with C1 ¼ 248 cm1 nm, C2 ¼ 0 cm1 and k ¼ 1, as reported in the literature for isolated nanotubes on a SiO2 substrate [9]. C1 ¼ 223.75 cm1 nm and C2 ¼ 0 cm1 have also been reported for suspended CNTs [10]. The cut-off of the notch filter in our Raman spectrometer is around 120 cm1, excluding the observation of SWNTs with diameters larger than 2 nm. 3. Results and discussion The SEM micrograph on the left side of Fig. 1 shows one of the CNTs horizontally grown on silicon dioxide substrate, bridging across the trenches. In this case, the iron catalyst has been deposited by spin coating the 0.4 mM iron nitrate/IPA solution onto the sample. Moreover, the SEM micrograph evidences a quite clean and particle free
Fig. 1. On the left SEM micrograph of an isolated CNT bridging across a trench of SiO2. CNTs have been grown by using Fe from salt solution (spin coating) as metal catalyst. On the right AFM measurement on a 1.25 1.25 mm2 area of an isolated SWNT bridging across two adjacent trenches, originating from Fe nanoparticles on SiO2. The edges of trenches are visible.
ARTICLE IN PRESS R. Angelucci et al. / Physica E 37 (2007) 11–15
oxide surface, whereas agglomerates of byproducts (likely determined by a mixture of iron and carbon) can be observed at the bottom of the trench. In order to gain further information on the suspended nanotubes, we carried out an AFM analysis by operating in tapping mode. On the right side of Fig. 1, the AFM image of the same sample confirms the existence of CNTs bridging the adjacent trenches, allowing to conclude that these nanotubes are tightly bounded at the extremities. From AFM inspection an upper limit of the nanotube diameter, equal to 5 nm, can be estimated. Analogous results have been obtained with the same spin coating deposition technique on the polysilicon mesa structures. As shown in the SEM micrograph on the left side of Fig. 2, also in this case a straight carbon nanotube protrudes between the mesas. A map of the Raman spectra, collected along the trenches of the same polysilicon sample, reveals the presence of RBM in the low frequency region.
13
This allows classifying the bridging CNTs as SWNTs. On the right side of Fig. 2, the Raman spectrum in the region of the RBM shows a narrow feature corresponding to a resonant SWNT of 1.77 nm diameter. The RBM features are resolved on a background of photoluminescence due to the scattering of impurities. The 303 cm1 feature comes from the Si substrate. Moreover, the straight nanotube gives clues on the absences of kinks in the honeycomb network of the bridging CNTs. As far as the other catalyst deposition techniques are concerned, it is possible to observe that both the e-gun evaporation of 1 nm thick iron film, and the simple immersion of the substrates in the 0.4 mM iron nitrate/ IPA solution, allow to synthesize SWNTs crossing the microfabricated structures. The Raman spectra of Fig. 3, D and G bands region and RBM region, confirm the excellent quality of the as-grown SWNTs for the case of 1 nm iron evaporated onto polysilicon structures. The feature and
Fig. 2. On the left SEM micrograph of an isolated CNT bridging between two poly-Si mesas. CNTs have been grown using Fe nanoparticles from a spin coated Fe salt solution. On the right Raman spectrum of one of the isolated SWNTs of the same sample.
Fig. 3. Raman spectra of isolated SWNTs grown on a poly-Si substrate after evaporating 1 nm of iron as a catalyst. The D, G+ and G bands are resolved. The low frequency region shows many features coming from RBM of SWNTs and the 303 cm1 feature of Si substrate.
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position of G and G+ peaks, 1569 and 1590 cm1, respectively, indicate the semiconducting nature of the resonant SWNTs. From the features of the RBM spectrum the corresponding SWNTs diameters of 1.78, 1.28 and 1.00 nm have been calculated. In the case of flat and clean surfaces as in the case of the CNTs grown by preparing the catalyst from a salt solution on silicon oxide, the AFM analysis can give good indications. The AFM topography of Fig. 4 shows SWNTs obtained on iron catalyst delivered by dip coating technique. In the 3D view it is possible to observe two crossed nanotubes. The section analysis of each structures allows to conclude that these nanotubes are SWNTs, with diameters d1 ¼ 1.270.3 nm and d2 ¼ 1.47 0.2 nm. The analysis of the height of the crossing of the two CNTs provides a value of 2.670.8 nm, that is consistent with the sum of the two diameters. The two techniques exploited for the iron deposition, determine a different density of the iron particles covering the structures. This results in a different density of CNTs, which is higher for the evaporated iron catalyst with respect to the iron from solution. Nonetheless, according to
the previous results, in our experimental conditions, there is no evidence of significant differences in the growth of suspended CNTs on the two types of substrate (silicon oxide and polysilicon). Concerning the nucleation of CNTs on those substrates, SEM investigation reveals that only a very small fraction of the iron catalyst particles, available on the surface, is effective for the CNTs growth. As an example, Fig. 5 shows CNTs grown on iron nanoparticles deposited by dip coating on a polysilicon sample. The higher magnification image on the right evidences that CNTs extrude from the catalyst nanoparticles located among polysilicon grains, and extend horizontally towards the polysilicon mesa edge. The large amount of iron that is inactive as a catalyst can be ascribed to several causes, such as the different phase or chemical status of the iron, the chemical interactions between the catalyst and the substrate and the process of metal nanoparticles formation that determines their distribution in size, so that only a small fraction of the particles has nanometric size and is effective for the nucleation of the CNTs.
Fig. 4. 3D AFM image of a 200 200 nm2 area of a CNTs sample deposited on SiO2 with dip coated Fe nanoparticles. Two crossing CNTs on the top of a SiO2 mesa are shown. The diameters of isolated and crossing tubes are reported below.
Fig. 5. On the left SEM micrograph of isolated SWNTs protruding from Fe particles deposited on poly-Si trench structures. On the right magnification of a selected area. The catalyst nanoparticles that originate CNTs are resolved.
ARTICLE IN PRESS R. Angelucci et al. / Physica E 37 (2007) 11–15
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Fig. 6. Series of SEM micrographs showing SWNTs protruding from Fe particles deposited on poly-Si trench structures. The pictures show us the interaction between nanotube and surface produces the stopping of tube elongation. SWNTs are able to bridge across trenches of distances varying from 1 to 5 mm.
A thorough investigation of the SEM micrographs allows us to consider a possible mechanism for the growth of the CNTs in the case of the polysilicon substrate. In Figs. 6a–c, three different cases of CNTs growth are shown. In Fig. 6a a nanotube that ends its growth on the bottom of the trench is reported. The brighter catalyst nanoparticle is evident at the tip of the tube. Fig. 6b shows a tube that has bridged the mesa structure after touching the substrate at the bottom of the trench. In Fig. 6c we report the case of a nanotube bridging the mesas across two adjacent trenches separated of about 5 mm. These micrographs refer to three different locations, though in the same grown substrate, they can be coherently interpreted as the result of a tip growth mechanism where the metal nanoparticle floats over the substrate while elongating during the growth process. This growth mechanism has been already reported in Refs. [11–13] and is known as the ‘‘kite mechanism’’. According to this mechanism, the catalytic nanoparticle hitting the substrate surface several times, increases the chance of its poisoning, thus stopping the CNTs growth. 4. Conclusions We have investigated the CVD growth process of horizontal nanotubes bridging across trenches on silicon oxide and polysilicon substrates. The use of evaporated as well as dispersed iron as a catalyst has been explored and the formation of SWNTs has been evidenced by SEM, Raman and AFM structural investigations. Our results prove that high quality SWNTs, with diameters as small as 1 nm, can be grown on our patterned structures, forming bridges across the trenches. A reduced density of CNTs has been achieved by replacing evaporated Fe with dispersed Fe as catalyst. A further density drop can be obtained by decreasing the Fe solution molarity so producing both isolated and low density network of CNTs. The prevailing semiconducting character can be considered for the fabrication of electronic devices such as CNT–FETs and interconnects.
In view of these micro- and nano-electronic applications, an electrical characterization of the grown isolated carbon nanotubes and CNTs networks, together with the development of a hybrid silicon–carbon nanotubes technology, is in progress.
Acknowledgments The authors wish to thank M. Torrisi and A. Sangrigoli of ST Catania for the SEM observations; P. Ruani of the CNR-ISMN of Bologna for the help in Raman measurements. We acknowledge the CNR-IMM of Catania for the access to the AFM equipments.
References [1] R. Saito, M.S. Dresselhaus, G. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. [2] S. Reich, C. Thomsen, J. Maultzsch, Carbon Nanotubes: Basic Concepts and Physical Properties, Wiley-VCH, USA, 2003. [3] S.J. Tans, A.R.M. Verschueren, C. Dekker, Nature 393 (1998) 49. [4] R. Martel, T. Schmidt, H. Shea, T. Hertel, Ph. Avouris, Appl. Phys. Lett. 73 (1998) 2447. [5] Y. Lu, S. Bangsaruntip, X. Wang, L. Zhang, Y. Nishi, H. Dai, J. Am. Chem. Soc. 128 (2006) 518. [6] H.W.Ch. Postma, T. Teepen, Z. Yao, M. Grifoni, C. Dekker, Science 293 (2001) 76. [7] M. Bockrath, D.H. Cobden, P.L. McEuen, N.G. Chopra, A.Z.A. Thess, R.E. Smalley, Science 275 (1997) 1922. [8] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, H. Lee, S.G. Kim, D.T. Colbert, G. Scuseria, D. Tomanek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [9] A. Jorio, R. Saito, J.H. Hafner, C.M. Lieber, M. Hunter, T. McClure, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. Lett. 86 (2001) 1118. [10] Y. Kobayashi, T. Yamashita, Y. Ueno, O. Niwa, Y. Homma, T. Ogino, Chem. Phys. Lett. 386 (2004) 153. [11] S. Huang, M. Woodson, R. Smalley, J. Liu, NanoLett. 4 (2004) 1025. [12] S. Huang, X. Cai, J. Liu, J. Am. Chem. Soc. 125 (2003) 5636. [13] L.X. Zheng, M.J. O’Connell, S.K. Doorn, X.Z. Liao, Y.H. Zhao, E.A. Akhadov, M.A. Hoffbauer, B.J. Roop, Q.X. Jia, R.C. Dye, D.E. Peterson, S.M. Huang, J. Liu, Y.T. Zhu, Nature 3 (2004) 673.
Physica E 37 (2007) 11–15 www.elsevier.com/locate/physe
Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates Renato Angeluccia,, Rita Rizzolia, Vincenzo Vinciguerrab, Maria Fortuna Bevilacquac, Sergio Guerria, Franco Corticellia, Mara Passinia a CNR-IMM Bologna, via Gobetti 101, Bologna 40129, Italy STMicroelectronics, 10 Science Park Road, #02-08, 117684, Singapore c STMicroelectronics, Piazzale Enrico Fermi 1, Porto del Granatello—Portici, Napoli 80055, Italy b
Available online 6 October 2006
Abstract In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly microfabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide ‘‘pillars’’, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De; 61.46.Fg; 73.63.Fg Keywords: Carbon nanotubes; Catalytic chemical vapor deposition; Iron
1. Introduction Carbon nanotubes (CNTs) [1,2] are novel allotropic 1D (one dimensional) forms of carbon consisting of hollow cylindrical-shaped molecules with graphitized walls that can be considered as molecular nanowires. CNTs have emerged in the field of molecular electronics because of their excellent conducting properties allowing the manufacturing of devices such as FETs (field effect transistors) [3–5] and SETs (single electron transistors) [6–8]. A strong key requirement for a successful CNTs integration with silicon is the development of a growth technology compatible with the standard manufacturing processes of the microelectronic industry. Moreover, the ability to grow CNTs with predictable and reproducible properties, i.e. controlling their semiconducting or metallic characteristics, on suitable substrates, at a fixed location Corresponding author. Tel.: +39 0516399120; fax: +39 0516399216.
E-mail address: [email protected] (R. Angelucci). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.07.043
and with a desired orientation on surfaces, is still a great challenge for the fabrication of hybrid carbon micro and nanocircuits. This ability, including the control over CNTs diameter and chirality, would allow the exploitation of their unique electrical, thermal and mechanical properties, in a wide range of electronic devices as emitters in flat panel displays, bio and DNA sensors and in the IC thermal management. Considering this scenario, we have developed a site-selective catalytic chemical vapor deposition (CCVD) synthesis of CNTs on silicon-based substrates, which is based on a standard process of the silicon technology, the CVD technique. In this paper, a modified CCVD process, exploiting iron nanoparticles as catalysts, methane as carbon gas feedstock and hydrogen as carrier gas at 900 1C, has been developed in order to get reproducible horizontally oriented nanotubes for FETs and other electronic devices. Modifications of the CCVD process are necessary in order to switch from multi-walled nanotubes (MWNTs) to single-walled nanotubes (SWNTs) and from a vertical to a horizontal growth of isolated SWNTs.
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Single horizontal CNTs satisfy both the requirements of an easier process to manufacture a CNT–FET and an unequivocal structural and electrical characterization of the single nanotube. 2. Experimental In this study, we investigate methods to control the directionality and location of nanotubes by means of patterned substrates and proper dispersion of iron nanoparticles. In the specific, substrates of silicon oxide and polysilicon have been patterned with trenched structures, about 500 nm deep, and their embossed surface has been used to pattern the iron-catalyst. The width of the trenches ranged between 1.4 and 5 mm. In the case of thermal SiO2, the trench patterns have been obtained by a dry etching process. In order to smooth the finishing SiO2 surface a wet etching process in NH4F and HF mixture and a soft cleaning process in ozone, have been performed. The polysilicon layer has been deposited by CVD of silane on a thermal silicon oxide substrate. The polysilicon mesa structures have been defined using an anisotropic dry etching process, avoiding the rounding effects by means of sacrificial oxide layers. Iron nanoparticles have been obtained both from an evaporated Fe film, deposited by e-gun at a rate of 5 A˚/s for 2 s, to get a nominal layer thickness of 1 nm and from iron nitrate solutions accurately dispersed on the substrates. In the case of catalyst delivered by iron solution, a liquid-phase catalyst, 0.4 mM of iron nitrate Fe(NO3)3 9H2O in isopropilic alcohol (IPA), was dispersed on the substrate both by spin coating and dip coating techniques. Hence, for spin coating, 100 ml of the solution was transferred onto the sample, spun at 500 rpm for 5 s and calcined in oven at 80 1C for 10 min; for dip coating the samples were immersed in a beaker with 200 ml of IPA iron solution for 4 h and then dried in air. The CNTs were grown by CCVD in a hot-wall quartz
furnace at 750 Torr, 900 1C and with a methane flow rate of 140 sccm. The catalyst nanoparticles were formed in H2 atmosphere during the heating up to the deposition temperature, (60 min in H2 ¼ 200 sccm) and the following annealing step (900 1C, 20 min, H2 ¼ 16 sccm). All the samples have been characterized by using the field emission scanning electron microscopy, FE-SEM, (Leo 1550, with 1 nm/4 nm lateral resolution) as the customary characterization technique for the morphology. Deeper and more accurate information on the CNTs structure have been carried out by means of the micro-Raman spectroscopy (Renishaw 1000 spectrometer with a HeNe laser source, at 633 nm and maximum output power of 15 mW) and AFM (dimension 3100, nanoscope III, Digital Instruments) technique. The Raman D-band, G-bands and radial breathing modes (RBM) features have been fitted by Lorentzian curves. The SWNTs diameters have been calculated by the RBM peak positions using the relation oRBM ¼
C1 dk
þ C2
with C1 ¼ 248 cm1 nm, C2 ¼ 0 cm1 and k ¼ 1, as reported in the literature for isolated nanotubes on a SiO2 substrate [9]. C1 ¼ 223.75 cm1 nm and C2 ¼ 0 cm1 have also been reported for suspended CNTs [10]. The cut-off of the notch filter in our Raman spectrometer is around 120 cm1, excluding the observation of SWNTs with diameters larger than 2 nm. 3. Results and discussion The SEM micrograph on the left side of Fig. 1 shows one of the CNTs horizontally grown on silicon dioxide substrate, bridging across the trenches. In this case, the iron catalyst has been deposited by spin coating the 0.4 mM iron nitrate/IPA solution onto the sample. Moreover, the SEM micrograph evidences a quite clean and particle free
Fig. 1. On the left SEM micrograph of an isolated CNT bridging across a trench of SiO2. CNTs have been grown by using Fe from salt solution (spin coating) as metal catalyst. On the right AFM measurement on a 1.25 1.25 mm2 area of an isolated SWNT bridging across two adjacent trenches, originating from Fe nanoparticles on SiO2. The edges of trenches are visible.
ARTICLE IN PRESS R. Angelucci et al. / Physica E 37 (2007) 11–15
oxide surface, whereas agglomerates of byproducts (likely determined by a mixture of iron and carbon) can be observed at the bottom of the trench. In order to gain further information on the suspended nanotubes, we carried out an AFM analysis by operating in tapping mode. On the right side of Fig. 1, the AFM image of the same sample confirms the existence of CNTs bridging the adjacent trenches, allowing to conclude that these nanotubes are tightly bounded at the extremities. From AFM inspection an upper limit of the nanotube diameter, equal to 5 nm, can be estimated. Analogous results have been obtained with the same spin coating deposition technique on the polysilicon mesa structures. As shown in the SEM micrograph on the left side of Fig. 2, also in this case a straight carbon nanotube protrudes between the mesas. A map of the Raman spectra, collected along the trenches of the same polysilicon sample, reveals the presence of RBM in the low frequency region.
13
This allows classifying the bridging CNTs as SWNTs. On the right side of Fig. 2, the Raman spectrum in the region of the RBM shows a narrow feature corresponding to a resonant SWNT of 1.77 nm diameter. The RBM features are resolved on a background of photoluminescence due to the scattering of impurities. The 303 cm1 feature comes from the Si substrate. Moreover, the straight nanotube gives clues on the absences of kinks in the honeycomb network of the bridging CNTs. As far as the other catalyst deposition techniques are concerned, it is possible to observe that both the e-gun evaporation of 1 nm thick iron film, and the simple immersion of the substrates in the 0.4 mM iron nitrate/ IPA solution, allow to synthesize SWNTs crossing the microfabricated structures. The Raman spectra of Fig. 3, D and G bands region and RBM region, confirm the excellent quality of the as-grown SWNTs for the case of 1 nm iron evaporated onto polysilicon structures. The feature and
Fig. 2. On the left SEM micrograph of an isolated CNT bridging between two poly-Si mesas. CNTs have been grown using Fe nanoparticles from a spin coated Fe salt solution. On the right Raman spectrum of one of the isolated SWNTs of the same sample.
Fig. 3. Raman spectra of isolated SWNTs grown on a poly-Si substrate after evaporating 1 nm of iron as a catalyst. The D, G+ and G bands are resolved. The low frequency region shows many features coming from RBM of SWNTs and the 303 cm1 feature of Si substrate.
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position of G and G+ peaks, 1569 and 1590 cm1, respectively, indicate the semiconducting nature of the resonant SWNTs. From the features of the RBM spectrum the corresponding SWNTs diameters of 1.78, 1.28 and 1.00 nm have been calculated. In the case of flat and clean surfaces as in the case of the CNTs grown by preparing the catalyst from a salt solution on silicon oxide, the AFM analysis can give good indications. The AFM topography of Fig. 4 shows SWNTs obtained on iron catalyst delivered by dip coating technique. In the 3D view it is possible to observe two crossed nanotubes. The section analysis of each structures allows to conclude that these nanotubes are SWNTs, with diameters d1 ¼ 1.270.3 nm and d2 ¼ 1.47 0.2 nm. The analysis of the height of the crossing of the two CNTs provides a value of 2.670.8 nm, that is consistent with the sum of the two diameters. The two techniques exploited for the iron deposition, determine a different density of the iron particles covering the structures. This results in a different density of CNTs, which is higher for the evaporated iron catalyst with respect to the iron from solution. Nonetheless, according to
the previous results, in our experimental conditions, there is no evidence of significant differences in the growth of suspended CNTs on the two types of substrate (silicon oxide and polysilicon). Concerning the nucleation of CNTs on those substrates, SEM investigation reveals that only a very small fraction of the iron catalyst particles, available on the surface, is effective for the CNTs growth. As an example, Fig. 5 shows CNTs grown on iron nanoparticles deposited by dip coating on a polysilicon sample. The higher magnification image on the right evidences that CNTs extrude from the catalyst nanoparticles located among polysilicon grains, and extend horizontally towards the polysilicon mesa edge. The large amount of iron that is inactive as a catalyst can be ascribed to several causes, such as the different phase or chemical status of the iron, the chemical interactions between the catalyst and the substrate and the process of metal nanoparticles formation that determines their distribution in size, so that only a small fraction of the particles has nanometric size and is effective for the nucleation of the CNTs.
Fig. 4. 3D AFM image of a 200 200 nm2 area of a CNTs sample deposited on SiO2 with dip coated Fe nanoparticles. Two crossing CNTs on the top of a SiO2 mesa are shown. The diameters of isolated and crossing tubes are reported below.
Fig. 5. On the left SEM micrograph of isolated SWNTs protruding from Fe particles deposited on poly-Si trench structures. On the right magnification of a selected area. The catalyst nanoparticles that originate CNTs are resolved.
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15
Fig. 6. Series of SEM micrographs showing SWNTs protruding from Fe particles deposited on poly-Si trench structures. The pictures show us the interaction between nanotube and surface produces the stopping of tube elongation. SWNTs are able to bridge across trenches of distances varying from 1 to 5 mm.
A thorough investigation of the SEM micrographs allows us to consider a possible mechanism for the growth of the CNTs in the case of the polysilicon substrate. In Figs. 6a–c, three different cases of CNTs growth are shown. In Fig. 6a a nanotube that ends its growth on the bottom of the trench is reported. The brighter catalyst nanoparticle is evident at the tip of the tube. Fig. 6b shows a tube that has bridged the mesa structure after touching the substrate at the bottom of the trench. In Fig. 6c we report the case of a nanotube bridging the mesas across two adjacent trenches separated of about 5 mm. These micrographs refer to three different locations, though in the same grown substrate, they can be coherently interpreted as the result of a tip growth mechanism where the metal nanoparticle floats over the substrate while elongating during the growth process. This growth mechanism has been already reported in Refs. [11–13] and is known as the ‘‘kite mechanism’’. According to this mechanism, the catalytic nanoparticle hitting the substrate surface several times, increases the chance of its poisoning, thus stopping the CNTs growth. 4. Conclusions We have investigated the CVD growth process of horizontal nanotubes bridging across trenches on silicon oxide and polysilicon substrates. The use of evaporated as well as dispersed iron as a catalyst has been explored and the formation of SWNTs has been evidenced by SEM, Raman and AFM structural investigations. Our results prove that high quality SWNTs, with diameters as small as 1 nm, can be grown on our patterned structures, forming bridges across the trenches. A reduced density of CNTs has been achieved by replacing evaporated Fe with dispersed Fe as catalyst. A further density drop can be obtained by decreasing the Fe solution molarity so producing both isolated and low density network of CNTs. The prevailing semiconducting character can be considered for the fabrication of electronic devices such as CNT–FETs and interconnects.
In view of these micro- and nano-electronic applications, an electrical characterization of the grown isolated carbon nanotubes and CNTs networks, together with the development of a hybrid silicon–carbon nanotubes technology, is in progress.
Acknowledgments The authors wish to thank M. Torrisi and A. Sangrigoli of ST Catania for the SEM observations; P. Ruani of the CNR-ISMN of Bologna for the help in Raman measurements. We acknowledge the CNR-IMM of Catania for the access to the AFM equipments.
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