Growth and evaluation of nanostructured carbon films for triode field

Growth and evaluation of nanostructured carbon films for triode field emitter application Kyung Ho Park, Hyung Jun Han, Seungho Choi, Kyung Moon Lee, Soonil Lee,a) and Ken Ha Koh Department of Molecular Science and Technology and Information Display Research Institute, Ajou University, Suwon 442-749, Korea

共Received 28 March 2002; accepted 28 October 2002; published 5 February 2003兲 To identify the deposition conditions that can minimize the nanotube density in the nanostructured carbon films without compromising the emission properties, we carried out a systematic investigation of the effect of deposition conditions on the emission properties and the structure of the nanostructured carbon films. Catalyst-layer thickness, methane concentration, deposition time, deposition pressure, and substrate temperature were the main deposition parameters we investigated. Within the parameter range for nanoparticle-dominant growth, substrate temperature and deposition time were the two factors that had the largest effect on the variation of the turn-on field. However, catalyst-layer thickness and methane concentration turned out to be the factors allowing the minimization of nanotube density with rather small concomitant variations of the turn-on field. A 50 h test showed that the emission stability of a nanoparticle film was better than that of a multiwall nanotube film with comparable emission characteristics. The feasibility of triode fabrication was verified by the successful deposition of a nanoparticle-film cathode of about 8 ␮m diameter using the conventional photolithography process. © 2003 American Vacuum Society. 关DOI: 10.1116/1.1531130兴

I. INTRODUCTION Since its discovery in 1991,1 carbon nanotube has emerged as a promising cathode material for field-electronemission applications due to its peculiar structural and electronic properties. There have been many reports confirming the excellent field emission from carbon-nanotube films.2– 6 However, the fabrication of triode-type emitters with carbonnanotube cathodes remains a difficult task due to the length and flexibility of the carbon nanotubes. Nanotubeincorporated triode-structure emitters are susceptible to the cathode-gate short or to a huge gate current. The aforementioned problems become more severe if one tries to fabricate a triode-type emitter with a nanotube cathode by using the conventional photolithography and the chemical-vapordeposition method. Therefore in the few cases that reported the successful fabrication of triode emitters with carbon nanotubes, either the cathode was formed by the screenprinting method, or a separately prepared mesh-type gate was incorporated.7–11 Previously, we reported that the emissions from nanostructured carbon films, which are the mixtures of nanotubes and nanoparticles, could be comparable to or even better than the emissions from carbon-nanotube films.12 The observation that the morphology and the electron-emission properties of nanostructured carbon films changed a great deal depending on the growth conditions motivated us to look for the deposition conditions that could ideally produce carbonnanoparticle films of excellent emitting properties without any nanotubes. Moreover, the selective growth of carbon a兲

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nanoparticles on the catalyst metals supported by the Cr layer led us to the possible scheme for triode-type-emitter fabrication using the conventional photolithography and hotfilament chemical-vapor deposition 共HFCVD兲 method, the feasibility of which we tested in this work. II. EXPERIMENT Using a Ni/Fe-alloy layer and a CH4 /H2-mixture gas as a catalyst and a precursor, respectively, nanoparticledominated carbon films were grown on silicon substrates via the HFCVD method. Prior to the carbon-film deposition, chrome and catalyst layers were deposited in succession by magnetron sputtering. The deposition rate for each metal layer was predetermined by measuring the thickness of a series of reference samples using a surface profiler 共model P-10, Tencor Corp.兲. The chrome layer was introduced to enhance the film adhesion and to promote the nanoparticle formation.13 Catalytic layers had smooth surface morphology with the average grain size of about 20 nm.14 Catalyst-layer thickness, methane concentration, deposition time, deposition pressure, and substrate temperature were the main deposition parameters, which we systematically investigated. The default values for these parameters were 60 nm, 10%, 20 min, 30 Torr, and 720 °C, respectively, and we changed only one parameter at a time, while keeping other parameters fixed. The surface morphology and the structure of the synthesized carbon films were examined using a Philips XL30FEG field-emission scanning electron microscope 共SEM兲 and a JEOL JEM-3011 and JEM-2000EXII transmission electron microscope 共TEM兲, respectively. Field-emission measurements were carried out using both a diode and a triode con-

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FIG. 1. Similar field emission from the two carbon films deposited using different conditions: 共a兲 emission-current density vs electric field and 共b兲 corresponding FN plots. Catalytic-layer thickness and methane concentration were 200 nm and 10% for sample 1 and 60 nm and 2% for sample 2, respectively.

figuration in a vacuum chamber maintained at a pressure of 10⫺7 Torr with an oil-free turbomolecular pump. In a diode configuration, a parallel anode plate, consisting of an indium tin oxide 共ITO兲 or a phosphor-coated ITO glass, was separated from the sample 共cathode兲 with a 110-␮m-thick spacer. For a simple triode-emission measurement, a mesh 200 copper TEM grid supported with a 55-␮m-thick spacer was used as a gate electrode. However, a conventional photolithography process was used to test the feasibility of nanoparticlecathode formation with a few-␮m-diameter gate hole. III. RESULT AND DISCUSSION We present typical examples of the similar electronemission curves 共Fig. 1兲 and the images of a pair of nanostructured carbon films deposited at different conditions 共Fig. 2兲. While keeping other deposition parameters fixed, we changed the catalytic-layer thickness and methane concentration for these two samples: 200 nm and 10% for sample 1, and 60 nm and 2% for sample 2, respectively. Note that not only the current density versus electric field (J-E) curves or the corresponding Fowler–Nordheim 共FN兲 plots in Fig. 1, but also the emission-site densities over the measured area of 28 mm2 were very similar. The particular images presented in Fig. 2 were taken at the applied field of ⬃7.0 V/␮m and corresponded to the emission current of ⬃320 ␮A. SEM images presented in Fig. 3 show that both of these

FIG. 2. Emission images of 共a兲 sample 1 at 6.9 V/␮m and 323 ␮A: and 共b兲 sample 2 at 7.0 V/␮m and 316 ␮A. These images were taken using a chargecoupled-device camera during the emission I–V measurement from the region of 6 mm diameter in a diode configuration. JVST B - Microelectronics and Nanometer Structures

FIG. 3. SEM images of 共a兲 sample 1 and 共b兲 sample 2; a nanotube segment in 共b兲 is marked with an arrow. Note that the size of carbon nanoparticles and the density of carbon nanotubes in sample 1 are about 2.5 and 15 times larger than their respective counterparts in sample 2.

films were nanoparticle-dominated films. Nevertheless, there were noticeable differences in the nanotube density and nanoparticle size between theses two films. Sample 1, which was deposited with the thicker catalytic layer and higher methane concentration, had the average nanoparticle diameter of ⬃240 nm, and there were about ⬃30 nanotubes per 100 ␮ m2. However, in sample 2, the average diameter of nanoparticles was only ⬃95 nm, and there were less than one nanotube per 50 ␮ m2. Moreover, the rare tube-like segments in sample 2 were much shorter compared to the nanotubes in sample 1. The observed comparable emission from the two nanoparticle-dominated carbon films with at least one order of tube-density difference motivated us to carry out a moredetailed investigation of the variations in tube morphology and emission properties within the parameter range for nanoparticle-dominant growth. Our goal was to identify the deposition conditions for the lowest nanotube density without compromising the emission properties, which would be ideal for triode fabrication, For example, if we use the carbon-film cathode with less than one nanotube per 50 ␮ m2, each triode pixel with a gate opening of less than 8 ␮m would be a lot less susceptible to gate-related failure caused by a carbon nanotube. In Fig. 4, we summarize the deposition-conditiondependent variations of nanotube density, nanoparticle size, and turn-on field. Because the variation of deposition parameters was generally restricted within the nanoparticledominant-growth range, the variation of the emission characteristics was quite limited. Therefore the turn-on field

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FIG. 4. Deposition-condition-dependent variation of nanotube density, nanoparticle size, and turn-on field. Turn-on field corresponds to the electric field needed to extract a current density of 10 nA/cm2.

which corresponded to the current density of 10 nA/cm2 was a good representation of the overall emission characteristics. As shown in Fig. 4共a兲, large turn-on fields over 5.7 V/␮m were observed at substrate temperatures below ⬃670 °C. However, the turn-on field decreased rapidly as the substrate temperature was raised and the field became smaller than 3.5 V/␮m at substrate temperatures over ⬃715 °C regardless of other deposition parameters. We attribute the poor emission characteristics at low substrate temperatures to the difficulty of the graphitic sheet formation,15 which nanoparticles and nanotubes consist of and are responsible for electron emission. The observed slight increases of nanotube density and nanoparticle size with substrate-temperature increase are the supporting evidence. Unlike the substrate temperature, the total pressure did not affect the emission significantly, as long as the pressure was kept in the range of 10–50 Torr. Within this pressure range the nanoparticle size did not show significant pressure dependence. Only a slight increase in tube density was observed as the pressure was raised from 10 to 30 Torr. Opposite variations of the turn-on field and nanoparticle size, similar to the substrate-temperature effect, were observed in the deposition-time dependence. As presented in Fig. 4共b兲, the nanoparticles were small, and the turn-on field was as large as ⬃3.3 V/␮m when the deposition time was too short. However, deposition time longer than 12 min resulted in the low and stable turn-on field of ⬃2.3 V/␮m together with the near saturation of the nanoparticle size to ⬃210 nm. When the other deposition parameters were properly maintained, the change of the catalytic-layer thickness and methane concentration resulted in a rather small variation in the turn-on field, as shown in Figs. 4共c兲 and 4共d兲. When the catalyst layer was thinner than 80 nm, the turn-on field was ⬃2.7 V/␮m, but decreased to ⬃2.1 V/␮m as the catalystlayer thickness was increased to 200 nm. Corresponding to the increase of methane concentration from 2% to 10%, the turn-on field varied from ⬃2.2 to ⬃2.8 V/␮m, but came down to ⬃2.4 V/␮m as the methane concentration was increased further to 20%. However, Figs. 4共c兲 and 4共d兲 show J. Vac. Sci. Technol. B, Vol. 21, No. 1, JanÕFeb 2003

FIG. 5. Typical TEM images of carbon nanoparticles show carbon layers encapsulating catalyst-metal and/or its carbide cores; catalyst-layer thicknesses were 120 共a兲 and 60 共b兲 nm, respectively. The scale bar in 共a兲 and 共b兲 corresponds to 20 nm. The high-resolution TEM image 共c兲 clearly shows graphitic-sheet edges.

that the same variation of catalytic-layer thickness and methane concentration resulted in about fivefold and eightfold increases in nanotube density, respectively. We also observed that in spite of the methaneconcentration variation in the 2%–20% range the average size of the nanoparticles remained nearly unchanged. However, the average size increased as thicker catalyst layers were used. The observed variation of nanoparticle size, together with the aforementioned deposition-time dependence, can be understood in relationship to the nanoparticle structure. TEM revealed that nanoparticles consisted of carbon layers encapsulating catalyst-metal and/or its carbide cores and that the core diameter increased roughly linearly as the catalyst-layer thickness was increased while the thickness increase of the outer carbon layer was limited only to the fraction of the core-size increase 共see Fig. 5兲. Therefore it is conceivable that after nanoparticles were formed with certain size cores at the early stage of the deposition, the thickening of the outer carbon layer proceeded only up to a certain level. Based on the systematic investigation, some results of which are shown in Fig. 4, we identified methane concentration of 2%, catalyst-layer thickness of 60 nm, deposition time of 15 min, total pressure of 30 Torr, and substrate temperature of 720 °C as the optimal deposition condition for the carbon cathode layers in a triode structure. It is worth emphasizing again that this deposition condition was selected solely from the practical point of view to facilitate reliable triode fabrication; nominally there would be less than one nanotube per 50 ␮ m2, and turn-on field would be ⬃2.5 V/␮m.

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FIG. 6. Emission-current fluctuation of the nanoparticle and the nanotube films. Emission current of the carbon-nanoparticle sample showed less than 10% fluctuation during the 50 h operation period.

Another important factor to consider in selecting the cathode material for practical tiode applications 共such as field emission display兲 is the emission stability. In Fig. 6, we compared the stability-test result of our choice of nanoparticle film with that of the multiwall-nanotube sample. 共That sample was also grown using the metal-catalyst-assisted HFCVD method.兲 For both samples, we monitored the temporal variations of the emission current at a fixed dc bias, which had been set to yield initial emission-current densities of slightly over 1 mA/cm2. Note that other than the initial loss, the emission-current density of the nanoparticle film remained stable. Over the test period of 50 h, current-density fluctuation over the average current density was about 10% and 26% for the nanoparticle film and the nanotube film, respectively. To test the modulation of the emission current from the nanoparticle-film cathode with the gate-voltage control, we constructed a simple triode using a copper TEM grid as a gate. The distance between the gate and the cathode and between the gate and the anode was 55 and 260 ␮m, respectively. As presented in Fig. 7, we were able to modulate the anode current by controlling the gate bias while keeping the anode bias fixed at 800 V. The gate current, which was about half of the anode current, showed very similar gate-bias dependence. The FN plot shown in the inset confirmed that the field emission was responsible for these currents. Figure 8共a兲 is the SEM image of the triode emitter with a nanoparticle-film cathode and an 8 ␮m gate hole. To fabricate this triode, we first deposited 100 nm chrome and 60 nm NiFe layers in succession by magnetron sputtering, followed by the deposition of the 1.5 ␮m silicon dioxide layer by CVD and the 200 nm chrome gate layer. The next two steps were the patterning of the gate hole using the conventional photolithography process and the wet etching of Cr and SiO2 layers. The etching process exposed the catalytic-metal layer and the subsequent HFCVD process resulted in the selective growth of the nanoparticle-film cathode onto the exposed catalytic-metal layer in the central area. Figure 8共b兲 shows the dependence of emission current on gate voltage at the anode voltage of 1000 V. The corresponding Fowler– JVST B - Microelectronics and Nanometer Structures

FIG. 7. Anode and gate currents measured ina triode structure with a nanoparticle-film cathode. We used a 200-mesh copper TEM grid as a gate. The distance between the gate and the cathode was 55 ␮m.

Nordheim plot and the emission image at the gate voltage of 60 V are presented in the inset. IV. CONCLUSION From the careful mapping of the morphology and the field-emission property of the nanoparticle-dominated carbon

FIG. 8. 共a兲 SEM image of the prototype triode emitter with a nanoparticlefilm cathode and a 8 ␮m gate hole: and 共b兲 the dependence of emission current on gate voltage at the anode voltage of 1000 V. The inset shows the corresponding Fowler–Nordheim plot and the emission image at the gate voltage of 60 V.

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films, the emission characteristics of which were good enough for practical applications, we were able to find the deposition conditions to minimize the nanotube density. We argue that the minimization of the nanotube density can greatly reduce the failure possibility of the conventional gated-triode structure incorporating the nanoparticledominated carbon films as cathode material; due to their length and flexibility, nanotubes in these films are the potential failure source for the micron-scale device. We also presented the temporal stability of the emission current from an optimized carbon-nanoparticle film and the modulation of the anode current by the gate-bias at the fixed anode bias from the simple triode structure. Finally, we demonstrated the successful deposition of a carbon-nanoparticle cathode in the triode structure with an 8 ␮m gate hole using the conventional photolithography process. ACKNOWLEDGMENT This work was supported by Korea Research Foundation Grant No. KRF-2001-015-DP0167. 1 2

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