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Field emission triode amplifier utilizing aligned carbon nanotubes
Diamond & Related Materials 14 (2005) 2069 – 2073 www.elsevier.com/locate/diamond
Field emission triode amplifier utilizing aligned carbon nanotubes Y.M. Wong a, W.P. Kang a,*, J.L. Davidson a, B.K. Choi a, W. Hofmeister b, J.H. Huang c a
c
Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA b Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235, USA Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Available online 26 October 2005
Abstract A vacuum field emission transistor utilizing aligned carbon nanotubes (CNTs) to form a triode configured as a common emitter amplifier is investigated. DC and AC performances of this triode amplifier are reported. The vertically aligned CNT emitters for the microtriodes with a convex surface profile were selectively synthesized utilizing microwave plasma chemical vapor deposition (MPCVD) with Ni or Co as catalysts. The single-mask micro-fabrication process achieved a CNT microtriode array (array size: 34 84, element dimension: 10 Am 10 Am square, element spacing: 20 Am) with self-aligned gate. Transistor curves for the CNT triode with anode current (I a) as a function of anode voltage (Va) for different gate voltages (V g) was measured, followed by AC characterization. The triode amplifier demonstrated gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of operation. A large DC gain or amplification factor of ¨350, transconductance of ¨2 AS, and a computed anode resistance of 182 MV are obtained. A large anode current of ¨3.5 AA or current density of ¨1.2 mA/cm2 was achieved at V g = 46 V and Va = 300 V. Saturation of the anode currents at Va > 80 V was also observed. The AC performance of the CNT triode amplifier was characterized by input voltage (v in) vs. output voltage (v out). The voltage gain of the triode amplifier, Av, is given by the ratio of v out/ v in. The estimated Av is calculated to be ¨3.13 with a phase shift of ¨180-, as expected. A larger Av could be attained if larger R L is applied. Preliminary frequency response of the triode amplifier is presented. The results obtained thus far demonstrate that the CNT triode amplifier can be a promising amplifier candidate. D 2005 Elsevier B.V. All rights reserved. Keywords: MPCVD; Carbon nanotubes; Field emission; Electrical properties characterization
1. Introduction Since the first field emission characterization of CNTs in 1995 [1,2], CNTs have been widely studied for various applications, including vacuum microelectronics. The high aspect ratio (often exceeding 1000) and good thermal and chemical stability make CNTs an outstanding contender for electron field emitters. Some of the potential applications of CNTs include field emission displays [3], tips for scanning microprobe [4], gas sensing elements [5], vacuum microelectronics [6], microwave power devices [6], and electron source for electron-beam lithography [6]. A field emission triode amplifier in microelectronic form is a three-terminal device capitalizing on microfabrication * Corresponding author. Vanderbilt University, VU Station B 351661, Nashville, TN 37235-1661, USA. Tel.: +1 615 322 0952; fax: +1 615 343 6614. E-mail address: [email protected] (W.P. Kang). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.025
techniques to bring the controlling gate in close proximity to the electron emitting cathode. These microfabricated vacuum triodes can be operated at low turn-on voltage, generally less than 50 V [7 –9]. As a result, the complexity of the peripheral electronics can be reduced significantly with better control of thermal and power management. When an AC small-signal is imposed on the input gate voltage and a load is connected to the output of the field emission triode, an AC voltage gain can be obtained. A triode amplifier based on field emission technology is particularly attractive for electronic applications that require radiation-hardness and temperature-immune capability such as space, aviation and telecommunication applications. Previously, we have successfully fabricated a CNT triode by thermal CVD in a furnace tube at atmospheric pressure [8]. The single-mask micro-fabrication process achieved a 3-band (25 Am 100 Am) CNT triode with self-aligned gate. The device demonstrated basic gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of
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operation. Since the CNTs grown were randomly oriented, an over-etched gate structure was adopted in order to avoid cathode-gate shorting problem. As a result, the large cathodegate spacing of ¨12 Am led to high turn-on voltage with reasonable triode characteristics (DC gain, l = 10 at I a = 1.0 AA, and transconductance, g m = 47 nS at Va = 198 V) when configured as a triode amplifier. In this work, vertically aligned CNT gated microcathodes were synthesized selectively utilizing microwave plasma chemical vapor deposition (MPCVD). Moreover, plasma pretreatment of the catalysts prior to the CNT synthesis was successfully employed to obtain a convex-shaped surface profile, which is important in prevention of cathode-gate ‘‘shorting’’. The DC and AC characteristics of the aligned CNTs field emission triode, configured in a common emitter amplifier were investigated. To the best of our knowledge, this is the first time AC small-signal performance of a CNT triode amplifier is being reported. 2. Experimental method In this work, a gated CNT field emitter array or triode device was fabricated utilizing microfabrication processes. A single-mask process achieved an array of CNT microcathodes with self-aligned gate (array size: 34 84, elemental array dimension: 10 Am 10 Am square, elemental array spacing: 20 Am). As illustrated in the fabrication process flow (Fig. 1), the process began with thermal oxidation of a highly doped n-type silicon (100) substrate followed by low-pressure CVD deposition of polysilicon as the gate electrode. The polysilicon was subsequently doped through a diffusion process utilizing spinon-diffusion source and high-temperature annealing in order to obtain good conductivity of the gate. The thickness of the thermal oxide and the polysilicon gate layers was ¨1.5 Am and 0.8 Am, respectively. After lithography patterning, the polysilicon gate was dryetched by a gas mixture of sulfur hexafluoride (SF6) and oxygen (O2) at 150 mTorr in a reactive-ion-etch system and the
Thermal oxidation and LPCVD of poly-Si gate
Catalyst deposition & PR liftoff
SiO 2
SiO 2
Patterning & RIE of poly-Si
MPCVD growth of aligned CNTs
SiO 2
SiO 2
Poly-Si Photoresist
SiO 2
SiO 2
n+-Si SiO2
SiO 2
Fig. 1. Schematic diagrams of the single-mask fabrication process for the aligned CNT field emission triode amplifier.
thermal oxide was isotropically wet-etched with bufferedoxide-etch (BOE) to obtain an oxide undercut structure, as illustrated in Fig. 1. Next, a thin layer of titanium (Ti), ¨20 nm, acting as the diffusion barrier layer, and nickel (Ni), ¨5– 10 nm (growth catalyst), were sputter-deposited in sequence utilizing a DC magnetron sputtering system. The sputter-deposition was performed at 7 mTorr of argon (Ar). Following that, the unwanted catalyst above the gate layer was removed with an acetone lift-off technique. Last, the mold was transferred to the MPCVD chamber (AsTex) for selective growth of CNTs inside the microtriode mold. Critically, a H2 plasma pretreatment (400 W microwave power at 400 -C) was performed in order to obtain the desired convex shape of the CNT cathode. The CNTs were synthesized at a working pressure of 20 Torr, substrate temperature of 650 -C, and microwave power of 1 kW. The flow ratio of the gas mixtures of methane (CH4) and H2 was maintained at 1:8 with a total flowrate of 135 sccm by massflow-controllers during CNT synthesis. The growth time of the CNTs ranges from 60 to 120 s, which is crucial in obtaining optimal height of the vertically aligned CNT cathodes. After that, the CNT triode sample was cooled to room temperature in H2 ambient and ready for subsequent device characterization. 3. Device characterization The CNT triode was first tested in a common emitter amplifier configuration for DC field emission characteristics. A schematic diagram of the DC large-signal test setup is shown in Fig. 2(a). The purpose of the DC characterization was to obtain optimum operating conditions, i.e. the corresponding gate and anode biasing voltages in saturation region for subsequent AC mode of operation. The measurements were performed at room temperature in a vacuum chamber evacuated to a base pressure of 106 Torr. A computerized data acquisition system equipped with Labview program (National Instruments) was employed for the measurements of the anode emission current, I a as a function of the anode voltages (Va) for different gate voltages (V g). A fixed voltage of 400 V was first applied to the anode to attract electrons induced by the positive gate voltage (V g). The anode emission current (I a) was then measured as a function of the effective anode voltages (Va) while holding the gate voltage constant. The measurements were then repeated for different gate voltages to obtain a family of I a – Va curves. Next, the AC performance of the CNT triode was characterized by superimposing an AC signal (sinusoidal) input voltage, v in on top of the applied DC gate voltage utilizing a function generator, as illustrated by the schematic diagram of the AC test circuit in Fig. 2(b). The corresponding AC output voltage, v out was measured across a load resistor, R L. Both input and output signals were then measured simultaneously with a dual-channel digital oscilloscope. The AC voltage gain, Av of the CNT vacuum amplifier is characterized by the ratio of v out to v in at a chosen operating frequency. Last, the AC frequency response of the CNT
Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
Ia
(a)
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Ra
Anode/ Collector
Rg SiO2
SiO2
Vg
Va
Vacuum 10-6 Torr
Ia
(b)
Vacuum 10-6 Torr
C +
Ra
Rg SiO2
RL
SiO2
vout
Va
Vg
Fig. 3. SEM micrographs of (a) a single 10 Am 10 Am aligned CNT microtriode and (b) cross-sectional view of the same.
_
CH1 CH2 Digital Oscilloscope Fig. 2. Schematic diagrams of the: (a) DC large-signal, and (b) AC small-signal test circuits for the aligned CNTs field emission triode amplifier.
vacuum triode was further investigated by varying the frequency of the AC signal input voltage. 4. Results and discussion The fabricated CNT field emitter array was examined with a Hitachi 4200 scanning electron microscope (SEM). The singlemask micro-fabrication process achieved an array of 10 um 10 Am square CNT gated microcathodes. The SEM micrographs in Fig. 3 show the array geometry and cross-sectional view of a single gated CNT microtriode. The pre-growth H2 plasma pretreatment of the sputtered catalysts was utilized successfully to achieve the gated CNT cathode with a convex-shaped surface profile, i.e. shorter nanotubes on the edges, as shown in Fig. 3(b). As mentioned in [10], this convex profile is important in preventing cathode-gate ‘‘shorting’’ without resorting to more complicated fabrication processes such as sidewall protector [11,12] or gate over-etching method [8,13] that result in higher turn-on voltages. The cathode-gate spacing was ¨2.0 Am, as estimated from the edge of the gate to the edge of the CNT cathode. In this study, a more uniform emission from the 10 Am 10 Am CNT emitters was made possible with a convex-shaped cathode surface profile. Simulation of electric field distribution on triode using CNTs as emitters [14,15] has demonstrated a
1.8
(a)
Vg = 45 V
1.6 1.4 1.2
Ia (µA)
vin
more quasi-uniform electric field, i.e. less field screening effect on CNTs with variable heights having shorter nanotubes on the circumferential region, which is similar to the CNT emitters with convex-shaped cathode surface profile than those with uniform heights. When tested in a common emitter configuration for DC characteristics, the CNT triode demonstrated good gatecontrolled modulation of the emission current with distinct cutoff, linear and saturation regions of operation, as displayed in Fig. 4. The microfabricated CNT triode amplifier showed a low gate turn-on voltage of ¨33 V. From the estimated cathode-gate spacing (¨2Am), the gate turn-on field of the device is ¨16.5 V/Am. In DC mode of operation, three important transistor parameters [8,16 –20] characterizing
Vg = 44 V
1.0 7 MΩ Loadline
100 M Ω Loadline
0.8 0.6
Vg = 43 V
0.4 0.2
Vg = 39 V Vg = 36 V
0.0 0
100
200
300
400
500
Va (V) Fig. 4. Plot of the measured anode currents, I a vs. the anode voltage, Va of the aligned CNTs field emission triode amplifier for different applied gate voltage, V g. Included are the 7 MV and 100 MV loadlines.
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the performance of a triode amplifier are DC gain or BVa amplification factor (l ¼ BV j ), transconductance g Ia ¼const BIa a (gm ¼ BVg jVa ¼const ) and anode resistance (ra ¼ BV BIa jVg ¼const ). In AC signal amplification, l and g m directly relates to the AC voltage gain, Av and cutoff frequency ( f T) of a triode amplifier. Also note that the anode resistance is related to the transconductance and the DC gain of the triode amplifier at an operating point by this expression, r a = l/g m. The device characteristic of field emission triode is found to obey Fowler – Nordheim theory [8,16 – 20] and not to follow Langmuir’s law [21,22]. In other words, the space-charge effect is ignored because in a field emission triode amplifier, the device generally operates in saturation region (do not confuse with thermionic triode amplifier that usually operates in spacecharge limited mode), where emitted electrons are mainly extracted by gate (V g) and the anode (Va) acted primarily as an electron collector. In addition, the proximity of the gate electrode to the cathode (on the order of a few Am) has effective field shielding from the distant anode (on the order of a few mm) [20]. In this case, the local electric field on the surface of the cathode, E can be expressed as [8,16 –20]: Vg þ Va =l E¼b : ð1Þ d where b and d are the field enhancement factor and cathodegate spacing, respectively. Since l is generally very large (usually greater than 100, and ¨352 in this case) for a good field emission amplifier, the second term in Eq. (1) has little influence on E. Hence, E å bV g/d. In general, deviations from FN theory could only occur in high field and high current density regions [23]. The DC transistor parameters were determined graphically from the DC characteristics curves in Fig. 4. A large DC gain of ¨352 at I a = 1.7 AA, and a transconductance g m ¨ 1.93 AS at Va = 400 V were obtained. The corresponding anode resistance was computed to be 182 MV. In addition, a large anode current of ¨3.5 AA or a current density of ¨1.2 mA/cm2 was also observed at V g = 46 V and Va = 300 V. Saturation of the anode currents at Va > 80 V was also observed. These DC transistor characteristics are substantially improved as compared to our previously reported results [8], where randomly oriented CNTs were grown by thermal CVD using an over-etched gate triode structure. The improvement in the triode characteristics is attributed to the vertically aligned CNTs with a convex-shaped surface profile, smaller cathode-gate spacing (reduced from 12 Am to 2 Am) and larger array size (increased from 3 bands of 25 Am 100 Am to an array of 2856 of 10 um 10 Am squares). The AC performance of the CNT triode amplifier was characterized by the AC signal input voltage (v in) vs. AC output voltage (v out) plots. The AC voltage gain of the triode amplifier, Av, is then given by the ratio of v out/v in. Fig. 5(a) shows the AC plots of the CNT triode amplifier operated at a frequency of 100 Hz with a 1 V peak-to-peak sinusoidal input signal, as recorded by the digital oscilloscope. The corresponding biasing conditions, as predetermined from the DC characteristics curve, are V g = 45 V, Va = 400 V, and I a = 1.7 AA. The anode (R a) and the load resistors (R L) adopted were both 7 MV.
(a)
vin = 1.09 V
vout = 3.44 V
f = 100 Hz, Av = 3.13
(b) vin = 0.94 V
vout = 2.09 V
f = 500 Hz, Av = 2.22
(c)
vin = 0.77 V
vout = 1.45 V
f = 1 kHz, Av = 1.89 Fig. 5. Plots of the ac small-signal input voltage (v in) vs. AC output voltage (v out) plots of the aligned CNTs field emission triode amplifier at an operating input frequency of (a) 100 Hz, (b) 500 Hz, and (c) 1 kHz.
From the AC plot (i.e., oscilloscope output screen shown in Fig. 5(a)), the estimated Av is measured to be ¨3.13 or 9.91 dB. As expected from a triode amplifier in a common-emitter configuration, the output signal is 179- out-of-phase with the input signal, close to the theoretical value of 180-. The small AC voltage gain in this case is not unanticipated, judging from the 7-MV loadline, as shown in Fig. 4. This load resistor is relatively small compared to the computed anode resistance, resulting in an almost vertical loadline which is detrimental for high Av output. A larger Av, however, is comfortably attainable by adopting a larger R L as evidenced by the 100 MV loadline in Fig. 4. In this case, the AC voltage gain is expected to improve to ¨30 or 29.54 dB. The AC frequency response of the CNT vacuum amplifier was further investigated at 500 Hz and 1 kHz. The corresponding Av was found to be lower, 2.22 (6.93 dB) and 1.89 (5.53 dB), respectively, as exhibited in Fig. 5(b) and (c). It is observed that the phase shift also reduced to 143- and 107-, respectively. Since this CNT triode amplifier configuration is not optimized for high frequency operation, this behavior is predictable. In fact, with a measured cathode-gate capacitance (C g) of ¨1 nF and calculated g m ¨ 1.93 AS, the theoretical
Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
cutoff frequency as given by the following expression, f T = g m/ (2pC g), [19,24] was computed to be only ¨300 Hz. The reduction in the phase shift at higher frequency needs further investigation. However, the DC characteristic curves (Fig. 4) and the measured AC voltage gain at 100 Hz (Fig. 5(a)) are concrete proofs that voltage amplification of the device has been demonstrated in this study. In order to increase the cutoff frequency of the triode amplifier, low cathode-gate capacitance in the order of pF is necessary. Lower C g is attainable by increasing the thickness of the cathode-gate dielectric or minimizing the gate area. Also, reduction in gate aperture and increase in array size and packing density will also result in higher g m and lower C g, and hence higher cutoff frequency could be attained. 5. Conclusions We have successfully fabricated and characterized a CNT field emission triode amplifier in both DC and AC modes of operation. The single-mask microfabrication process achieved a field emitter array of 10 Am 10 Am CNT microtriode with selfaligned gate. From the DC characteristic curves, the microfabricated CNT triode amplifier showed a low gate turn-on voltage of ¨33 V, a large DC gain of ¨352 at I a = 1.7 AA, and a transconductance of ¨1.93 AS at Va = 400 V. The CNT field emission triode device demonstrated excellent gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of operation. AC amplifier function of the triode was successfully performed by biasing the gate and anode voltages within the saturation regions. An AC voltage gain of ¨3.13 or 9.91 dB was obtained for a load resistor of 7 MV with a sinusoidal 1.0-V peak-to-peak input signal, operating at I a = 1.7 AA, V g = 45 V and Va = 400 V. Frequency response of the triode amplifier up to 1 kHz was also demonstrated. Overall, our studies suggest that future device optimization to achieve high power, high gain and high frequency operation of CNT field emission triode amplifier is feasible. References [1] A.G. Rinzler, J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kim, D. Tomanek, P. Nordlander, D.T. Colbert, R.E. Smalley, Science 269 (1995) 1550.
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[2] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [3] W.B. Choi, D.S. Chung, J.H. Kang, H.Y. Kim, Y.W. Jin, I.T. Han, Y.H. Lee, J.E. Jung, N.S. Lee, G.S. Park, J.M. Kim, Appl. Phys. Lett. 75 (1999) 3129. [4] J. Hafner, C. Cheung, C. Lieber, Nature 398 (1999) 761. [5] Y.M. Wong, W.P. Kang, J.L. Davidson, A. Wisitsora-at, K.L. Soh, Sens. Actuators, B 93 (2003) 326. [6] J. Robertson, W.I. Milne, K.B.K. Teo, M. Chhowalla, XVI International Winterschool on Electronic Properties of Novel Materials, 2 – 9 March, Kirchberg, Austria, 2002, p. 537. [7] Y.M. Wong, W.P. Kang, J.L. Davidson, W. Hofmeister, S. Wei, J.H. Huang, Diamond Relat. Mater. 14 (2005) 697. [8] Y.M. Wong, W.P. Kang, J.L. Davidson, W. Hofmeister, S. Wei, J.H. Huang, J. Vac. Sci. Technol., B 23 (868) (2005). [9] W.P. Kang, J.L. Davidson, A. Wisitsora-at, Y.M. Wong, R. Takalkar, K. Subramania, D.V. Kerns, W.H. Hofmeister, Diamond Relat. Mater. 14 (2005) 685. [10] Y.M. Wong, W.P. Kang, J.L. Davidson, B.K. Choi, W. Hofmeister, J.H. Huang, Abstract of the 8th International Conference on Applications of Diamond and Related Materials/1st NanoCarbon Joint Conference, Argonne, IL, USA, May 15 – 19, 2005. [11] Y. Huh, J.Y. Lee, J.H. Lee, T.J. Lee, S.C. Lyu, C.J. Lee, Chem. Phys. Lett. 375 (2003) 388. [12] D.S.Y. Hsu, Appl. Phys. Lett. 80 (2002) 2988. [13] K.J. Chen, W.K. Hong, C.P. Lin, K.H. Chen, L.C. Chen, H.C. Cheng, IEEE Electron Device Lett. 22 (2001) 516. [14] D. Nicolaescu, L.D. Filip, S. Kanemaru, J. Itoh, J. Vac. Sci. Technol., B 21 (2003) 366. [15] D. Nicolaescu, L.D. Filip, S. Kanemaru, J. Itoh, Jpn. J. Appl. Phys. 43 (2004) 485. [16] A. Wisitsora-at, W.P. Kang, J.L. Davidson, D.V. Kerns, T. Fisher, J. Vac. Sci. Technol., B 21 (2003) 614. [17] A. Wisitsora-at, W.P. Kang, J.L. Davidson, C. Li, D.V. Kerns, M. Howell, J. Vac. Sci. Technol., B 21 (2003) 1665. [18] W.J. Orvis, C.F. McConaghy, D.R. Ciarlo, J.H. Yee, E.W. Hee, IEEE Trans. Electron Devices 36 (1989) 2651. [19] R.E. Neidert, P.M. Phillips, S.T. Smith, C.A. Spindt, IEEE Trans. Electron Devices 38 (1991) 661. [20] H.H. Busta, J.E. Pogemiller, B.J. Zimmerman, IEEE Trans. Electron Devices 40 (1993) 1537. [21] C.D. Child, Phys. Rev. 32 (1911) 492. [22] I. Langmuir, Phys. Rev. 2 (1913) 450. [23] G. Fursey, Field Emission in Vacuum Microelectronics, Kluwer Academic/ Plenum Publishers, New York, 2005. [24] C. Bower, D. Shalom, W. Zhu, D. Lopez, G.P. Kochanski, P.L. Gammel, S. Jin, IEEE Trans. Electron Devices 49 (2002) 1478.
Field emission triode amplifier utilizing aligned carbon nanotubes Y.M. Wong a, W.P. Kang a,*, J.L. Davidson a, B.K. Choi a, W. Hofmeister b, J.H. Huang c a
c
Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA b Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235, USA Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Available online 26 October 2005
Abstract A vacuum field emission transistor utilizing aligned carbon nanotubes (CNTs) to form a triode configured as a common emitter amplifier is investigated. DC and AC performances of this triode amplifier are reported. The vertically aligned CNT emitters for the microtriodes with a convex surface profile were selectively synthesized utilizing microwave plasma chemical vapor deposition (MPCVD) with Ni or Co as catalysts. The single-mask micro-fabrication process achieved a CNT microtriode array (array size: 34 84, element dimension: 10 Am 10 Am square, element spacing: 20 Am) with self-aligned gate. Transistor curves for the CNT triode with anode current (I a) as a function of anode voltage (Va) for different gate voltages (V g) was measured, followed by AC characterization. The triode amplifier demonstrated gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of operation. A large DC gain or amplification factor of ¨350, transconductance of ¨2 AS, and a computed anode resistance of 182 MV are obtained. A large anode current of ¨3.5 AA or current density of ¨1.2 mA/cm2 was achieved at V g = 46 V and Va = 300 V. Saturation of the anode currents at Va > 80 V was also observed. The AC performance of the CNT triode amplifier was characterized by input voltage (v in) vs. output voltage (v out). The voltage gain of the triode amplifier, Av, is given by the ratio of v out/ v in. The estimated Av is calculated to be ¨3.13 with a phase shift of ¨180-, as expected. A larger Av could be attained if larger R L is applied. Preliminary frequency response of the triode amplifier is presented. The results obtained thus far demonstrate that the CNT triode amplifier can be a promising amplifier candidate. D 2005 Elsevier B.V. All rights reserved. Keywords: MPCVD; Carbon nanotubes; Field emission; Electrical properties characterization
1. Introduction Since the first field emission characterization of CNTs in 1995 [1,2], CNTs have been widely studied for various applications, including vacuum microelectronics. The high aspect ratio (often exceeding 1000) and good thermal and chemical stability make CNTs an outstanding contender for electron field emitters. Some of the potential applications of CNTs include field emission displays [3], tips for scanning microprobe [4], gas sensing elements [5], vacuum microelectronics [6], microwave power devices [6], and electron source for electron-beam lithography [6]. A field emission triode amplifier in microelectronic form is a three-terminal device capitalizing on microfabrication * Corresponding author. Vanderbilt University, VU Station B 351661, Nashville, TN 37235-1661, USA. Tel.: +1 615 322 0952; fax: +1 615 343 6614. E-mail address: [email protected] (W.P. Kang). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.09.025
techniques to bring the controlling gate in close proximity to the electron emitting cathode. These microfabricated vacuum triodes can be operated at low turn-on voltage, generally less than 50 V [7 –9]. As a result, the complexity of the peripheral electronics can be reduced significantly with better control of thermal and power management. When an AC small-signal is imposed on the input gate voltage and a load is connected to the output of the field emission triode, an AC voltage gain can be obtained. A triode amplifier based on field emission technology is particularly attractive for electronic applications that require radiation-hardness and temperature-immune capability such as space, aviation and telecommunication applications. Previously, we have successfully fabricated a CNT triode by thermal CVD in a furnace tube at atmospheric pressure [8]. The single-mask micro-fabrication process achieved a 3-band (25 Am 100 Am) CNT triode with self-aligned gate. The device demonstrated basic gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of
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Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
operation. Since the CNTs grown were randomly oriented, an over-etched gate structure was adopted in order to avoid cathode-gate shorting problem. As a result, the large cathodegate spacing of ¨12 Am led to high turn-on voltage with reasonable triode characteristics (DC gain, l = 10 at I a = 1.0 AA, and transconductance, g m = 47 nS at Va = 198 V) when configured as a triode amplifier. In this work, vertically aligned CNT gated microcathodes were synthesized selectively utilizing microwave plasma chemical vapor deposition (MPCVD). Moreover, plasma pretreatment of the catalysts prior to the CNT synthesis was successfully employed to obtain a convex-shaped surface profile, which is important in prevention of cathode-gate ‘‘shorting’’. The DC and AC characteristics of the aligned CNTs field emission triode, configured in a common emitter amplifier were investigated. To the best of our knowledge, this is the first time AC small-signal performance of a CNT triode amplifier is being reported. 2. Experimental method In this work, a gated CNT field emitter array or triode device was fabricated utilizing microfabrication processes. A single-mask process achieved an array of CNT microcathodes with self-aligned gate (array size: 34 84, elemental array dimension: 10 Am 10 Am square, elemental array spacing: 20 Am). As illustrated in the fabrication process flow (Fig. 1), the process began with thermal oxidation of a highly doped n-type silicon (100) substrate followed by low-pressure CVD deposition of polysilicon as the gate electrode. The polysilicon was subsequently doped through a diffusion process utilizing spinon-diffusion source and high-temperature annealing in order to obtain good conductivity of the gate. The thickness of the thermal oxide and the polysilicon gate layers was ¨1.5 Am and 0.8 Am, respectively. After lithography patterning, the polysilicon gate was dryetched by a gas mixture of sulfur hexafluoride (SF6) and oxygen (O2) at 150 mTorr in a reactive-ion-etch system and the
Thermal oxidation and LPCVD of poly-Si gate
Catalyst deposition & PR liftoff
SiO 2
SiO 2
Patterning & RIE of poly-Si
MPCVD growth of aligned CNTs
SiO 2
SiO 2
Poly-Si Photoresist
SiO 2
SiO 2
n+-Si SiO2
SiO 2
Fig. 1. Schematic diagrams of the single-mask fabrication process for the aligned CNT field emission triode amplifier.
thermal oxide was isotropically wet-etched with bufferedoxide-etch (BOE) to obtain an oxide undercut structure, as illustrated in Fig. 1. Next, a thin layer of titanium (Ti), ¨20 nm, acting as the diffusion barrier layer, and nickel (Ni), ¨5– 10 nm (growth catalyst), were sputter-deposited in sequence utilizing a DC magnetron sputtering system. The sputter-deposition was performed at 7 mTorr of argon (Ar). Following that, the unwanted catalyst above the gate layer was removed with an acetone lift-off technique. Last, the mold was transferred to the MPCVD chamber (AsTex) for selective growth of CNTs inside the microtriode mold. Critically, a H2 plasma pretreatment (400 W microwave power at 400 -C) was performed in order to obtain the desired convex shape of the CNT cathode. The CNTs were synthesized at a working pressure of 20 Torr, substrate temperature of 650 -C, and microwave power of 1 kW. The flow ratio of the gas mixtures of methane (CH4) and H2 was maintained at 1:8 with a total flowrate of 135 sccm by massflow-controllers during CNT synthesis. The growth time of the CNTs ranges from 60 to 120 s, which is crucial in obtaining optimal height of the vertically aligned CNT cathodes. After that, the CNT triode sample was cooled to room temperature in H2 ambient and ready for subsequent device characterization. 3. Device characterization The CNT triode was first tested in a common emitter amplifier configuration for DC field emission characteristics. A schematic diagram of the DC large-signal test setup is shown in Fig. 2(a). The purpose of the DC characterization was to obtain optimum operating conditions, i.e. the corresponding gate and anode biasing voltages in saturation region for subsequent AC mode of operation. The measurements were performed at room temperature in a vacuum chamber evacuated to a base pressure of 106 Torr. A computerized data acquisition system equipped with Labview program (National Instruments) was employed for the measurements of the anode emission current, I a as a function of the anode voltages (Va) for different gate voltages (V g). A fixed voltage of 400 V was first applied to the anode to attract electrons induced by the positive gate voltage (V g). The anode emission current (I a) was then measured as a function of the effective anode voltages (Va) while holding the gate voltage constant. The measurements were then repeated for different gate voltages to obtain a family of I a – Va curves. Next, the AC performance of the CNT triode was characterized by superimposing an AC signal (sinusoidal) input voltage, v in on top of the applied DC gate voltage utilizing a function generator, as illustrated by the schematic diagram of the AC test circuit in Fig. 2(b). The corresponding AC output voltage, v out was measured across a load resistor, R L. Both input and output signals were then measured simultaneously with a dual-channel digital oscilloscope. The AC voltage gain, Av of the CNT vacuum amplifier is characterized by the ratio of v out to v in at a chosen operating frequency. Last, the AC frequency response of the CNT
Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
Ia
(a)
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Ra
Anode/ Collector
Rg SiO2
SiO2
Vg
Va
Vacuum 10-6 Torr
Ia
(b)
Vacuum 10-6 Torr
C +
Ra
Rg SiO2
RL
SiO2
vout
Va
Vg
Fig. 3. SEM micrographs of (a) a single 10 Am 10 Am aligned CNT microtriode and (b) cross-sectional view of the same.
_
CH1 CH2 Digital Oscilloscope Fig. 2. Schematic diagrams of the: (a) DC large-signal, and (b) AC small-signal test circuits for the aligned CNTs field emission triode amplifier.
vacuum triode was further investigated by varying the frequency of the AC signal input voltage. 4. Results and discussion The fabricated CNT field emitter array was examined with a Hitachi 4200 scanning electron microscope (SEM). The singlemask micro-fabrication process achieved an array of 10 um 10 Am square CNT gated microcathodes. The SEM micrographs in Fig. 3 show the array geometry and cross-sectional view of a single gated CNT microtriode. The pre-growth H2 plasma pretreatment of the sputtered catalysts was utilized successfully to achieve the gated CNT cathode with a convex-shaped surface profile, i.e. shorter nanotubes on the edges, as shown in Fig. 3(b). As mentioned in [10], this convex profile is important in preventing cathode-gate ‘‘shorting’’ without resorting to more complicated fabrication processes such as sidewall protector [11,12] or gate over-etching method [8,13] that result in higher turn-on voltages. The cathode-gate spacing was ¨2.0 Am, as estimated from the edge of the gate to the edge of the CNT cathode. In this study, a more uniform emission from the 10 Am 10 Am CNT emitters was made possible with a convex-shaped cathode surface profile. Simulation of electric field distribution on triode using CNTs as emitters [14,15] has demonstrated a
1.8
(a)
Vg = 45 V
1.6 1.4 1.2
Ia (µA)
vin
more quasi-uniform electric field, i.e. less field screening effect on CNTs with variable heights having shorter nanotubes on the circumferential region, which is similar to the CNT emitters with convex-shaped cathode surface profile than those with uniform heights. When tested in a common emitter configuration for DC characteristics, the CNT triode demonstrated good gatecontrolled modulation of the emission current with distinct cutoff, linear and saturation regions of operation, as displayed in Fig. 4. The microfabricated CNT triode amplifier showed a low gate turn-on voltage of ¨33 V. From the estimated cathode-gate spacing (¨2Am), the gate turn-on field of the device is ¨16.5 V/Am. In DC mode of operation, three important transistor parameters [8,16 –20] characterizing
Vg = 44 V
1.0 7 MΩ Loadline
100 M Ω Loadline
0.8 0.6
Vg = 43 V
0.4 0.2
Vg = 39 V Vg = 36 V
0.0 0
100
200
300
400
500
Va (V) Fig. 4. Plot of the measured anode currents, I a vs. the anode voltage, Va of the aligned CNTs field emission triode amplifier for different applied gate voltage, V g. Included are the 7 MV and 100 MV loadlines.
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Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
the performance of a triode amplifier are DC gain or BVa amplification factor (l ¼ BV j ), transconductance g Ia ¼const BIa a (gm ¼ BVg jVa ¼const ) and anode resistance (ra ¼ BV BIa jVg ¼const ). In AC signal amplification, l and g m directly relates to the AC voltage gain, Av and cutoff frequency ( f T) of a triode amplifier. Also note that the anode resistance is related to the transconductance and the DC gain of the triode amplifier at an operating point by this expression, r a = l/g m. The device characteristic of field emission triode is found to obey Fowler – Nordheim theory [8,16 – 20] and not to follow Langmuir’s law [21,22]. In other words, the space-charge effect is ignored because in a field emission triode amplifier, the device generally operates in saturation region (do not confuse with thermionic triode amplifier that usually operates in spacecharge limited mode), where emitted electrons are mainly extracted by gate (V g) and the anode (Va) acted primarily as an electron collector. In addition, the proximity of the gate electrode to the cathode (on the order of a few Am) has effective field shielding from the distant anode (on the order of a few mm) [20]. In this case, the local electric field on the surface of the cathode, E can be expressed as [8,16 –20]: Vg þ Va =l E¼b : ð1Þ d where b and d are the field enhancement factor and cathodegate spacing, respectively. Since l is generally very large (usually greater than 100, and ¨352 in this case) for a good field emission amplifier, the second term in Eq. (1) has little influence on E. Hence, E å bV g/d. In general, deviations from FN theory could only occur in high field and high current density regions [23]. The DC transistor parameters were determined graphically from the DC characteristics curves in Fig. 4. A large DC gain of ¨352 at I a = 1.7 AA, and a transconductance g m ¨ 1.93 AS at Va = 400 V were obtained. The corresponding anode resistance was computed to be 182 MV. In addition, a large anode current of ¨3.5 AA or a current density of ¨1.2 mA/cm2 was also observed at V g = 46 V and Va = 300 V. Saturation of the anode currents at Va > 80 V was also observed. These DC transistor characteristics are substantially improved as compared to our previously reported results [8], where randomly oriented CNTs were grown by thermal CVD using an over-etched gate triode structure. The improvement in the triode characteristics is attributed to the vertically aligned CNTs with a convex-shaped surface profile, smaller cathode-gate spacing (reduced from 12 Am to 2 Am) and larger array size (increased from 3 bands of 25 Am 100 Am to an array of 2856 of 10 um 10 Am squares). The AC performance of the CNT triode amplifier was characterized by the AC signal input voltage (v in) vs. AC output voltage (v out) plots. The AC voltage gain of the triode amplifier, Av, is then given by the ratio of v out/v in. Fig. 5(a) shows the AC plots of the CNT triode amplifier operated at a frequency of 100 Hz with a 1 V peak-to-peak sinusoidal input signal, as recorded by the digital oscilloscope. The corresponding biasing conditions, as predetermined from the DC characteristics curve, are V g = 45 V, Va = 400 V, and I a = 1.7 AA. The anode (R a) and the load resistors (R L) adopted were both 7 MV.
(a)
vin = 1.09 V
vout = 3.44 V
f = 100 Hz, Av = 3.13
(b) vin = 0.94 V
vout = 2.09 V
f = 500 Hz, Av = 2.22
(c)
vin = 0.77 V
vout = 1.45 V
f = 1 kHz, Av = 1.89 Fig. 5. Plots of the ac small-signal input voltage (v in) vs. AC output voltage (v out) plots of the aligned CNTs field emission triode amplifier at an operating input frequency of (a) 100 Hz, (b) 500 Hz, and (c) 1 kHz.
From the AC plot (i.e., oscilloscope output screen shown in Fig. 5(a)), the estimated Av is measured to be ¨3.13 or 9.91 dB. As expected from a triode amplifier in a common-emitter configuration, the output signal is 179- out-of-phase with the input signal, close to the theoretical value of 180-. The small AC voltage gain in this case is not unanticipated, judging from the 7-MV loadline, as shown in Fig. 4. This load resistor is relatively small compared to the computed anode resistance, resulting in an almost vertical loadline which is detrimental for high Av output. A larger Av, however, is comfortably attainable by adopting a larger R L as evidenced by the 100 MV loadline in Fig. 4. In this case, the AC voltage gain is expected to improve to ¨30 or 29.54 dB. The AC frequency response of the CNT vacuum amplifier was further investigated at 500 Hz and 1 kHz. The corresponding Av was found to be lower, 2.22 (6.93 dB) and 1.89 (5.53 dB), respectively, as exhibited in Fig. 5(b) and (c). It is observed that the phase shift also reduced to 143- and 107-, respectively. Since this CNT triode amplifier configuration is not optimized for high frequency operation, this behavior is predictable. In fact, with a measured cathode-gate capacitance (C g) of ¨1 nF and calculated g m ¨ 1.93 AS, the theoretical
Y.M. Wong et al. / Diamond & Related Materials 14 (2005) 2069 – 2073
cutoff frequency as given by the following expression, f T = g m/ (2pC g), [19,24] was computed to be only ¨300 Hz. The reduction in the phase shift at higher frequency needs further investigation. However, the DC characteristic curves (Fig. 4) and the measured AC voltage gain at 100 Hz (Fig. 5(a)) are concrete proofs that voltage amplification of the device has been demonstrated in this study. In order to increase the cutoff frequency of the triode amplifier, low cathode-gate capacitance in the order of pF is necessary. Lower C g is attainable by increasing the thickness of the cathode-gate dielectric or minimizing the gate area. Also, reduction in gate aperture and increase in array size and packing density will also result in higher g m and lower C g, and hence higher cutoff frequency could be attained. 5. Conclusions We have successfully fabricated and characterized a CNT field emission triode amplifier in both DC and AC modes of operation. The single-mask microfabrication process achieved a field emitter array of 10 Am 10 Am CNT microtriode with selfaligned gate. From the DC characteristic curves, the microfabricated CNT triode amplifier showed a low gate turn-on voltage of ¨33 V, a large DC gain of ¨352 at I a = 1.7 AA, and a transconductance of ¨1.93 AS at Va = 400 V. The CNT field emission triode device demonstrated excellent gate-controlled modulation of the emission current with distinct cutoff, linear and saturation regions of operation. AC amplifier function of the triode was successfully performed by biasing the gate and anode voltages within the saturation regions. An AC voltage gain of ¨3.13 or 9.91 dB was obtained for a load resistor of 7 MV with a sinusoidal 1.0-V peak-to-peak input signal, operating at I a = 1.7 AA, V g = 45 V and Va = 400 V. Frequency response of the triode amplifier up to 1 kHz was also demonstrated. Overall, our studies suggest that future device optimization to achieve high power, high gain and high frequency operation of CNT field emission triode amplifier is feasible. References [1] A.G. Rinzler, J.H. Hafner, P. Nikolaev, L. Lou, S.G. Kim, D. Tomanek, P. Nordlander, D.T. Colbert, R.E. Smalley, Science 269 (1995) 1550.
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