Effect of electrical aging on field emission from carbon nanotube field ...

Effect of electrical aging on field emission from carbon nanotube field emitter arrays Je Hwang Ryu Department of Information Display and Advanced Display Research Center, Kyung Hee University, Seoul 130-701, Korea

Ki Seo Kim Department of Physics and Advanced Display Research Center, Kyung Hee University, Seoul 130-701, Korea

Chang Seok Lee, Jin Jang, and Kyu Chang Parka兲 Department of Information Display and Advanced Display Research Center, Kyung Hee University, Seoul 130-701, Korea

共Received 4 September 2007; accepted 28 January 2008; published 1 April 2008兲 We studied the effect of bias aging on the field emission properties of selectively patterned carbon nanotube field emitter arrays 共CNT兲 grown using the resist-assisted patterning process. After electrical aging using an electric field of 6.87 V / ␮m for 40 h, it was observed that the electron emission properties and uniformity were remarkably improved. X-ray photoelectron spectroscopy spectra show a shift of 0.2 eV in the O1s peak and the concentration of oxygen is reduced. Also, transmission electron microscopy measurements showed that Ni catalyst was removed from CNT tips after aging. Therefore, after electrical aging, we obtained enhanced and stable electron emission over a wide current range. © 2008 American Vacuum Society. 关DOI: 10.1116/1.2884757兴

I. INTRODUCTION The remarkable properties of carbon nanotubes 共CNTs兲 make them attractive for nanoelectronic device applications,1 especially for electron emitters. CNTs have several superior characteristics, such as high aspect ratio, high thermal conductivity, and low chemical reactivity.2 These characteristics make CNTs suitable for the fabrication of x-ray tubes3 and electron guns.4 High and stable field emission properties are key requirements of carbon nanotubes in electron emission source applications. To enhance electron emission from carbon nanotube emitters, oxidation,5 doping6 and laser irradiation processes7 were proposed during the fabrication of CNT emitters. Also, it was reported that stable electron emission of CNTs was achieved by removing surface adsorbates.8 In this article, the electrical aging effect on electron emission properties of resist-assisted patterning 共RAP兲 grown CNTs were studied. An electric field of 6.87 V / ␮m was applied to the CNT-field emitter arrays 共FEAs兲 on silicon substrates over a period of 40 h, resulting in an initial increase in emission current followed by saturation. After 1400 min, the electron emission current was stable at about 1.0 mA/ cm2. Hence, CNTs grown using RAP produced using electrical aging showed improved and stable electron emission. The origin of enhanced and stable electron emission current was investigated using both transmission electron microscopy 共TEM兲 and X-ray photoelectron spectroscopy 共XPS兲 analyses, and the structural relationship between the CNT-FEAs and their electron emission properties was studied using TEM. Furthermore, XPS was used to analyze a兲

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the ratio of carbon and oxygen concentrations in the CNTs plus bonding configurations before and after the aging process. II. EXPERIMENT The CNT electron emitter arrays were grown using a triode-type plasma enhanced chemical vapor deposition process. Prior to CNT growth, the substrate was dipped into a HF solution to remove the surface oxide layer. CNT growth was performed at 580 ° C using a pressure of 2.0 Torr in 40:60 mixture of C2H2 and NH3, respectively. The RAP process was used to form CNT electron emitter arrays with a 40 ␮m pitch island.9 The diameter of each island was fixed at 5 ␮m. The field emission properties of as grown and biasaged CNTs were investigated using a diode-type electron emission measurement system under vacuum 共⬃1 ⫻ 10−7 Torr兲. The distance between the anode and cathode was fixed at 150 ␮m during measurements. Electrical aging was performed using an electric field strength of 6.87 V / ␮m and the stability of electron emission was determined, as a function of field strength. Also, the relationship between structural change and electron emission properties was determined using XPS analysis. III. RESULTS AND DISCUSSION Scanning electron microscopy 共SEM兲 images of the grown CNT electron emitter arrays are shown in Fig. 1. Selective growth was attained using the RAP process and the uniformity of each island was examined using a SEM. The resultant vertically aligned CNT-FEAs were grown at specific locations with excellent uniformity.

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FIG. 3. Comparision of the emission current of as grown and aged CNT-FEAs.

FIG. 1. SEM images of 共a兲 the CNT electron emitter arrays and 共b兲 a magnified image of one emitter.

The electron emission current of the as grown CNTs was not stable; therefore, they required further treatment to stabilize the emission current. Curves showing the time variation of electron emission current during aging over a period of 40 h are presented in Fig. 2. When the electric field is 6.87 V / ␮m 共J = 100 ␮A / cm2兲, the emission current increases initially and then saturates. After stabilizing, the emission current is approximately 1.0 mA/ cm2. Figure 3 shows the electron emission characteristics of the CNTs and their Fowler-Nordheim 共FN兲 plots as a function of aging. The electron emission current density of CNT-FEAs after aging is 1.55 mA/ cm2 at a field strength of 8 V / ␮m. This electron emission current is almost six times greater than as grown CNT-FEAs. Furthermore, the turn on field decreases from 4.9 to 1.3 V / ␮m at an emission current den-

FIG. 2. Electron emission current density as a function of aging time. JVST B - Microelectronics and Nanometer Structures

sity of 1 ␮A / cm2. The FN plot indicates that the emission current is governed by a tunneling process. After aging, the field enhancement factor of CNT-FEAs is 9 ⫻ 104. For this calculation, the work function of the CNTs was assumed to be 4.6 eV. The time dependence of the electron emission current density is presented in Fig. 4. Initially, the current density was set at 0.05 共E = 4 V / ␮m兲, 0.15 共E = 5 V / ␮m兲 and 1.62 mA/ cm2 共E = 9 V / ␮m兲. After electrical aging, the current density remained constant for 60 min. Thus, after electrical aging, the electron emission remains stable over any current range. Enhanced electron emission from the emitter array is related to structural change in the CNT emitters. Figure 5 shows TEM images of CNT emitter tips. Interestingly, we observed that the tip of each CNT was sharpened during the aging process. Also, the TEM image clearly indicates the removal of the Ni catalyst during aging. Therefore, it is believed that the Ni catalyst is removed by Joule heating during aging.10 The removal of the metal catalyst and the modification of the tip from a closed to an open structure are responsible for the enhancement of the electron emission current.11,12 XPS was used to analyze the change of bonding configuration after electrical aging. The x-ray beam of the XPS sys-

FIG. 4. Time dependence of the electron emission current density after CNT high field aging.

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FIG. 6. XPS spectra of as grown and aged CNTs: 共a兲 full range XPS spectrum, 共b兲 C1s peak, and 共c兲 O1s peak.

FIG. 5. TEM images of 共a兲 as grown CNTs and 共b兲 CNTs after aging.

tem could not focus on individual patterned CNT emitters. Therefore, to remove the bare Si surface effects from the XPS analysis, the CNT emitters were grown over the whole unpatterned substrate surface. Figure 6 shows the XPS spectra of as grown and aged CNTs. The existence of a peak at 284.75 eV in both CNTs reveals the presence of amorphous carbon 共in the form of C u H and C v H兲 arising from C1s.13 This measurement was performed without etching the surface of the CNT. From the TEM image, the graphite is surrounded by amorphous carbon. Furthermore, it was noted that the pristine tube structure is not affected by aging. However, we observed that the O1s peak had shifted after the aging process, since it was observed that the as grown CNT has only one peak between 532.7 and 532.8 eV which represents the C u O bonding configuration.14 The oxygen bond peak position was shifted to higher binding energy after bias aging. This higher binding energy peak appears to be due to O u H bonding. Thus, we strongly believe that the enhancement of electron emission is due to replacement of O with H during aging. To study the structural changes in the graphite sheet, the CNT surfaces were etched using ion beam apparatus in the XPS system. Figure 7 shows the measured XPS spectra after ion etch. From the XPS spectra, it is observed that both the as grown and aged CNTs have a C-graphite peak at 284.3 eV denoted by C1s. This experiment confirms the presence of J. Vac. Sci. Technol. B, Vol. 26, No. 2, Mar/Apr 2008

FIG. 7. XPS spectra inside the CNT after a surface etch with Ar+ ions: 共a兲 C1s peak and 共b兲 O1s peak.

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TABLE I. Relative atomic concentration of C and O in CNTs.

C1s / O1s

Measurement

As grown

After aging

No etched 共surface兲 Etched 共inside兲

1.092

1.925

1.399

2.058

hydrogenated amorphous carbon at the surface of the CNT by contrast with C graphite inside the CNT; therefore, we expect enhanced electron emission when the hydrogenated amorphous carbon on the outer surface of the CNT is removed. From the XPS spectra, no shifting in the O1s peak was observed for as grown CNT-FEAs, however, some shifting was observed for aged CNTs. The energy difference between 532.4 and 532.6 eV, for the O1s peak, indicates the presence of O u H and O u C bonding, respectively, after aging.15 Table I shows a change in the atomic concentration of carbon and oxygen atoms due to aging after ion etching. The ratio of atomic concentration of carbon to oxygen atoms increased after aging. This study appears to suggest that enhanced electron emission current is due to removal of oxygen atoms from the CNT-FEAs; the removal of oxygen increases the carbon concentration. Oxygen atoms are generally known to degrade the electron emission current of CNTs. Thus, electron emission was enhanced due to the removal of oxygen. IV. CONCLUSION The effect of aging treatment on the electron emission properties of CNTs has been carried out. Electron emission is enhanced from a current density of 0.3 to 1.5 mA/ cm2 at a field strength of 8.0 V / ␮m as a result of electrical aging;

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TEM and XPS were used in the analysis of the CNT-FEAs. Changes in the structure of the CNTs, due to tip sharpening and Ni catalyst removal, plus the removal of impurities after aging were responsible for the emission enhancement. Consequently, electrical aging, improved the stability of electron emission over a wide current range. Moreover, aging is a very simple and effective process for postgrowth treatment of CNT emitters. This can be used in the fabrication of electron emission sources and it can reduce the fabrication cost. ACKNOWLEDGMENTS This work was supported by the Seoul Research and Business Development program 共Grant No. CR070054兲. The authors would like to thank G. P. Kennedy for kind discussion during correction of this article. K. B. K. Teo et al., J. Vac. Sci. Technol. B 20, 116 共2002兲. Z. Yao, C. L. Kane, and C. Dekker, Phys. Rev. Lett. 84, 2941 共2002兲. 3 W. S. Chang, H. Y. Choi, and J. U. Kim, Jpn. J. Appl. Phys., Part 1 459, 7175 共2002兲. 4 M. A. Guillorn et al., J. Vac. Sci. Technol. B 22, 35 共2004兲. 5 S. C. Chin, K. C. Hwang, and I. N. Lin, Appl. Phys. Lett. 80, 4819 共2002兲. 6 R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and R. E. Smalley, Nature 共London兲 388, 255 共1997兲. 7 W. J. Zhao, A. Sawada, and M. Takai, Jpn. J. Appl. Phys., Part 1 41, 4314 共2002兲. 8 V. Semet et al., Appl. Phys. Lett. 81, 343 共2002兲. 9 K. C. Park, J. H. Ryu, K. S. Kim, Y. Y. Yu, and J. Jang, J. Vac. Sci. Technol. B 25, 1261 共2007兲. 10 S. T. Purcell, P. Vincent, C. Journet, and V. T. Binh, Phys. Rev. Lett. 88, 105502 共2002兲. 11 S. M. Yoon, J. S. Chae, and J. S. Seo, Appl. Phys. Lett. 84, 825 共2004兲. 12 D. W. Kang and J. S. Suh, J. Appl. Phys. 96, 5234 共2004兲. 13 J. C. Lascovich, R. Giorgi, and S. Scaglione, Appl. Surf. Sci. 47, 17 共1991兲. 14 S. D. Gardner, C. S. K. Singamsetty, G. L. Booth, and G. R. He, Carbon 33, 587 共1995兲. 15 C. Jones and E. Sammann, Carbon 28, 509 共1990兲. 1 2