J. Phys. Chem. B 2006, 110, 9923-9926
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Gate-Controlled Rectifying Behavior in C70@SWNT Networks Ao Guo,†,§ Yunyi Fu,*,†,§ Jia Liu,†,§ Lunhui Guan,‡ Zujin Shi,*,‡ Zhennan Gu,‡ Ru Huang,†,§ and Xing Zhang†,§ Department of Microelectronics, Peking UniVersity, Beijing 100871, P. R. China, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China, and Key Laboratory for the Physics and Chemistry of NanodeVices, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: January 11, 2006; In Final Form: March 28, 2006
We report the gate-controlled rectification behavior in C70@SWNT networks at room temperature in air. The electrical transport characteristics can be fitted well with the conventional Schottky diode model. The origin of the rectifying behavior in fullerene peapod networks device is qualitatively discussed. This paper demonstrates a strategy for diode fabrication based on peapod networks.
Introduction Single-walled carbon nanotubes (SWNTs) filled with fullerene or metallofullerene molecules, generally named peapods, have become emerging materials in nanoelectronics.1-3 The interaction between fullerene molecules and nanotubes is expected to influence structure and electronic properties of SWNT. It has been demonstrated both experimentally and theoretically that the band gap of semiconducting SWNTs can be modulated greatly by encapsulating fullerene and metallofullerene molecules,4-6 which strongly suggests that nanoscale peapods can be used for the nanoelectronic devices such as field-effect transistors (FETs),7-8 memory,9 and diodes.10-13 Solid-state diodes, as fundamental cells in electronics, are widely used in rectification, switching, and photonic devices. Recently, nanoscale diodes have attracted much attention. Both p-n diodes and Schottky diodes have been reported based on SWNTs.10-13 However, rectifying behaviors based on SWNT peapods have not been reported so far. Here, we present our studies on gate-controlled rectification in random networks of C70 peapods. We argue that current rectification results from highly asymmetric intermolecular barriers between semiconducting peapods (i.e., the channel for a device) and the source and drain electrodes. Experimental Section The SWNTs used were grown by the arc-discharge method.14 The SWNTs were filled with fullerene molecules in a vapordiffusion method by mixing SWNTs with C70 and heating the mixture at 500 °C in a vacuum of 10-5 Torr for over 48 hours.15 The microstructure characterization of as-synthesized fullerene nanotube peapods was carried by high-resolution transmission electron microscopy (HRTEM, Hitachi, HR-9000). The C70 peapod network devices in the present paper were fabricated using the back-gate FET configuration. A heavily doped silicon substrate, covered by 300-nm-thick thermally * Corresponding authors. E-mail:
[email protected] (Y.F.); zjshi@ pku.edu.cn (Z.S.). † Department of Microelectronics. ‡ Department of Chemistry. § Key Laboratory for the Physics and Chemistry of Nanodevices.
grown SiO2, was used as a back gate. Ti/Au (10:100 nm) source and drain electrodes, with a spacing of 300-1500 nm, were defined by optical lithography and lift-off technique. After patterning metal electrodes, the fullerene peapods, dispersed in DMF solution, were deposited on a substrate surface. The peapods distribute randomly to form conducting pathways across a pair of electrodes. To decrease the contact resistances, the as-prepared devices were annealed in Ar atmosphere at 350 °C for 5 min. The device micrographs were examined by a focused ion beam workstation (FIB, Strata DB235). Electric transports of devices were performed on a HP 4156B precision semiconductor parameter analyzer. Electrical transport measurements were carried out at room temperature in air. Results and Discussion Figure 1a shows a typical HRTEM image of C70 fullerene peapods. The nanotube bundles are filled with C70 molecules. The parallel lines correspond to the walls of SWNTs. The space between the lines is about 1.4 nm, consistent with the diameter of a typical single-walled carbon nanotube, as revealed by HRTEM and Raman spectroscopy.14 Ring-shaped objects, with diameter close to 0.7 nm, are attributed to individual C70 molecules that are aligned almost linearly along the tube axis, forming a one-dimensional chainlike structure. Long chains of C70 molecules may offer additional conductive paths for carriers and form new energy bands derived from the C70 LUMO. The schematic of a peapod networks device is shown in Figure 1b, and the inset gives the FE-SEM image of a C70 peapods device. The C70 peapods, in the form of random bundles networks, were in electric contact with the predefined electrodes (A and B, denoted in the inset of Figure 1b) and used as the channel of a field-effect transistor. The random networks can form various junctions in both peapod-peapods and peapodelectrodes. We examined IDS-VDS characteristics of the C70@SWNTs networks device in ambient conditions. The pristine device depicts almost linear IDS-VDS characteristics with the series resistance of about 50 kΩ, which shows nearly ohmic contact between C70 peapod networks and Ti/Au electrodes (inset of Figure 2). The current-voltage curve is symmetric, which does not show any rectifying behavior. It also indicates that the
10.1021/jp0602068 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006
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Figure 3. IDS-VDS curve plotted in absolute magnitude of current (VGS ) -30 V). The fit to the diode eq 1 using IS ) 4.5 × 10-11A, RS ) 1.2 × 107 Ω, and n ) 2.8.
gate voltage from -30 V to 30 V, the device could be switched on or off under forward bias. The IDS-VDS rectifying characteristics can be well fitted to the following diode equation:13
IDS ) IS[eq(V-IRS)/(nkT) - 1]
Figure 1. HRTEM image of C70 fullerene peapods (a) and schematic of a device with peapod networks across a pair of predefined electrodes (b). Inset: FE-SEM image of a C70 peapod networks device.
Figure 2. Gate-controlled rectifying IDS-VDS characteristics after electrical breakdown. The device switch on at VGS ) -30 V and off at VGS ) 30 V under forward bias. Inset: almost linear IDS-VDS curve for pristine device.
C70@SWNTs networks electrically conduct like a twodimensional electronic material due to the low peapod-peapod or peapod-electrode contact resistance. After following the electrical breakdown technique proposed by P. G. Collins et al.,16 and burning the metallic nanotubes, we observed strongly asymmetric IDS-VDS characteristics (Figure 2). The rectifying behavior is well controlled by gate voltage. By modulating the
(1)
where IS is the reverse saturation current, V is the source-drain biasing voltage, RS is the total series resistance, n is the ideality factor (n ) 1 for an ideal diode), q is the electron charge, k is Boltzmann’s constant, and T ) 300 K. Figure 3 shows comparison of absolute value of IDS versus VDS curves between measured data at VGS ) -30 V and the fitted result from eq 1 for our IDS-VDS rectifying characteristics. The two curves exhibit good agreement, especially in the low bias range, with fitting parameters IS ) 4.5 × 10-11 A, RS ) 1.2 × 107 Ω, and n ) 2.8, which suggests that the C70 peapod networks device can function well as a Schottky diode. We here provide a qualitative explanation for the origin of rectifying behavior in C70@SWNTs networks. The pristine C70@SWNTs networks exhibit typically metallic transport feature because of the coexistence of semiconducting and metallic peapods, where only metallic ones may contribute to the current transport (Figure 4a and inset of Figure 2). From the IDS-VDS curves, it also indicates that there are no evident Schottky barriers between peapod networks and the metal electrodes, which behave as a nearly ohmic contact. While after electrical breakdown, the transfer characteristics show p-type field-effect behavior with a high on/off ratio (∼104), suggesting the current is mainly carried by semiconducting peapods. Furthermore, the low off current (∼10-12 A) indicates that the metallic current paths, which consist only of metallic peapods, are almost broken after electrical breakdown. It is well demonstrated that electrical breakdown is an effective method to destroy metallic peapods in peapods networks, i.e., break down the metallic transport paths. The predominant semiconducting peapods, which may contain neglectable metallic ones, function as the channel of the field-effect transistor. The destroyed metallic peapods give rise to intermolecular (or interpeapod) barriers, which may build Schottky barriers between the semiconducting channel and the metal electrodes. We argue that the field-effect characteristics and the current rectification may result from the Schottky barriers between channel and source or drain electrodes.
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Figure 4. Electron transport characteristics of the C70 peapod networks device at VDS ) 1 V: (a) IDS-VGS of the networks device before and after electrical breakdown. (b) Different transfer characteristics of C70@SWNT FET by only exchanging source and drain electrodes. (c,d) Energy band diagrams of the asymmetric barriers corresponding to one of electrodes (e.g., electrode B) positively biased(c) or grounded (d), respectively.
If we exchange the source and drain electrodes, the transfer characteristics of the devices are apparently different (Figure 4b). Both the on-current and on/off ratio have changed greatly. We can infer that the Schottky barriers around the source and drain electrodes are highly asymmetric. According to Freitag et al., for Schottky barrier SWNT-FETs, the barrier around the positively biased electrode (i.e., with higher potential) probably dominates the field-effect behaviors.17 So when one electrode (e.g., electrode B) was positively biased and the other electrode (electrode A) grounded, the Schottky barrier close to electrode B is in charge of the transport behavior (energy band diagrams shown in the Figure 4c). Contrarily, if the potential of electrode A is higher than that of electrode B, the Schottky barrier close to A will be dominant (energy band diagrams shown in the Figure 4d). This leads to the different transfer characteristics in Figure 4b. And also, it is just the highly asymmetric Schottky barriers around source and drain electrodes that lead to the gatecontrolled rectifying behaviors in the breakdown-induced C70@SWNT network diode. If we keep the scanning direction of bias voltage, i.e., from negative to positive, the rectifying behavior in Figure 2a can be explained as follows. For VGS ) -30 V, when VDS is in the range of -1 to 0 V, the potential of electrode A is higher than B. The Schottky barrier close to A is higher than that of B, which suppresses the drain current. When VDS is in the range of 0-1 V, the drain current then increases exponentially with the drain voltage due to the lower Schottky barrier close to B. While for VGS ) 30 V, the barriers close to electrode A and B are so high for hole, the device then is fully switch off. On the basis of the above analysis, we can configure the general structure of the device after breakdown. The channel (i.e., the predominant semiconducting peapods) connects the
source and drain electrodes via asymmetric Schottky barriers, which may be originated from the different contact configurations between channel and source or drain electrodes. Generally, there may exist three possible contact configurations around source and drain electrodes: (a) The channel contacting directly with source and drain electrodes, that is to say, the semiconducting and metallic transports are simultaneous in pristine devices. The electrical breakdown only destroys individual metallic transport paths, while most of the semiconducting ones remained unaffected, which function as the channel. Because the peapods in our device are in the form of network, the contacts between channel and source/drain electrodes are possibly different in this case, which thus may result in the asymmetric Schottky barriers around source and drain electrodes. (b) The channel forming indirect contacts with two electrodes via destroyed metallic peapods. Because the electric broken process is a random one, the number of destroyed metallic peapods around source and drain electrodes by breakdown are different, which may construct different Schottky barriers around source and drain electrodes. (c) The channel forming direct contact with one electrode, while indirectly contacting via the destroyed metallic peapods with another electrode. It is evident that the contacts between the semiconducting channel and source/drain electrodes are quite different in this case, which may also be responsible for the rectifying characteristics. In fact, the actual structure of the device after breakdown is very complicated. To reproduce the Schottky diode reliably and intended for practical use, we should identify the exact structure of the device after breakdown, especially with experimental method, which remains under investigation. We then estimate the height of the Schottky barrier from the fitting value of reverse saturation current (IS). According to the
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transport characteristics of a conventional Schottky diode, the IS can be calculated by the following equation:18
IS ) JST‚A ) A*‚T2 exp(-qφB/kT)‚A
(2)
where A* ) (4πqm*k2)/h3 is the effective Richardson constant, JST is the reverse saturation current density, A is the cross section area of the semiconducting channel, ΦB is the height of Schottky barrier, m* is the effective mass of carbon nanotubes, m* ∼ 0.06m0 (m0 is the free electron mass) for CNTs with a diameter of 1.4 nm,19 q is the electron charge, k is Boltzmann’s constant, h is the Planck’s constant, and T ) 300 K. For the peapod Schottky diode we presented here, we assumed the currents transport through an individual peapod from source to drain. The cross section area of the peapod is about π(d/2)2, where d ∼ 1.4 nm is the diameter of a single peapod. According to the fitting parameter (IS ) 4.5 × 10-11 A) in Figure 3, we can extract the height of Schottky barrier for hole transport eΦBp ∼ 0.14 eV at room temperature. What we should notice is that the performances of peapod networks diodes deviate away from the ideal Schottky diode such as the lower current-carrying capacity, the higher series resistance, and the relatively large ideality factor (n ) 2.8). One of the key factors to improve the performances of the device is to decrease the series resistance, which mainly relates to an interpeapod tunneling barrier induced by the electrical breakdown. Conclusion We demonstrate an effective and reliable way to fabricate nanoscale gate-controlled diodes from one-dimensional C70@ SWNTs networks using an electrical breakdown technique at room temperature in air. The current rectification characteristics are in good agreement with the conventional Schottky diode model. We argue that current rectification results from highly asymmetric Schottky barriers between semiconducting peapods and the S/D electrodes. The height of Schottky barrier for hole transport eΦBp is estimated to be 0.14 eV at room temperature.
Acknowledgment. We gratefully acknowledge Prof. A. Narlikar for helpful discussion and financial support from the National Natural Science Foundation of China (nos. 90207004, 90206048, 90406024, and 20371004), the State Key Fundamental Research Project, and the National Center for Nanoscience & Nanotechnology China and Instrumental Analysis Fund of Peking University. References and Notes (1) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (2) Suenaga, K.; Tence, M.; Mory, C.; Colliex, C.; Kato, H.; Okazaki, T.; Shinohara, H.; Hirahara, K.; Bandow, S.; Iijima, S. Science 2000, 290, 2280. (3) Hirahara, K.; Suenaga, K.; Bandow, S.; Kato, H., Okazaki, T.; Shinohara, H.; Iijima, S. Phys. ReV. Lett. 2000, 85, 5384. (4) Lee, J.; Kim, H.; Kahng, S.-J.; Kim, G.; Son, Y.-W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; Kuk Y. Nature 2002, 415, 1005. (5) Hornbaker, D. J.; Kahng, S.-J.; Misra, S.; Misra, S.; Smith, B. W.; Johnson, A. T.; Mele, E. J.; Luzzi, D. E.; Yazdani A. Science 2002, 295, 828. (6) Rochefort A. Phys. ReV. B 2003, 67, 115401. (7) Chiu, P. W.; Gu, G.; Kim, G. T.; Philipp, G.; Roth, S.; Yang, S. F.; Yang, S. Appl. Phys. Lett. 2001, 79, 3845. (8) Shimada, T.; Okazake, T.; Taniguchi, R.; Sugai, T.; Shinohara, H.; Suenaga, K.; Ohno, Y.; Mizuno, S.; Kishimoto, S.; Mizutani, T. Appl. Phys. Lett. 2002, 81, 4067. (9) Lee, C. H.; Kang, K. T.; Park, K. S.; Kim, M. S.; Kim, H. S.; Kim, H. G.; Fishcher, J. E.; Johnson, A. T. Jpn. J. Appl. Phys. 2003, 42, 5392. (10) Antonov, R. D.; Johnson, A. T. Phys. ReV. Lett. 1999, 83, 3274. (11) Zhou, Y.; Gaur, A.; Hur, S.-H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4, 2031. (12) Lee, J. U.; Gipp, P. P.; Heller, C. M. Appl. Phys. Lett. 2004, 85, 145. (13) Manohara, H. M.; Wong, E. W.; Schlecht, E.; Hunt, B. D.; Siegel, P. H. Nano Lett. 2005, 5, 1469. (14) Shi, Z. J., Lian, Y. F.; Zhou, X. H.; Gu, Z. N.; Zhang, Y. G.; Iijima, S.; Zhou, L. X.; Yue, K. T.; Zhang, S. X. Carbon 1999, 37, 1449. (15) Guan, L. H.; Li, H. J.; Shi, Z. J.; You, L. P.; Gu, Z. N. Solid State Commun. 2005, 133, 333. (16) Collins, P. G.; Arnold, M. S.; Avouris, Ph. Science 2001, 292, 706. (17) Freitag, M.; Radosavljevic, M.; Zhou, Y.; Johnson, A. T.; Smith, W. F. Appl. Phys. Lett. 2001, 79, 3326. (18) Sze, S. M. Physics of Semiconductor DeVices; Wiley: New York, 1981. (19) Appenzeller, J.; Radosavljevic, M.; Knoch, J.; Avouris, Ph. Phys. ReV. Lett. 2004, 92, 048301.
