Thin Solid Films 409(2002) 120–125

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00114-1

NH effect on the growth of carbon nanotubes on glass substrate in3

plasma enhanced chemical vapor deposition

Jae-Hee Han , Chong Hyun Lee , Duk-Young Jung , Chul-Woong Yang , Ji-Beom Yoo *, Chong-a b b a a,

Yun Park , Ha Jin Kim , SeGi Yu , Whikun Yi , Gyeong Su Park , I.T. Han , N.S. Lee , J.M. Kimc d d d e d f d

School of Metallurgy and Materials Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon, 440-746, South Koreaa

Department of Chemistry, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon, 440-746, South Koreab

Department of Physics, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon, 440-746, South Koreac

FED Project, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon, 440-600, South Koread

AE Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon, 440-600, South Koreae

Department of Advanced Materials Engineering, Sejong University, Seoul, 143-740, South Koreaf

Abstract

The effects of NH on the growth characteristics of carbon nanotubes(CNTs) were systematically investigated in plasma3

enhanced chemical vapor deposition(PECVD) with C H as a carbon source. CNTs were not formed when N was used instead2 2 2

of NH . Carbon films instead of CNTs were observed without NH . As the growth of CNTs with only C H proceeded after the3 3 2 2

initial growth of CNTs with NH and C H , films were formed with rugged surface morphology instead of nanotube-features.3 2 2

High-resolution transmission electron microscopy revealed that carbon films consisted of graphite carbons, and 20–30 nm graphiteonions covered with amorphous layers. NH was found to be essential to the growth of CNTs in the PECVD system. NH may3 3

provide additional etching effects for the growing carbon surface, and suppression of carbon supply.� 2002 Elsevier ScienceB.V. All rights reserved.

Keywords: NH ; Carbon nanotubes; Plasma enhanced chemical vapor deposition3

1. Introduction

Carbon nanotubes(CNTs) have received considerableattention since the first discovery of CNTs in 1991w1x,because CNTs offer the prospect of both new fundamen-tal science and many technological applications. CNTsare especially promising candidates for cold cathodefield emitters because of their unique electrical proper-ties, their high aspect ratios and small radii of curvatureat their tips. Numerous papers have been reported onthe growth characteristics and properties of CNTs suchas their growth mechanismw2–5x, and their structureand alignmentw6–9x using various methods. Among thevarious methods for CNT growth, plasma enhancedchemical vapor deposition(PECVD) has several advan-tages over other methods such as low temperature, easyvertical alignment of CNTs, high yield, and easy scale-up for large area and mass production. In the PECVD

*Corresponding author.E-mail address: jbyoo@yurim.skku.ac.kr(J.-B. Yoo).

system where NH and C H(or CH ) gas were usually3 2 2 4

used for synthesizing CNTs, the effects of reactant gaseson the growth of CNTs are very important for controllingthe density, diameter, length and morphologies of CNTs.In particular, a study on the role of NH as a catalyst in3

the PECVD system is of more importance. However,the role of NH in the PECVD system is not fully3

understood. There has been little report on the effectsof NH on the growth of CNTs so far. In this study, the3

effects of NH gas on growth characteristics were3

systematically investigated and the role of NH in the3

growth of CNTs using PECVD was identified.

2. Experiments

In this study, growth of vertically aligned carbonnanotubes(CNTs) was performed on nickel-coated glasswith and without a buffer layer(such as Cr and SiN)xat a temperature below 5508C by PECVD. An acetylene(C H ) gas was used as a carbon source, and an2 2

ammonia gas was used as a catalyst and dilution gas. A

121J.-H. Han et al. / Thin Solid Films 409 (2002) 120–125

Fig. 1. Surface morphologies of carbon films grown without NH :(a) carbon films grown with only C H for 5 min after pre-etching of Ni layer3 2 2

with NH for 4 min; (b) carbon films grown with NqC H for 14 min after pre-etching of an Ni layer with N for 1 min.3 2 2 2 2

Fig. 2. Microstructure of the carbon film grown only with C H and2 2

N : (a) TEM image showing that the graphite carbon and graphite2

onions; and(b) high resolution TEM image showing the nano-sizedgraphite onions covered with an amorphous layer.

Fig. 3. EDS pattern of CNTs:(a) as-grown;(b) grown with onlyC H ; and(c) grown with N qC H .2 2 2 2 2

d.c. plasma was used in this work to grow verticallyaligned CNTs. A thin nickel layer was deposited onglass by d.c. magnetron sputtering. For the depositionof the nickel film, the base pressure and operatingpressure of the chamber were kept below 7=10 andy5

5=10 torr, respectively. The operating current wasy3

0.3 A, the flow rate of an argon gas was 10 sccm, andthe deposition was carried out at room temperature. Thethickness of the nickel film was 300 A. Prior to CNT˚growth, the substrate was cleaned in trichloroethylene,acetone and methanol for 10 min, and rinsed in deioni-zed water. The substrate was transferred to the chamberand pumped to below 2=10 torr by a mechanicaly5

and diffusion pump. NH was subsequently introduced3

into the chamber. After the working pressure had beenstabilized, the infrared(IR) lamp and d.c. power supplywas turned on. The pre-treatment for surface etching ofthe nickel layer was conducted by NH plasma for 43

min. C H was introduced into the chamber for the2 2

growth of CNTs. Growth of CNTs was carried out undervarious conditions to investigate the effect of NH on3

growth characteristics. CNTs grown with C H and2 2

NH after NH pretreatment were regarded as the stan-3 3

dard process for comparison. Instead of NH , pre-etching3

of the Ni catalyst and the growth of CNTs were carriedout with N . The growth of CNTs was performed with2

only C H . To clarify the effect of NH on the growth2 2 3

of CNTs in our PECVD system, the overgrowth of CNTs

122 J.-H. Han et al. / Thin Solid Films 409 (2002) 120–125

Fig. 4. SEM images of CNTs with five different secondary growth times. CNTs were first grown with NHqC H for 10 min after 4 min NH3 2 2 3

pre-etching. Then, secondary growth of CNTs with only C H for different times:(a) 0; (b) 1; (c) 3; (d) 5; and(e) 9 min.2 2

was conducted with only C H after the initial growth2 2

of CNTs with C H and NH for 10 min.2 2 3

Field emission scanning electron microscopy(FESEM) was used for the analysis of surface mor-phology and cross-section of the grown samples. X-Raydiffraction with CuKa at a wavelength of 1.54056 A˚and energy dispersive spectroscopy was employed forstructural and compositional analysis of grown filmsunder various conditions. Transmission electron micros-copy (TEM) was adopted for microstructural analysisof grown films. Raman spectroscopy was employed forexamining the binding state of grown carbon films andCNTs.

3. Results and discussion

Vertically aligned CNTs were grown under pretreat-ment of the Ni catalytic layer with NH for 4 min, and3

exposure of the substrate to mixed reactant gases(NH qC H ) for 10 min. In order to investigate NH3 2 2 3

effects on the growth of CNTs, we have grown CNTsunder various conditions without NH andyor with N3 2

instead of NH . As shown in Fig. 1a, CNTs were not3

observed but the carbon film with the rough surface wasobserved when NH pretreatment was carried out for 43

min, and only C H was subsequently supplied during2 2

the CNT growth. Although C H was used as a carbon2 2

source and plasma intensity was large enough to growCNTs, CNTs were not formed without NH . Dissociation3

of NH produced nitrogen and hydrogen-related active3

species such as NH, NH , H and N. Among the various2

species, the effects of nitrogen-related species on thegrowth of CNTs were examined. Growth of CNTs wasperformed using C H and N instead of C H and2 2 2 2 2

NH . An SEM image of the grown film is shown in3

Fig. 1b, which is similar to Fig. 1. This result suggeststhat nitrogen-related species do not play a critical rolein the growth of CNTs in our d.c. PECVD system.

To identify the structure of the film deposited on thesubstrate using C H and N , X-ray diffraction was2 2 2

taken. In the X-ray diffraction pattern of the film(notshown here), peaks corresponding to graphite and car-bon nitride were not observed, but peaks related to theamorphous layer were found. TEM analysis was carriedout to obtain detailed information on the microstructureof the film. Fig. 2a,b shows the TEM(a) and highresolution TEM (HRTEM) image (b) of the film,respectively. Fig. 2a shows that the film is needle shapedwith a graphite layer and small-sized particles. From theHRTEM, nanometer sized graphite onions and amor-phous graphite layers which surrounds the onions wereobserved. This result indicates that the graphitization of

123J.-H. Han et al. / Thin Solid Films 409 (2002) 120–125

Fig. 5. Cross-sectional SEM images of CNTs which are the same samples used in Fig. 4.

carbon atoms supplied from the C H could take place2 2

without NH . It may be proposed that NH plays a3 3

major role in the formation of nanotube rather than thegraphitization of carbon. Additional experiments weresystematically carried out to identify the role of NH in3

the CNTs growth. After pretreatment of the substratewith N instead of NH , NH was subsequently supplied2 3 3

during the growth of CNTs, which resulted in theformation of CNTs. It was also found that CNTs with alarge diameter were grown even without pretreatment,only if NH was provided with C H during the growth3 2 2

of CNTs. The pretreatment of the Ni layer with NH or3

N provided the nucleation site for CNT growth, how-2

ever, that was not essential to the formation of CNTs.These results suggest that NH gas may be essential to3

the growth of CNTs in our PECVD system.Compositions of films were examined by energy

dispersive spectroscopy(EDS). Fig. 3a shows the EDSfor CNTs grown using C H and NH as a reference. A2 2 3

small amount of nitrogen was found in the CNTs grownwith C H and NH . The incorporation of nitrogen2 2 3

originated from the NH plasma, but nitrogen was not3

found in the carbon films grown with only C H(Fig.2 2

3b). However, a large amount of nitrogen was found inthe carbon film grown with C H and N instead of2 2 2

NH (Fig. 3c). A large Al peak in Fig. 3c is expected3

to come from the aluminum sample holder and not fromthe film itself. Based on these results, it is proposed thatnitrogen may not play a critical role in the growth ofCNTs in our PECVD system with C H and NH .2 2 3

CNTs were vertically grown when the CNTs growthwas performed for 10 min with NH and C H as shown3 2 2

in Fig. 4a. After the initial 10 min of growth of CNTs,we continued to grow CNTs with only C H by changing2 2

the second growth time. The Fig. 4b corresponds to themorphology of CNTs grown for 1 min without a NH3gas. The diameter of CNTs, especially at the tip ofCNTs, becomes larger than those of CNTs shown inFig. 4a. However, most of CNTs still maintained nano-tube-feature. The Fig. 4c shows that the verticallyaligned CNTs grown at the initial growth were changedinto a planar film with the rugged surface when thesecondary growth time without NH is 3 min. As the3

secondary growth time without NH increased, the3

124 J.-H. Han et al. / Thin Solid Films 409 (2002) 120–125

Fig. 6. Raman spectra of CNTs grown under different conditions:(a)pre-etching with N and CNTs grown with NqC H ; (b) pre-etching2 2 2 2

with NH and CNTs grown with only C H ;(c) pre-etching with3 2 2

NH and CNTs grown with NHqC H for 10 min, and additional3 3 2 2

CNTs grown with only C H for 5 min; and(d) pre-etching with2 2

NH and CNTs grown with NHqC H for 10 min. The pre-etching3 3 2 2

time for all samples were fixed at 4 min, irrespective of gas types.

surface roughness of films increased. From these results,even though CNTs were already grown under NH and3

C H , they transformed into films with a rough surface2 2

as the growth of CNTs without NH proceeded.3

Cross-sectional SEM images of the sample were takenand shown in Fig. 5. Most of CNTs remained(Fig. 4b)after secondary growth without NH for 1 min but some3

part of CNTs was covered with carbon films as shownin Fig. 5b. As growth without NH proceeded, film3

growth mechanism became dominant. Fig. 5d, which isthe SEM image of 9 min secondary growth, clearlypresents the film growth process. At the initial stage offilm growth, carbon atoms attached to CNTs mainly atthe top of CNTs, but some of the carbon atoms adheredto the sidewall of CNTs, resulting in an increase in thethickness of CNTs and formation of large tips. The largetips on the CNTs continued to grow and merged togetherto form small carbon column-like grain growth in bulkmaterials. Carbon columns grew at the expense of smallcarbon column nearby with the supply of carbon atomsfrom C H . As shown in Fig. 5d, carbon films with a2 2

rough surface were eventually formed. NH was disso-3

ciated under plasma, leading to the supply of a largeamount of hydrogen. High concentration of hydrogenreduces the dissociation of C H , resulting in suppres-2 2

sion of the carbon supply to the growing surface.Incorporation of carbon atoms to the proper siteenhanced the formation of CNTs. Additionally, hydrogensupplied from NH provided the additional etching3

effects, which enhanced the formation of CNTs.Raman spectra using the argon laser excitation wave-

length of 514.5 nm(Renishaw-3000) as shown in Fig.6, indicate the difference in peak intensity and widthaccording to various growth times without NH . In our3

previous workw10x, there had been two peaks at 1592.4and 1366.6 cm . Similarly, in the case of as-growny1

CNTs indicated by(d), two weak features were observedat 1593 and 1368 cm . The peak at 1593 cmy1 y1

corresponds to the E longitudinal optic(LO) compo-2g

nent of the G mode in graphite at 1582 cm , while they1

peak at approximately 1368 cm (D line) was attrib-y1

uted to disorders that were caused by the low tempera-ture PECVD process in our system. Raman spectra ofvarious samples grown under different conditions wereshown in Fig. 6. A Peak near the G line was observedin all samples, indicating that there is little difference invibrational states. For the sample grown with onlyC H , peak intensities at approximately 1596(G line)2 2

and 1350 cm (D line) became stronger. A strongy1

intensity at approximately 1596 cm (G line) wasy1

usually interpreted as a peak from highly orientedpyrolytic graphite; however, in this study amorphousfilms were obtained so that the origin of this enhance-

ment in peak intensity requires further investigation.However, strong intensity at approximately 1350 cmy1

(D line) suggested an increase in disorder with growthtime without NH , which may be due to defects or3

amorphous carbon. Raman spectra measured on thesample obtained with NqC H and C H only, was2 2 2 2 2

shown in Fig. 6a,b. Compared to Fig. 6d, the intensityand width of the peaks at the two main frequencies(G-and D line) was reduced and broadened.

4. Conclusion

The effects of NH on growth characteristics of carbon3

nanotubes (CNTs) were systematically investigated.CNTs were grown with and without NH in the pretreat-3

ment of the Ni catalytic layer and growth. The growthof CNTs without NH resulted in the formation of3

carbon films instead of CNTs. HRTEM revealed thatcarbon films were composed of graphite carbons andgraphite onions of 20–30 nm in diameter. As secondarygrowth with only C H proceeded after initial growth of2 2

CNTs, these CNTs changed to a carbon film with ruggedsurface morphology instead of nanotube-feature. NH3

was found to be essential to the growth of CNTs in thePECVD system. It may be due to the fact that NH3

under plasma provided a large amount of hydrogen,resulting in the suppression of carbon supply, andplaying an important role in the formation of tubesrather than a carbon film. Furthermore, the additionaletching effect of hydrogen from NH may enhance the3

formation of CNTs.

125J.-H. Han et al. / Thin Solid Films 409 (2002) 120–125

Acknowledgments

This work was supported by the Korean Ministry ofCommerce, Industry and Energy.

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