Density control for carbon nanotube arrays synthesized by ICP-CVD using AAO/Si as a nanotemplate

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Density Control for Carbon Nanotube Arrays Synthesized

by ICP-CVD Using AAO

ÕSi as a Nanotemplate

Jung-Hsien Yen, Ing-Chi Leu,zMin-Tao Wu, Chien-Chih Lin, and Ming-Hsiung Hon

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Carbon nanotubes共CNTs兲 with tunable density have been grown directly by inductively coupled plasma chemical vapor deposi-tion共ICP-CVD兲 using anodic aluminum oxide 共AAO兲 on silicon as a substrate. The density of CNTs (106to 109CNTs/cm2兲 can be controlled by changing the length of catalytic Ni nanowires embedded in AAO. The one-dimensional CNTs-Ni nanowire heterojunction without observable defects by transmission electron microscopy characterization ensures good contact between the CNT and the Ni nanowire. The realization of combining AAO on silicon and controlling the density of carbon nanotubes by a plasma process is significant for application of CNTs as field emitters and nanoelectronic devices.

Manuscript submitted August 19, 2003; revised manuscript received December 8, 2003. Available electronically July 12, 2004.

Since first discovered by Ijima in 1991,1 carbon nanotubes 共CNTs兲 have attracted considerable interest because of their unique physical properties and many potential applications. Due to their high aspect ratio, good emission stability, long emitter lifetime, and high current density, CNTs have been regarded as the most efficient electron emitter among various candidate materials. Many research-ers have fabricated densely packed CNTs on the electrode for a proposed field emission application. However, such densely packed CNTs have a negative effect on field enhancement due to the field screening effect caused by the high spatial density of CNT arrays.

Although changing the catalyst particle size and growth time may easily control the diameter and length of the aligned CNTs, control on the site density 共number of CNTs per unit area兲 is challenging.2There are some methods proposed to control the site density of CNTs such as varying the concentration of catalyst in the solution,3NH3plasma treatment of the catalyst layer,4electroplating the catalyst by pulse-current deposition techniques,5and using dif-ferent aspect ratios of anodic alumina oxide共AAO兲 nanochannels.6 For exploring further applications, it is important to integrate the controlled growth of CNT arrays to Si devices and to implement compatible processing steps with Si-related processing. However, attempts to control different packing densities of CNTs from AAO thin-film template deposited on silicon substrate have met with little success. If an AAO template can be applied to a silicon substrate and control the packing density of CNT growth through appropriate processing parameters, then isolated CNTs with uniform diameters can be arranged to meet device requirements. Besides, there are many methods proposed for the synthesis of CNTs, but only a few reports for the growth of CNTs on an AAO/Si substrate with plasma-enhanced chemical vapor deposition 共PECVD兲 have been presented.7The advantages of using PECVD for CNT growth in-clude low-temperature growth, easy vertical alignment, and large area growth.7

Here, we report a simple method to control the packing density of aligned CNTs in AAO integrated to Si wafer by controlling the Ni nanowire catalyst length, as well as providing a better methodology for electrical contacts for CNTs. The deposition of CNTs was con-ducted on an inductively coupled plasma-enhanced chemical vapor deposition共ICP-CVD兲 system.

Experimental

The AAO nanotemplate used for CNT growth was prepared by anodizing the Al film deposited on Si wafer through a multilayer scheme共Al/Au/Ti/Si兲, where Al top layer, Au conducting layer, and Ti adhesive layer were deposited successively on the silicon wafer by E-beam evaporation. The layer thicknesses of Al, Au, and Ti were 1200, 10, and, 5 nm, respectively. Anodization was first

con-ducted in a bath of 0.3 M oxalic acid solution with cooling circula-tion at a constant voltage of 40 V and 13°C. After anodizacircula-tion through the entire Al film thickness, the specimen was then im-mersed into a 5 wt % H3PO4solution to remove the barrier layer at the oxide/Au interface. Ni nanowires used as catalyst for CNT growth were electrodeposited at the bottom of the nanoholes by three-probe direct current 共dc兲 in 330 g/L NiSO4•6H2O, 45 g/L NiCl2•6H2O, and 35 g/L H3BO3solution at 23°C. The length of Ni nanowires was controlled by the deposition time.

After depositing the Ni nanowires with different lengths in an AAO template on silicon, the sample was loaded into the CVD chamber; CNTs were then synthesized using an ICP plasma source. The carrier and reaction gases were hydrogen共80 sccm兲 and meth-ane共20 sccm兲, respectively. The parameters for CNT growth are radio-frequency共rf兲 power 250 W, dc bias ⫺400 V, pressure 3 Torr, and temperature 660°C for 40 min. The obtained Ni nanowires and CNTs were characterized by field-emission scanning electron mi-croscopy共FESEM兲共Philips XL-40EFG兲 and high-resolution trans-mission electron microscope共HRTEM, Hitachi HF-2000兲.

Results and Discussion

Figure 1 shows a scanning electron microscopy共SEM兲 image of Ni nanowires in the AAO by electroplating at⫺1.0 V for 30 s. The Al layer is completely anodized and transformed to AAO, and straight nanohole arrays, 60 nm in diameter, are arranged perpen-dicularly on the Au/Ti/Si substrate. Only uniform length Ni nanow-ires共300 nm兲 are embedded at the bottom of all nanoholes and no excessive Ni is deposited on the surface of AAO. Due to the high resistance of Si employed in this study, it is hard to deposit metal nanowires or oxide in AAO/Si by dc electroplating. To solve this problem, Iwasaki et al.8used Nb as the intermediate layer, but they found it necessary to conduct thermal pretreatment at high tempera-ture共500°C兲 to reduce the insulating Nb2O5to the semiconducting NbO2. The homogeneity of the Ni nanowires deposited by dc elec-troplating may be improved by introduction of the Au/Ti intermedi-ate layer. The Au layer enhances the conductivity during electroplat-ing and the Ti layer is used as an adhesive buffer layer. This process is also a simple and easy method to apply to other nanowire arrays on Si by dc electroplating.

To obtain different length Ni nanowires embedded in the AAO, the electrodeposition was carried out at a constant potential for dif-ferent deposition times. Figure 2 shows the length of Ni nanowires deposited at⫺1.0 V for different deposition times in which approxi-mately linear variation with the deposition time 共30-140 s兲 is ob-tained. The deposition rate is about 8.4 nm/s. This result also shows that the length of Ni nanowires can be easily controlled by properly selecting the electroplating time.

After the different length Ni nanowire arrays were deposited in the AAO on Si, CNTs were grown by ICP-CVD using the Ni nanowires as catalyst. The advantage of using ICP-CVD to grow z_{E-mail: [email protected]}

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CNTs is that the ICP source has a higher ion density (1012/cm3) than conventional dc or rf parallel plate capacitive discharge system and it is easy to scale up.9 In the ICP-CVD system, the inductive rf power and dc bias were used separately to control the hydrocarbon radical dissociation and their subsequent transportation to the sub-strate for CNTs growth. Figure 3 shows that the density of CNTs changes dramatically as a function of the pore length of AAO with Ni nanowires at an appropriate length. At a AAO pore length of 1.2 ␮m, the density of CNTs is about 1 ⫻ 106

CNTs/cm2. When the pore length is reduced to 300 nm, the density of CNTs increases to 1⫻ 109CNTs/cm2. Figure 4a-d show the SEM images for different density CNT arrays obtained from an AAO nanotemplate with dif-ferent Ni nanowire lengths. The diameter of the CNTs is almost the same as that of the AAO pore diameter and all the CNTs are grown perpendicularly to the Si substrate. Other researchers reported that during the growth of CNTs from AAO by thermal CVD, the length of the CNTs increased and then the CNTs became curved with an increase in growth time.6However the present study demonstrates a better control on the growth characteristics of CNTs by ICP-CVD. The orientation of CNTs perpendicular to the substrate is due to the AAO confinement at the initial growth stage. After the nanotube grows out of the AAO pore, the orientation is influenced by self-bias potential established on the immersed substrate surface in high-frequency plasma.10Besides, the Ni nanowire-CNT heterojunctions show good electrical contact and good field-emission properties共not shown here兲.

There are several important characteristics of this method. First, the present scheme for integrating Si and AAO to grow CNTs arrays with controllable density is facile and flexible. The other merit in-cludes the more precise adjustment of the aspect ratio of the AAO on silicon, which is controlled by varying the electroplating voltage and current. This method does not require nanolithography for

cata-lyst patterning and is therefore simple and inexpensive. Moreover, the growth can be potentially scaled up by properly combining the Si-compatible processing steps with ICP-CVD for CNT growth. The above-mentioned properties are important to the fundamental char-acterization and potential applications of CNT-based nanoelectronic devices.

The mechanism of CNT growth in AAO channel is still specula-tive. If using catalyst-embedded AAO nanotemplate, the growth of CNTs may be a more complex process, which may involve competi-tive hydrocarbon decomposition by both catalyst at the bottom of pores and the AAO template itself. In our experimental conditions, the catalyst is located at the tip of the nanotube, and the CNTs are grown only from the nanochannels of the AAO template instead of being grown from the top surface of the AAO template. The CNT growth in this system should belong to the tip growth mode.11 Ac-cording to previous reports the growth of CNTs by thermal CVD involves共i兲 decomposition of hydrocarbon gas, (ii) incorporation of carbon atoms into the transition metals, and (iii) diffusion of carbon atoms in the transition metals and then to form nucleation seed for the nanotube as a result of supersaturation of carbon species.11For this study using nanowire as a catalyst, the interaction of C-containing species and the Ni nanowire cause a localized flow 共and/or melting兲 at the tip of Ni nanowire which subsequently

de-Figure 2. Dependence of Ni nanowire length on deposition time at an

ap-plied voltage of⫺1.0 V and solution temperature of 23°C.

Figure 3. Dependence of CNT packing density on the pore length of AAO

on silicon substrate.

Figure 4. SEM images of CNTs grown in the AAO template with different

pore lengths of共a兲 1, 共b兲 0.75, 共c兲 0.45, and 共d兲 0.3 ␮m on silicon substrate from a gas mixture of 20% CH4 and 80% H2 at 660°C for 40 min by ICP-CVD.

Figure 1. SEM image of an AAO template embedding Ni nanowires by

electroplating at⫺1.0 V for 30 s.

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taches from the Ni nanowire and forms as a nanoparticle catalyst for CNT growth. Figure 5 shows one-dimensional CNT-Ni nanowire heterojunction observed under TEM, which clearly demonstrates the formation of elongated Ni nanowire embedded in the hollow core of

the CNT. As mentioned previously, the AAO itself and Ni can act as a catalyst for CNT growth. From the observation of Ni-CNT hetero-junction and the microstructure of CNT, it is believed that the active Ni nanowires used as a catalyst in this system play a more important role than the AAO in controlling the growth behavior and the con-sequent packing density of the CNTs. But a detailed mechanism for CNT growth in the AAO template requires still further investigation.

Conclusion

In summary, a simple method to better control the density and electrical contact of aligned CNTs on silicon is proposed. The activ-ity of Ni nanowire is more important than AAO in controlling CNT growth in this system. This method may also successfully grow aligned and isolated straight CNTs with the assistance of AAO on Si as a template by ICP-CVD. This approach can open novel possibili-ties for the fundamental characterization and potential applications of nanoscale devices that can be integrated with Si microelectronics.

Acknowledgments

The authors acknowledge the financial support from the National Science Council of Taiwan, Republic of China, under contract no. NSC 91-2216-E-006-028. Technical assistance on ICP plasma source from Professor F. C. N. Hong is greatly appreciated.

The National Science Council assisted in meeting the publication costs of this article.

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Figure 5. 共a兲 TEM 共b兲 HRTEM images of 1D carbon nanotube-Ni nanowire

heterojunction.

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