Costel Sorin Cojocaru

Dohyung Kim

Didier Pribat

Jean Eric

Jean-Eric Boure

Synthesis of multi-walled carbon nanotubes by combining hot-wire and dc plasma-enhanced chemical vapor deposition

Keywords:

published or not. The documents may come

Synthesis of multi-walled carbon nanotubes by combining hot-wire and dc plasma-enhanced chemical vapor deposition

Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991 [1], there has been widespread interest in their use in a multitude of applications in nanoscale devices. Numerous studies have shown that multi-walled carbon nanotubes (MWCNTs) always exhibit metallic behavior and, due to their high aspect ratio (length/radius), can emit electrons in the neighborhood of their tips. These interesting electronic properties allow MWCNTs to be useful as new materials for the development of electron-field-emitting flat panel displays [2,3]. Although aligned CNTs are not strictly necessary for these applications, the use of aligned/micropatterned forms of CNTs has been shown to offer additional advantages for the development of low threshold field (electric field required for an electron field emission) nanotube-based flat-panel displays.

In the beginning, CNTs have been produced by arcdischarge or laser ablation. These high-temperature methods were the most useful because they could produce large amounts of nanotubes, but their common drawback was the number of impurities incorporated in the material, whose type and amount depended on the deposition technique. Along with controlling the CNT morphology and structure, many practical applications require as well the precise control of their orientation, alignment and density. Thus, the hightemperature methods have been replaced by several lowtemperature chemical vapor deposition (CVD) techniques (< 800 -C), which are most promising in terms of CNT deterministic growth. These techniques include thermal CVD [4,5], direct current (dc) plasma-enhanced CVD (dc PECVD) [6,7], microwave PECVD [8 -10], hot-wire (HW) dc PECVD [11 -13], and HWCVD [14 -16]. In all these techniques, substrates (c-Si or SiO 2 coated c-Si) were coated with a suitable transition metal catalyst, primarily Fe, Co or Ni.

The present paper focuses on the synthesis of MWCNTs in a reactor combining both HWCVD and dc PECVD techniques, using the C 2 H 2 /NH 3 /H 2 gas mixture. The idea is that by comparing these two CVD processes separately or not, under identical deposition conditions, it should be possible to get a new insight into the understanding of the complex mechanism of CNT nucleation and growth. Thus, optimized growth conditions should be found, providing aligned MWCNTs with uniform diameters and lengths. Moreover, using patterning techniques such as optical or electron lithography in order to precisely locate the catalyst on the surface, this work shows that patterned arrays of aligned nanotubes can be deposited to ensure a significant electron emission current.

Experimental details

A stainless steel reactor (Fig. 1) containing one tungsten filament (0.38 mm in diameter) and two molybdenum electrodes has been designed for the growth of carbon nanotubes. The tungsten filament coil is powered by a direct current (dc) source around 13 A (250 W), allowing the filament temperature to stabilize at 1800 -C. The cathode (substrate-holder) can be negatively dc-biased compared to the anode (Mo grid) up to -450 V (with current densities between 1.5 and 4 mA/cm 2 ). The distance between the tungsten wire and the cathode is fixed to 15 mm. An Ni -Cr heater, in thermal contact with the cathode but electrically For all samples, the substrate consisted of Ni (7 nm)/TiN (10 nm)/Si(100), and the growth time was set to 15 min. a V A -C : potential drop between the anode and the cathode; J C : current density of ions impinging on the cathode. insulated from it, can heat the cathode up to 700 -C. The base pressure of the deposition chamber is as low as 10 -7 mbar. In the gas mixture used, acetylene is the deposition gas whereas ammonia and hydrogen are the etching gases.

During the growth, a pressure ranging from 1 to 5 mbar is maintained in the reactor. The nickel catalyst (7 nm thick) and the TiN diffusion barrier (10 nm thick) films were deposited onto doped silicon substrates. The diffusion barrier layer prevents the formation of NiSi x via Ni diffusion above 300 -C and maintains Ni particles for the catalytic nucleation and subsequent growth of the carbon nanotubes. Upon annealing to the growth temperature in the range 500 to 700 -C, under H 2 flow, with the help of the thermally activated HWCVD process (W filament heated at 1800 -C), the nickel thin film was found to break up into nanoparticles which seed the growth of the MWCNTs. Owing to the studies of other groups [5,7,9], it has been shown that the thickness of the initial Ni thin film and the growth temperature determine the size and density of the nanoparticles formed after annealing, and hence control the diameter and density of the nanotubes. After reaching the working temperature, the nanotube growth was initiated by introducing NH 3 and C 2 H 2 into the chamber and igniting the glow discharge plasma using a 1 kW dc generator between the anode and the heated substrate-holder, the distance between these electrodes being 8 mm.

The surface morphology of the grown MWCNTs was investigated with a FEG high resolution scanning electron microscope whereas the carbon bonding and the micro- Intensity (a.u.) structure were assessed by Raman spectroscopy and transmission electron microscopy, respectively. Electron field emission was carried out using a triode set-up in a vacuum chamber working at 10 -6 Torr.

Results and discussion

Different types of growth processes, namely dc PECVD, HWCVD and combined HW dc PECVD were investigated as function of gas mixture composition, total pressure, activation power and geometrical configuration. Various C 2 H 2 /NH 3 /H 2 ratios were used with a total gas flow rate of 100 sccm (see the growth conditions in Table 1, samples 1 to 14). All syntheses were performed at 620 -C and 1 mbar total pressure with 15 min deposition time.

The dc PECVD growth mode was first studied for gas mixtures containing 10% acetylene and an increasing ammonia content diluted in hydrogen (see Table 1, samples 1 to 5). The dc PECVD process generates radicals and ions impinging on the cathode. Moreover, the dissociation energy for NH 3 is lower than for H 2 (a bias of 325V [17] is sufficient), so that atomic H is easily produced by the plasma. The current density of ions impinging on the cathode has been maintained to a fixed value of 1.5 mA/cm 2 while the potential drop between the electrodes could vary freely (current regulation mode) between 360 V and 480 V. Fig. 2A is the SEM image corresponding to sample 1 prepared with no ammonia and high bias voltage: the deposited layer exhibits a large roughness with scarce whiskers. The associated Raman spectrum (Fig. 3) exhibits two intense D and G peaks around 1350 and 1590 cm -1 with two additional small bands around 1160 and 1500 cm -1 , indicative of many sp 3 C defects. The graphitic contribution may be ascribed to the high voltage which generates some etching species. When 20% ammonia is introduced in the gas mixture (sample 2, Table 1), formation of bundles of carbon nanotubes with diameters ranging from 15 to 50 nm and lengths of a few hundreds of nanometers can be observed, scarcely distributed on the surface (see SEM micrograph of Fig. 2B): a thinner carbon layer is deposited and Ni nanoparticules are identified on tips of CNTs. The associated Raman spectrum (Fig. 3) exhibits weaker and broadened D and G peaks, compared with the peaks of sample 1. The smaller amount of carbon deposition as well as the growth of some bundles of CNTs can be correlated with the higher concentration of the activated etching species (H, NH 2 ) generated by the presence of ammonia which is easily decomposed by the dc discharge, as discussed previously. By increasing the amount of ammonia introduced in the gas mixture (samples 3 to 5, Table 1), an increased formation of carbon nanotubes is observed: Fig. 2C shows, for the sample 5 prepared with 90% NH 3 , a surface uniformly covered by aligned carbon nanotubes with diameters ranging from 15 to 100 nm and lengths ranging from 100 to 300 nm. The corresponding Raman shift (cm -1 ) Fig. 3. Sequence of Raman spectra recorded for samples 1 to 14 (see Table 1).

Raman spectra (Fig. 3) confirm, through the decrease of the I D /I G ratio when [NH 3 ] increases, the improvement of the CNT structure: both the amount of amorphous carbon and of defective structure in CNTs decrease. The HWCVD growth mode was studied for gas mixtures containing 10% C 2 H 2 and an increasing amount of NH 3 diluted in H 2 (Table 1, samples 6 to 8). The HWCVD process generates atomic hydrogen and C n H n radicals with n = 1,2 [18], depending on the filament temperature T f . When T f is set at 1800 -C, the various gases are mostly dissociated at the contact of the tungsten wire or close to the wire. For these samples, dense tangled and curled up nanotubes are observed by SEM (Fig. 2D). The diameters of the nanotubes range from 15 to 60 nm and their lengths are several micrometers. The corresponding Raman spectra (Fig. 3) show, first the highly graphitic nature of the deposited samples, then the increase of the I D /I G ratio when [NH 3 ] increases, which could suggest that NH 2 radicals generated at high concentration deteriorate the CNT structure.

The use of both the dc plasma and the hot-wire leads to a high rate of transformation of gas molecules in the gas mixture into ions, radicals and activated molecules. So with the HW dc PECVD process, we first investigated the influence of the filament temperature (samples 9 to 11), taking as C 2 H 2 /NH 3 /H 2 ratio the ratio of sample 2 (Table 1). Apart from the fact that sample 9 is equivalent to sample 2 because NH 3 and H 2 molecules are not significantly dissociated when the filament temperature T f is as low as 1100 -C, increase of T f results in the formation of CNTs with a small amount of amorphous carbon due to the efficient etching effect of H radicals, as observed in the Raman spectra (Fig. 3). Setting T f at 1800 -C and increasing the H 2 /NH 3 ratio improves the CNT growth: Fig. 2E and 2F corresponding to samples 12 and 13, respectively, show the SEM micrographs of dense aligned carbon nanotubes with lengths ranging from 500 to 700 nm and diameters ranging from 15 to 100 nm. The interpretation of these HW dc PECVD results is very similar to that of the HWCVD results. The length of the nanotubes has been observed to increase rapidly when the current density impinging on the cathode (correlated with the total pressure) increases: for instance, carbon nanotubes of 5 Am length are obtained using 3 mbar pressure corresponding to a current density in the dc-discharge of 3 to 4 mA/cm 2 .

Using HW dc PECVD process at 3 mbar pressure and T s = 660 -C, we have performed growth of CNTs on patterned substrates with Ni dots of different sizes. For sample 15 (Table 1) deposited on a patterned substrate with 1 Am Ni dots realized by UV lithography, we observed several MWCNTs with diameters less than 100 nm growing on each dot (Fig. 4A), whereas for sample 16 deposited on a patterned substrate with 100 nm Ni dots realized by electron-beam lithography, we observed only one MWCNT on each Ni dot (Fig. 4B). The first I -V characteristics performed on sample 16, consisting of an array of individual MWCNTs, gave an onset electric field of 6 V/Am and a maximum emission current density of 0.6 A/cm 2 for a field of 16 V/Am. Further work is necessary, taking into account the uniformity of the nanotube length, the spacing between nanotubes, etc., in order to get efficient electron emission useful for cold-cathode flat panel FEDs or vacuum microelectronic sources [2,3,19].

Conclusions

Since the first paper dealing with the preparation of CNTs by an activated CVD process [4], many workers tried to control the growth of aligned CNTs by the use of various CVD methods and to understand the mechanisms of CNT nucleation and growth.

The present study has been restricted to the use of two specific CVD methods: the hot-wire CVD and the dc plasma-enhanced CVD, working separately or in a combined action. Fixing the diffusion barrier thickness, the catalyst thickness, the substrate temperature and the pressure in the reactor, only the relative composition of the gases, the filament temperature and the potential drop between the anode and the cathode could vary in the experiments. In these conditions, we observed that using the dc PECVD process exclusively, with a potential drop of around 400 V, for which radicals and ions impinging on the cathode were generated, growth of aligned carbon nanotubes was favored. Moreover, the CNT structure was improved when ammonia replaced molecular hydrogen as the etching gas. When the HWCVD process was used exclusively with a filament temperature of 1800 -C, for which a high concentration of atomic hydrogen was generated, dense tangled and curled up localized carbon nanotubes have been grown. However, contrary to what was observed with dc PECVD, the CNT structure was deteriorated when increasing ammonia content. Thus, combining HWCVD and dc PECVD, we observed that growth of dense aligned carbon nanotubes could be obtained if ammonia content was minimized.

The last part of this work was focused on the possibility to grow aligned MWCNTs on patterned substrates, using the HW dc PECVD process, in order to get rid of the screening effect which is deleterious for the field emission properties. First attempts of growth have been realized on an array of 100 nm Ni catalyst dots realized by electron beam lithography: individual MWCNTs have been grown on each dot, thus a maximum emission current density of 0.6 A/cm 2 has been measured for a field of 16 V/Am. Further work should be necessary for these results to be useful for electronic applications.

Fig. 1 .

 1 Fig. 1. Schematic cross-section of the experimental HW dc PECVD set-up.

Fig. 2 .

 2 Fig. 2. Sequence of SEM images (observation angle 45-) on: (A) sample 1; (B) sample 2; (C) sample 5; (D) sample 7; (E) sample 12; (F) sample 13.

8

 8

Fig. 4 .

 4 Fig. 4. Sequence of SEM images (observation angle 45-) on: (A) sample 15; (B) sample 16.

Table 1 Growth conditions for dc PECVD, HWCVD and combined HW dc PECVD processes

 1

	a	2


Costel Sorin Cojocaru

Dohyung Kim

Didier Pribat

Jean Eric Bouree

Eric Minoux

Laurent Gangloff

Pierre Legagneux

Jean-Eric Boure ´e

Laurent Anglo

Study of electron eld emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

Keywords: Catalysis, Chemical vapor deposition, Plasma deposition, Scanning electron microscopy, Carbon-based nanotubes

Study of electron field emission from arrays of multi-walled carbon nanotubes synthesized by hot-wire dc plasma-enhanced chemical vapor deposition

Introduction

Carbon nanotubes (CNTs) are graphitic tubules with nanometer-size diameters and have a very large aspect ratio (length/radius). They exhibit highly anisotropic physical properties, speci cally excellent eld emission properties .

(high local electric eld due to nanometer-size radius of curvature at the CNT's apex), which make them attractive as potential electron sources in various vacuum electronic applications. To take optimal bene t of the eld enhancement factor, de ned by the ratio (local electric eld)/ (applied eld), relatively low densities of small-diameter (<50 nm) multi-walled nanotubes (MWCNTs) are required, as opposed to thick mats of tubes. In other words, the ability to grow a few discrete and separate small-diameter nanotubes within a speci ed area, in the plane of the growth substrate, is essential for realizing low threshold eld nanotube-based electronic devices.

The technique used for CNT growth is often some form of catalyst assisted chemical vapor deposition (CVD), in which the growth process involves the use of ne particles of some transition metal catalyst (for instance Ni) on a heated substrate (typically 700-900 °C) that is exposed to a gaseous carbon source such as methane, ethylene or acetylene. CVD methods have replaced high-temperature methods like arc-discharge (in an atmosphere of hydrogen or argon) or laser ablation because they are more promising in terms of CNT deterministic growth (morphology, structure and density). They include thermal CVD [1], direct current plasma-enhanced CVD (dc PECVD) [2], microwave PECVD [3], hot-wire (HW) CVD [4] and HW dc PECVD [5,6].

The present paper had two objectives. First, it was necessary to demonstrate the possibility of controlling the synthesis of well-aligned MWCNTs in a reactor combining both HWCVD and dc PECVD techniques, using the C 2 H 2 /NH 3 /H 2 gas mixture. This problem was solved recently [7]. The second objective was the control of the size and location of catalyst nanoparticles, and thus the control of the CNT diameters and their arrangement in regular arrays in order to maximize the electron emission from such CNT arrays. Up to now, most attempts have been made, based on the synthesis of regular arrays of oriented nanotubes on mesoporous silica [8] or on porous silicon [9], using thermal CVD. This is why growth using HW dc PECVD coupled with various catalyst patterning techniques should be attempted.

Experimental details

A schematic description of the experimental HW dc PECVD set-up has been made earlier [7]. The tungsten lament (0.38 mm in diameter) is powered by a dc source around 13 A (250 W), allowing the lament temperature, measured with an infrared pyrometer, to stabilize at rv1800 °C. In the gas mixture injected into the reactor, acetylene (C 2 H 2 ) is the deposition gas, ammonia (NH 3 ) and molecular hydrogen (H 2 ) are the etching gases. During the growth, the reactor pressure is maintained in the range 1-5 mbar, and the base pressure is as low as 10 -7 mbar.

The synthesis process of MWCNTs in the HW dc PECVD reactor is based on formation of nickel nanoparticles by HWCVD, followed by CNT growth [7]. For the catalyst patterning, we focused on UV lithography to produce micron-size dots and on electron-beam lithography to provide submicron-size dots (from 500 nm down to l00 nm). For the latter case, positive resin was spin coated onto the substrate and care was taken to adjust the resin layer thickness and the electron-beam dose in order to control the dot size.

The thermal SiO 2 or TiN di usion barrier (10 nm thick) was deposited onto 300 lm thick highly n-doped (1 0 0) silicon substrate. Then the nickel catalyst (7 nm thick) was deposited. The di usion barrier layer prevents the formation of NiSi x via Ni di usion above 300 °C and maintains su cient Ni particles for the catalytic nucleation and subsequent growth of the CNTs. CNT diameters depend upon the size of the metal catalyst particles [10,11].

The surface morphology of the grown MWCNTs was investigated with a FEG high resolution scanning electron microscope and the carbon bonding was assessed by Raman spectroscopy using 632.8 nm laser excitation [7]. Electron eld emission was carried out using a triode setup in a vacuum chamber working at 10 -6 Torr. The distance between the cathode and the grid is set to 110 lm. The measured emitted current is the sum of the anode current and the grid current.

Results and discussion

First we demonstrate that it is possible to realize the CVD synthesis of aligned and bundled MWCNTs without utilizing mesoporous silica [8] or porous silicon [9], containing catalyst nanoparticles embedded in the pores. In a recent work, using a reactor combining both HWCVD and dc PECVD techniques, similar to that of Ren et al. [5], we have shown that mechanisms of CNT nucleation and growth can be approached by comparing these two CVD processes working separately or in a combined action, under identical deposition conditions [7]. We observed that with the dc PECVD process, yielding radicals and ions impinging on the cathode, growth of aligned CNTs was favored. The electric eld in the cathode plasma sheath was equivalent to rv0.1 V/lm. When the HWCVD process was used exclusively with a lament temperature of 1800 °C, for which a high concentration of atomic hydrogen was generated, dense tangled and curled up localized CNTs were grown. Moreover the substitution of ammonia for H 2 had contradictory e ects on the CNT structure for both processes, so that by using the HW dc PECVD process, optimum conditions for the gas mixture were necessary to obtain growth of dense (rv4 • 10 9 cm -2 ) aligned CNTs, which corresponded to a minimization of the NH 3 content [7]. Fig. 1 shows the SEM image of such a sample deposited under conditions described in Table 1: 5% NH 3 and 85% H 2 with a total gas ow rate of 100 sccm under a 1 mbar pressure, a potential drop of 420 V and a current density of ions impinging on the cathode of 1.5 mA/cm 2 , and a lament temperature T f of 1800 °C. The CNT diameter ranges from 30 to 100 nm and the CNT length from 1.8 to 2.2 lm. Compared to the self-oriented arrays of CNTs synthesized on patterned porous silicon substrate where the nanotubes within each block interact via van der Waals forces and where the diameter distribution is narrow (16 ± 2 nm) [9], we observe a broad diameter distribution (65 ± 35 nm). Ni can be seen at the apex of CNTs in Fig. 1, which is the signature of a tip growth mode and which indicates weak interactions at the interfaces between the CNTs, in contrast to porous silicon where a base growth mode dominates [9]. 1 for growth conditions).

In the second part of this work, we focus on the synthesis of arrays of a few discrete and separate CNTs within a speci ed area, which should be useful for many nanotubebased electronic devices. Some numerical studies on the assessment of the local electric eld in the vicinity of the CNT tips for an array of aligned CNTs consider two factors: one internal factor describing the CNT structure (sensitive to the CNT diameter), one external factor describing the in uence of the neighboring CNTs [12,13]. It was calculated, for a given CNT length, that the emission current density from such an array of CNTs increases rapidly when their diameter decreases from 80 nm to 20 nm and when the ratio (nanotube spacing)/(nanotube length) approaches 2.3 [13]. Experimental results concerning arrays of nanotube blocks (ultrahigh-density rv10 11 cm -2 ) synthesized on patterned porous silicon [9] con rm this analysis: electric elds about 6 V/lm were necessary to reach current densities as low as 10 mA/cm 2 , which means that the eld screening e ect plays a dominant role in these CNT blocks. Thus, we tried to get high electron eld emission from arrays of MWCNTs grown by HW dc PECVD. For this purpose, square arrays of Ni catalyst dots, of di erent sizes, with various lattice translation vectors (pitches) were realized, using two patterning techniques: optical UV lithography (with a resolution of 1 lm) and electron-beam lithography (with a resolution of 40 nm).

Fig. 2(A) is the SEM image of sample 2 deposited under conditions similar to those of sample 1 (see Table 1). UV lithography was used with a pitch of 4 lm. For each dot, there are rv25 almost aligned C nanotubes whose diameter

(A) SEM image of sample 2 (see Table 1 for growth conditions). (B) Emission current density versus applied eld characteristic of sample 2.

The insert shows the tting of J-E characteristics according to the Fowler-Nordheim equation. The substrate consisted of Ni(7 nm)/TiN(10 nm)/Si(1 0 0) for all samples except sample 2 for which substrate was Ni(7 nm)/SiO2(10 nm)/Si(1 0 0). a V A-C : potential drop between the anode and the cathode; J C : current density of ions impinging on the cathode. and length is in the range 60-120 nm and 2-2.6 lm, respectively. No CNTs have been observed between dots. In Fig. 2(B) is plotted the emission current versus the applied eld: the threshold electric eld is 16 V/lm and the maximum emission current density is 40 mA/cm 2 for a eld of 33 V/lm. The eld enhancement factor b, as deduced from the Fowler-Nordheim t, is around 240. The weak electron emission obtained is attributable partly to the large CNT diameters and their broad length distribution, and partly to the screening e ect inside each Ni dot. A limitation of the emission current is also expected due to the resistivity of the silica barrier layer used for this sample. Fig. 3(A) is the SEM image of sample 3 deposited under conditions of Table 1. Compared to sample 2, the pressure was lower (1.5 mbar) and the V A-C was higher (440 V), which favors smaller CNT diameters, narrowing of length distribution and better alignment. For this sample, for which UV lithography was used with a pitch of 4 lm, the maximum emission current density is found to be 200 mA/cm 2 for a eld of 18 V/lm (Fig. 3(B)). Thus, in sample 3, in comparison to sample 2, several CNT tips contribute to the emission of electrons, and thus reinforce the emission current, even though the screening e ect between these CNTs is always dominant. Fig. 4(A) is the SEM image of sample 4 deposited under conditions of Table 1. Electron-beam lithography with a resolution of 100 nm was used with a pitch of 10 lm. On each Ni dot, we observe the growth of a single MWCNT with a length of 1.5 lm and a diameter of 50 nm at the apex. In Fig. 4(B) is plotted the emission current versus the applied eld: the threshold electric eld is 8 V/lm and the maximum emission current density is 0.4 A/cm 2 for a eld of 20 V/lm. The eld enhancement factor b, as deduced from the Fowler-Nordheim t, is around 430. Compared to sample 2 (Fig. 2), the lower threshold eld and the higher electron emission current are attributable to perfectly aligned CNTs without any screening e ect (the spacing between nanotubes is about six times as big as the nanotube length). Similar eld emission results have been obtained through the realization of individual MWCNTs under slightly di erent deposition conditions (di erent gas mixture, see sample 5 in Table 1), but using electronbeam lithography with the same resolution (100 nm) and with the same pitch (10 lm).

A more systematic work consisting of decreasing step by step the pitch between individual nanotubes, while keeping the same growth conditions is in process of completion, in order to get stable emission current densities in excess of 1 A/cm 2 , which is of interest for speci c eld emission displays such as the new generation of microwave ampli ers working at UHF (>30 GHz).

Conclusion

This work has shown that it is possible to control the synthesis of well-aligned MWCNTs in a reactor combining both HWCVD and dc PECVD techniques, using the C 2 H 2 / NH 3 /H 2 gas mixture. Optimum conditions for the growth of MWCNTs have been found, corresponding to a minimization of the NH 3 content at rv5%, with rv85% H 2 . Keeping these growth conditions, we have demonstrated, with the use of electron-beam lithography (100 nm resolution), that square arrays of individual MWCNTs (50 nm in diameter) allowed to get high eld emission current density (rv0.4 A/ cm 2 ). Further work is necessary in order to get higher emission density useful for at panel displays or vacuum microelectronic sources.

Fig. 1 .

 1 Fig. 1. SEM image (observation angle 45°) of sample 1 (seeTable 1 for growth conditions).

Fig.

  Fig. 2. (A) SEM image of sample 2 (see Table 1 for growth conditions). (B) Emission current density versus applied eld characteristic of sample 2.The insert shows the tting of J-E characteristics according to the Fowler-Nordheim equation.

Fig. 3 .

 3 Fig. 3. (A) SEM image of sample 3 (seeTable 1 for growth conditions). (B) Emission current density versus applied eld characteristic of sample 3.

Fig. 4 .

 4 Fig. 4. (A) SEM image of sample 4 (see Table 1 for growth conditions). (B) Emission current density versus applied eld characteristic of sample 4.The insert shows the tting of J-E characteristics according to the Fowler-Nordheim equation.

Table 1

 1 Growth conditions for combined HW dc-PECVD processes

	Sample	Gas mixture,	Pressure	T S (°C)	dc-PE activation,	HW activation,	Growth time
		C 2 H 2 :NH 3 :H 2 (%)	(mbar)		2 cm )	P(W)/T f (°C)	(min)
	1	10:05:85	1	620	420 • 1.5	250/1800	15
	2	10:05:85	2.5	650	400 • 4.4	300/1850	20
	3	10:02:88	1.5	660	440 • 3.3	250/1800	15
	4	10:02:88	2	660	390 • 4.4	250/1800	30
	5	15:05:80	2.5	660	390 • 8.9	250/1800	15

a V A-C (V) • J C (mA/

Acknowledgements

This work was supported by the CANAST project driven by the French Research Network in Micro and Nano Technologies and by the CNRS-KOSEF international joint research project, under Contract No. F01-2004-000-10240-0.


C S Cojocaru

F Le Normand

On the role of activation mode in the plasma-and hot filamentsenhanced catalytic chemical vapour deposition of vertically aligned carbon nanotubes

Keywords: Carbon nanotubes, Carbon nanoparticles, Catalytically grown carbon, Chemical vapour deposition, Transmission electron microscopy

Catalytic chemical vapor deposition (CCVD) with different activation modes (thermal; hot filaments-enhanced; direct current plasma-enhanced and both hot filament and direct current plasma-enhanced) are achieved in order to grow vertically aligned carbon nanotubes (VA CNTs). By widely varying the power of the different activation sources of the gas (plasma, hot filaments, substrate heating) while keeping identical the substrate temperature (973 K) and the catalyst preparation, the results point out the important role of ions in the nucleation of carbon nanotubes (CNTs), as well as the etching behaviour of highly activated radicals such as H˙ in the selective growth of vertically aligned films of CNTs. Moreover, it is demonstrated that, within the deposition conditions (temperature, pressure, flow rate) used in this study, oriented carbon nanotubes can be grown only when both ions, mainly generated by the gas discharge plasma, and highly reactive radicals, mainly formed by the hot filaments, are produced in the gas phase. We propose that highly energetic ions are needed to nucleate the carbon nanotubes by increasing the carbon concentration gradient whereas the highly reactive radicals allow the selective growth of vertically aligned CNTs by preventing carbon deposition on the whole surface through chemical etching of edge carbons in graphene sheets.

Introduction

Carbon nanotubes (CNTs) are currently produced by many pathways. Among them generally low temperature preparation techniques by chemical vapor deposition (CVD) are considered as the most convenient ones for applications in electron field emission and for any applications required to grow vertically aligned nanotubes on a flat film [START_REF] Dresselhaus | Carbon Nanotubes: Synthesis, Structure, Properties and Applications[END_REF]. In these methods, nanoparticles of a transition metal like Fe, Co, Ni, Mo, etc., catalyze the CNT growth. The gas mixture is either thermally decomposed on the substrate surface, or is activated by processes such as hot filaments (HF), direct current discharge (DC), microwave (MW) plasma, or by any combination of these processes of gaseous decomposition. For a review of the thermal and plasma-activated CVD processes, the reader can report to Refs. [START_REF] Dresselhaus | Carbon Nanotubes: Synthesis, Structure, Properties and Applications[END_REF][START_REF] Ebbesen | Carbon Nanotubes: Preparation and Properties[END_REF] and [3 -5], respectively. On a fundamental point of view, the mechanism of CNT nucleation and growth

has not yet been firmly assessed: either growth starting from the top (tip growth mechanism) or starting from the base (root growth mechanism) of the growing CNT has been yet proposed [START_REF] Ebbesen | Carbon Nanotubes: Preparation and Properties[END_REF]. However, the deposition conditions, as well as the preparation of the catalysts, are too widely varying in the literature for useful and rational comparisons. In this paper, several catalytic CVD (CCVD) depositions of carbon nanos-tructures with different modes of gas activation have been carried out to grow CNTs, while keeping constant the substrate temperature to 973 K, the initial gas composition, flow rate, pressure and catalyst preparation. The gas activation is achieved by thermal heating of the substrate (T CCVD); or by heating by hot filaments (HF CCVD); or by a direct current discharge plasma (DC CCVD) or finally both by hot filaments and by direct current discharge plasma (DC HF CCVD). Identical preparation of Co catalytic nanoparticles is carried out by ultra high vacuum evaporation at high temperature. By qualitatively comparing these different gas activation modes (nature and concentration of neutral, activated and ionized species, energy distribution, etc.), it is expected to provide new insight into the understanding of the complex CNTs nucleation and growth. The catalyst and the deposited carbon were characterized by high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and Raman spectroscopy.

Experimental

Catalyst preparation

A SiO 2 layer of 8 nm thickness was deposited by an ECR plasma process on a Si(100) sample (Sb n-doped with q = 3 mV cm). The SiO 2 /Si(100) sample was then transferred into a stainless steel ultra high vacuum (UHV) preparation chamber (base vacuum 10 -10 mbar). Co metal (grade 99.995) was evaporated with an OMICRON EFM3 effusive source at a pressure within 0.7 -2 x 10 -9 mbar on the sample heated at 925 K T 20 K during 30 min. Co nanoparticles with a narrow size distribution within 10 -15 nm were obtained. Co is in a metallic state, as checked by in situ X-ray photoemission spectroscopy (XPS).

Growth of carbon nanostructures

The sample was further in situ transferred to an UHV CVD chamber for the growth of the carbon nanostructures, which has been described elsewhere in more details [6]. The UHV base pressure is lower than 10 -9 mbar. The sample is heated with H 2 to 973 K at a constant rate of 10 K/min. The variable experimental parameters are summarized in Table 1. The gas mixture (C 2 H 2 + H 2 ) was thermally activated by heating of the substrate with an infrared lamp on the rear of the sample, or by heating with four tungsten hot filaments at 8 mm above the sample (diameter 15/100 mm; filament power P F = 150 W corresponding to a filament temperature T F � 2173 K). The gas mixture can also be kinetic energyactivated by a direct current discharge plasma ignited between two independent electrodes at V p = 310 T 10 V. A small negative extraction voltage V e (between -5 and -20 V) was set between the cathode and the sample. This allowed to withdraw in a controlled way an extraction current I e from the polarisation plasma that was focussed onto the substrate. The activation power ( P e = I e x V e ) on the sample, less than 1 W, was too small to contribute to the sample heating. As displayed in Table 1, the temperature of the sample, and the CVD gas conditions (pressure, gas composition, flow rate) were set constant for all experiments, while the modes of gas activation were allowed to continuously vary.

CNTs characterization

The samples were examined by HRTEM on a TOPCON 002B microscope operating at 200 kV. SEM observations were performed on a XL30S-FEG PHILIPS generally working at 3 KV. Raman spectra were recorded on a Renishaw spectrometer using a He -Ne laser at k = 632.8 nm (hm = 1.96 eV) equipped with a Notch filter and working in the backscattering geometry.

Results

Thermal CCVD

The thermal CCVD is the first growth mode studied. The gaseous mixture is decomposed only over the heated surface of the sample. The SEM image shows however that the surface is rather uniform (Fig. 1A) with a very weak carbon deposition, as checked by XPS. The TEM images of the sample reveal that the Co particles are encapsulated by turbostratic graphitic carbon (Fig. 2A). Lower magnitude images show that carbon strips connect these metallic particles, whereas the support is mostly let free of carbon. Very weak carbon contributions are recorded in the noisy Raman spectrum and D and G bands of carbon are hardly evidenced (Fig. 3A).

HF CCVD

In the HF CCVD mode, the gas mixture is activated both by the hot filaments and by the thermal heating of the sample. As the hot filaments are heated to around 2173 K, it is expected that most of the hydrocarbon and part of the hydrogen is decomposed into radicals at this temperature. The intensity collected onto the sample with the plasma switched off is around 10 AA, and it can be inferred that the concentration of ions is very weak. Stripes of curved and closed graphitic shells encapsulating the metallic particles are well evidenced by HRTEM. Both SEM (Fig. 1B) and TEM (Fig. 2B) at low b I p and V p are not operating parameters. They depend on the quality of the coating of the electrodes, the cleanliness of the walls, the electrodes, the substrate and filaments holders, and they may shortly vary in the course of the process. contribution around 1595 cm -1 and another even smaller one around 1500 cm -1 (Fig. 3B). In conclusion, the activation of the gas phase by hot filaments promotes a catalytic activity of the metallic nanoparticles for carbon decomposition leading to their encapsulation or to large stripes of high quality graphene planes. However the Raman spectrum suggests that the nature of carbon deposited is more complex than either graphite compound or graphene sheets. Anyway carbon nanotubes cannot be observed while using such activation mode.

V p (V) Ip (mA) b - - 320 300 T 10 - - 10

DC CCVD

In the DC CCVD mode, the gas mixture is mainly activated by high direct current discharge plasma maintained between a set of electrodes above the sample, giving rise to plasma with formation of ions through impact collisions between electrons and molecules, and formation of radicals and activated molecules through collisions between molecules, ions and electrons. Some of these ions are extracted through the small negative polarisation V e set between the cathode and the sample and impinge the surface of the sample. It is clear by SEM examination that a thick and uniform carbon layer is now deposited (Fig. 1C), of thickness estimated larger than 10 nm from additional XPS measurements. Sometimes we do also observe some sharp and short vertically aligned whiskers that could be identified to carbon nanotubes or carbon nanofibers merging from the surface, but they are scarcely spread on the surface as evidenced by SEM observations (Fig. 1C). The nature of the carbon is mostly amorphous (Fig. 2C). This is confirmed by the Raman spectrum with two large and broad D and G lines at 1329 cm -1 and 1599 cm -1 , indicative of many defects of sp 3 -type in the intense amorphous carbon layer (Fig. 3C). In conclusion, the DC CCVD process mainly leads to a widespread and amorphous deposition of carbon.

DC HF CCVD

In the DC HF CCVD mode, both the formation of plasma and the decomposition of the gas on the heated filaments lead to a high rate of reactive ions, radicals and activated molecules in the gas mixture above the sample. Within these conditions, strong modifications occur in the nature of the deposited carbon. The SEM (Fig. 1D) as well as the TEM images (Fig. 2D) now show clearly a film of aligned and long carbon nanotubes or carbon nanofibers vertically oriented relative to the surface. Moreover the tubes are of the bamboo-type with always a metallic particle on top of the tubes. These metallic particles exhibit now an anisotropic shape rather than the isotropic shape observed in the previous CCVD processes. In addition, the high resolution shows some defects within the graphitic shells. The lateral size is around 15 -20 nm with a narrow size distribution. This size distribution is quite similar to the initial Co catalytic nanoparticles. The Raman spectrum (Fig. 3D) exhibits very sharp D and G bands with many narrow substructures within these band domains. The sharp and narrow (FWHM = 13 cm -1 ) G band at 1591 cm -1 has two weak substructures on each side at 1555 cm -1 and 1606 cm -1 , respectively, quite characteristic of the occurrence of CNTs [START_REF] Dresselhaus | Carbon Nanotubes: Synthesis, Structure, Properties and Applications[END_REF]. The small FWHM of band G and the weak D band are both qualitative indications of the absence of any other carbon deposit as well as the absence of many defects within the nanotubes. In conclusion, vertically aligned carbon nanotubes are observed only when combining, within the experimental conditions used in this study (sample preparation, CVD conditions, ..etc.), gas phase activation mode with hot filaments and plasma discharge (DC HF CCVD process).

Discussion

As ascertained by SEM, TEM and Raman spectroscopy, the nature of the deposited carbon in the CCVD process is strongly dependent of the activation mode of the gas phase. The literature provides a great deal of reports of CNTs growth using either TCCVD [7,8], or HF CCVD [9 -11], or DC CCVD [12 -17], or DC HF CCVD [18 -21] activation processes. However, it must be recalled that the experimental conditions are so different from one report to another one, as far as the CVD deposition conditions (substrate temperature, pressure, nature and concentration of the gas mixture) and the catalyst preparation (nature of the catalyst and substrate, density and size of the catalytic nanoparticles, etc.) are concerned, that it is difficult to extract some rationale for the optimization of the nucleation and growth of carbon nanotubes. The occurrence and the optimization of vertically aligned CNTs might concern both the preparation of catalysts with the desired size, as the size of the CNT is in direct relationship with the size of the catalyst, and the subsequent CVD conditions of nucleation and growth. Thus, successful growth of CNTs or CNFs was reported at 973 K by thermal CCVD or HF CCVD in other reports, but with different preparation of the catalysts and CVD overall growth conditions. The reasons why CNTs cannot be grown here by TCCVD or HF CCVD can be explained by the very low pressure (15 mbar instead of 1000 mbar) and the low carbon concentration (diluted by 20% instead of around 100%) in the gas mixture. In addition, the CNTs or CNFs deposited are generally not vertically aligned, especially when the gas mixture is thermally or hot filaments activated. The alignment has been explained by the electric field created between the anode and the sample working as a cathode in a plasmaactivated process [22]. No electric field is present in the T CCVD or the HF CCVD processes. Forests of aligned singlewall CNTs were recently grown however by thermal CCVD [7,8]. In these cases the alignment can be obtained through mutual van der Waals interactions between the growing CNTs and with a high density of the small sized catalytic nanoparticles. Finally, there are recent reports that directly compare DC CCVD and DC HF CCVD processes with the development of a gas phase model [16,17]. The conclusions were that the hot filaments play a minor role. However ammonia was used in the gas phase instead of hydrogen in the present work. In a partial conclusion, it must be reminded that the different gas phase activation pathways here used might not correspond to an optimized process to growth carbon nanotubes in each of the CCVD mode used here. The main interest of this study is to provide a ''photography'' of different activation modes for the growth of vertically aligned carbon nanotubes under strictly identical catalyst preparation and overall CVD conditions, with some qualitative elements of comparison between different gas phase activations. At T = 973 K, P = 15 mbar, 20% C 2 H 2 mixed with H 2 decomposed on 10-to 15-nm-sized Co particles, it is thus found that the growth of nanotubes needs the presence of both ions with high kinetic energy, generated by the DC plasma, and highly activated radicals, mostly generated by the hot filaments. We believe ions play a crucial role in the nucleation electric field as nanotubes are growing in the extraction plasma sheath, maintain an orientation of the CNT in the direction normal to the surface. It is proposed that these ions sputter carbon atoms that can merge outside of the surface of the catalytic Co nanoparticles. Thus, it is generally observed that the nanotubes contain the amount of graphitic shells just required by the shadowing of the top nanoparticle [24]. But now, why are there no or very few CNTs observed when the DC CCVD mode is used whereas ions are also formed in the gas phase? This behaviour indicates that the presence of highly reactive radical species, like atomic hydrogen produced by hot filaments, is also required for the growth of CNTs. The difference between the DC CCVD and DC HF CCVD modes can be relied to the deposition of a large amount of amorphous carbon in the former case that spreads out on the surface of the substrate, whereas growth of CNTs only localized to the Co nanoparticle area occurs in the DC HF CCVD case. This could be relied to an important chemical etching of highly reactive radicals such as H˙ that preferentially react on edge carbon of graphene sheets. The edge carbons in a graphene sheet (prismatic carbon) are fivefold more reactive to H˙ radicals than carbon inside a graphene sheet [25]. Therefore, the spreading of carbon, which first develops at the interface of the metallic nanoparticle and the substrate, is now forbidden over the whole surface of the substrate.

From these results, as well as from the results of other experiments where either the plasma power or the power of the hot filaments are continuously varied [26], it is possible to plot a schematic 3D carbon phase diagram with the three modes of gas phase activation as parameters (Fig. 4). This phase diagram points out that, at a very low power of hot filaments, with hydrogen in the gas mixture, amorphous carbon will be formed rather than CNTs. At very low temperatures or very low plasma power, Co particles are encapsulated and thus poisoned for carbon growth. At high plasma power, filled carbon nanofibers, without central hollow channel, are produced only. Growth of vertically aligned CNTs or CNFs thus proceeds in a narrow window of activation parameters. Such qualitative carbon phase diagram has been recently reported, but with a C 2 H 2 / NH 3 gas mixture [5].

Plasma

of the nanotubes by comparing the carbon deposition in the HF CCVD and the DC HF CCVD modes, respectively. In the former case, carbon encapsulates the nanoparticle and poisons it on its whole surface for further growth. Reversely, in the late case, the particles take now an oblong shape and the top surface of the particle remains available for hydrocarbon dissociation and for subsequent CNT growth according to the model first proposed by Baker [23]. This difference can be explained by the high kinetic energy of the ions impinging the surface of the nanoparticle (non-thermalized surface) that induces a gradient of carbon concentration between the top surface and the bottom surface at the interface between the

Conclusion

As main results of this study, we demonstrate that, with identical CVD deposition conditions and preparation of the Co catalysts, oriented carbon nanotubes can be grown only when using both ions, mainly produced both by a gas discharge plasma, and highly reactive radicals, mainly produced by hot filaments (DC HF CCVD process). A synergy in the effect of these two gas species is observed in the nucleation and growth of CNTs. We propose that highly energetic ions are needed for the nucleation of the CNTs by increasing the carbon concentration gradient. Reversely, highly reactive radicals as H˙ prevent the spreading of carbon over the whole surface of the substrate by preferential chemical etching, and thus allow the selective growth of CNTs.

  most of the carbon accumulates around the metallic particles, which are clearly visible. The V e conditions are: substrate temperature = 973 K; [C 2 H 2 ] = 20%; flow rate D = 100 sccm; P = 15 mbar; time of growth, t = 15 min. a d CS is the distance between the bottom couple of filaments and the substrate.

Fig. 1 .

 1 Fig. 1. Sequence of SEM images on: (A) the T CCVD (the straight line above is due to the scratching of the deposit for TEM examinations); (B) the HF CCVD; (C) the DC CCVD; and (D) the DC HF CCVD sample.

Fig. 2 .

 2 Fig. 2. Sequence of HRTEM images (high magnitude) on: (A) T CCVD (the arrows indicate tubular graphitic planes escaping from the metallic particles); (B) HF CCVD; (C) DC CCVD; and (D) DC HF CCVD samples, respectively.

Fig. 3 .

 3 Fig. 3. Sequence of Raman spectra on: (A) the T CCVD; (B) the HF CCVD; (C) the DC CCVD; and (D) the DC HF CCVD sample. The contributions are fitted with Gaussian-lorentzian line shapes.

substrate

  Fig.4. A schematic phase diagram of the carbon species grown according to the activation modes of the gas phase containing hydrogen: thermal heating from the substrate; thermal heating by hot filaments above the sample and direct current discharge plasma.

Table 1

 1 Parameters of the CNTs growth for the different CVD processes

	Process	T CCVD HF CCVD DC CCVD DC HF CCVD
	P F (W)	-	150	-	150
	T F (K)	-	2173	-	2173
	d CS (mm) a	-	5	5	5
	b

Acknowledgements

C.S.C. acknowledges the ''Re ´gion Alsace'' for a grant. J. Faerber and G. Ehret are kindly acknowledged for SEM and TEM observations, respectively.


Costel Didier Pribat

Sorin Cojocaru

Manoraham Gowtham

Laurent Eude

Paolo Bondavalli

Pierre Legagneux

C S Cojocaru

Carbon Nanotubes and Semiconductor Nanowires for Active Matrix Backplanes

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

Active matrix displays are based on planar arrays of thin film transistors (TFTs). Today, the most mature, simple and cost effective TFT technology makes extensive use of hydrogenated am orphous silicon (a-Si:H), as millions of squared meters of glass substrates are coated with this material. However, due to some drawbacks, such as gate instabilities [START_REF] Powel | Time and temperature dependence of instability mechanisms in amorphous silicon TFTs[END_REF] (particularly when the duty cycle is high), other TFT technologies are currently needed and have been developed, especially for addressing organic light emitters (OLEDs) or for fabricating flexible displays. Low temperature polysilicon technology (LTPS) is one of those, but its acceptance in industry is still low, because it is complex [START_REF] Pribat | An overview of laser crystallisation processes and techniques for LTPS[END_REF] (and expensive) and its productivity (the throughput in particular) is lower than that achieved in AMLCD factories with a-Si:H. Also, organic semiconductors and TFTs made of such materials [START_REF] Dimitrakopoulos | Organic thin-film transistors : A review of recent advances[END_REF] are currently being studied, but device stability is still an issue.

In this paper, we will present some novel approaches to TFT fabrication (compatible with glass substrates) that use carbon nanotubes (CNTs) and semiconductor nanowires (essentially Si and Ge). We will also emphasise the possible use of carbon nanotubes films (or rather mats) as transparent electrode material in replacement of ITO or ZnO-based materials.

CARBON NANOTUBES

Single wall carbon nanotubes (SWNTs) can be viewed as resulting from rolling-up a graphene sheet into a seamless cylinder with a diameter between ~ 0.7 and several nanometers (nm). Depending on the way the C atoms connect when the cylinder is formed, SWNTs can be metallic or semiconductor [START_REF] Saito | Physical Properties of Carbon Nanotubes[END_REF]. For semiconductor specimens, the forbidden band gap is inversely proportional to the diameter d (Eg ~ 0.9 eV/d, with d expressed in nm). There is another category of CNTs, named multiwall carbon nanotubes (MWNTs). These are made up of several concentric cylinders and they are metallic [START_REF] Saito | Physical Properties of Carbon Nanotubes[END_REF]. They are materials of choice for field emission devices [START_REF] Minoux | Achieving high-current carbon nanotube emitters[END_REF].

Various synthesis methods are used for carbon nanotubes [START_REF] Loiseau | Understanding Carbon Nanotubes[END_REF], with growth temperatures down to ~ 450°C [START_REF] Min | Low-temperature growth of singlewalled carbon nanotubes by water plasma chemical vapor deposition[END_REF]. However, as far as SWNTs are concerned, all these methods produce mixtures of metallic and semiconductor specimens. Complex post-growth separation procedures have to be used [START_REF] Haddon | Purification and separation of carbon nanotubes[END_REF] if one wants one type of SWNT only (metal or semiconductor).

Moreover, SWNTs tend to group into bundles under the action of van der Waals forces, which complicates their separation. Perhaps the simpler way of separating them is to eliminate the metallic ones by electrical breakdown, once a device has been fabricated [9].

Transistors made from individual semiconductor (s-) SWNTs are invariably p-type in air and they exhibit impressive properties, with hole mobility values of 3000 cm 2 /Vs, very low threshold voltages and subthreshold slope values of ~ 70 mV per decade of current [START_REF] Javey | High-dielectrics for advanced carbon nanotube transistors and logic gates[END_REF].

Ballistic transport has also been demonstrated in short channel devices [START_REF] Javey | Ballistic carbon nanotube fieldeffect transistors[END_REF][START_REF] Javey | Ten-to 50-nm-long quasi-ballistic carbon nanotube devices obtained without complex lithography[END_REF]. The switching mechanism in these transistors is different from that of conventional CMOS devices, since carrier transport is controlled by Schottky barriers at the s-SWNT/metal contact [START_REF] Heinze | Carbon nanotubes as Schottky barrier transistors[END_REF]. As a consequence, p-type devices can transform to n-type transistors after annealing under vacuum [START_REF] Derycke | Carbon nanotube inter and intra molecular logic gates[END_REF], because the Fermi level alignment at the contacts (i.e., the Schottky barrier heights) is modified by the out-gasing of adsorbed oxygen molecules from the contact regions. Controlling (or rather finding) the position and in-plane orientation of an individual SWNT is therefore a complicated task, which is not amenable to mass fabrication. Moreover, once a SWNT spotted on the substrate and contacted, its conductivity has to be measured in order to assess whether it is semiconducting or not. Finally, for semiconducting specimens, small variations in diameter (ie., in the band gap) will induce irreproducibility in transistor characteristics.

As an alternative to the unrealistic process of using individual SWNTs for device fabrication, researchers have recently investigated the possible use of randomly organised CNT networks. Such networks are generally obtained by spraying, filtration, spin coating, etc from a precursor suspension of CNTs in a solvent. They yield reproducible electrical results because of the averaging of physical characteristics over a large number of CNTs.

However, the fabrication of CNT suspensions is by no means an easy task, since as grown CNTs are not soluble in most solvents (see e.g., ref. 17 for water-based suspensions). Figure 1 schematically shows the various operations necessary for the fabrication of a suspension. In brief, after growth and purification (operations usually performed by the company selling CNTs), the CNTs are mixed with the appropriate solvent (and surfactant), then they are sonicated, in order to break the bundles and yield individual CNTs. The mixture is subsequently submitted to ultra-centrifugation in order to separate individual SWNTs from the remaining bundles and catalyst impurities. Finally, the supernatant is carefully recuperated.

Once the suspension obtained, the next step is to optimise the deposition conditions, i.e., chose the most convenient spreading method in order to get an easy control over the thickness or areal density of CNTs on the substrate, once the solvent eliminated. As quoted above (see fig. 1), various spreading methods have been used in the literature, such as spin coating, spraying, dipping etc, and even stamping [18]. The areal density of deposited SWNTs has to be controlled carefully, because above the threshold for percolation, for high areal densities (5 to 10 tubes/µm 2 ), the conduction is practically ohmic [START_REF] Snow | Random networks of carbon nanotubes as an electronic material[END_REF][START_REF] Kumar | Percolating conduction in finite nanotube networks[END_REF][START_REF] Kumar | Performance of carbon nanotube-dispersed thin-film transistors[END_REF] with no gating effect [START_REF] Snow | Random networks of carbon nanotubes as an electronic material[END_REF].

Recent studies show that the conductivity in the networks is controlled by the Schottky barriers at the contacts between metallic and semiconducting tubes [START_REF] Bekyarova | Electronic properties of single-walled carbon nanotube networks[END_REF]. As the thickness of the random CNT network increases, the number of intertube contacts with a low Schottky barrier also increases, providing easy conduction paths and leading to quasi ohmic behaviour [START_REF] Bekyarova | Electronic properties of single-walled carbon nanotube networks[END_REF]. When the films are ohmic, they can be used as transparent electrodes, exhibiting properties close (though still inferior) to ITO [START_REF] Wu | Transparent, conductive carbon nanotube films[END_REF][START_REF] Lee | Carbon nanotube films for flexible transparent conducting electrodes[END_REF]. 2-b) was deposited from a CNT suspension in DMF (dimethylformamide, C 3 H 7 NO). For these devices, the gold electrodes were deposited first (i.e, before the CNTs), which is not ideal for obtaining a low contact resistance. The bottom insulator is a 50 nm thick thermal SiO 2 film and the doped Si substrate is used as gate electrode. Of course, this can be translated to a glass or a plastic substrate, using a deposited metallic thin film for the gate electrode and a deposited insulating thin film as a gate insulator.

Figure 3 shows the corresponding TFT characteristics. These characteristics will be improved in the future, by depositing the source and drain electrodes on top of the CNT random network. Also, it is well known that the gate coupling is more efficient with a hightop gate structure [START_REF] Javey | High-dielectrics for advanced carbon nanotube transistors and logic gates[END_REF][START_REF] Dai | Electrical transport properties and field effect transistors of carbon nanotubes[END_REF] or with a very thin bottom gate insulator, such as an organic selfassembled monolayer (SAM) [START_REF] Weitz | [END_REF]. Last but not least, TFTs fabricated from random CNT networks are highly bendable, which makes them ideal for flexible substrates [START_REF] Hur | Extreme bendability of single-walled carbon nanotube networks transferred from high temperature growth substrates to plastic and their use in thin-film transistors[END_REF] and they can be made transparent using indium tungsten oxide as contact electrodes [START_REF] Takenobu | High-performance transparent flexible transistors using carbon nanotube films[END_REF].

SEMICONDUCTOR NANOWIRES

Semiconductor (column IV elements) nanowires (NWs) are currently being extensively studied for the replacement of traditional CMOS devices. Si or Ge NWs are usually grown by the vapour-liquid-solid (VLS) technique (using e.g., gold catalyst particles), which was developed 40 years ago for growing single crystals [START_REF] Wagner | Whisker technology[END_REF]. A vapour-solid-solid (VSS) process has recently been demonstrated, using Al as catalyst material [START_REF] Wang | Epitaxial growth of silicon nanowires using an aluminium catalyst[END_REF]. The VLS process (that can be tailored to be temperature-compatible with glass substrates) has recently been applied to the growth of NWs and high performance field effect transistors (made with individual NWs) have been demonstrated, exhibiting mobility values up to ~ 1300 cm 2 /Vs [START_REF] Cui | High performance silicon nanowire field effect transistors[END_REF][START_REF] Duan | High performance thin-film transistors using semiconductor nanowires and nanoribbons[END_REF][START_REF] Goldberger | Silicon vertically integrated nanowire field effect transistors[END_REF].

The problem here again is to organise these NWs on the surface of a substrate, in order to control their placement and in-plane organisation. For this purpose, fluidic methods based on the use of Langmuir-Blodgett films have been developed [START_REF] Yang | Wires on water[END_REF], but they do not provide end-toend registration.

We are currently developing a novel template-based method for the collective organisation of nanowires [START_REF] Cojocaru | Conformal anodic oxidation of aluminium thin films[END_REF][START_REF] Pribat | Lateral templates for the controlled fabrication of carbon nanotube and nanowire field effect transistors[END_REF]. This method makes extensive use of porous anodic alumina as a template material. However, the pores are synthesised parallel to the surface of the substrate (instead of perpendicular as usually done), which facilitates the contacting operations for a three terminal device (planar-type technology). Once the lateral template synthesised, the bottom of the pores can be filled with gold catalyst particles (using an electrodeposition process, fig. 4-c) and the VLS growth of Si NWs can subsequently be achieved inside the pores (fig. 4-d). Figure 4 The feasibility of the VLS growth of Si NWs inside the pores of anodic alumina has already been shown for vertical templates [START_REF] Pribat | Lateral templates for the controlled fabrication of carbon nanotube and nanowire field effect transistors[END_REF][START_REF] Lew | Growth characteristics of silicon nanowires synthesised by VLS growth in nanoporous alumina templates[END_REF]. Figure 5 shows Si NWs grown in such a way.. We are now adapting the process to the lateral situation depicted in figure 4-c &d. In particular, due to the fact that the Au catalyst surface exposed to the growth nutrients (e.g., SiH 4 for Si NW synthesis) is much smaller than for a vertical membrane, large amounts of atomic hydrogen have to be supplied to the deposition area in order to avoid parasitic Si deposition on the membrane. Such parasitic deposition would clog the pores, thus inhibiting further NW growth.

Figure 6 shows a preliminary result of Si NW growth in a lateral anodic alumina membrane. The VLS growth has been performed at 550°C with a mixture of 5% SiH 4 diluted in H 2 at a total pressure of 5mbar for a 100sccm total flow. Clearly, the process is not uniform yet and only few NWs emerge from the lateral membrane. Also, parasitic Si deposition is apparent, particularly on fig. 6-c, which means that the H 2 decomposition rate (which supplies atomic hydrogen) at the growth temperature is too low. We are presently studying the possibility of increasing this decomposition rate by fitting the CVD reactor with a hot tungsten filament, upstream of the deposition zone. We emphasise that only 4 masks are used for the fabrication of the AMLCD backplane and that the TFT structure is fabricated during only one pump-down operation, as with the current a-Si:H TFT technology [START_REF] Pribat | The use of thin silicon films in flat panel displays[END_REF].

CONCLUSIONS

We have reviewed some properties of carbon nanotube random networks and emphasised their interest in the area of active matrix backplanes. We have also presented a novel TFT concept, based on the use of semiconductor (Si, Ge, ..) nanowires and shown its interest for the fabrication of an AMLCD backplane with only 4 masks. This concept can also be applied to AMOLEDs with a reduced mask number, as well as to large area X-ray sensors [START_REF] Pribat | Nanowire-based active matrix backplanes for the control of large-area X-ray imagers[END_REF].

For a recent review

  on the properties of field-effect transistors based on individual s-SWNT, see refs. 15 and 16.

Figure 1 :

 1 Figure 1: Schematics of the various steps needed for the preparation of a CNT suspension. The operations depicted in the upper box on the left-hand side of the picture are usually performed by the CNT supplier.

Figure 2 :

 2 Figure 2: (a): Interdigited bottom gate transistor structure (W/L = 15µm/700µm) . (b) Detail of the CNT random network between two adjacent contact (gold) electrodes. The bottom gate insulator and gate contact are respectively a 50 nm-thick thermal SiO 2 film and a doped Si substrate.

Figure 2 -

 2 Figure 2-a shows bottom gate test transistor structures with interdigited gold electrodes

7 , 2 /

 72 Some characteristics recently published in the literature show that such devices can outperform most TFT technologies (except LTPS, see ref. 2), with On/Off ratio of 10 subthreshold slope of 250 mV/decade, threshold voltage of 1V and an effective mobility above 10 cm Vs [26, 27].

Figure 3 :

 3 Figure 3: Typical characteristics of a TFT (W/L = 100µm/15µm) made from a CNT random network. (a): output characteristics (I DS -V DS ). (b) transfer characteristics (I DS -V GS ).

  summarises the overall process (see refs. 36 & 37 for details).

Figure 4 :

 4 Figure 4: Major process steps used for the fabrication of lateral templates based on porous anodic alumina, followed by VLS growth of semiconductor nanowires. The bottom sketch (e) shows a planar TFT structure making use of the nanowires after their growth inside the lateral template.

.

Figure 5 :

 5 Figure 5: (a): top view of Si nanowires grown in a vertical anodic alumina membrane. For easy observation, the growth time has been prolonged purposely, which results in NWs growing out of the membrane. (b): Close view of a nanowire emerging from a pore. (c) and (d): transmission electron microscope views of a nanowire. Crystallographic planes can be observed on (d).

Figure 6 :

 6 Figure 6: Growth of Si NWs in a lateral porous alumina membrane. (a): View of the membrane and SiO 2 cap, with some NWs emerging. (b): close up view of an emerging NW.

Figure 7 :

 7 Figure 7: The first two masks used for the fabrication of an AMLCD backplane (see text for details).

Figure 8 :

 8 Figure 8 : The last two masks used for the fabrication of an AMLCD backplane (see text for details).

Figure 1 :

 1 Figure 1: Schematics of the various steps needed for the preparation of a CNT

Figure 2 :

 2 Figure 2: (a): Interdigited bottom gate transistor structure (W/L = 15µm/700µm). (b)

Figure 3 :

 3 Figure 3: Typical characteristics of a TFT (W/L = 100µm/15µm) made from a CNT

Figure 4 :

 4 Figure 4: Major process steps used for the fabrication of lateral templates based on

Figure 5 :

 5 Figure 5: (a): top view of Si nanowires grown in a vertical anodic alumina membrane.

Figure 6 :Figure 7 :

 67 Figure 6: Growth of Si NWs in a lateral porous alumina membrane. (a): View of the

Figure 8 :

 8 Figure 8 : The last two masks used for the fabrication of an AMLCD backplane (see