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Effect of substrate biasing on structural and field-emissive properties of carbon nanotubes synthesized by ICP-CVD method
Thin Solid Films 469–470 (2004) 142 – 148 www.elsevier.com/locate/tsf
Effect of substrate biasing on structural and field-emissive properties of carbon nanotubes synthesized by ICP-CVD method Chang-Kyun Park, Jong-Pil Kim, Young-Do Kim, Hyun-Seok Uhm, Jin-Seok Park* Department of Electrical Engineering, Hanyang University, 1271, Sa 1-dong, Sangrok-ku, Ansan, Kyonggi-do 426-791, South Korea
Abstract Both negative and positive substrate bias effects on structural and field-emissive properties of carbon nanotubes (CNTs) are investigated. The CNTs are grown on Ni catalysts employing an inductively coupled plasma chemical vapor deposition (ICP-CVD) method by varying substrate bias from 550 to 400 V. Characterization using various techniques, such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Auger spectroscopy (AES), and Raman spectroscopy, shows that the physical dimension as well as the crystal quality of grown CNTs can be changed and controlled by the application of substrate bias during CNT growth. It is for the first time observed that the prevailing growth mechanism of CNTs which is either due to tip-driven growth or basedon-catalyst growth may be influenced by the application of substrate bias. It is also seen that negative bias would be more effectual for vertical-alignment of CNTs compared with positive bias, whereas the CNTs grown under positive bias display much better electron emission capabilities than those grown under negative bias or without bias. The reasons for all the measured data regarding the structural properties of CNTs are discussed to confirm the correlation with the observed field-emissive properties. D 2004 Published by Elsevier B.V. Keywords: Carbon nanotubes (CNTs); Substrate-bias effect; Nanostructure; Field-emissive property; Inductively coupled plasma chemical vapor deposition (ICP-CVD)
1. Introduction Electron-emitters based on carbon nanotubes (CNTs) are purported to be an ideal candidate for the realization of a variety of cold cathodes, point electron sources, and flat panel displays due to their remarkable physical properties [1]. In particular, the high aspect ratio (i.e., small diameters of several nanometers and relatively long length of several micrometers) of CNTs can generate a large electric field enhancement to obtain electron emission at low electric field [2]. It has thus been generally recognized that one of the most important factors is to grow the nanotubes in such a way that they are aligned perpendicular to the substrate surface; hence, the huge geometric electric field enhancement of sharp tip structures can be exploited. Several methods for achieving aligned CNTs were suggested by
* Corresponding author. Tel.: +82 31 400 5166; fax: +82 31 419 3042. E-mail address: [email protected] (J.-S. Park). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.08.102
means of embedding nanoparticles of the catalyst in Si nanopores, utilizing very dense tube growth which forces the tubes to align parallel to each other, and growing the tubes under plasma conditions. Recently, it has been proposed that aligned CNTs can also be achieved by the application of electric bias to the substrate during growth [3]. Avigal and Kalish [4] firstly reported the bias effect such that the nanotube alignment occurred only when a positive bias was applied to the substrate whereas no aligned growth occurred under a negative bias and no tube growth was observed without bias. On the contrary, several researchers reported some different observations [5–7] that aligned nanotubes could also be grown under negative substrate biases. This discrepancy as for the effect of positive and negative bias may indicate that the bias effect is not fully understood yet. Most recently, it has also been reported [8] that controlling the plasma potential during CNT-growth by means of substrate-biasing techniques may control the diameter and the crystal quality of CNTs and furthermore, nanotubes
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grown under positive bias can possess better field emission characteristics. However, this issue has failed to be profoundly investigated. Here we present more systematic, experimental investigation to elucidate the effects of electric biasing (including both positive and negative biasing) on nanostructures and growth behaviors of CNTs as well as their electron-emission capabilities. Several new observations regarding the bias effect are introduced. Furthermore, the relation between the structural properties and the field-emissive characteristics of CNTs is discussed in terms of the bias effect.
2. Experimental details A patterned TiN film (150-nm thickness), which serves as a barrier material and an electrically conductive layer, is deposited on a SiO2-coated Si substrate by dc magnetron sputtering. Using a photolithography process, the TiN layer with 600600 arrays of disk cell (5-Am diameter, 10-Am pitch) is patterned in order to minimize the field-screening effect [9]. A Ni catalyst layer is formed on the patterned TiN layer by RF magnetron sputtering. Prior to growth of CNTs, NH3-plasma etching has been performed for 5 min. This process may inhibit the generation of amorphous carbon in the initial stage of synthesis and thereby protect the Ni particles from being covered by amorphous carbon, resulting in a high density of nucleation sites for CNT growth [10]. CNTs are subsequently grown at 600 8C using an ICPCVD system with gas mixture of C2H2/NH3 (200/25 sccm). During CNT growth, dc bias was applied to the substrate in the voltage range from 550 to 400 V. Both the positive and negative bias conditions used are listed in Table 1, corresponding to samples A to H. Sample I indicates the nanotube grown with no bias (i.e., floating). Other conditions used in CNT growth are given as follows; RF power of 300 W, working pressure of 2.6102 Pa, and process time of 20 min. The morphologies of all the CNTs grown have been monitored by field-emission scanning electron microscopy (FESEM, JSM-6330F, JEOL). The high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL) images are also measured to identify the prevailing growth Table 1 The bias conditions in the growth of nanotubes and the resultant values of b, E th, J max CNT A B C D E F G H I
V dc (V) 550 450 350 0 100 200 300 400 Floating
b
Eth (V/Am)
477 230 – 253 460 844 4043 – 498
4.42 4.35 – 4.16 3.23 3.01 1.79 – 4.25
J max (AA/cm2) 15.0 10.7 – 456 1210 1490 2450 – 618
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mechanism for CNTs, as a function of the substrate bias voltage. The carbon structure and the crystal quality of grown CNTs are analyzed by using triple Raman spectroscopy (T64000, Jobin Yvon). The chemical analysis of the CNTs is performed by Auger spectroscopy (AES, PHI680, Physical Electronics). The AES spectra have been taken in a derivative mode since a large background count due to secondary electrons may be included in the spectra. For all the grown CNTs, the electron-emission currents are measured using a compactly designed field-emission measurement system, as a function of the electric field applied [11]. In this measurement, the spacing between cathode (i.e., substrate) and anode was fixed to be 200 Am and the vacuum pressure inside the measurement chamber was about 6.010 6 Pa.
3. Results and discussion The FESEM cross-sectional morphologies pictured from the CNTs grown with bias are shown in Fig. 1, as a function of the bias conditions used (samples A–H), along with the image of the nanotube grown with no bias (sample I). The morphologies are different with the grown CNTs, implying that the morphology may be sensitive to bias effect. In more detail, the CNTs, when they are grown at relatively large negative biases (samples A, B), display the vertically aligned structure. In contrast, the nanotubes, when they are grown at 0 V (sample D) or with no bias (sample I), or at relatively low positive biases (samples E, F, G), reveal the tangled or randomly oriented structure. The CNTs, when they are prepared either at a low negative bias (sample C) or at a large positive bias (sample H), seem to be hardly grown and have relatively short lengths. This may also indicate that negative bias would be more effectual for the verticalalignment of CNTs rather than positive bias. Diameters of the grown CNTs have been measured from the FESEM images shown in Fig. 1 and the resultant values are depicted in Fig. 2, where the ranges of error bars and the positions of symbols indicate the minimum-to-maximum ranges and the most frequently measured values, respectively. It is generally recognized that the diameter as well as the growth rate and the areal density of CNTs may be controlled by the size and the thickness of the catalyst particles used [12,13]. The average size of the Ni catalyst islands, which was measured just after the NH3 pretreatment was performed, was approximately 30 nm. It is noted that all the process conditions used for the growth of CNTs were identical, only with the electric bias condition varied. Therefore, the changes of diameter observed in the CNTs are believed to be due to application of substrate bias. As the applied bias is increased toward a larger positive voltage, the diameter of CNTs is slightly decreased. The similar phenomenon was previously reported [8]. Here, it is worthy of being noted that the diameters are significantly reduced when the CNTs are grown at large negative biases
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Fig. 1. The FESEM cross-sectional and surface images pictured from the nanotubes grown with bias-enhancement, as a function of the various bias conditions. The alphabets A to I refer to the sample IDs, as shown in Table 1.
such as 450 and 550 V. This may be ascribed due to the etching effect of NH3 plasma during CNT growth [14] since enhancing the bias negatively would accelerate the excited NH3 species toward the substrate. The behavior of nitrogencontained species due to substrate bias has also been confirmed by the Auger spectra obtained from the corresponding CNTs, which is depicted in Fig. 6. In addition, the CNTs grown with dc bias applied, regardless of polarity, possess larger diameters compared with the sample grown with no bias. This could result from the agglomeration of the fragmented Ni islands by receiving heat which may be generated due to bombardment of high density ions or electrons. As for the growth mechanism of CNTs, two models, tipdriven growth [15] and based-on-catalyst growth are widely known to be dominant. It was reported by previous HRTEM study on bias effects [8] that the application of bias during growth hardly altered the nanostructure of the grown CNTs nor affected the growth mechanism. In contrast, our HRTEM study has revealed some different observations. Fig. 3(a) and (c) shows the HRTEM images for the CNTs grown under a negative bias ( 550 V) and a positive bias (300 V), respectively. The enlarged images
corresponding to those are also displayed in Fig. 3(b) and (d), respectively. The results of Fig. 3 clearly indicate that the prevailing mechanism for CNT growth may be affected by the application of substrate bias. The nanotube grown at 550
Fig. 2. The variation of diameter measured from the FESEM images of Fig. 1.
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Fig. 3. The HRTEM images of CNTs formed with the bias voltage of (a) shown in (b) and (d), respectively.
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550 and (c) 300 V. The magnification of the HRTEM micrographs of CNTs grown as
V (see Fig. 3(b)) possesses such a shape that the Ni catalyst particle is located at the tip with the tube encapsulated by the ambient carbons, which indicates a tip-driven growth. On the other hand, the nanotube grown at 300 V (see Fig. 3(d)) exhibits a multi-walled, bamboo-like structure and has a closed tip with the catalyst particle not encapsulated, which means a based-on-catalyst growth [16]. This also allows conjecturing that substrate-biasing may cause a transformation between the two distinguishable mechanisms for CNT growth. The critical bias voltage where the transformation may take place is hard to be accurately determined though. It has been reported that the adhesion between catalyst and substrate would be a crucial factor in dominating the growth mechanism of CNTs [17]. In our experiment, the nanotube grown at 550 V has revealed the smallest diameter, implying that the Ni catalyst particles would be significantly etched by the surplus species of excited NH3. This could deteriorate the adhesion between the Ni catalyst and the substrate. And further, after the supersaturation occurs in the catalyst during growth, graphitic shells can rapidly be formed beneath the catalyst due to attachment of the carbon species supplied. This reaction may support the reason why the Ni catalyst stays at the top of the CNTs. On the other hand, the nanotube grown at 300 V could have relatively better adhesion and hence the
nanotube is grown, based on the catalyst which is left on the substrate. In addition, the Ni catalyst particles located at the top of the tip-driven nanotube are negatively charged and attracted to the anode when a negative bias is applied to the substrate. As the result, the nanotube is grown up along the direction of the electric field applied. This is believed to be the reason why negative-biasing has revealed a stronger effect on the vertical-alignment of CNTs rather than positive-biasing. The Raman spectra have been measured to investigate the change in crystal quality of the grown CNTs due to substrate-biasing, which is shown in Fig. 4. The G-band at around 1350 cm 1 corresponds to the symmetric E 2g vibrational mode in a graphite-like material, while the Dband at around 1600 cm 1 is activated in the first-order scattering process of sp 2 carbons by the presence of substitutional heteroatoms, vacancies, grain boundary, or other defects, all of which lower the crystalline symmetry of the quasi-infinite lattice [18]. Normally, the intensity of the D-peak increases with an increase in the amount of unorganized carbon in the samples and with a decrease in the graphite crystal size. In addition, the relative intensity ratio (i.e., I D/I G) is used as a fitting parameter for predicting the quality of CNTs, a smaller I D/I G ratio corresponding to a more graphitized structure.
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Fig. 5 depicts the change of I D/I G intensity ratio according to the substrate bias voltage applied. It is found that the bias-enhanced grown CNTs reveal a lower I D/I G ratio rather than the tube grown with no bias. The I D/I G values measured are approximately 1.04 for the nanotube grown with no bias, less than 1.00 for the nanotube grown at 550 V, and 0.85 (the lowest) for the nanotube grown at 300 V. Increasing negative bias would provide the carbon species with sufficient energy that they could easily be diffused though the catalyst, which may consequently enhance the graphitic quality of grown CNTs. In addition, increasing positive bias would help more electrons to be accelerated toward the substrate. The electrons then strike the CNTs and as the result, the relatively weak amorphous phases included in the carbon network of the CNTs would gradually be removed while the CNTs are being grown. The reduced diameter observed for the CNTs grown at large positive bias, which has been shown in Fig. 2, may support this result. In addition, the exceptionally large I D/I G observed for the nanotube grown at 400 V reflects that the
nanotube may contain a large density of defects, which is believed to be due to excessive collision. In most CVD processes used for CNT-growth, it has been known that N2 or NH3 gas environments would play a key role for the vertical alignment of CNTs [19]. Fig. 6 shows the AES data measured from the CNTs grown under the different bias conditions, such as (a) 550 V, (b) 0 V, and (c) 300 V, respectively. Three distinguishable Auger peaks can be viewed from the spectra; C(KLL) at 270 eV, N(KLL) at 380 eV, and O(KLL) at 510 eV. It is emphasized that the Auger intensity of the nitrogen-related peak is noticeably increased by the application of negative bias ( 550 V). This may support that negative-biasing appears more advantageous in vertical alignment of CNTs rather than positive-biasing, as has already been monitored by FESEM (see Fig. 1). Fig. 7 shows the emission current density (log( J)) versus the applied electric field (E) characteristics measured from all the grown CNTs. The Fowler–Nordheim (F–N) plots (ln( J/E 2) vs. 1/E) [20] are also depicted in the inset of Fig. 7. All the F–N plots obtained are well fitted to a straight line, which supports electron emission through tunneling. The field enhancement factor (b) can be evaluated from the F–N plot by assuming the work function of CNT to be the same as graphite (4.6 eV). The threshold field (E th, V/Am) for electron emission is defined as the electric filed at which the emission current density approached 1 AA/cm2. The maximum current density ( J max, AA/cm2) has also been measured at the applied field of 5.5 V/Am. All the resultant values of b, E th, and J max are listed in Table 1. It is clearly seen that the CNTs grown with positive bias reveal much better electron emission characteristics rather than those grown with negative bias or with no bias. And furthermore, the contribution of biasing to the field emission enhancement appears to be the strongest for the nanotube grown at 300 V, which has revealed the best emission characteristics, such as b=4034, E th=1.75 V/Am, and J max=2.45 mA/cm2.
Fig. 5. The relative variation of I D/I G intensity ratio under various bias conditions.
Fig. 6. The AES spectra of the CNTs grown under (a) 300 V, (b) 0 V, and (c) 550 V.
Fig. 4. The Raman spectra obtained from the grown CNTs, as a function of the various bias conditions.
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Fig. 7. The current density ( J)–electric field (E) characteristics measured from all the grown CNTs. The inset shows the Fowler–Nordheim (F–N) plots.
The field-emission result shows that positive bias would be more favorable for improving electron-emission capabilities of CNTs, compared with negative bias. The reduced b of the CNTs grown at the bias condition of 550 V (sample H) may be due to an electrostatic screening effect provoked by the proximity of neighboring tubes although the CNTS had a relatively small diameter and a large length as shown in Figs. 1 and 2. In contrast, it is also noted that the CNTs grown at the positive bias conditions, such as 200 V (sample F) and 300 V (sample G), revealed better b values despite relatively larger diameters. The reason adequate for the improved b and better emission may be related to the body emission, which has frequently been observed in randomly oriented or less vertically oriented CNTs [21]. It can be conjectured from the result shown in Fig. 7 that the body emission through the outer wall may prevail over the emission through the tips of vertically aligned CNTs. This effect is noticeably observed for the nanotube grown at 300 V. And besides, as shown in Fig. 3(b), the growth ends of the CNTs grown under a negative bias ( 550 V) are terminated by the Ni catalyst particles, which may prevent the electron emission from the tip.
positive bias. On the other hand, the better field-emission properties, such as reduced threshold fields, enlarged field enhancement factors, and increased emission currents, have been achieved for the CNTs grown under positive bias rather than those grown under negative bias. It is confirmed from this work that the substrate-biasing technique would be a way of improving the structural capability as well as the field-emissive capability of CNTs. Nevertheless, it is also suggested that further studies on the change of plasma potentials due to substrate-biasing and its effect on the growth behavior of CNTs may still be required. Concerning this issue, our recent data is being presented elsewhere [22].
Acknowledgement This research was partially supported by a grant in aid of the center for Electronic Materials and Components (EM&C) at Hanyang University (Ansan campus).
References 4. Conclusion Field-emissive capabilities and structural properties of CNTs grown by ICP-CVD have been characterized, in terms of substrate-bias effect. The FESEM and HRTEM analyses indicate that applying substrate bias change the diameter of grown CNTs and furthermore, there may exist a critical electric bias for causing the transformation between growth mechanisms such as tip-driven growth and based-oncatalyst growth. It is observed that negative bias is more profitable for obtaining the vertically aligned CNTs than
[1] J.M. Bonard, M. Croci, C. Klinke, R. Kurt, O. Noury, N. Weiss, Carbon 40 (2002) 1715. [2] W.I. Milne, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, S.B. Lee, D.G. Hasko, H. Ahmed, O. Groening, P. Legagneux, L. Gangloff, J.P. Schnell, G. Pirio, D. Pribat, M. Castignolles, A. Loiseau, V. Semet, V.T. Binh, Diamond Relat. Mater. 12 (2003) 422. [3] Q. Yang, C. Xiao, W. Chen, A.K. Singh, T. Asai, A. Hirose, Diamond Relat. Mater. 12 (2003) 1482. [4] Y. Avigal, R. Kalish, Appl. Phys. Lett. 78 (2001) 2291. [5] M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [6] Y. Chen, D.T. Shaw, L. Guo, Appl. Phys. Lett. 76 (2000) 2469.
148
C.-K. Park et al. / Thin Solid Films 469–470 (2004) 142–148
[7] K.S. Kim, H.J. Ryu, G.E. Jang, Diamond Relat. Mater. 12 (2003) 1717. [8] C.L. Tsai, C.F. Chen, Diamond Relat. Mater. 12 (2003) 1615. [9] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett. 76 (2000) 2071. [10] Y.T. Jang, J.H. Ahn, Y.H. Lee, B.K. Ju, Chem. Phys. Lett. 372 (2003) 745. [11] S.H. Seo, T.H. Lee, Y.D. Kim, C.K. Park, J.S. Park, Thin Solid Films 447 (2003) 212. [12] Y.T. Feng, S.Z. Deng, J. Chen, N.S. Xu, Ultramicroscopy 95 (2003) 93. [13] E.F. Kukovitsky, S.G. L’vov, N.A. Sainov, V.A. Shustov, L.A. Chernozatonskii, Chem. Phys. Lett. 355 (2002) 497. [14] J.H. Choi, T.Y. Lee, S.H. Choi, J.H. Han, J.B. Yoo, C.Y. Park, T.W. Jung, S.G. Yu, W.K. Yi, I.T. Han, J.M. Kim, Thin Solid Films 435 (2003) 318.
[15] M. Tanemura, K. Iwata, K. Takahashi, Y. Fujimoto, F. Okuyama, H. Sugie, V. Filip, J. Appl. Phys. 90 (2001) 1529. [16] C.J. Lee, J.H. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175. [17] R.T.K. Baker, Carbon 27 (1989) 315. [18] M. Sveningsson, R.E. Morjan, O.A. Nerushev, Y. Sato, J. Backstrom, E.E.B. Campbell, F. Rohmund, Appl. Phys., A 73 (2001) 409. [19] T.Y. Kim, K.R. Lee, K.Y. Eun, K.H. Oh, Chem. Phys. Lett. 372 (2003) 603. [20] M. Hirakawa, S. Sonoda, C. Tanaka, H. Murakami, H. Yamakawa, Appl. Surf. Sci. 169 (2001) 662. [21] C.M. Hsu, C.H. Lin, H.L. Chang, C.T. Kuo, Thin Solid Films 420 (2002) 225. [22] C.K. Park, J.P. Kim, Y.D. Kim, H.S. Uhm, J.S. Park, ICNDST-9, 2004.
Effect of substrate biasing on structural and field-emissive properties of carbon nanotubes synthesized by ICP-CVD method Chang-Kyun Park, Jong-Pil Kim, Young-Do Kim, Hyun-Seok Uhm, Jin-Seok Park* Department of Electrical Engineering, Hanyang University, 1271, Sa 1-dong, Sangrok-ku, Ansan, Kyonggi-do 426-791, South Korea
Abstract Both negative and positive substrate bias effects on structural and field-emissive properties of carbon nanotubes (CNTs) are investigated. The CNTs are grown on Ni catalysts employing an inductively coupled plasma chemical vapor deposition (ICP-CVD) method by varying substrate bias from 550 to 400 V. Characterization using various techniques, such as field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Auger spectroscopy (AES), and Raman spectroscopy, shows that the physical dimension as well as the crystal quality of grown CNTs can be changed and controlled by the application of substrate bias during CNT growth. It is for the first time observed that the prevailing growth mechanism of CNTs which is either due to tip-driven growth or basedon-catalyst growth may be influenced by the application of substrate bias. It is also seen that negative bias would be more effectual for vertical-alignment of CNTs compared with positive bias, whereas the CNTs grown under positive bias display much better electron emission capabilities than those grown under negative bias or without bias. The reasons for all the measured data regarding the structural properties of CNTs are discussed to confirm the correlation with the observed field-emissive properties. D 2004 Published by Elsevier B.V. Keywords: Carbon nanotubes (CNTs); Substrate-bias effect; Nanostructure; Field-emissive property; Inductively coupled plasma chemical vapor deposition (ICP-CVD)
1. Introduction Electron-emitters based on carbon nanotubes (CNTs) are purported to be an ideal candidate for the realization of a variety of cold cathodes, point electron sources, and flat panel displays due to their remarkable physical properties [1]. In particular, the high aspect ratio (i.e., small diameters of several nanometers and relatively long length of several micrometers) of CNTs can generate a large electric field enhancement to obtain electron emission at low electric field [2]. It has thus been generally recognized that one of the most important factors is to grow the nanotubes in such a way that they are aligned perpendicular to the substrate surface; hence, the huge geometric electric field enhancement of sharp tip structures can be exploited. Several methods for achieving aligned CNTs were suggested by
* Corresponding author. Tel.: +82 31 400 5166; fax: +82 31 419 3042. E-mail address: [email protected] (J.-S. Park). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.08.102
means of embedding nanoparticles of the catalyst in Si nanopores, utilizing very dense tube growth which forces the tubes to align parallel to each other, and growing the tubes under plasma conditions. Recently, it has been proposed that aligned CNTs can also be achieved by the application of electric bias to the substrate during growth [3]. Avigal and Kalish [4] firstly reported the bias effect such that the nanotube alignment occurred only when a positive bias was applied to the substrate whereas no aligned growth occurred under a negative bias and no tube growth was observed without bias. On the contrary, several researchers reported some different observations [5–7] that aligned nanotubes could also be grown under negative substrate biases. This discrepancy as for the effect of positive and negative bias may indicate that the bias effect is not fully understood yet. Most recently, it has also been reported [8] that controlling the plasma potential during CNT-growth by means of substrate-biasing techniques may control the diameter and the crystal quality of CNTs and furthermore, nanotubes
C.-K. Park et al. / Thin Solid Films 469–470 (2004) 142–148
grown under positive bias can possess better field emission characteristics. However, this issue has failed to be profoundly investigated. Here we present more systematic, experimental investigation to elucidate the effects of electric biasing (including both positive and negative biasing) on nanostructures and growth behaviors of CNTs as well as their electron-emission capabilities. Several new observations regarding the bias effect are introduced. Furthermore, the relation between the structural properties and the field-emissive characteristics of CNTs is discussed in terms of the bias effect.
2. Experimental details A patterned TiN film (150-nm thickness), which serves as a barrier material and an electrically conductive layer, is deposited on a SiO2-coated Si substrate by dc magnetron sputtering. Using a photolithography process, the TiN layer with 600600 arrays of disk cell (5-Am diameter, 10-Am pitch) is patterned in order to minimize the field-screening effect [9]. A Ni catalyst layer is formed on the patterned TiN layer by RF magnetron sputtering. Prior to growth of CNTs, NH3-plasma etching has been performed for 5 min. This process may inhibit the generation of amorphous carbon in the initial stage of synthesis and thereby protect the Ni particles from being covered by amorphous carbon, resulting in a high density of nucleation sites for CNT growth [10]. CNTs are subsequently grown at 600 8C using an ICPCVD system with gas mixture of C2H2/NH3 (200/25 sccm). During CNT growth, dc bias was applied to the substrate in the voltage range from 550 to 400 V. Both the positive and negative bias conditions used are listed in Table 1, corresponding to samples A to H. Sample I indicates the nanotube grown with no bias (i.e., floating). Other conditions used in CNT growth are given as follows; RF power of 300 W, working pressure of 2.6102 Pa, and process time of 20 min. The morphologies of all the CNTs grown have been monitored by field-emission scanning electron microscopy (FESEM, JSM-6330F, JEOL). The high-resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL) images are also measured to identify the prevailing growth Table 1 The bias conditions in the growth of nanotubes and the resultant values of b, E th, J max CNT A B C D E F G H I
V dc (V) 550 450 350 0 100 200 300 400 Floating
b
Eth (V/Am)
477 230 – 253 460 844 4043 – 498
4.42 4.35 – 4.16 3.23 3.01 1.79 – 4.25
J max (AA/cm2) 15.0 10.7 – 456 1210 1490 2450 – 618
143
mechanism for CNTs, as a function of the substrate bias voltage. The carbon structure and the crystal quality of grown CNTs are analyzed by using triple Raman spectroscopy (T64000, Jobin Yvon). The chemical analysis of the CNTs is performed by Auger spectroscopy (AES, PHI680, Physical Electronics). The AES spectra have been taken in a derivative mode since a large background count due to secondary electrons may be included in the spectra. For all the grown CNTs, the electron-emission currents are measured using a compactly designed field-emission measurement system, as a function of the electric field applied [11]. In this measurement, the spacing between cathode (i.e., substrate) and anode was fixed to be 200 Am and the vacuum pressure inside the measurement chamber was about 6.010 6 Pa.
3. Results and discussion The FESEM cross-sectional morphologies pictured from the CNTs grown with bias are shown in Fig. 1, as a function of the bias conditions used (samples A–H), along with the image of the nanotube grown with no bias (sample I). The morphologies are different with the grown CNTs, implying that the morphology may be sensitive to bias effect. In more detail, the CNTs, when they are grown at relatively large negative biases (samples A, B), display the vertically aligned structure. In contrast, the nanotubes, when they are grown at 0 V (sample D) or with no bias (sample I), or at relatively low positive biases (samples E, F, G), reveal the tangled or randomly oriented structure. The CNTs, when they are prepared either at a low negative bias (sample C) or at a large positive bias (sample H), seem to be hardly grown and have relatively short lengths. This may also indicate that negative bias would be more effectual for the verticalalignment of CNTs rather than positive bias. Diameters of the grown CNTs have been measured from the FESEM images shown in Fig. 1 and the resultant values are depicted in Fig. 2, where the ranges of error bars and the positions of symbols indicate the minimum-to-maximum ranges and the most frequently measured values, respectively. It is generally recognized that the diameter as well as the growth rate and the areal density of CNTs may be controlled by the size and the thickness of the catalyst particles used [12,13]. The average size of the Ni catalyst islands, which was measured just after the NH3 pretreatment was performed, was approximately 30 nm. It is noted that all the process conditions used for the growth of CNTs were identical, only with the electric bias condition varied. Therefore, the changes of diameter observed in the CNTs are believed to be due to application of substrate bias. As the applied bias is increased toward a larger positive voltage, the diameter of CNTs is slightly decreased. The similar phenomenon was previously reported [8]. Here, it is worthy of being noted that the diameters are significantly reduced when the CNTs are grown at large negative biases
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Fig. 1. The FESEM cross-sectional and surface images pictured from the nanotubes grown with bias-enhancement, as a function of the various bias conditions. The alphabets A to I refer to the sample IDs, as shown in Table 1.
such as 450 and 550 V. This may be ascribed due to the etching effect of NH3 plasma during CNT growth [14] since enhancing the bias negatively would accelerate the excited NH3 species toward the substrate. The behavior of nitrogencontained species due to substrate bias has also been confirmed by the Auger spectra obtained from the corresponding CNTs, which is depicted in Fig. 6. In addition, the CNTs grown with dc bias applied, regardless of polarity, possess larger diameters compared with the sample grown with no bias. This could result from the agglomeration of the fragmented Ni islands by receiving heat which may be generated due to bombardment of high density ions or electrons. As for the growth mechanism of CNTs, two models, tipdriven growth [15] and based-on-catalyst growth are widely known to be dominant. It was reported by previous HRTEM study on bias effects [8] that the application of bias during growth hardly altered the nanostructure of the grown CNTs nor affected the growth mechanism. In contrast, our HRTEM study has revealed some different observations. Fig. 3(a) and (c) shows the HRTEM images for the CNTs grown under a negative bias ( 550 V) and a positive bias (300 V), respectively. The enlarged images
corresponding to those are also displayed in Fig. 3(b) and (d), respectively. The results of Fig. 3 clearly indicate that the prevailing mechanism for CNT growth may be affected by the application of substrate bias. The nanotube grown at 550
Fig. 2. The variation of diameter measured from the FESEM images of Fig. 1.
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Fig. 3. The HRTEM images of CNTs formed with the bias voltage of (a) shown in (b) and (d), respectively.
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550 and (c) 300 V. The magnification of the HRTEM micrographs of CNTs grown as
V (see Fig. 3(b)) possesses such a shape that the Ni catalyst particle is located at the tip with the tube encapsulated by the ambient carbons, which indicates a tip-driven growth. On the other hand, the nanotube grown at 300 V (see Fig. 3(d)) exhibits a multi-walled, bamboo-like structure and has a closed tip with the catalyst particle not encapsulated, which means a based-on-catalyst growth [16]. This also allows conjecturing that substrate-biasing may cause a transformation between the two distinguishable mechanisms for CNT growth. The critical bias voltage where the transformation may take place is hard to be accurately determined though. It has been reported that the adhesion between catalyst and substrate would be a crucial factor in dominating the growth mechanism of CNTs [17]. In our experiment, the nanotube grown at 550 V has revealed the smallest diameter, implying that the Ni catalyst particles would be significantly etched by the surplus species of excited NH3. This could deteriorate the adhesion between the Ni catalyst and the substrate. And further, after the supersaturation occurs in the catalyst during growth, graphitic shells can rapidly be formed beneath the catalyst due to attachment of the carbon species supplied. This reaction may support the reason why the Ni catalyst stays at the top of the CNTs. On the other hand, the nanotube grown at 300 V could have relatively better adhesion and hence the
nanotube is grown, based on the catalyst which is left on the substrate. In addition, the Ni catalyst particles located at the top of the tip-driven nanotube are negatively charged and attracted to the anode when a negative bias is applied to the substrate. As the result, the nanotube is grown up along the direction of the electric field applied. This is believed to be the reason why negative-biasing has revealed a stronger effect on the vertical-alignment of CNTs rather than positive-biasing. The Raman spectra have been measured to investigate the change in crystal quality of the grown CNTs due to substrate-biasing, which is shown in Fig. 4. The G-band at around 1350 cm 1 corresponds to the symmetric E 2g vibrational mode in a graphite-like material, while the Dband at around 1600 cm 1 is activated in the first-order scattering process of sp 2 carbons by the presence of substitutional heteroatoms, vacancies, grain boundary, or other defects, all of which lower the crystalline symmetry of the quasi-infinite lattice [18]. Normally, the intensity of the D-peak increases with an increase in the amount of unorganized carbon in the samples and with a decrease in the graphite crystal size. In addition, the relative intensity ratio (i.e., I D/I G) is used as a fitting parameter for predicting the quality of CNTs, a smaller I D/I G ratio corresponding to a more graphitized structure.
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Fig. 5 depicts the change of I D/I G intensity ratio according to the substrate bias voltage applied. It is found that the bias-enhanced grown CNTs reveal a lower I D/I G ratio rather than the tube grown with no bias. The I D/I G values measured are approximately 1.04 for the nanotube grown with no bias, less than 1.00 for the nanotube grown at 550 V, and 0.85 (the lowest) for the nanotube grown at 300 V. Increasing negative bias would provide the carbon species with sufficient energy that they could easily be diffused though the catalyst, which may consequently enhance the graphitic quality of grown CNTs. In addition, increasing positive bias would help more electrons to be accelerated toward the substrate. The electrons then strike the CNTs and as the result, the relatively weak amorphous phases included in the carbon network of the CNTs would gradually be removed while the CNTs are being grown. The reduced diameter observed for the CNTs grown at large positive bias, which has been shown in Fig. 2, may support this result. In addition, the exceptionally large I D/I G observed for the nanotube grown at 400 V reflects that the
nanotube may contain a large density of defects, which is believed to be due to excessive collision. In most CVD processes used for CNT-growth, it has been known that N2 or NH3 gas environments would play a key role for the vertical alignment of CNTs [19]. Fig. 6 shows the AES data measured from the CNTs grown under the different bias conditions, such as (a) 550 V, (b) 0 V, and (c) 300 V, respectively. Three distinguishable Auger peaks can be viewed from the spectra; C(KLL) at 270 eV, N(KLL) at 380 eV, and O(KLL) at 510 eV. It is emphasized that the Auger intensity of the nitrogen-related peak is noticeably increased by the application of negative bias ( 550 V). This may support that negative-biasing appears more advantageous in vertical alignment of CNTs rather than positive-biasing, as has already been monitored by FESEM (see Fig. 1). Fig. 7 shows the emission current density (log( J)) versus the applied electric field (E) characteristics measured from all the grown CNTs. The Fowler–Nordheim (F–N) plots (ln( J/E 2) vs. 1/E) [20] are also depicted in the inset of Fig. 7. All the F–N plots obtained are well fitted to a straight line, which supports electron emission through tunneling. The field enhancement factor (b) can be evaluated from the F–N plot by assuming the work function of CNT to be the same as graphite (4.6 eV). The threshold field (E th, V/Am) for electron emission is defined as the electric filed at which the emission current density approached 1 AA/cm2. The maximum current density ( J max, AA/cm2) has also been measured at the applied field of 5.5 V/Am. All the resultant values of b, E th, and J max are listed in Table 1. It is clearly seen that the CNTs grown with positive bias reveal much better electron emission characteristics rather than those grown with negative bias or with no bias. And furthermore, the contribution of biasing to the field emission enhancement appears to be the strongest for the nanotube grown at 300 V, which has revealed the best emission characteristics, such as b=4034, E th=1.75 V/Am, and J max=2.45 mA/cm2.
Fig. 5. The relative variation of I D/I G intensity ratio under various bias conditions.
Fig. 6. The AES spectra of the CNTs grown under (a) 300 V, (b) 0 V, and (c) 550 V.
Fig. 4. The Raman spectra obtained from the grown CNTs, as a function of the various bias conditions.
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Fig. 7. The current density ( J)–electric field (E) characteristics measured from all the grown CNTs. The inset shows the Fowler–Nordheim (F–N) plots.
The field-emission result shows that positive bias would be more favorable for improving electron-emission capabilities of CNTs, compared with negative bias. The reduced b of the CNTs grown at the bias condition of 550 V (sample H) may be due to an electrostatic screening effect provoked by the proximity of neighboring tubes although the CNTS had a relatively small diameter and a large length as shown in Figs. 1 and 2. In contrast, it is also noted that the CNTs grown at the positive bias conditions, such as 200 V (sample F) and 300 V (sample G), revealed better b values despite relatively larger diameters. The reason adequate for the improved b and better emission may be related to the body emission, which has frequently been observed in randomly oriented or less vertically oriented CNTs [21]. It can be conjectured from the result shown in Fig. 7 that the body emission through the outer wall may prevail over the emission through the tips of vertically aligned CNTs. This effect is noticeably observed for the nanotube grown at 300 V. And besides, as shown in Fig. 3(b), the growth ends of the CNTs grown under a negative bias ( 550 V) are terminated by the Ni catalyst particles, which may prevent the electron emission from the tip.
positive bias. On the other hand, the better field-emission properties, such as reduced threshold fields, enlarged field enhancement factors, and increased emission currents, have been achieved for the CNTs grown under positive bias rather than those grown under negative bias. It is confirmed from this work that the substrate-biasing technique would be a way of improving the structural capability as well as the field-emissive capability of CNTs. Nevertheless, it is also suggested that further studies on the change of plasma potentials due to substrate-biasing and its effect on the growth behavior of CNTs may still be required. Concerning this issue, our recent data is being presented elsewhere [22].
Acknowledgement This research was partially supported by a grant in aid of the center for Electronic Materials and Components (EM&C) at Hanyang University (Ansan campus).
References 4. Conclusion Field-emissive capabilities and structural properties of CNTs grown by ICP-CVD have been characterized, in terms of substrate-bias effect. The FESEM and HRTEM analyses indicate that applying substrate bias change the diameter of grown CNTs and furthermore, there may exist a critical electric bias for causing the transformation between growth mechanisms such as tip-driven growth and based-oncatalyst growth. It is observed that negative bias is more profitable for obtaining the vertically aligned CNTs than
[1] J.M. Bonard, M. Croci, C. Klinke, R. Kurt, O. Noury, N. Weiss, Carbon 40 (2002) 1715. [2] W.I. Milne, K.B.K. Teo, M. Chhowalla, G.A.J. Amaratunga, S.B. Lee, D.G. Hasko, H. Ahmed, O. Groening, P. Legagneux, L. Gangloff, J.P. Schnell, G. Pirio, D. Pribat, M. Castignolles, A. Loiseau, V. Semet, V.T. Binh, Diamond Relat. Mater. 12 (2003) 422. [3] Q. Yang, C. Xiao, W. Chen, A.K. Singh, T. Asai, A. Hirose, Diamond Relat. Mater. 12 (2003) 1482. [4] Y. Avigal, R. Kalish, Appl. Phys. Lett. 78 (2001) 2291. [5] M. Chhowalla, K.B.K. Teo, C. Ducati, N.L. Rupesinghe, G.A.J. Amaratunga, A.C. Ferrari, D. Roy, J. Robertson, W.I. Milne, J. Appl. Phys. 90 (2001) 5308. [6] Y. Chen, D.T. Shaw, L. Guo, Appl. Phys. Lett. 76 (2000) 2469.
148
C.-K. Park et al. / Thin Solid Films 469–470 (2004) 142–148
[7] K.S. Kim, H.J. Ryu, G.E. Jang, Diamond Relat. Mater. 12 (2003) 1717. [8] C.L. Tsai, C.F. Chen, Diamond Relat. Mater. 12 (2003) 1615. [9] L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett. 76 (2000) 2071. [10] Y.T. Jang, J.H. Ahn, Y.H. Lee, B.K. Ju, Chem. Phys. Lett. 372 (2003) 745. [11] S.H. Seo, T.H. Lee, Y.D. Kim, C.K. Park, J.S. Park, Thin Solid Films 447 (2003) 212. [12] Y.T. Feng, S.Z. Deng, J. Chen, N.S. Xu, Ultramicroscopy 95 (2003) 93. [13] E.F. Kukovitsky, S.G. L’vov, N.A. Sainov, V.A. Shustov, L.A. Chernozatonskii, Chem. Phys. Lett. 355 (2002) 497. [14] J.H. Choi, T.Y. Lee, S.H. Choi, J.H. Han, J.B. Yoo, C.Y. Park, T.W. Jung, S.G. Yu, W.K. Yi, I.T. Han, J.M. Kim, Thin Solid Films 435 (2003) 318.
[15] M. Tanemura, K. Iwata, K. Takahashi, Y. Fujimoto, F. Okuyama, H. Sugie, V. Filip, J. Appl. Phys. 90 (2001) 1529. [16] C.J. Lee, J.H. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175. [17] R.T.K. Baker, Carbon 27 (1989) 315. [18] M. Sveningsson, R.E. Morjan, O.A. Nerushev, Y. Sato, J. Backstrom, E.E.B. Campbell, F. Rohmund, Appl. Phys., A 73 (2001) 409. [19] T.Y. Kim, K.R. Lee, K.Y. Eun, K.H. Oh, Chem. Phys. Lett. 372 (2003) 603. [20] M. Hirakawa, S. Sonoda, C. Tanaka, H. Murakami, H. Yamakawa, Appl. Surf. Sci. 169 (2001) 662. [21] C.M. Hsu, C.H. Lin, H.L. Chang, C.T. Kuo, Thin Solid Films 420 (2002) 225. [22] C.K. Park, J.P. Kim, Y.D. Kim, H.S. Uhm, J.S. Park, ICNDST-9, 2004.