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Nano-structured nitrogenated carbon films — morphology and field emission
Diamond and Related Materials 10 Ž2001. 1962᎐1967
Nano-structured nitrogenated carbon films ᎏ morphology and field emission R. KurtU , J.-M. Bonard, A. Karimi Department of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland Received 3 July 2000; accepted 24 January 2001
Abstract Crystalline nitrogenated carbon ŽC:N. films have been successfully synthesised by plasma-enhanced hot filament chemical vapour deposition ŽPE-HF-CVD.. Nitrogen gas ŽN2 . and ammonia ŽNH 3 . were used as sources of atomic nitrogen whereas methane ŽCH 4 . acted as a carbon precursor. Structure analysis reveals the growth of a new type of C:N film characterised by various polymorphs including worm- and foil-like microstructures at the nanometer scale. The effects of bias voltage and filament temperature on the film morphology are investigated in detail. Using pure Si as a substrate results in growth of homogeneous nanostructured films, whereas arrays of nanotubes were deposited on Ni-coated substrates. Field emission in vacuum was observed on C:N films deposited on pure Si above applied fields of 15 Vrm. The onset field could be decreased below 4 Vrm with Ni-coated substrates due to the presence of well-separated nano-tubular structures. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Nitrogenated carbon; Nano-structures; Field emission
1. Introduction In the last number of years, a lot of effort has been made to study the influence of nitrogen in carbon-based materials. The crystalline -C 3 N4 phase was tried to synthesise, e.g. w1᎐3x due to predicted interesting mechanical properties w4x. Furthermore, nitrogen doping in diamond was studied with regard to the electrical properties of the resulting film w5x and the morphological change w6x. The influence of nitrogen in amorphous CN x and diamond-like ŽDLC. materials on the mechanical properties was also analysed, e.g. w7,8x. Miyamoto et al. w9x showed that tubule forms of graphitic carbon nitrides can be stabilised by displacing C atoms out of the plane in the honeycomb network of graphite. U
Corresponding author. Tel.: q41-21-693-4515; fax: q41-21-6934470. E-mail address: [email protected] ŽR. Kurt..
However, the role of nitrogen for structure formation as well as for modification of the material properties is not completely clear. One of the applications of carbon-based materials studied over the last few years has been the production of flat cold cathodes which can serve as field emitters with low turn-on voltages and high emission currents. The first carbon-based films that attracted attention in terms of their field emission ŽFE. properties were aC:H, a-C:N and DLC coatings w10᎐17x. However, after the discovery of carbon nanotubes by Iijima in 1991 w18x there has been an increasing interest in their field emission characteristics w19᎐21x. Only a few works deal with two-dimensional arrays of aligned and well-distributed nanotubes w22,23x. Recently, Ren et al. w24x reported on fabrication patterned growth of freestanding and highly oriented carbon nanotubes but with dispersion in length and thickness affecting their FE properties. Nevertheless, until now there is a lack in develop-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 3 8 8 - 0
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
ment of fast techniques for the production of macroscopic quantities of nano-tubular structures andror for the deposition of homogeneous flat arrays characterised by a large emitter density. In this work we propose a method for the fast production of uniform C:N films characterised by wormand foil-like graphite sheets. The influence of deposition parameters on structure formation will be discussed. Under certain conditions remarkable FE properties of the nanostructured films were observed.
2. Experimental details 2.1. Film deposition The experimental set-up used for deposition was described in Kurt and Karimi w25x. A resistively heated tungsten ŽW. filament Ž⭋ s 500 m. was used for film deposition after complete crystallisation Žtransformation to WC.. A molybdenum ŽMo. substrate holder placed 8 mm underneath the filament allowed the substrate temperature Tsub s 700⬚C to be controlled independently from the filament temperature Tfil . The background pressure typically reached in the deposition chamber was 10y5 Pa, whereas the working pressure p was kept constant at 300 Pa. A feed mixture of 100 sccm CH 4 q 5 sccm N2 q 1 sccm NH 3 was used. After complete transformation of the filament to tungsten carbide WC Žas described in Kurt and Karimi w25x. C:N films were deposited onto clean n-type Si Ž100. wafer and onto Si substrates covered by a 2 m thick Ni layer. Various filament temperatures between Tfil s 1500 and 2300⬚C as measured by means of a two-colour pyrometer were applied. Positive or negative d.c.-bias potential could be applied independently to the substrate through the Mo substrate holder as well as to the filament. Both types of experiments, the hot filament chemical vapour deposition ŽHF-CVD. and the plasma enhanced hot filament chemical vapour deposition PE-HF-CVD, were performed. The later one proceeds as follows. First, a negative d.c. voltage of 200 V was applied to the substrate. Under these conditions no ion current could be detected reflecting the difficulty of inducing a gaseous breakdown under negative biasing conditions. After that, a positive bias was applied to the filament, and its magnitude was gradually increased until a sudden increase of current was detected accompanied by a small drop of voltage Žin the range of 10᎐12 mArcm2 at a voltage of approx. 150 V.. A purple glow could be observed near the substrate surface under such conditions. 2.2. Characterisation techniques Scanning electron microscopy ŽSEM. was used to
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analyse the microstructure of the C:N films in plan view. A Philips XL 30 microscope equipped with a field emission gun ŽFEG. operating at an acceleration voltage of 5 kV, a working distance of typically 10 mm, and a secondary electron ŽSE. image mode was used. The growth morphology of the films and their crystalline structure were controlled by analytical transmission electron microscopy ŽTEM.. For this purpose a Hitachi HF-2000 field emission microscope equipped with a Gatan image plate operating at 200 kV Žpoint resolution 0.23 nm. was used. For the performed cross-section analysis the film-substrate composite was freshly cleaved prior to the investigations. In this way any structural modifications of the C:N films were prevented as they occur in the case of mechanical or chemical thinning or ion milling. In order to analyse the chemical composition electron energy loss spectroscopy ŽEELS. measurements in transmission geometry were carried out in the same FEG-TEM using a Gatan parallel detection EEL spectrometer Žmodel 666.. The field emission measurements were performed in a vacuum of 10y5 Pa by using the examined samples as cathodes. The emitted electrons were collected on a highly polished stainless steel spherical counterelectrode of 1 cm diameter, which corresponds to an emission area of ; 0.007 cm2 . The distance was initially adjusted to 125 m and was reduced to 50 or 25 m when no emission was observed below 1000 V. A Keithley 237 source-measure unit was used for sourcing the voltage Žup to 1000 V. and measuring the current with pA sensitivity, allowing the characterisation of current᎐voltage Ž I᎐V . behaviour.
3. Results 3.1. Structure analysis Optically the films consist of black or dark brown deposit. The experiments were performed in a deposition time t s 30 min resulting in film growths covering the entire substrate The thickness profile as obtained by laser interferometer measurements of the film surfaces differs slightly with the applied deposition parameter. In contrast to other methods very high deposition rates R in the range of 100 nmrmin were reached. With increasing filament temperature Tfil the deposition rate R increases and the deposition zones become smaller. NH 3 addition results in a reduction of R down to approximately 15% of R. Biasing induces a smaller reduction of approximately 3᎐5%. When we examined the surface underneath the filament by SEM, interlaced worm- or foil-like nanostructures were found ŽFig. 1.. Varying the filament temperature exerts a strong influence on film microstructure. The results discussed here were observed
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R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
at films deposited by HF-CVD, but the same tendency was found using PE-HF-CVD. Fig. 1a᎐d represent surface morphologies of films deposited on Si substrate at Tfil ranging from 1800 to 2100⬚C. For lower filament temperatures only a negligible film growth was detected. Increasing Tfil goes together with a marked coarsening of the morphology. At 1800⬚C ŽFig. 1a. small disordered needle-like structures were agglomerated together, whereas at 1900⬚C ŽFig. 1b. a porous network of worm-like or tubular structures was formed. Films deposited at Tfil s 2000⬚C ŽFig. 1c. are characterised by larger but twisted lamellae. Finally in Fig. 1d Ž2100⬚C. straight sheets showing branching are clearly seen. Note that all mentioned polymorphs are in the nanometer range. Finally films are formed which are characterised by a large surface roughness and a large surface area. But on macroscopic scale homogeneous films in terms of thickness and smoothness are obtained. Moreover, the existence of these nano-structured carbon-based materials was confirmed all over the growth region, i.e. there are homogeneous areas that are much larger than several square millimetres. Cross-section TEM investigations were performed to study the microstructure of samples deposited at Tfil s 2000⬚C in more detail. In Fig. 2a an example of the film surface as typically obtained by HF-CVD is shown. A lamellar structure with a preferential in-growth-direction Žperpendicular to the substrate . was observed. The film surface being always on top is characterised by a large number of tips. On average the tip-to-tip distance is less than 50 nm. A detailed view reveals a weak contrast beside the needle-like structures, which is probably due to thin electron transparent foils. Plasma-enhanced deposition ŽPE-HF-CVD. results in formation of smaller and more randomly oriented lamellae as compared in Fig. 2b. In that case the surface morphology is characterised by more dis-
ordered structures always similar to Fig. 1a even for higher Tfil . The crystal structure of the films was examined from a selected area electron diffraction ŽSAD. pattern obtained from its TEM specimen indicating disordered graphite sheets. No residual amorphous phase was detected by SAD. In fact, the lattice images of graphite layers are observed in high resolution TEM micrographs at the film surface ŽHF-CVD., as shown in Fig. 2c. The number of graphite layers is two or five for the thinnest cases and several tens for thick cases. Locally the planes are parallel to each other but on a larger scale they show bending and a certain degree of disorder. Fig. 2d shows a high resolution image of a PE-HF-CVD sample. A hollow nanotube is clearly seen. The diameter of such tubes was estimated to be in the order of 10 nm, whereas their length reaches several hundred nm. As a role they show a large degree of disorder, namely the lattice fringes are buckled and the inner and outer wall surface is rough. It is worth noting, that even the part of Fig. 2c᎐d with low contrast lattice fringes become visible by changing the focusing conditions. Therefore, the films are characterised as crystalline nitrogenated carbon ŽC:N. films. The nitrogen loading of the films was analysed by EELS reaching 1᎐2 at.% N and approximately 2᎐3 at.% N for HF-CVD and PE-HF-CVD, respectively. However, quantification of such small nitrogen concentrations is very difficult due to the dominant C᎐K edge making the small N᎐K edge hardly visible. In agreement with other works on CN x materials, e.g. w7x, nitrogen is assumed to be substitutionally incorporated into the graphitic lattice originating from the bending of the fringes as seen in Fig. 2c᎐d. The choice of substrates for growth of nano-structured material is known to play an important role. Consequently, Si wafers were covered with Ni layers before deposition. Annealing resulted in crystallisation
Fig. 1. SEM plane view micrographs of nitrogenated carbon films deposited by HF-CVD on pure Si. With increasing filament temperature Tfil Ža. 1800, Žb. 1900, Žc. 2000 Žd. 2100⬚C the surface morphology changes from small disordered needle-like clusters vs. a network of worm-like structures to larger and straighter formed plates or foils. All images show the same magnification.
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
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However, the emission started at significantly lower fields Žapprox. 3.7 Vrm. for the twisted tubular C:N structures of the samples deposited on Ni-coated substrates. In the case of HF-CVD the turn-on and threshold fields amounted to 4.7 and 7.8 Vrm, respectively. Table 1 gives the field emission characteristics of numerous C:N films. For both techniques, HF-CVD and PE-HF-CVD, the samples deposited at Tfil s 2000⬚C reveal the best FE properties Žlowest fields.. As a role HF-CVD provides slightly better values. As a function of the used substrates large differences were also observed for the field amplification factor . This parameter depends on the geometrical shape of the emitter only and can be extracted by fitting the I᎐V curves with the Fowler᎐Nordheim model for field emission w26x. With a work function of 5 eV Žwhich is a reasonable assumption for carbon-based materials.,  amounts to 675 and 450 ŽTable 1. for the samples deposited onto Ni-coated wafer by HF and PE-HFCVD, respectively. Fig. 2. TEM cross-section micrographs of C:N film surfaces being always on top. The shape of the grown needles or foils depends strongly on the deposition parameter ŽTfil s 2000⬚C.: Ža,c. without bias; Žb,d. plasma-enhanced deposition. HR-TEM Žc,d. clearly indicates the crystalline structure due to the lattice planes of disordered graphite being visible. PE-HF-CVD results in formation of hollow nanotubes Žd..
of numerous large grains. Fig. 3a represents a SEM picture at low magnification after deposition of C:N film using HF-CVD at Tfil s 2000⬚C. The coverage of substrate characterised by brightness differs strongly in dependence on the Ni grain orientation. A tilted view at higher magnification reveals that the films consist of twisted nano-tubes ŽFig. 3b. which are well separated from each other. Performing PE-HF-CVD. ŽTfil s 2000⬚C. also results in tubular growth as seen in Fig. 3c. The comparison between Fig. 3b,c Žboth are of the same magnification. exhibits strong differences in the local distribution. The plasma applied during deposition probably originates an enhanced nucleation density. 3.2. Field emission Fig. 4 shows typical I᎐V curves acquired on C:N films deposited at Tfil s 2000⬚C on pure Žright. and on Ni-coated Si wafer Žleft., respectively. Field emitted current was observed only for fields higher than 15 Vrm on the former samples. For example, the turn-on and threshold fields needed to produce a current density of 10 Arcm2 and 10 mArcm2 were 27 and 58 Vrm for the film deposited by PE-HF-CVD Žas indicated in Fig. 4.. HF-CVD clearly decreases the required fields Žsee also Table 1. and corresponding values of 21 and 34 Vrm were measured.
Fig. 3. SEM overview reveals Ža. a different degree of coverage in dependence on the Ni grain orientation. A detailed view Žb. clearly shows the formation of twisted carbon nano-tubes being well separated from each other performing HF-CVD at Tfil s 2000⬚C. The sample in Fig. 3b is slightly tilted to improve the contrast. PE-HFCVD at Tfil s 2000⬚C Žc. results in tubular growth, too, but with a dense packing at the surface.
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
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Table 1 Emission characteristics of various C:N films in dependence of the main deposition parameter a Type of CVD
Tfil w⬚Cx
Substrate
Ei wVrmx
Eto wVrmx
Ethr wVrmx

HF HF HF HF PE-HF PE-HF PE-HF PE-HF HF PE-HF
1800 1900 2000 2100 1800 1900 2000 2100 2000 2000
Si Si Si Si Si Si Si Si Ni-coated Ni-coated
54 36 18 29 70 60 23 34 3.7 6.2
84 49 21 34 89 80 27 48 4.7 7.8
170 95 34 63 140 135 58 104 7.8 13
40 80 125 115 20 55 100 75 675 450
Ei is the electric field needed to extract a current density of 10 nA cmy2 Žonset field.. Eto and Ethr are the turn-on and threshold fields, respectively, corresponding to current densities of 10 A cmy2 and 10 mA cmy2 after training.  is the field amplification factor extracted from the low current I᎐V characteristics after training. a
4. Discussion The film morphology changed from a network of worm-like structures to interlaced graphite sheets, when the filament temperature was increased. This effect is seen as a consequence of increased thermal radiation and a higher dissociation of the gas phase. The twisted and straight lenticular shaped microstructures which were typically observed, may be interpreted as facets of preferentially growing crystallographic orientations, namely disordered stacking of graphene. This interpretation was confirmed by Raman spectroscopy w25x and it is in accordance with discussions about MoS2 films where a similar morphology was detected w27x. Ando et al. w28x reported about growth of petal-like graphite
Fig. 4. Current density J vs. applied electrical field E for typical C:N films deposited at Tfil s 2000⬚C onto Ni-coated wafer Ža,b. and onto pure Si Žc,d.. In general HF-CVD Ža. and Žc. provides lower fields in comparison to PE-HF-CVD, which were applied for samples Žb. and Žd.. From a mathematical fit using the Fowler᎐Nordheim model field amplification factors  of 675 Ža., 450 Žb., and 125 Žc. were determined.
sheets showing similarities to structures observed in this work. In addition to their remarks concerning the influence of hydrogen we assume an important role of the dilution gas N2 because varying the gas mixture results in different morphologies. Anyhow, nucleation and the growth mechanism of nitrogenated carbon nano-tubes and graphite sheets must be quite different. Small additions of NH 3 as well as the use of catalytic Ni substrate favour the tubular growth. The effects of nitrogen precursors on the formed structure was analysed by HR-TEM and EELS in detail and will be presented in a further paper w29x. The spectacular difference in field emission properties of the C:N films can be ascribed to the differences in morphology. The structures of Fig. 2a,b are sharp, but the dense packing and small height difference between the tip of the lamellae and the average film surface greatly reduce the field amplification and lead to high emission voltages. The nanostructures found on the C:N films obtained with Ni-coated substrates do not show these drawbacks and emit consequently at far lower fields with field amplification higher by a factor of f 5. Furthermore, plasma-enhanced deposition leads to a strong increase of the measured turn-on field due to the smoother surface as seen in Fig. 2b and the higher nucleation density for nanotubes ŽFig. 3c.. Finally, we note that the C:N films are slightly less efficient emitters than nano-tube films w30x as typical turn-on fields are approximately 3 Vrm for the latter. They emit, however, at lower fields than films realised with graphitic nanofibers of 30 nm diameter w31x despite their larger diameter. We believe that their larger height over the substrate Ž; 2 m as compared to a few tens of nm. as well as the relatively large separation between adjacent structures enhance field amplification and reduces screening between neighbouring emitters, leading to lower emission fields. In fact, elec-
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
trostatic calculations show that the optimal interemitter distance to obtain a maximal current density is approximately twice the height of the emitter w32x, which corresponds approximately to what is observed on our samples Žsee Fig. 3b..
5. Conclusions Decomposition of CH 4 in the nitrogen-based atmosphere by HF-CVD results in a fast and homogeneous film growth. These deposits are characterised by an interesting nano-structured morphology, namely interlaced foils and tubular structures, the large surface area making the material probably interesting for any catalytic or storage application. Quantitative analysis such as hydrogen and nitrogen adsorption experiments is under progress. Correlating the structure analysis and the field emission results suggests that the good FE properties can be related to a certain degree of substrate coverage by nano-tubes.
Acknowledgements The Swiss National Science Foundation ŽSNSF. is acknowledged for the financial support of the project. The electron microscopy was performed at the Centre Interdepartmental de Microscopie Electronique of ´ EPFL. The authors are grateful to P. Stadelmann for EELS analysis. References w1x A.K.M.S. Chowdhury, D.C. Cameron, M.S.J. Hashmi, J.M. Gregg, J. Mater. Res. 14 Ž6. Ž1999. 2359. w2x Y. Fahmy, T.D. Shen, D.A. Tucker, R.L. Spontak, C.C. Koch, J Mater. Res. 14 Ž6. Ž1999. 2488. w3x J. Martin-Gil, F.J. Martin-Gil, M. Sarikaya, M. Qian, M.J. Yacaman, A. Rubio, J. Appl. Phys. 81 Ž6. Ž1997. 2555. w4x A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 Ž1989. 841. w5x M. Park, A.T. Sowers, C. Lizzul Rinne et al., J. Vac. Sci. Technol. B 17 Ž2. Ž1999. 734.
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w6x V. Baranauskas, S.F. Durrant, M.C. Tosin, A.C. Peterlevitz, B.B. Li, S.G. Castro, Thin Solid Films 355r356 Ž1999. 157. w7x N. Hellgren, M.P. Johansson, E. Broitman, L. Hultman, J.-E. Sundgren, Phys. Rev. B 59 Ž7. Ž1999. 5162. w8x R. Kurt, R. Sanjines, ´ A. Karimi, F. Levy, ´ Diamond Relat. Mater. 9 Ž3᎐6. Ž2000. 566. w9x Y. Miyamoto, M.L. Cohen, S.G. Loui, Solid State Commun. 102 Ž8. Ž1997. 605. w10x F.J. Himpsel, J.S. Knapp, J.A.V. Vechten, D.E. Eastman, Phys. Rev. B 20 Ž1979. 624. w11x B.B. Pate, Surf. Sci. 165 Ž1986. 83. w12x G.A.J. Amaratunga, S.R.P. Silva, Appl. Phys. Lett. 68 Ž1996. 2529. w13x J.O. Choi, J.W. Huh, Y.H. Choi, M.J.K.H. Kim, Y.R. Cho, H.S. Jeong, J. Vac. Sci. Technol. B 16 Ž1998. 1199. w14x S.R.P. Silva, R.G. Forbes, Ultramicroscopy 73 Ž1998. 51. w15x K. Okano, A. Hiraki, T. Yamada, S.K.J. Itoh, Ultramicroscopy 73 Ž1998. 43. w16x E.J. Chi, J.Y. Shim, D.J. Choi, H.K. Baik, J. Vac. Sci. Technol. B 16 Ž1998. 1219. w17x J.P. Zhao, Z.Y. Chen, X. Wang et al., Mater. Lett. 35 Ž1998. 157. w18x S. Iijima, Nature 354 Ž1991. 56. w19x Y. Saito, K. Hamaguchi, K. Hata et al., Ultramicroscopy 73 Ž1998. 1. w20x J.-M. Bonard, F. Maier, T. Stockli et al., Ultramicroscopy 73 Ž1998. 7. w21x S. Lee, B. Chung, T.Y. Ko, D. Jeon, K.R. Lee, K.Y. Eun, Ultramicroscopy 73 Ž1998. 17. w22x Z.P. Huang, J.W. Xu, Z.F. Ren, J.H. Wang, M.P. Siegal, P.N. Provencio, Appl. Phys. Lett. 73 Ž26. Ž1998. 3845. w23x D. Xu, G. Guo, L. Gui et al., Appl. Phys. Lett. 75 Ž4. Ž1999. 481. w24x Z.F. Ren, Z.P. Huang, D.Z. Wang et al., Appl. Phys. Lett. 75 Ž8. Ž1999. 1086. w25x R. Kurt, A. Karimi, Thin Solid Films 377᎐378 Ž2000. 163. w26x J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 Ž1973. 487. w27x M. Regula, C. Ballif, J.H. Moser, F. Levy, ´ Thin Solid Films 280 Ž1996. 67. w28x Y. Ando, X. Zhao, M. Ohkohchi, Carbon 35 Ž1. Ž1997. 153. w29x R. Kurt, A. Karimim, Chem. Phys. Lett. 335 Ž5᎐6. Ž2001. 545. w30x J.-M. Bonard, H. Kind, T. Stockli, L.-O. Nilsson, Solid State ¨ Elec. Ž2001. in print. w31x J.-M. Bonard, J.-P. Salvetat, T. Stockli, L. Forro, ¨ ´ K. Kern, A. Chatelain, Appl. Phys. A69 Ž1999. 245. ˆ w32x L. Nilsson, O. Groning, C. Emmenegger et al., Appl. Phys. Lett. ¨ 76 Ž2000. 2071.
Nano-structured nitrogenated carbon films ᎏ morphology and field emission R. KurtU , J.-M. Bonard, A. Karimi Department of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland Received 3 July 2000; accepted 24 January 2001
Abstract Crystalline nitrogenated carbon ŽC:N. films have been successfully synthesised by plasma-enhanced hot filament chemical vapour deposition ŽPE-HF-CVD.. Nitrogen gas ŽN2 . and ammonia ŽNH 3 . were used as sources of atomic nitrogen whereas methane ŽCH 4 . acted as a carbon precursor. Structure analysis reveals the growth of a new type of C:N film characterised by various polymorphs including worm- and foil-like microstructures at the nanometer scale. The effects of bias voltage and filament temperature on the film morphology are investigated in detail. Using pure Si as a substrate results in growth of homogeneous nanostructured films, whereas arrays of nanotubes were deposited on Ni-coated substrates. Field emission in vacuum was observed on C:N films deposited on pure Si above applied fields of 15 Vrm. The onset field could be decreased below 4 Vrm with Ni-coated substrates due to the presence of well-separated nano-tubular structures. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Nitrogenated carbon; Nano-structures; Field emission
1. Introduction In the last number of years, a lot of effort has been made to study the influence of nitrogen in carbon-based materials. The crystalline -C 3 N4 phase was tried to synthesise, e.g. w1᎐3x due to predicted interesting mechanical properties w4x. Furthermore, nitrogen doping in diamond was studied with regard to the electrical properties of the resulting film w5x and the morphological change w6x. The influence of nitrogen in amorphous CN x and diamond-like ŽDLC. materials on the mechanical properties was also analysed, e.g. w7,8x. Miyamoto et al. w9x showed that tubule forms of graphitic carbon nitrides can be stabilised by displacing C atoms out of the plane in the honeycomb network of graphite. U
Corresponding author. Tel.: q41-21-693-4515; fax: q41-21-6934470. E-mail address: [email protected] ŽR. Kurt..
However, the role of nitrogen for structure formation as well as for modification of the material properties is not completely clear. One of the applications of carbon-based materials studied over the last few years has been the production of flat cold cathodes which can serve as field emitters with low turn-on voltages and high emission currents. The first carbon-based films that attracted attention in terms of their field emission ŽFE. properties were aC:H, a-C:N and DLC coatings w10᎐17x. However, after the discovery of carbon nanotubes by Iijima in 1991 w18x there has been an increasing interest in their field emission characteristics w19᎐21x. Only a few works deal with two-dimensional arrays of aligned and well-distributed nanotubes w22,23x. Recently, Ren et al. w24x reported on fabrication patterned growth of freestanding and highly oriented carbon nanotubes but with dispersion in length and thickness affecting their FE properties. Nevertheless, until now there is a lack in develop-
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 3 8 8 - 0
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
ment of fast techniques for the production of macroscopic quantities of nano-tubular structures andror for the deposition of homogeneous flat arrays characterised by a large emitter density. In this work we propose a method for the fast production of uniform C:N films characterised by wormand foil-like graphite sheets. The influence of deposition parameters on structure formation will be discussed. Under certain conditions remarkable FE properties of the nanostructured films were observed.
2. Experimental details 2.1. Film deposition The experimental set-up used for deposition was described in Kurt and Karimi w25x. A resistively heated tungsten ŽW. filament Ž⭋ s 500 m. was used for film deposition after complete crystallisation Žtransformation to WC.. A molybdenum ŽMo. substrate holder placed 8 mm underneath the filament allowed the substrate temperature Tsub s 700⬚C to be controlled independently from the filament temperature Tfil . The background pressure typically reached in the deposition chamber was 10y5 Pa, whereas the working pressure p was kept constant at 300 Pa. A feed mixture of 100 sccm CH 4 q 5 sccm N2 q 1 sccm NH 3 was used. After complete transformation of the filament to tungsten carbide WC Žas described in Kurt and Karimi w25x. C:N films were deposited onto clean n-type Si Ž100. wafer and onto Si substrates covered by a 2 m thick Ni layer. Various filament temperatures between Tfil s 1500 and 2300⬚C as measured by means of a two-colour pyrometer were applied. Positive or negative d.c.-bias potential could be applied independently to the substrate through the Mo substrate holder as well as to the filament. Both types of experiments, the hot filament chemical vapour deposition ŽHF-CVD. and the plasma enhanced hot filament chemical vapour deposition PE-HF-CVD, were performed. The later one proceeds as follows. First, a negative d.c. voltage of 200 V was applied to the substrate. Under these conditions no ion current could be detected reflecting the difficulty of inducing a gaseous breakdown under negative biasing conditions. After that, a positive bias was applied to the filament, and its magnitude was gradually increased until a sudden increase of current was detected accompanied by a small drop of voltage Žin the range of 10᎐12 mArcm2 at a voltage of approx. 150 V.. A purple glow could be observed near the substrate surface under such conditions. 2.2. Characterisation techniques Scanning electron microscopy ŽSEM. was used to
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analyse the microstructure of the C:N films in plan view. A Philips XL 30 microscope equipped with a field emission gun ŽFEG. operating at an acceleration voltage of 5 kV, a working distance of typically 10 mm, and a secondary electron ŽSE. image mode was used. The growth morphology of the films and their crystalline structure were controlled by analytical transmission electron microscopy ŽTEM.. For this purpose a Hitachi HF-2000 field emission microscope equipped with a Gatan image plate operating at 200 kV Žpoint resolution 0.23 nm. was used. For the performed cross-section analysis the film-substrate composite was freshly cleaved prior to the investigations. In this way any structural modifications of the C:N films were prevented as they occur in the case of mechanical or chemical thinning or ion milling. In order to analyse the chemical composition electron energy loss spectroscopy ŽEELS. measurements in transmission geometry were carried out in the same FEG-TEM using a Gatan parallel detection EEL spectrometer Žmodel 666.. The field emission measurements were performed in a vacuum of 10y5 Pa by using the examined samples as cathodes. The emitted electrons were collected on a highly polished stainless steel spherical counterelectrode of 1 cm diameter, which corresponds to an emission area of ; 0.007 cm2 . The distance was initially adjusted to 125 m and was reduced to 50 or 25 m when no emission was observed below 1000 V. A Keithley 237 source-measure unit was used for sourcing the voltage Žup to 1000 V. and measuring the current with pA sensitivity, allowing the characterisation of current᎐voltage Ž I᎐V . behaviour.
3. Results 3.1. Structure analysis Optically the films consist of black or dark brown deposit. The experiments were performed in a deposition time t s 30 min resulting in film growths covering the entire substrate The thickness profile as obtained by laser interferometer measurements of the film surfaces differs slightly with the applied deposition parameter. In contrast to other methods very high deposition rates R in the range of 100 nmrmin were reached. With increasing filament temperature Tfil the deposition rate R increases and the deposition zones become smaller. NH 3 addition results in a reduction of R down to approximately 15% of R. Biasing induces a smaller reduction of approximately 3᎐5%. When we examined the surface underneath the filament by SEM, interlaced worm- or foil-like nanostructures were found ŽFig. 1.. Varying the filament temperature exerts a strong influence on film microstructure. The results discussed here were observed
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R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
at films deposited by HF-CVD, but the same tendency was found using PE-HF-CVD. Fig. 1a᎐d represent surface morphologies of films deposited on Si substrate at Tfil ranging from 1800 to 2100⬚C. For lower filament temperatures only a negligible film growth was detected. Increasing Tfil goes together with a marked coarsening of the morphology. At 1800⬚C ŽFig. 1a. small disordered needle-like structures were agglomerated together, whereas at 1900⬚C ŽFig. 1b. a porous network of worm-like or tubular structures was formed. Films deposited at Tfil s 2000⬚C ŽFig. 1c. are characterised by larger but twisted lamellae. Finally in Fig. 1d Ž2100⬚C. straight sheets showing branching are clearly seen. Note that all mentioned polymorphs are in the nanometer range. Finally films are formed which are characterised by a large surface roughness and a large surface area. But on macroscopic scale homogeneous films in terms of thickness and smoothness are obtained. Moreover, the existence of these nano-structured carbon-based materials was confirmed all over the growth region, i.e. there are homogeneous areas that are much larger than several square millimetres. Cross-section TEM investigations were performed to study the microstructure of samples deposited at Tfil s 2000⬚C in more detail. In Fig. 2a an example of the film surface as typically obtained by HF-CVD is shown. A lamellar structure with a preferential in-growth-direction Žperpendicular to the substrate . was observed. The film surface being always on top is characterised by a large number of tips. On average the tip-to-tip distance is less than 50 nm. A detailed view reveals a weak contrast beside the needle-like structures, which is probably due to thin electron transparent foils. Plasma-enhanced deposition ŽPE-HF-CVD. results in formation of smaller and more randomly oriented lamellae as compared in Fig. 2b. In that case the surface morphology is characterised by more dis-
ordered structures always similar to Fig. 1a even for higher Tfil . The crystal structure of the films was examined from a selected area electron diffraction ŽSAD. pattern obtained from its TEM specimen indicating disordered graphite sheets. No residual amorphous phase was detected by SAD. In fact, the lattice images of graphite layers are observed in high resolution TEM micrographs at the film surface ŽHF-CVD., as shown in Fig. 2c. The number of graphite layers is two or five for the thinnest cases and several tens for thick cases. Locally the planes are parallel to each other but on a larger scale they show bending and a certain degree of disorder. Fig. 2d shows a high resolution image of a PE-HF-CVD sample. A hollow nanotube is clearly seen. The diameter of such tubes was estimated to be in the order of 10 nm, whereas their length reaches several hundred nm. As a role they show a large degree of disorder, namely the lattice fringes are buckled and the inner and outer wall surface is rough. It is worth noting, that even the part of Fig. 2c᎐d with low contrast lattice fringes become visible by changing the focusing conditions. Therefore, the films are characterised as crystalline nitrogenated carbon ŽC:N. films. The nitrogen loading of the films was analysed by EELS reaching 1᎐2 at.% N and approximately 2᎐3 at.% N for HF-CVD and PE-HF-CVD, respectively. However, quantification of such small nitrogen concentrations is very difficult due to the dominant C᎐K edge making the small N᎐K edge hardly visible. In agreement with other works on CN x materials, e.g. w7x, nitrogen is assumed to be substitutionally incorporated into the graphitic lattice originating from the bending of the fringes as seen in Fig. 2c᎐d. The choice of substrates for growth of nano-structured material is known to play an important role. Consequently, Si wafers were covered with Ni layers before deposition. Annealing resulted in crystallisation
Fig. 1. SEM plane view micrographs of nitrogenated carbon films deposited by HF-CVD on pure Si. With increasing filament temperature Tfil Ža. 1800, Žb. 1900, Žc. 2000 Žd. 2100⬚C the surface morphology changes from small disordered needle-like clusters vs. a network of worm-like structures to larger and straighter formed plates or foils. All images show the same magnification.
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However, the emission started at significantly lower fields Žapprox. 3.7 Vrm. for the twisted tubular C:N structures of the samples deposited on Ni-coated substrates. In the case of HF-CVD the turn-on and threshold fields amounted to 4.7 and 7.8 Vrm, respectively. Table 1 gives the field emission characteristics of numerous C:N films. For both techniques, HF-CVD and PE-HF-CVD, the samples deposited at Tfil s 2000⬚C reveal the best FE properties Žlowest fields.. As a role HF-CVD provides slightly better values. As a function of the used substrates large differences were also observed for the field amplification factor . This parameter depends on the geometrical shape of the emitter only and can be extracted by fitting the I᎐V curves with the Fowler᎐Nordheim model for field emission w26x. With a work function of 5 eV Žwhich is a reasonable assumption for carbon-based materials.,  amounts to 675 and 450 ŽTable 1. for the samples deposited onto Ni-coated wafer by HF and PE-HFCVD, respectively. Fig. 2. TEM cross-section micrographs of C:N film surfaces being always on top. The shape of the grown needles or foils depends strongly on the deposition parameter ŽTfil s 2000⬚C.: Ža,c. without bias; Žb,d. plasma-enhanced deposition. HR-TEM Žc,d. clearly indicates the crystalline structure due to the lattice planes of disordered graphite being visible. PE-HF-CVD results in formation of hollow nanotubes Žd..
of numerous large grains. Fig. 3a represents a SEM picture at low magnification after deposition of C:N film using HF-CVD at Tfil s 2000⬚C. The coverage of substrate characterised by brightness differs strongly in dependence on the Ni grain orientation. A tilted view at higher magnification reveals that the films consist of twisted nano-tubes ŽFig. 3b. which are well separated from each other. Performing PE-HF-CVD. ŽTfil s 2000⬚C. also results in tubular growth as seen in Fig. 3c. The comparison between Fig. 3b,c Žboth are of the same magnification. exhibits strong differences in the local distribution. The plasma applied during deposition probably originates an enhanced nucleation density. 3.2. Field emission Fig. 4 shows typical I᎐V curves acquired on C:N films deposited at Tfil s 2000⬚C on pure Žright. and on Ni-coated Si wafer Žleft., respectively. Field emitted current was observed only for fields higher than 15 Vrm on the former samples. For example, the turn-on and threshold fields needed to produce a current density of 10 Arcm2 and 10 mArcm2 were 27 and 58 Vrm for the film deposited by PE-HF-CVD Žas indicated in Fig. 4.. HF-CVD clearly decreases the required fields Žsee also Table 1. and corresponding values of 21 and 34 Vrm were measured.
Fig. 3. SEM overview reveals Ža. a different degree of coverage in dependence on the Ni grain orientation. A detailed view Žb. clearly shows the formation of twisted carbon nano-tubes being well separated from each other performing HF-CVD at Tfil s 2000⬚C. The sample in Fig. 3b is slightly tilted to improve the contrast. PE-HFCVD at Tfil s 2000⬚C Žc. results in tubular growth, too, but with a dense packing at the surface.
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Table 1 Emission characteristics of various C:N films in dependence of the main deposition parameter a Type of CVD
Tfil w⬚Cx
Substrate
Ei wVrmx
Eto wVrmx
Ethr wVrmx

HF HF HF HF PE-HF PE-HF PE-HF PE-HF HF PE-HF
1800 1900 2000 2100 1800 1900 2000 2100 2000 2000
Si Si Si Si Si Si Si Si Ni-coated Ni-coated
54 36 18 29 70 60 23 34 3.7 6.2
84 49 21 34 89 80 27 48 4.7 7.8
170 95 34 63 140 135 58 104 7.8 13
40 80 125 115 20 55 100 75 675 450
Ei is the electric field needed to extract a current density of 10 nA cmy2 Žonset field.. Eto and Ethr are the turn-on and threshold fields, respectively, corresponding to current densities of 10 A cmy2 and 10 mA cmy2 after training.  is the field amplification factor extracted from the low current I᎐V characteristics after training. a
4. Discussion The film morphology changed from a network of worm-like structures to interlaced graphite sheets, when the filament temperature was increased. This effect is seen as a consequence of increased thermal radiation and a higher dissociation of the gas phase. The twisted and straight lenticular shaped microstructures which were typically observed, may be interpreted as facets of preferentially growing crystallographic orientations, namely disordered stacking of graphene. This interpretation was confirmed by Raman spectroscopy w25x and it is in accordance with discussions about MoS2 films where a similar morphology was detected w27x. Ando et al. w28x reported about growth of petal-like graphite
Fig. 4. Current density J vs. applied electrical field E for typical C:N films deposited at Tfil s 2000⬚C onto Ni-coated wafer Ža,b. and onto pure Si Žc,d.. In general HF-CVD Ža. and Žc. provides lower fields in comparison to PE-HF-CVD, which were applied for samples Žb. and Žd.. From a mathematical fit using the Fowler᎐Nordheim model field amplification factors  of 675 Ža., 450 Žb., and 125 Žc. were determined.
sheets showing similarities to structures observed in this work. In addition to their remarks concerning the influence of hydrogen we assume an important role of the dilution gas N2 because varying the gas mixture results in different morphologies. Anyhow, nucleation and the growth mechanism of nitrogenated carbon nano-tubes and graphite sheets must be quite different. Small additions of NH 3 as well as the use of catalytic Ni substrate favour the tubular growth. The effects of nitrogen precursors on the formed structure was analysed by HR-TEM and EELS in detail and will be presented in a further paper w29x. The spectacular difference in field emission properties of the C:N films can be ascribed to the differences in morphology. The structures of Fig. 2a,b are sharp, but the dense packing and small height difference between the tip of the lamellae and the average film surface greatly reduce the field amplification and lead to high emission voltages. The nanostructures found on the C:N films obtained with Ni-coated substrates do not show these drawbacks and emit consequently at far lower fields with field amplification higher by a factor of f 5. Furthermore, plasma-enhanced deposition leads to a strong increase of the measured turn-on field due to the smoother surface as seen in Fig. 2b and the higher nucleation density for nanotubes ŽFig. 3c.. Finally, we note that the C:N films are slightly less efficient emitters than nano-tube films w30x as typical turn-on fields are approximately 3 Vrm for the latter. They emit, however, at lower fields than films realised with graphitic nanofibers of 30 nm diameter w31x despite their larger diameter. We believe that their larger height over the substrate Ž; 2 m as compared to a few tens of nm. as well as the relatively large separation between adjacent structures enhance field amplification and reduces screening between neighbouring emitters, leading to lower emission fields. In fact, elec-
R. Kurt et al. r Diamond and Related Materials 10 (2001) 1962᎐1967
trostatic calculations show that the optimal interemitter distance to obtain a maximal current density is approximately twice the height of the emitter w32x, which corresponds approximately to what is observed on our samples Žsee Fig. 3b..
5. Conclusions Decomposition of CH 4 in the nitrogen-based atmosphere by HF-CVD results in a fast and homogeneous film growth. These deposits are characterised by an interesting nano-structured morphology, namely interlaced foils and tubular structures, the large surface area making the material probably interesting for any catalytic or storage application. Quantitative analysis such as hydrogen and nitrogen adsorption experiments is under progress. Correlating the structure analysis and the field emission results suggests that the good FE properties can be related to a certain degree of substrate coverage by nano-tubes.
Acknowledgements The Swiss National Science Foundation ŽSNSF. is acknowledged for the financial support of the project. The electron microscopy was performed at the Centre Interdepartmental de Microscopie Electronique of ´ EPFL. The authors are grateful to P. Stadelmann for EELS analysis. References w1x A.K.M.S. Chowdhury, D.C. Cameron, M.S.J. Hashmi, J.M. Gregg, J. Mater. Res. 14 Ž6. Ž1999. 2359. w2x Y. Fahmy, T.D. Shen, D.A. Tucker, R.L. Spontak, C.C. Koch, J Mater. Res. 14 Ž6. Ž1999. 2488. w3x J. Martin-Gil, F.J. Martin-Gil, M. Sarikaya, M. Qian, M.J. Yacaman, A. Rubio, J. Appl. Phys. 81 Ž6. Ž1997. 2555. w4x A.Y. Liu, M.L. Cohen, Phys. Rev. B 41 Ž1989. 841. w5x M. Park, A.T. Sowers, C. Lizzul Rinne et al., J. Vac. Sci. Technol. B 17 Ž2. Ž1999. 734.
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