Effects of metal buffer layers on the hot filament chemical vapor ... - AVS

Effects of metal buffer layers on the hot filament chemical vapor deposition of nanostructured carbon films Kyung Moon Lee, Hyung Jun Han, Seungho Choi, Kyung Ho Park, Soo-ghee Oh, Soonil Lee,a) and Ken Ha Koh Department of Molecular Science and Technology and Information Display Research Institute, Ajou University, Suwon 442-749, Korea

共Received 28 March 2002; accepted 24 June 2002; published 5 February 2003兲 We examined how the addition of different metal buffer layers between the Ni/Fe-alloy-catalyst layer and the silicon substrate affected the growth of nanostructured carbon films; Cr, Ti, Ta, and W were tested as buffer layers. Even when the sputter-deposition of catalytic-metal layers and the hot filament chemical vapor deposition of carbon films were carried out under the identical conditions, different buffer layers resulted in substantially different carbon-film growth. More specifically, carbon-nanoparticle films were produced with the Cr and the W buffer layers, and carbon-nanotube films were produced with the Ti and the Ta buffer layers. X-ray diffraction 共XRD兲 showed a significant and systematic difference between the carbon-nanoparticle and carbon-nanotube films. In the case of the carbon-nanoparticle films deposited with either the Cr or the W buffer layer, the peaks corresponding to the catalytic metal, the carbide phases of the catalytic metal, and the carbide phases of the respective buffer metal were observed. However, in the case of the carbon-nanotube films deposited with either the Ti or the Ta buffer layer, the peaks corresponding to the carbide phases of the catalytic metal and the silicide phases of the respective buffer metal were observed. Moreover, scanning electron microscopy 共SEM兲 images of the cross sections of the films showed the difference in the interface structure and its deposition-time-dependent change. Based on the XRD and cross-section SEM observations, we proposed a model that could account for the growth of different nanostructured carbon films on the different sets of buffer layers. © 2003 American Vacuum Society. 关DOI: 10.1116/1.1524136兴

I. INTRODUCTION Good adhesion of carbon electron-emitting layers to substrates is a prerequisite for the fabrication of practical cold cathodes for electron emitting devices. However, nanostructured carbon films with good electron-emitting properties, as well as nanotube films, showed either modest or poor adhesion properties when the metal catalyst was deposited directly on common substrates such as silicon wafers or sodalime glasses. Previously, we reported that the adhesion of nanostructured carbon films could be greatly improved by adding chrome buffer layers underneath the catalytic metal layers.1,2 Note that the chrome buffer layers can also be used as electrodes for device operation. In this work, we tested metals such as titanium, tantalum, and tungsten as buffer layers in addition to chrome, and examined, in particular, how the growth of composite films of carbon nanotubes and carbon nanoclusters was affected by the presence of different metal buffer layers. On the contrary to the reports by other groups, which suggested that the morphology of metal catalyst determined the carbon-nanotube growth,3–5 our preliminary study showed a substantial difference in the nanotube growth when the metal catalyst with similar surface morphology was supported by different metal buffer layers. Therefore we carried out a systematic investigation on the effect of metal buffer layers. This study was a兲

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also motivated by the reports that nanoparticles of various metals or carbides encapsulated by carbon layers could be formed using the modified arc discharge method.6 –10 As far as we know, similar nanoparticles were never synthesized using the hot filament chemical vapor deposition 共HFCVD兲 method. II. EXPERIMENT Using the predetermined deposition rate, we first deposited 100-nm-thick chrome, titanium, tantalum, and tungsten buffer layers on separate silicon substrates by sputtering. Onto each of the metal buffer layers, a 60-nm-thick Ni/Fealloy-catalyst layer was subsequently deposited using identical sputtering conditions. Nanostructured-carbon films were prepared by the HFCVD method using the mixtures of methane and hydrogen as source gases. Note that to avoid the uncontrolled variation in the HFCVD conditions, the carbon films were deposited by putting four substrates with different buffer layers in the reaction chamber simultaneously. The carbon deposition was carried out at the working pressure of 30 Torr, substrate temperature of 700 °C, and the filament temperature of 2050 °C. Scanning electron microscopy 共SEM兲 and atomic force microscopy 共AFM兲 were employed to probe the surface morphology of bare buffer layers before the catalytic-metal deposition and also the morphology of the metal-catalyst surface. The cross sections of the films were examined using SEM to investigate the buffer-metal-dependent difference

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FIG. 1. SEM images of Ni/Fe-alloy catalyst layers deposited on 共a兲 Cr, 共b兲 Ti, 共c兲 W, and 共d兲 Ta buffer layers. Catalyst layers were deposited using identical sputtering conditions. The bar corresponds to 500 nm.

and the deposition-time-dependent variation in the interface structure. X-ray diffraction 共XRD兲 was used to probe the structure of the samples before and after the carbon-film deposition. XRD patterns were measured in 0.02° steps with the scanning speed of 5°/min using Cu K ␣ radiation. III. RESULT AND DISCUSSION Figure 1 shows the SEM images of metal catalyst layers deposited using identical conditions on Cr, Ti, Ta, and W buffer layers. Nominal thickness of the buffer layer and the catalyst layer was 100 and 60 nm, respectively. As evident from these images, all catalytic layers deposited on different metal buffer layers showed similarly smooth surface morphology; close examination of the grain size produced average sizes of about 20, 37, 22, and 17 nm for Cr, Ti, Ta, and W buffer layers, respectively. Root-mean-square 共rms兲 surface roughness measurement using AFM also produced similar results. The rms roughness determined from the area of 1⫻1 ␮ m2 was, respectively, 16, 42, 21, and 16 Å for the catalytic layer deposited on Cr, Ti, Ta, and W buffer layers. Figure 2 shows the typical SEM images of the surfaces of the nanostructured carbon films grown under the identical

FIG. 2. SEM images of the surfaces of the nanostructured carbon films grown with 共a兲 Cr, 共b兲 Ti, 共c兲 W, and 共d兲 Ta buffer layers for 5, 15, 3, and 5 min, respectively. All other deposition conditions were identical. The four images were taken at the same magnification. J. Vac. Sci. Technol. B, Vol. 21, No. 1, JanÕFeb 2003

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conditions. It was interesting to find a wide variation in the structures of carbon films, in spite of the similarity in the morphologies of catalytic-metal layers. The most striking difference was the type of carbon species formed with different buffer layers; nanoparticles were deposited with either the Cr or the W buffer layer, but nanotubes were deposited with either the Ti or the Ta buffer layer. More specifically, nanoparticles of average diameter of ⬃65 nm completely covered the film surface deposited with the Cr buffer layer. In the case of the W buffer layer the size distribution of nanoparticles was broader with the larger average diameter of ⬃80 nm, and the nanoparticles did not completely cover the surface. Comparison of the nanotubes grown with the Ti and the Ta buffer layer showed that the tubes were either more curly or coiled with the Ti-buffer layer, but that the tubes grown with Ta buffer layer had a spaghetti-like morphology. The nanoparticles formed with the Cr-buffer layer did not show a significant growth-time dependence. The particle– size distribution became a little bit broader, but both the average particle diameter and the density remained more or less the same. However, prolonged carbon deposition resulted in an amorphous carbon layer on top of the nanoparticles. In the case of the W buffer layer, there was an increase in the average diameter of nanoparticles from ⬃80 to ⬃100 nm and a noticeable decrease in the nanoparticle density as the deposition time was increased from 3 to 15 min. For the Ti and Ta buffer layers, the increase of growth time produced a substantial increase in the nanotube density without a significant change in the average tube diameter; the cross section of the nanotube film deposited using the Ta buffer layer for 15 min resembled a rug of compact texture. In Fig. 3共a兲, we compared the x-ray diffraction patterns of the four samples, of which the SEM images were shown in Fig. 1. Note that all four samples showed peaks corresponding to the respective metal-buffer layer and a broad peak corresponding to the Ni3 Fe phase at about 43.3°. However, as presented in Fig. 3共b兲, a significant and systematic change was observed after 15 min of carbon-film deposition. First, peaks corresponding to the Ni3 Fe phase were observed only from the samples with the Cr or the W buffer layer. Second, peaks corresponding to the Cr3 C2 phase and the WC and W2 C phases were observed together with the various carbide phases of Fe and Ni from the samples with the Cr or the W buffer layer. Third, peaks corresponding to the titanium- and tantalum-silicide phases were observed together with the various Fe- and Ni-carbide phases from the samples with the Ti or the Ta buffer layer. Supporting evidence for the formation of carbide and silicide phases of buffer metals was provided by the SEM images of the film cross section. Figure 4共a兲 shows the cross section of the film deposited for 3 min with the W buffer layer. Note that there was an interface layer about 130-nm-thick underneath the carbonparticle layer, which we assigned as the W carbide and/or W layer; there was a little bit of contrast difference between the upper and the lower part of the interface. An interface layer was also observed from the cross section of the film depos-

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FIG. 3. X-ray diffraction patterns of the four samples with Cr, Ti, W, and Ta buffer layers: 共a兲 before the carbon deposition and 共b兲 after the carbon deposition for 15 min. For all four samples, peaks corresponding to buffer metal, catalyst metal, catalyst-metal carbide, buffer-metal carbide, and buffer-metal silicide are marked with a closed circle, a closed diamond, a closed inverted triangle, a closed square, and an open circle, respectively. The broad and intense diffraction peak at ⬃70° is that of silicon substrates.

ited for 5 min with the Cr buffer layer as is shown in Fig. 4共b兲. The thickness of the interface layer underneath the carbon-particle layer was about 140 nm. In this case, the upper and the lower part of the interface showed different morphology, and we assigned them as Cr carbide and Cr, respectively. However, the cross section of the film deposited for 15 min with the Cr buffer layer showed a single interface of ⬃170 nm with the morphology similar to the part we assigned as Cr carbide. The cross section of the carbonnanotube films presented in Fig. 5 also showed interface layers. However, the morphology and the thickness of the interfaces’ layers were the same for both the Ti and the Ta buffer layer regardless of the deposition time. We assign the interface layer underneath the nanotubes deposited with either the Ti or the Ta buffer layer as the silicide of the respective buffer metal. Based on the XRD and cross-section SEM observations, we propose the following model for the growth of either carbon-nanoparticle films or carbon-nanotube films with different metal-buffer layers underneath the catalyst layer. In the case of the Cr and W buffer layers, the catalyst and the buffer metals shared the finite decomposition products of carbonaceous molecules at the beginning of the deposition. Therefore chrome carbide and tungsten carbide started to form from the upper part of the buffer layer even in the early stages of deposition.11 Moreover, the depletion of the availJVST B - Microelectronics and Nanometer Structures

FIG. 4. SEM images of the cross sections of the carbon-nanoparticle films grown with 共a兲 a W buffer layer for 3 min, 共b兲 a Cr buffer layer for 5 min, and 共c兲 a Cr buffer layer for 15 min. All other deposition conditions were identical.

able carbon resulted in insufficient carbon supply to catalytic metals so that the growth of carbon nanotubes was hindered, and carbon nanoparticles were formed instead. Note that the preferred formation of carbon nanoparticles with metal cores by quenching the evaporated metal-carbon in modified arc discharge was reported previously.7 Once a certain thickness of carbon layers was formed around the catalyst-metal cores, the cores were completely isolated from the carbon supply and lost their catalytic ability. Therefore the nanoparticle size increased only slightly when the deposition time was increased, but the catalyst core remained in the metallic state. However, the formation of chrome carbide and tungsten carbide continued steadily until all the buffer layers turned into their carbide phases. After the buffer-carbide formation stopped, the continued carbon supply resulted in only the deposition of amorphous-carbon layers. In the case of the Ti and Ta buffer layers, the formation of

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plied carbons exclusively to form nanotubes through the widely accepted catalyst-assisted process. However, the absence of the catalytic-metal peaks in the XRD pattern suggested that the catalyst-metals themselves turned into their carbide phases when the nanotube growth was terminated. IV. CONCLUSION We reported on the growth of two very different kinds of nanostructured carbon films onto the same metal-catalyst layers which were deposited under the identical sputtering conditions, but supported by different buffer layers. In particular, the growth of carbon-nanoparticle films with the Cr and the W buffer layers, both of which turned into carbide phases, and the growth of carbon-nanotube films with the Ti and the Ta buffer layers, both of which turned into silicide phases, were reported. Moreover, we presented a plausible model that can account for the observed difference in the carbon-film growth with different metal buffer layers. ACKNOWLEDGMENT This work was supported by Korea Research Foundation Grant No. KRF-2001-015-DP0167. 1

FIG. 5. SEM images of the cross sections of the carbon-nanotube films grown with 共a兲 a Ta buffer layer for 5 min, 共b兲 a Ti buffer layer for 3 min, and 共c兲 a Ti buffer layer for 15 min. All other deposition conditions were identical.

the silicide phases of the buffer metals were favored over the formation of the carbide phases of the buffer metals.11 In particular, it seemed that the silicide formation occurred at the very early stages of the deposition process. Therefore it was conceivable that the catalytic metals consumed the sup-

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K. H. Park, K. M. Lee, S. Choi, S. Lee, and K. H. Koh, J. Vac. Sci. Technol. B 19, 946 共2001兲. 2 K. H. Park, S. Choi, K. M. Lee, S.-g. Oh, S. Lee, and K. H. Koh, J. Korean Phys. Soc. 37, L153 共2001兲. 3 Y. Y. Wei, G. Eres, V. I. Merkulov, and D. H. Lowndes, Appl. Phys. Lett. 78, 1394 共2001兲. 4 W. Z. Li, D. Z. Wang, S. X. Yang, J. G. Wen, and Z. F. Ren, Chem. Phys. Lett. 335, 141 共2001兲. 5 Y. C. Choi, Y. M. Shin, Y. H. Lee, B. S. Lee, G.-S. Park, W. B. Choi, N. S. Lee, and J. M. Kim, Appl. Phys. Lett. 76, 2367 共2000兲. 6 J. Jiao, S. Scraphin, X. Wang, and J. C. Withers, J. Appl. Phys. 80, 103 共1996兲. 7 S. Seraphin, D. Zhou, and J. Jiao, J. Appl. Phys. 80, 2097 共1996兲. 8 Y. Saito, K. Nishikubo, K. Kawabata, and T. Matsumoto, J. Appl. Phys. 80, 3062 共1996兲. 9 R. S. Ruoff, D. C. Lorents, B. Chan, R. Malhotra, and S. Subramoney, Science 259, 346 共1993兲. 10 S. Seraphin, J. Electrochem. Soc. 142, 290 共1995兲. 11 Because of the complexity of the carbide and silicide formation of Cr, W, Ti, and Ta, it was not straightforward to account for the favored formation of one phase over other phases. However, it was interesting to note that there was a correlation between the thermal expansion coefficients of buffer metals and the synthesis of carbon nanoconstituents; the thermal expansion coefficients of Ti and Ta are larger than that of Si, but those of Cr and W are a little bit smaller than that of Si. A further investigation to elucidate the origin of the difference in the carbon-nanoconstituent synthesis between two sets of buffer metals is under progress.