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Investigation of carbon nanotube field emitter geometry for increased current density
Solid-State Electronics 54 (2010) 1543–1548
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Investigation of carbon nanotube field emitter geometry for increased current density Jeremy L. Silan a,b, Darrell L. Niemann a,b, Bryan P. Ribaya a,b, Mahmud Rahman b,*, M. Meyyappan a, Cattien V. Nguyen c a b c
NASA Ames Research Center, Moffett Field, CA 94035, United States Electron Devices Laboratory, Electrical Engineering Department, Santa Clara University, Santa Clara, CA 95053, United States ELORET Corporation, NASA Ames Research Park, Moffett Field, CA 94035, United States
a r t i c l e
i n f o
Article history: Available online 12 August 2010 The review of this paper was arranged by Prof. A. Zaslavsky Keywords: Carbon nanotube Field emission Edge effect Emitter array
a b s t r a c t In this work we present field emission characteristics of four geometrically distinct carbon nanotube pillar arrays. Each cathode has a unique geometric configuration with different structural parameters such as number of edges and vertices. We present experimental data demonstrating a carbon nanotube cold field emitter with an emitted current density of 31.8 mA/cm2 at an applied field of 11 V/lm. The performance of these cathodes can be directly attributed to the electric field being enhanced along the edges and vertices of the structures. We investigated this phenomenon experimentally by changing the geometry of the carbon nanotube pillar structure. We show that by increasing the number of edges and vertices of a structure to keep the electric field screening to a minimum, the emission current can be increased. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs), a great deal of research has been devoted to studying their extraordinary properties and applications [1,2]. The high aspect ratio structure of CNTs, their chemical stability, high thermal conductivity, and mechanical strength have led to a host of potential applications that have been proven in many laboratories [3]. These attributes make CNTs excellent cold field emitters, achieving longer lifetime and higher current densities at lower applied fields, which makes them highly attractive for use in field emission applications requiring high current densities [3,4]. However, up to this point, it has been very difficult to fabricate stable and robust CNT cold field emitters while at the same time achieving economical manufacturing. Furthermore, the high current density required for certain applications such as traveling wave tube amplifiers remain a big challenge. Currently, a handful of applications requiring relatively low current density employing CNT emitters have been demonstrated in the laboratory, such as X-ray sources for biomedical imaging, e-beam lithography, lighting, microwave sources, as well as field emission displays (FEDs) [5–9]. Despite the progress made in recent years on understanding the fundamentals of CNT emitters and on how to implement CNTs in applications requiring electron sources, wide scale adoption re* Corresponding author. Tel.: +1 408 554 4175. E-mail address: [email protected] (M. Rahman). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.07.004
mains elusive. There are still many factors that contribute to the CNT emission process which are not fully understood, particularly with respect to achieving optimal emission current and emitter stability. Phenomena such as field screening, local field enhancement and the edge effect have been explored in the last few years [10–12]. For many applications where CNT emitters can be implemented several of the above mentioned phenomena have not been fully explored. In this paper, we explore the structural effects of CNT emitter arrays on the field emission properties of the CNT cathodes. Previously we have demonstrated that CNT pillar arrays (CPAs) offer a practical solution for the fabrication of stable and easy to manufacture CNT cathodes. A CNT pillar is described as a uniform, highly dense, vertically aligned, compact bundle of CNTs [13]. Vertical self-alignment of CNTs results from the van der Waals interaction between neighboring CNTs and contributes to the excellent structural stability of CNT pillars [14]. These CNT pillar structures exhibit lower turn-on fields and a higher current density when compared to that of continuous CNT films. The geometry of each pillar was specifically chosen to investigate the effect of the cathode geometry on the electric field enhancement at the edges of the structure. This enhancement effect is called the edge effect and can be described as a phenomenon where the electric field is enhanced along the edges of a CNT pillar structure. By varying the geometry of the pillar structures, we have a means to increase emission current density. It should be pointed out that CPAs were invented to overcome fabrication challenges
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that have been encountered with large scale manufacturing of CNT emitters. Previously a pillar and its geometric counterpart, a toroid, were modeled using finite element simulations to investigate the behavior of the electric field in close proximity to the structures [14]. Through these simulations it was validated that the electric field was enhanced along the inner and outer edges as opposed to just the outer edge for the toroid and pillar structure, respectively. 2. Experimental set-up Our CNT field emitter (CNTFE) fabrication begins with a 1 cm 1 cm Si substrate. These substrates were then sputtered with a 10 nm thick Al diffusion barrier and a 4 nm thick Fe catalytic layer. The cathodes were then patterned with a conventional photolithographic process using Clariant AZ 5412E photoresist and a Quintel Q-4000 series mask aligner. The patterned substrates were subsequently sputtered with a 10 nm Cr layer and 15 nm Mo layer to selectively prevent growth in the Cr/Mo-coated areas. CNT growth was carried out via thermal CVD at 750 °C with process gas flow rates of 200 SCCM of H2 and 800 SCCM of C2H4. The furnace ramp up and ramp down were both carried out with gas flow rates of 200 SCCM of H2 and 800 SCCM of Ar. Field emission (FE) data were collected using a diode configuration at base pressure of 107 Torr. The gold anode was polished and kept parallel to the cathode surface. The anode to cathode separation gap was precisely controlled using a Huntington Labs L-2111 linear positioner. For all experiments presented here the gap was fixed at a distance, d, equal to 100 lm, measured from the top of the CNT array to the polished gold anode surface. Data were col-
lected using a Keithley 237 SMU controlled by a custom LabVIEW program communicating over the General Purpose Interface Bus (GPIB). Here, the turn-on field refers to the macroscopic field necessary to draw 1 nA of current from each sample [15,16]. Current–voltage (I–V) data were collected only after preconditioning the cathodes by sweeping them from 0 V to three times their initial turn-on voltage for five sweeps. The cathodes were then swept to the maximum supply voltage of 1.1 kV for five more sweeps. The final reported values were the last of these five sweeps. Following the I–V characterization, cathode stability was studied by investigating the I–V hysteresis while the cathodes were being swept up then back down in voltage 0 V–1.1 kV and 1.1 kV–0 V, respectively, for an additional five times. The FE data were analyzed using the standard Fowler– Nordheim (FN) equation given in the following equation.
" # lbU3=2 I ¼ kAaU F exp F 1 2
ð1Þ
Here, I is the emitted current in amperes, a = 1.5413434 106 A eV V2 and b = 8.830888 109 eV3/2 V m1 are the universal FN constants, k and l are generalized correction factors, A is the emission area in cm2, U is the local work function (approximated to be fixed at 5 eV), and F is the field at the CNT tip [17–19]. For the modeling here, F t ¼ bV, where b and V are the geometric field enhancement factor of the array with units of m1 and the applied voltage, respectively [12,19,20]. The geometric field enhancement factor for the CNTFE was extracted from the Fowler–Nordheim plot, ln(I/V2) versus 1/V, by taking the slope at the start of the linear region to the first knee of the curve. The linear region of the plot
(a)
(b)
(c)
(d)
Fig. 1. SEM images at a tilt angle view of 45° of the (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA carbon nanotube arrays showing highly dense, uniform growth.
J.L. Silan et al. / Solid-State Electronics 54 (2010) 1543–1548
exhibits Fowler–Nordheim behavior which signifies field emission. After calculating the geometric field enhancement factor, b, the field enhancement factor c can be determined from the relation c ¼ bV=F m , where F m ¼ V=d [12]. In order to take into account field screening effects and geometrical spacing, the current density was calculated by dividing the total emitted current by the entire cathode area, including the Cr/Mo-coated region where there is no CNT growth, which was 0.0314 cm2 for each cathode.
Fig. 2. High resolution SEM image showing the typical CNT diameter distribution present when viewed from the tops of the CNTFEs.
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3. Results and discussion Four different array geometries consisting of a CNT pillar array (CPA), toroid pillar array (tCPA), hexagon pillar array (hCPA) and hollow hexagon pillar array (h2CPA) were fabricated. The pillar had a diameter of 30 lm, the toroid had an outer diameter of 30 lm and an inner diameter of 10 lm, the hexagon had side lengths of 15.7 lm and the hollow hexagon had side lengths of 15.7 lm and an inner diameter of 10 lm. The perimeter of each individual pillar structure, whether it is a circle or a hexagon, measured 94.2 lm. For these cathodes the CVD process was carried out for 60 s resulting in high density uniform growth of multi-walled carbon nanotubes in the predefined shapes. SEM images showing typical areas of the different emitter geometries with a pitch of 15 lm between structures are shown in Fig. 1. In order to understand the diameter distribution of the CNTs which comprise the FE structure we collected high magnification SEM images as shown in Fig. 2. Investigation of this data reveals CNT diameters in the range from 10–20 nm. Although the growth time was kept constant for each structure, there were variations in heights between CNTFEs. The difference in heights between CNTs of these emitters can be attributed to the variability of the CNT growth process due to surface contamination. As seen in Fig. 3a and c, the CPA and tCPA were approximately 20–25 lm in height, respectively, whereas in Fig. 3b and d, the hCPA and h2CPA were approximately 5–10 lm in height, respectively. This difference in heights proves useful when comparing FE data for the CNTFEs. Since the heights of the CNTs for the CPA
(a)
(b)
(c)
(d)
Fig. 3. SEM (2500) images at a tilt angle view of 45°: (a) CPA, (b) tCPA, (c) hCPA, and (d) h2CPA.
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and tCPA are greater than the heights of the hCPA and h2CPA, based on their higher aspect ratios one would expect higher emission current due to an increase in field enhancement along the edges of the structures. However, according to our FE data shown in Fig. 4 the opposite is true. Although the CPA and tCPA exhibit lower turn-on fields, the hCPA and h2CPA ultimately achieve higher current density at large applied fields. As seen in Fig. 4, the CPA and tCPA have an emitting current of 0.62 and 0.75 mA at 11 V/lm, and turn-on fields of 1.42 and 1.39 V/lm, respectively. The hCPA and h2CPA, despite having smaller aspect ratios, generate more emission current than their geometric counterparts. The hCPA and h2CPA have an emission current of 0.68 and 0.98 mA at 11 V/ lm, though their turn-on fields of 1.80 and 1.42 V/lm, respectively, are higher. Initial and post-conditioning field enhancement factors, c, as well as turn-on voltages were extracted from the slope of the FN plot, before the first knee, as show in Fig. 5. The CPA, tCPA, hCPA
Current (mA)
1 0.9
CPA
0.8
tCPA hCPA
0.7
hhCPA
0.6 0.5 0.4 0.3 0.2 0.1 0 0
200
400
600
800
1000
Volts (V) Fig. 4. Experimental I–V curves for the CPA, tCPA, hCPA, and h2CPA emitters during the field emission process.
-20 CPA tCPA hCPA hhCPA
LN(I/V2)
-22 -24 -26 -28 -30 -32 0.0009
0.0011
0.0013
0.0015
0.0017
0.0019
0.0021
1/V Fig. 5. Fowler–Nordheim plots of the I–V characteristics of the different geometric CNTFEs exhibiting Fowler–Nordheim field emission.
and h2CPA had post-conditioning c and turn-on fields of 1042, 1108, 676, 829 and 1.42, 1.39, 1.80, 1.42 V/lm, respectively. These high field enhancement factors for the CNTFEs can be directly attributed to a multistage enhancement effect coming from the field enhancement factor of the pillar geometry and the field enhancement factor of the individual CNTs in the pillar [21,22]. The higher c values for the CPA and tCPA as compared to the hCPA and h2CPA can be attributed to their higher aspect ratios as shown in the SEM images in Fig. 3. When comparing the FE characteristics of the solid geometries and hollow geometries, the CPA and tCPA have a lower turn-on field than the hCPA and h2CPA, respectively. The lower turn-on field demonstrated by the CPA and tCPA also can be attributed to these structures having a higher aspect ratio. Although the CPA and tCPA had a higher aspect ratio and lower turn-on voltage than the hCPA and h2CPA, the hCPA and h2CPA still have a higher emitting current at the same applied field of 11.0 V/lm than their geometric counterparts. This increase in current density for the hCPA and h2CPA can be attributed to an increase in the number of sharp vertices. Note that when comparing a CPA to an hCPA, the hCPA not only takes advantage of the edge effect, but also has six sharp vertices which also further contribute to electric field enhancement. The h2CPA, like its geometric counterpart the tCPA, is hollow and therefore has two instances of the edge effect: the outer edge and the inner edge. The h2CPA outperforms the other cathodes in terms of current density because it also features the six sharp field enhancing vertices. This suggests that the increase in emitting current from the hCPA and h2CPA emitters can be directly attributed to the electric field enhancement at the sharp vertices of these two emitter geometries. A comparison of the field enhancement factor c and turn-on fields for all the CNTFE geometries are shown in Table 1. It should be pointed out that the initially high field enhancement factors and resulting low turn-on voltages seen in Table 1 for each CNTFE, can be attributed to those CNTs which are taller than the rest of the CNTs in the pillar structure. As the CNTFEs go through a conditioning process local emission hot spots will burn off thus lowering the CNTFE’s field enhancement factors and increasing their turn-on voltages. This is evident when comparing the initial to post field enhancement factors and initial to post turn-on fields for each CNTFE. In order to study the structural stability of each of the CNT cold field emitters the I–V hysteresis was examined. This hysteresis results from those individual CNTs which are taller than the majority of the CNTs in the pillar burning off during field emission. Here the cathodes were swept up and then down in voltage from 0 V to 1.1 kV and 1.1 kV to 0 V, respectively for a total of five sweeps. Comparing the different CNT array geometries, hysteresis observed in the initial voltage sweep was greatest for the hCPA and the least for the h2CPA as seen in Fig. 6b and d, respectively. Fig. 6a and c shows that both the CPA a tCPA have little hysteresis. During subsequent sweeps, the hysteresis increased for the hCPA, but remained small for the CPA, tCPA and h2CPA emitters. After the fifth sweep, the final measured emitted current densities for the CPA, hCPA, tCPA and h2CPA were 19.4, 20.7, 22.2, and 31.8 mA/ cm2, respectively, at an applied electric field of 11 V/lm. It should be noted that although there is I–V hysteresis for the four different
Table 1 Summary of field emission results for geometrically different CNTFEs. Emitter array
Growth time (s)
Array heights (lm)
Initial c
Initial turn-on field (V/lm)
Post-conditioning c
Post-conditioning turn-on field (V/lm)
CPA tCPA hCPA h2CPA
60 60 60 60
20–25 20–25 5–10 5–10
4219 3260 4381 940
0.95 0.77 1.07 1.31
1042 1108 676 829
1.42 1.39 1.80 1.42
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(a)
(b)
CPA
0.7
Initial Voltage Sweep (Blue)
0.6 0.5
Current (mA)
Current (mA)
Final Voltage Sweep (Red)
0.4 0.3 0.2 0.1
hCPA
0.7 0.6
Initial Voltage Sweep (Blue)
0.5
Final Voltage Sweep (Red)
0.4 0.3 0.2 0.1
0.0 0
200
400
600
800
0.0
1000
0
200
Voltage (V) tCPA
0.80
(d)
0.70
Initial Voltage Sweep (Blue)
0.60
Final Voltage Sweep (Red)
Current (mA)
Current (mA)
(c)
0.50 0.40 0.30 0.20 0.10 0.00 0
200
400
600
400
600
800
1000
Voltage (V)
800
1000
h2CPA
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Initial Voltage Sweep (Blue) Final Voltage Sweep (Red)
0
Voltage (V)
200
400
600
800
Voltage (V)
Fig. 6. I–V curves comparing the stability of the (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA after conditioning.
(a)
(b)
(c)
(d)
Fig. 7. Post field emission SEM (1000) images showing the top view of the: (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA.
1000
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CNTFE geometries, there was little or no large scale damage to the CNT arrays during field emission as shown in Fig. 7. 4. Conclusion In conclusion, we compared the field emission characteristics of four geometrically different CNT arrays, the CPA, tCPA, hCPA, and h2CPA. We have demonstrated that an increase in emission current density can be achieved by increasing the number of emitting edges and vertices for a given CNTFE. Our experiments support this finding because the h2CPA structure, which had the greatest number of emitting edges and vertices, achieved the highest emitted current density of 32 mA/cm2 at an applied field of 11 V/lm. We also show that the high aspect ratio CPA and tCPA structures had lower maximum emission currents when compared to the low aspect ratio hCPA and h2CPA structures. At the same applied field of 11 V/lm, the CPA and tCPA achieved currents of 0.62 and 0.75 mA while the hCPA and h2CPA reached 0.68 and 0.98 mA, respectively. This suggests that the emitting edges and vertices of a CNTFE have a more dramatic effect on the increase of emission current density than the emitter’s aspect ratio. As the next step toward optimizing CNT cold field emitters and achieving stable higher emission current densities, our group is investigating the effect the thickness between the inner and outer edges of hollow structures has on the emission current density for the tCPA and h2CPA CNTFEs. We believe that there exist minimum thicknesses that will maximize the electric field along the inner and outer edges so that the emission current density will be optimized while still sustaining the structural integrity of each structure. By exploring the effects of aspect ratio and structural geometries, coupled with field emission phenomena such as field screening and electric field enhancement along the edges and vertices of a structure we anticipate that CNTFEs will be able to deliver the high current required for many applications. References [1] Endo M, Muramatsu H, Hayashi T, Kim YA, Terrones M, Dresselhaus MS. Buckypaper from coaxial nanotubes. Nature 2005;433:476. [2] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56.
[3] de Heer WA, Chatelain A, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270:1179–80. [4] Zhu W, Bower C, Zhou O, Kochanski G, Jin S. Large current density from carbon nanotube field emitters. Appl Phys Lett 1999;75:873–5. [5] Choi HY, Chang WS, Kim HS, Park YH, Kim JU. Acquisition of X-ray images by using a CNT cold emitter. Phys Lett A 2006;357:36–41. [6] Teo KBK, Chhowalla M, Amartunga GAJ, Milne WI, Legagneux P, Pirio G, et al. Fabrication and electrical characteristics of carbon nanotube-based microcathodes for use in a parallel electron-beam lithography system. J Vac Sci Technol B 2003;21:693–8. [7] Kim YC, Yoo EH. Printed carbon nanotube field emitters for backlight applications. Jpn J Appl Phys, Part 2 2005;44 L:454–6. [8] Fan S, Chapline MG, Franklin NR, Tombler TW, Cassell AM, Dai H. Self-oriented regular arrays of carbon nanotubes and their field emission properties. Science 1999;283:512–5. [9] Teo KBK, Minoux E, Hudanski L, Peauger F, Schnell J-P, Gangloff L, et al. Carbon nanotubes as cold cathodes. Nature 2005;437:968. [10] Jeong HJ, Lim SC, Kim KS, Lee YH. Edge effect on the field emission properties from vertically aligned carbon nanotube arrays. Carbon 2004;42:3036–8. [11] Suh JS, Jeong KS, Lee JS, Han I. Study of the field-screening effect of highly ordered carbon nanotube arrays. Appl Phys Lett 2002;80:2392–4. [12] Edgecombe CJ, Valdre U. Microscopy and computational modeling to elucidate the enhancement factor for field electron emitters. J Microsc 2001;203: 188–94. [13] Silan JL, Niemann DL, Ribaya BP, Rahman M, Meyyappan M, Nguyen CV. Carbon nanotube pillar arrays for achieving high emission current densities. Appl Phys Lett 2009;95:133111–3. [14] Fujii S, Honda S, Machida H, Kawai H, Ishida K, Katayama M, et al. Efficient field emission from an individual aligned carbon nanotube bundle enhanced by edge effect. Appl Phys Lett 2007;90:153108–13. [15] Rao AM, Jacques D, Haddon RC, Zhu W, Bower C, Jin S. In situ-grown carbon nanotube array with excellent field emission characteristics. Appl Phys Lett 2000;76:3813–5. [16] Groning O, Kuttel OM, Emmenegger Ch, Groning P, Schlapbach L. Field emission properties of carbon nanotubes. J Vac Sci Technol B 2000;18: 665–78. [17] Forbes R, Deane J, Hamid N, San Sim H. Extraction of emission area from Fowler–Nordheim plots. J Vac Sci Technol B 2004;22:1222–6. [18] Forbes R. Field emission: new theory for the derivation of emission area from the Folwer–Nordhiem plot. J Vac Sci Technol B 1999;12:526–32. [19] Fowler RH, Nordheim L. Electron emission in intense electric fields. Proc Roy Soc Lond Ser A 1928;119:173–81. [20] Bonard J-M, Dean K, Coll B, Klinke C. Field emission of individual carbon nanotubes in the scanning electron microscope. Phys Rev Lett 2002;89: 197602–4. [21] Niemann D, Ribaya B, Gunther N, Rahman M, Leung J, Nguyen C. Effects of cathode structure on the field emission properties of individual multi-walled carbon nanotube emitters. Nanotechnology 2007;18:485702–9. [22] Seelaboyina R, Boddepalli S, Noh K, Jeon M, Choi W. Enhanced field emission from aligned multistage carbon nanotube emitter arrays. Nanotechnology 2008;19:065605–9.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Investigation of carbon nanotube field emitter geometry for increased current density Jeremy L. Silan a,b, Darrell L. Niemann a,b, Bryan P. Ribaya a,b, Mahmud Rahman b,*, M. Meyyappan a, Cattien V. Nguyen c a b c
NASA Ames Research Center, Moffett Field, CA 94035, United States Electron Devices Laboratory, Electrical Engineering Department, Santa Clara University, Santa Clara, CA 95053, United States ELORET Corporation, NASA Ames Research Park, Moffett Field, CA 94035, United States
a r t i c l e
i n f o
Article history: Available online 12 August 2010 The review of this paper was arranged by Prof. A. Zaslavsky Keywords: Carbon nanotube Field emission Edge effect Emitter array
a b s t r a c t In this work we present field emission characteristics of four geometrically distinct carbon nanotube pillar arrays. Each cathode has a unique geometric configuration with different structural parameters such as number of edges and vertices. We present experimental data demonstrating a carbon nanotube cold field emitter with an emitted current density of 31.8 mA/cm2 at an applied field of 11 V/lm. The performance of these cathodes can be directly attributed to the electric field being enhanced along the edges and vertices of the structures. We investigated this phenomenon experimentally by changing the geometry of the carbon nanotube pillar structure. We show that by increasing the number of edges and vertices of a structure to keep the electric field screening to a minimum, the emission current can be increased. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs), a great deal of research has been devoted to studying their extraordinary properties and applications [1,2]. The high aspect ratio structure of CNTs, their chemical stability, high thermal conductivity, and mechanical strength have led to a host of potential applications that have been proven in many laboratories [3]. These attributes make CNTs excellent cold field emitters, achieving longer lifetime and higher current densities at lower applied fields, which makes them highly attractive for use in field emission applications requiring high current densities [3,4]. However, up to this point, it has been very difficult to fabricate stable and robust CNT cold field emitters while at the same time achieving economical manufacturing. Furthermore, the high current density required for certain applications such as traveling wave tube amplifiers remain a big challenge. Currently, a handful of applications requiring relatively low current density employing CNT emitters have been demonstrated in the laboratory, such as X-ray sources for biomedical imaging, e-beam lithography, lighting, microwave sources, as well as field emission displays (FEDs) [5–9]. Despite the progress made in recent years on understanding the fundamentals of CNT emitters and on how to implement CNTs in applications requiring electron sources, wide scale adoption re* Corresponding author. Tel.: +1 408 554 4175. E-mail address: [email protected] (M. Rahman). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.07.004
mains elusive. There are still many factors that contribute to the CNT emission process which are not fully understood, particularly with respect to achieving optimal emission current and emitter stability. Phenomena such as field screening, local field enhancement and the edge effect have been explored in the last few years [10–12]. For many applications where CNT emitters can be implemented several of the above mentioned phenomena have not been fully explored. In this paper, we explore the structural effects of CNT emitter arrays on the field emission properties of the CNT cathodes. Previously we have demonstrated that CNT pillar arrays (CPAs) offer a practical solution for the fabrication of stable and easy to manufacture CNT cathodes. A CNT pillar is described as a uniform, highly dense, vertically aligned, compact bundle of CNTs [13]. Vertical self-alignment of CNTs results from the van der Waals interaction between neighboring CNTs and contributes to the excellent structural stability of CNT pillars [14]. These CNT pillar structures exhibit lower turn-on fields and a higher current density when compared to that of continuous CNT films. The geometry of each pillar was specifically chosen to investigate the effect of the cathode geometry on the electric field enhancement at the edges of the structure. This enhancement effect is called the edge effect and can be described as a phenomenon where the electric field is enhanced along the edges of a CNT pillar structure. By varying the geometry of the pillar structures, we have a means to increase emission current density. It should be pointed out that CPAs were invented to overcome fabrication challenges
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J.L. Silan et al. / Solid-State Electronics 54 (2010) 1543–1548
that have been encountered with large scale manufacturing of CNT emitters. Previously a pillar and its geometric counterpart, a toroid, were modeled using finite element simulations to investigate the behavior of the electric field in close proximity to the structures [14]. Through these simulations it was validated that the electric field was enhanced along the inner and outer edges as opposed to just the outer edge for the toroid and pillar structure, respectively. 2. Experimental set-up Our CNT field emitter (CNTFE) fabrication begins with a 1 cm 1 cm Si substrate. These substrates were then sputtered with a 10 nm thick Al diffusion barrier and a 4 nm thick Fe catalytic layer. The cathodes were then patterned with a conventional photolithographic process using Clariant AZ 5412E photoresist and a Quintel Q-4000 series mask aligner. The patterned substrates were subsequently sputtered with a 10 nm Cr layer and 15 nm Mo layer to selectively prevent growth in the Cr/Mo-coated areas. CNT growth was carried out via thermal CVD at 750 °C with process gas flow rates of 200 SCCM of H2 and 800 SCCM of C2H4. The furnace ramp up and ramp down were both carried out with gas flow rates of 200 SCCM of H2 and 800 SCCM of Ar. Field emission (FE) data were collected using a diode configuration at base pressure of 107 Torr. The gold anode was polished and kept parallel to the cathode surface. The anode to cathode separation gap was precisely controlled using a Huntington Labs L-2111 linear positioner. For all experiments presented here the gap was fixed at a distance, d, equal to 100 lm, measured from the top of the CNT array to the polished gold anode surface. Data were col-
lected using a Keithley 237 SMU controlled by a custom LabVIEW program communicating over the General Purpose Interface Bus (GPIB). Here, the turn-on field refers to the macroscopic field necessary to draw 1 nA of current from each sample [15,16]. Current–voltage (I–V) data were collected only after preconditioning the cathodes by sweeping them from 0 V to three times their initial turn-on voltage for five sweeps. The cathodes were then swept to the maximum supply voltage of 1.1 kV for five more sweeps. The final reported values were the last of these five sweeps. Following the I–V characterization, cathode stability was studied by investigating the I–V hysteresis while the cathodes were being swept up then back down in voltage 0 V–1.1 kV and 1.1 kV–0 V, respectively, for an additional five times. The FE data were analyzed using the standard Fowler– Nordheim (FN) equation given in the following equation.
" # lbU3=2 I ¼ kAaU F exp F 1 2
ð1Þ
Here, I is the emitted current in amperes, a = 1.5413434 106 A eV V2 and b = 8.830888 109 eV3/2 V m1 are the universal FN constants, k and l are generalized correction factors, A is the emission area in cm2, U is the local work function (approximated to be fixed at 5 eV), and F is the field at the CNT tip [17–19]. For the modeling here, F t ¼ bV, where b and V are the geometric field enhancement factor of the array with units of m1 and the applied voltage, respectively [12,19,20]. The geometric field enhancement factor for the CNTFE was extracted from the Fowler–Nordheim plot, ln(I/V2) versus 1/V, by taking the slope at the start of the linear region to the first knee of the curve. The linear region of the plot
(a)
(b)
(c)
(d)
Fig. 1. SEM images at a tilt angle view of 45° of the (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA carbon nanotube arrays showing highly dense, uniform growth.
J.L. Silan et al. / Solid-State Electronics 54 (2010) 1543–1548
exhibits Fowler–Nordheim behavior which signifies field emission. After calculating the geometric field enhancement factor, b, the field enhancement factor c can be determined from the relation c ¼ bV=F m , where F m ¼ V=d [12]. In order to take into account field screening effects and geometrical spacing, the current density was calculated by dividing the total emitted current by the entire cathode area, including the Cr/Mo-coated region where there is no CNT growth, which was 0.0314 cm2 for each cathode.
Fig. 2. High resolution SEM image showing the typical CNT diameter distribution present when viewed from the tops of the CNTFEs.
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3. Results and discussion Four different array geometries consisting of a CNT pillar array (CPA), toroid pillar array (tCPA), hexagon pillar array (hCPA) and hollow hexagon pillar array (h2CPA) were fabricated. The pillar had a diameter of 30 lm, the toroid had an outer diameter of 30 lm and an inner diameter of 10 lm, the hexagon had side lengths of 15.7 lm and the hollow hexagon had side lengths of 15.7 lm and an inner diameter of 10 lm. The perimeter of each individual pillar structure, whether it is a circle or a hexagon, measured 94.2 lm. For these cathodes the CVD process was carried out for 60 s resulting in high density uniform growth of multi-walled carbon nanotubes in the predefined shapes. SEM images showing typical areas of the different emitter geometries with a pitch of 15 lm between structures are shown in Fig. 1. In order to understand the diameter distribution of the CNTs which comprise the FE structure we collected high magnification SEM images as shown in Fig. 2. Investigation of this data reveals CNT diameters in the range from 10–20 nm. Although the growth time was kept constant for each structure, there were variations in heights between CNTFEs. The difference in heights between CNTs of these emitters can be attributed to the variability of the CNT growth process due to surface contamination. As seen in Fig. 3a and c, the CPA and tCPA were approximately 20–25 lm in height, respectively, whereas in Fig. 3b and d, the hCPA and h2CPA were approximately 5–10 lm in height, respectively. This difference in heights proves useful when comparing FE data for the CNTFEs. Since the heights of the CNTs for the CPA
(a)
(b)
(c)
(d)
Fig. 3. SEM (2500) images at a tilt angle view of 45°: (a) CPA, (b) tCPA, (c) hCPA, and (d) h2CPA.
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and tCPA are greater than the heights of the hCPA and h2CPA, based on their higher aspect ratios one would expect higher emission current due to an increase in field enhancement along the edges of the structures. However, according to our FE data shown in Fig. 4 the opposite is true. Although the CPA and tCPA exhibit lower turn-on fields, the hCPA and h2CPA ultimately achieve higher current density at large applied fields. As seen in Fig. 4, the CPA and tCPA have an emitting current of 0.62 and 0.75 mA at 11 V/lm, and turn-on fields of 1.42 and 1.39 V/lm, respectively. The hCPA and h2CPA, despite having smaller aspect ratios, generate more emission current than their geometric counterparts. The hCPA and h2CPA have an emission current of 0.68 and 0.98 mA at 11 V/ lm, though their turn-on fields of 1.80 and 1.42 V/lm, respectively, are higher. Initial and post-conditioning field enhancement factors, c, as well as turn-on voltages were extracted from the slope of the FN plot, before the first knee, as show in Fig. 5. The CPA, tCPA, hCPA
Current (mA)
1 0.9
CPA
0.8
tCPA hCPA
0.7
hhCPA
0.6 0.5 0.4 0.3 0.2 0.1 0 0
200
400
600
800
1000
Volts (V) Fig. 4. Experimental I–V curves for the CPA, tCPA, hCPA, and h2CPA emitters during the field emission process.
-20 CPA tCPA hCPA hhCPA
LN(I/V2)
-22 -24 -26 -28 -30 -32 0.0009
0.0011
0.0013
0.0015
0.0017
0.0019
0.0021
1/V Fig. 5. Fowler–Nordheim plots of the I–V characteristics of the different geometric CNTFEs exhibiting Fowler–Nordheim field emission.
and h2CPA had post-conditioning c and turn-on fields of 1042, 1108, 676, 829 and 1.42, 1.39, 1.80, 1.42 V/lm, respectively. These high field enhancement factors for the CNTFEs can be directly attributed to a multistage enhancement effect coming from the field enhancement factor of the pillar geometry and the field enhancement factor of the individual CNTs in the pillar [21,22]. The higher c values for the CPA and tCPA as compared to the hCPA and h2CPA can be attributed to their higher aspect ratios as shown in the SEM images in Fig. 3. When comparing the FE characteristics of the solid geometries and hollow geometries, the CPA and tCPA have a lower turn-on field than the hCPA and h2CPA, respectively. The lower turn-on field demonstrated by the CPA and tCPA also can be attributed to these structures having a higher aspect ratio. Although the CPA and tCPA had a higher aspect ratio and lower turn-on voltage than the hCPA and h2CPA, the hCPA and h2CPA still have a higher emitting current at the same applied field of 11.0 V/lm than their geometric counterparts. This increase in current density for the hCPA and h2CPA can be attributed to an increase in the number of sharp vertices. Note that when comparing a CPA to an hCPA, the hCPA not only takes advantage of the edge effect, but also has six sharp vertices which also further contribute to electric field enhancement. The h2CPA, like its geometric counterpart the tCPA, is hollow and therefore has two instances of the edge effect: the outer edge and the inner edge. The h2CPA outperforms the other cathodes in terms of current density because it also features the six sharp field enhancing vertices. This suggests that the increase in emitting current from the hCPA and h2CPA emitters can be directly attributed to the electric field enhancement at the sharp vertices of these two emitter geometries. A comparison of the field enhancement factor c and turn-on fields for all the CNTFE geometries are shown in Table 1. It should be pointed out that the initially high field enhancement factors and resulting low turn-on voltages seen in Table 1 for each CNTFE, can be attributed to those CNTs which are taller than the rest of the CNTs in the pillar structure. As the CNTFEs go through a conditioning process local emission hot spots will burn off thus lowering the CNTFE’s field enhancement factors and increasing their turn-on voltages. This is evident when comparing the initial to post field enhancement factors and initial to post turn-on fields for each CNTFE. In order to study the structural stability of each of the CNT cold field emitters the I–V hysteresis was examined. This hysteresis results from those individual CNTs which are taller than the majority of the CNTs in the pillar burning off during field emission. Here the cathodes were swept up and then down in voltage from 0 V to 1.1 kV and 1.1 kV to 0 V, respectively for a total of five sweeps. Comparing the different CNT array geometries, hysteresis observed in the initial voltage sweep was greatest for the hCPA and the least for the h2CPA as seen in Fig. 6b and d, respectively. Fig. 6a and c shows that both the CPA a tCPA have little hysteresis. During subsequent sweeps, the hysteresis increased for the hCPA, but remained small for the CPA, tCPA and h2CPA emitters. After the fifth sweep, the final measured emitted current densities for the CPA, hCPA, tCPA and h2CPA were 19.4, 20.7, 22.2, and 31.8 mA/ cm2, respectively, at an applied electric field of 11 V/lm. It should be noted that although there is I–V hysteresis for the four different
Table 1 Summary of field emission results for geometrically different CNTFEs. Emitter array
Growth time (s)
Array heights (lm)
Initial c
Initial turn-on field (V/lm)
Post-conditioning c
Post-conditioning turn-on field (V/lm)
CPA tCPA hCPA h2CPA
60 60 60 60
20–25 20–25 5–10 5–10
4219 3260 4381 940
0.95 0.77 1.07 1.31
1042 1108 676 829
1.42 1.39 1.80 1.42
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(a)
(b)
CPA
0.7
Initial Voltage Sweep (Blue)
0.6 0.5
Current (mA)
Current (mA)
Final Voltage Sweep (Red)
0.4 0.3 0.2 0.1
hCPA
0.7 0.6
Initial Voltage Sweep (Blue)
0.5
Final Voltage Sweep (Red)
0.4 0.3 0.2 0.1
0.0 0
200
400
600
800
0.0
1000
0
200
Voltage (V) tCPA
0.80
(d)
0.70
Initial Voltage Sweep (Blue)
0.60
Final Voltage Sweep (Red)
Current (mA)
Current (mA)
(c)
0.50 0.40 0.30 0.20 0.10 0.00 0
200
400
600
400
600
800
1000
Voltage (V)
800
1000
h2CPA
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Initial Voltage Sweep (Blue) Final Voltage Sweep (Red)
0
Voltage (V)
200
400
600
800
Voltage (V)
Fig. 6. I–V curves comparing the stability of the (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA after conditioning.
(a)
(b)
(c)
(d)
Fig. 7. Post field emission SEM (1000) images showing the top view of the: (a) CPA, (b) hCPA, (c) tCPA, and (d) h2CPA.
1000
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CNTFE geometries, there was little or no large scale damage to the CNT arrays during field emission as shown in Fig. 7. 4. Conclusion In conclusion, we compared the field emission characteristics of four geometrically different CNT arrays, the CPA, tCPA, hCPA, and h2CPA. We have demonstrated that an increase in emission current density can be achieved by increasing the number of emitting edges and vertices for a given CNTFE. Our experiments support this finding because the h2CPA structure, which had the greatest number of emitting edges and vertices, achieved the highest emitted current density of 32 mA/cm2 at an applied field of 11 V/lm. We also show that the high aspect ratio CPA and tCPA structures had lower maximum emission currents when compared to the low aspect ratio hCPA and h2CPA structures. At the same applied field of 11 V/lm, the CPA and tCPA achieved currents of 0.62 and 0.75 mA while the hCPA and h2CPA reached 0.68 and 0.98 mA, respectively. This suggests that the emitting edges and vertices of a CNTFE have a more dramatic effect on the increase of emission current density than the emitter’s aspect ratio. As the next step toward optimizing CNT cold field emitters and achieving stable higher emission current densities, our group is investigating the effect the thickness between the inner and outer edges of hollow structures has on the emission current density for the tCPA and h2CPA CNTFEs. We believe that there exist minimum thicknesses that will maximize the electric field along the inner and outer edges so that the emission current density will be optimized while still sustaining the structural integrity of each structure. By exploring the effects of aspect ratio and structural geometries, coupled with field emission phenomena such as field screening and electric field enhancement along the edges and vertices of a structure we anticipate that CNTFEs will be able to deliver the high current required for many applications. References [1] Endo M, Muramatsu H, Hayashi T, Kim YA, Terrones M, Dresselhaus MS. Buckypaper from coaxial nanotubes. Nature 2005;433:476. [2] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56.
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