NANO LETTERS
Transport Phenomena and Conduction Mechanism of Single-Walled Carbon Nanotubes (SWNTs) at Y- and Crossed-Junctions
2006 Vol. 6, No. 12 2821-2825
Do-Hyun Kim,† Jun Huang,† Hoon-Kyu Shin,‡ Somenath Roy,† and Wonbong Choi*,† Department of Mechanical and Materials Engineering, Florida International UniVersity, Miami, Florida 33174, and National Center for Nanomaterials Technology, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea Received August 22, 2006; Revised Manuscript Received October 31, 2006
ABSTRACT This letter illustrates the transport phenomena associated with single-walled carbon nanotube (SWNT) junctions of Y- and cross-configurations. Localized gating effect exhibited by Y- and crossed-junctions suggests the resemblance of their electrical characteristics with ambipolar and unipolar p-type FETs, respectively. Temperature dependence of the I−V characteristics reveals that the conduction mechanism in the said SWNT junctions is governed by thermionic emission at temperatures above 100 K and by tunneling at T < 100 K. In-depth analysis of current transport through the crossed- and Y-junction SWNTs is significant in view of their predominant influence on the electrical performance of carbon nanotube networks (CNT-mat).
Single-walled carbon nanotubes (SWNTs) are ideal building blocks for novel nanoelectronic devices because of their unique electrical properties. Depending on the chirality and diameter of the nanotubes, they can be characterized as metallic (M) or semiconducting (S) SWNTs.1-3 SWNTs have already been exploited as heterojunctions, negative differential resistors, field-effect transistors, nonvolatile memory, and single electron transistors.4-8 Although SWNTs exhibit potential in terms of future electronics applications, the current technology only allows the growth of nanotubes with mixed chirality and limited control of directionality. Of late, randomly dispersed SWNTs consisting of overlapped M- and S-SWNTs (hereafter denoted as CNT-mat) are considered for immediate device applications because of their ease of synthesis. CNT-mat-based transistors and gas sensors are envisioned as future nanodevices.9-11 Although CNT-mat is useful for next-generation high-performance electronics, the electrical transport behavior of this structure is not fully understood because of its complex network configuration composed of crossed- and Y-junction SWNTs. Analogous to microelectronic ICs, in which 2D networks are composed of multiple nodes, nanoelectronic devices are also predicted to be based on a cluster of various nanojunctions. Hence, * Corresponding author. Tel: 1-305-348-1973. Fax: 1-305-348-1932. E-mail:
[email protected]. Web: http://web.eng.fiu.edu/choiwweb/. † Florida International University. ‡ Pohang University of Science and Technology. 10.1021/nl061977q CCC: $33.50 Published on Web 11/20/2006
© 2006 American Chemical Society
the junctions between nanotubes, or between nanotube and metal contacts, play a crucial role in dictating the overall circuit performance.12,13 Therefore, study of the electrical properties of Y-junction and crossed-junction SWNTs is essential and will provide insight into the transport mechanism of CNT-mat or network structure. Recently Khakani et al. reported ambipolar behavior from suspended CNTmat, which they suggest to be a cumulative effect of the multiple SWNT junctions.14 However, the said report still left the scope for probing into the conduction mechanism associated with various nanotube junctions. The objective of this study, therefore, is to understand the specific role of individual CNT junctions in monitoring the overall electrical characteristics of a CNT-mat nanosystem. We have analyzed in detail the electrical characteristics and conduction mechanism of two principal configurations, Y-junction and crossed-junction, respectively, the building blocks of a singlewalled nanotube network. Y-SWNTs are grown reproducibly by a thermal chemical vapor deposition (CVD) method with a Fe/Mo catalyst. We confirmed the presence of SWNT through Raman analysis and TEM. The experimental details on growth and characterization of the Y-SWNTs are presented in our previous report.15 After fabricating alignment markers on a SiO2/p-Si substrate by electron-beam lithography, a drop of homogeneously dispersed SWNTs in an ethanol solution was deposited, followed by drying in a nitrogen flow to yield
Figure 1. AFM (scanning area 1 × 1 µm2) and SEM micrographs of (a) Y-SWNT and (b) crossed-SWNT contacted by Ti/Au electrodes. The numbers in the SEM images represent the electrodes for electrical measurement in FET mode. From the height measurement, the diameter of the stem of the Y-SWNT is characterized to be about 1.9 nm, and one branch of the Y-SWNT has a diameter of about 1.0 nm while the other branch has a diameter of about 1.1 nm. The diameters of the nanotubes forming the crossed junction are 1.6 and 1.7 nm, respectively. A kink is formed at the crossed junction that might affect the charge transport in the individual nanotubes.
isolated or overlapping SWNTs on the patterned device. The patterns for electrical leads were generated by e-beam lithography (JEOL 7000 and Nanometer Pattern Generation System) onto the branches of the Y- and crossed-junction SWNTs, and then Ti (10 nm)/Au (80 nm) was deposited by sputtering followed by a lift-off process. To reduce the metal/ nanotube contact resistance, the device was annealed in Ar atmosphere for 60 s at 500 °C by rapid thermal annealing (RTA). Several local-gate CNT field-effect transistors (FETs) were fabricated and characterized in this study, and a statistical homogeneity was observed between the identical set of samples. Figure 1a and b shows the atomic force microscope (AFM) and scanning electron microscopy (SEM) images of Y-junction and crossed SWNTs, respectively. Semiconducting and metallic branches of the Y-SWNT are distinguished by the room-temperature two-probe resistance measurement at zero gate voltage. To verify the metalnanotube contact resistance, two-probe measurements were performed at the metal contact pads fabricated at the termini of a linear SWNT (not shown here) on the same chip under the identical process conditions. The ohmic nature of the electrical contacts was justified by their linear I-V characteristics. The electrical conductance measured between electrodes 1 (source) and 2 (drain), (Figure 1a) shows asymmetric nature from -1 V through 1 V. The current-voltage characteristics between the electrode 1 (source) and 3 (gate), as well as that between 2 (drain) and 3 (gate), exhibit rectification characteristics that are analogous to the Schottky barrier effect. However, a current-tail in the reverse bias region appears in the latter case. From the above set of two-probe measurements, it is apparent that the stem (electrode 2) of the Y-SWNT and its one branch, addressed by electrode 1, are 2822
of semiconducting nature. Another branch of the Y-SWNT, which is contacted by electrode 3, is metal-like, which is evidenced by the rectification behavior when the measurements are made between the two branches (1 and 3). The conductivity difference among various segments can be attributed to the band-gap difference,16 which evolves from the difference in diameter between the stem and branches of Y structure, as is evidenced by the AFM micrograph (Figure 1). A speculation can be made that a metal-semiconductortype interface is formed at the three-point junction of Y-SWNT, which is manifested by the ambipolar characteristics presented below. It is worth noting at this point that the metal-CNT interfaces play a trivial role in modulating the current-voltage characteristics of the system. Although the probability of formation of a Schottky barrier (SB) between the CNT and the metal electrode cannot be ruled out, RTA treatment might have reduced the contact barrier height to an extent that an appreciable amount of current can flow through it. The ohmic I-V curve obtained between the metal contacts to the ends of a linear SWNT fragment located on the same substrate precludes the possibility of high contact barrier formation at the metal-CNT interfaces. Hence, it can be inferred that the unique current-voltage characteristics recorded in this study and presented below are originated from the heterojunction between the stem and the branches of the Y-SWNT and not from the metal-CNT interfaces. The above set of observations and logical discussions also reason in favor of choosing terminal 3 as the gate (G) electrode and terminals 1 and 2 as source (S) and drain (D), respectively, in Figure 1a. Referring to Figure 1b, in which the termini of the crossedjunction SWNTs are addressed electrically by e-beam Nano Lett., Vol. 6, No. 12, 2006
Figure 2. Source-drain current as a function of gate voltage at different drain voltages. (a) Ambipolar characteristics of Y-junction SWNTs. The ambipolar I-Vd curves resemble that of an n-type semiconductor at a positive gate potential, and a p-type semiconductor at a negative gate potential. (b) Highly asymmetric I-V characteristics of crossed junction. The forward bias threshold voltage is almost unchanged but the source-drain current was strongly modulated by the gate bias. The forward bias current increases with the negative gate voltage, indicating p-type gate response. The data are taken at 4.3 K (liquid He temperature). The results at room temperature are of similar nature. Inset of part a: schematic energy band diagram explaining the ambipolar characteristics. The metallic SWNT Fermi level is located in the middle of the narrow band gap semiconductor-SWNT; the direction of band bending would be dependent on the polarity of the gate bias. Inset of part b: Schematic of the p-p isotype heterojunction band-edge diagram for crossed junction CNTs. Under forward bias (dashed lines); the Fermi energy splits by eVbias ) e (V1 + V2), reducing the barrier to hole transport.
patterned Ti/Au contact electrodes, the electrical conductance of each SWNT shows a semiconducting I-V characteristic with different current levels (electrode 1 and 4: ∼pA, electrode 2 and 3: ∼nA at 1 V). Figure 2 a shows the ambipolar characteristics of a Y-SWNT for a source-drain voltage ranging from -1.0 V through 1.0 V at various constant gate voltages between +1.5 V and -1.5 V. Although Figure 2a presents the data recorded at 4.3 K, the ambipolar behavior is essentially exhibited by the Y-SWNT FET at all of the measurement temperatures up to 300 K. The ambipolar nature can be speculated to be originated from the metal/p-semiconductor interface at the nanojunction of the Y-shape branching, in conformity with our previous results.17 It can be explained by the schematic of the energy band diagram as shown in the inset of Figure 2a. With the positive gate bias, the energy band of the S-SWNTs bends downward, resulting in electron tunneling from the M-SWNT to the S-SWNT. This leads to electron conduction and n-type behavior. When the energy band bends upward, it induces hole tunneling to the S-SWNT and leads to p-type behavior with negative gate bias. The crossed junction SWNTs show reproducible unipolar p-type FET characteristics (Figure 2b). As shown in Figure 1b, the current is forced to pass through the crossed junction (terminals 1 and 2) and a third terminal (3) is employed to explore the gating effect. A strong dependence of the sourceto-drain current (Id) was observed on the applied gate bias (VG) between -0.5 and +0.5 V. It conforms to the literature in which p-type semiconducting SWNTs are found to be conducting at negative VG and insulating at positive VG.18 The rectification effect across the crossover is probably due to the nanotube-nanotube heterojunction.19-21 When two dissimilar semiconducting SWNT’s are brought into contact to form a junction, the Fermi level will be lined up by Nano Lett., Vol. 6, No. 12, 2006
transferring holes from the large-band-gap side to the smallband-gap side, which causes band bending and the formation of a barrier to the holes in the valence band of the largeband-gap nanotube (Figure 2b, inset). When a negative gate voltage is applied through terminal 3 to the junction, it induces holes to the terminal 2 side of the heterojunction, and thereby diminishes the barrier height. Hence, the current through the crossed junction increases dramatically. We also performed a quantitative analysis of the junction behavior using the continuity condition. Under relatively large forward bias, the current-voltage characteristics follow Anderson’s model and can be expressed by a simple relation: I ∝ exp[qV/ηkBT],22 where q is the electronic charge, kB is the Boltzmann constant, T is the absolute temperature, and η is a parameter that depends on the material properties. According to this model, the current should vary exponentially with the bias voltage under large forward bias conditions, which is observed in the case of crossed SWNT junctions. From a typical fit of I-V characteristics, we found η ) 19.7 (at 150 K), which is in the range of 16 to 23, predicted theortically.23 This result is in synchronism with the speculation that the crossed junction is formed by two dissimilar p-type semiconducting heterojunctions.24 The carrier transport through the interface between metal contacts with CNTs of various conformations is further substantiated by the low-temperature measurements, which allow us to explore the properties of one-dimensional (1D) nanotube-nanotube junctions in depth. To determine the effective transport characteristics, I(V, T) measurements are performed from 300 down to 4.3 K with voltage swept between -1 and +1 V. Figure 3a and b shows the currentvoltage characteristics of Y-junction and crossed-junction SWNTs from 4.3 to 300 K in 50 K increments when the gate voltage, VG, equals to zero. In the case of Y-SWNTs, 2823
Figure 3. (a and b) I-V characteristics at different temperatures. (a) The channel current of Y-junction SWNTs is symmetric at positive and negative bias due to the formation of two symmetric junction barriers, confirmed by the calculated barrier heights. (b) The temperature dependence of I-V characteristics of crossed junction SWNTs. As bias is increased the current raises in a nonlinear fashion, displaying rectifying behavior throughout the temperature range. It is due to the heterojunction consisting of two crossed SWNTs. Inset of Figure 3a and b: Arrhenius plots [ln(I) vs 1/T] of Y-junction and crossed-junction SWNTs devices. In the forward as well as reverse biased mode both types of devices essentially exhibit a thermionic regime at temperatures above 100 K. However, a tunneling contribution can be observed below T ) 100 K. (c and d) Plots of the slope of (ln(I/T2) vs V0.5). The thermal barrier height of Y-junction and crossed-junction SWNTs can be deduced from the intercept of the straight line.
without the gate bias, the current propagates primarily through the semiconducting channel composed of its stem and the branch, addressed by terminal 2. The symmetric and slightly nonlinear pattern might have evolved from the difference in channel diameter at the stem and at the semiconducting branch of the Y-SWNT. In contrary, a high rectification nature of current flow is noticed through the channel of crossed junction device. This observation further supports our arguments in favor of formation of a p-p isotype heterojunction at the crossover. To elucidate the dominant specific mechanism and effective barriers, Arrhenius plots [ln(I) vs 1/T] of Y-junction and crossed-junction SWNT devices are shown in the inset of Figure 3a and b, respectively, which exhibit temperature dependence in the slopes of ln(I) versus 1/T at different bias. For both the forward and reverse bias, a clear thermionic emission regime is observed at temperatures above 100 K. Alternatively, below T ) 100 K there is no significant temperature dependence at different biases. Therefore, these results suggest that the channel current is attributed to the charge transport via tunneling below ∼100 K. 2824
To determine the energetic barriers (Φ), we plot the slope of [ln(I/T2) vs V0.5] in Figure 3c and d. In the case of Y-SWNTs, the interception of the linear fit gives thermionic emission barriers of Φ ) 118 and 108 meV for the forward and reverse bias modes, respectively. The corresponding values for the crossed junctions are 152 and 236 meV, which reiterate the rectification behavior of the heterojunction. Also, no significant temperature dependence of the characteristics is observed below about 100 K, thus indicating the absence of thermal activation as shown in Figure 3c and d. Therefore, it reveals that the conduction mechanism is tunneling, either direct or Folwer-Nordheim. Direct tunneling happens when the applied bias is less than the barrier height (V < Φ/e), whereas Fowler-Nordheim tunneling is dominant when the applied bias becomes larger than the barrier height (V > Φ/e).25 Analysis of ln(I/V2) versus 1/V shows significant voltage dependence, indicating obvious F-N tunneling mechanism. In fact, the measured barrier heights for both electrons and holes in the case of ambipolar Y-SWNTs are very small, at least 2.7 times lower than expected (∼110 meV instead of ∼300 meV).26 Therefore, the Schottky barrier Nano Lett., Vol. 6, No. 12, 2006
in the strong accumulation of either holes or electrons is so thin that the junction is quasi-transparent for carrier injection, that is, there is a very efficient tunneling through the barrier.27 A closer look at the Figure 3a inset reveals that over the entire temperature range below 100 K the carrier transport is governed by the Fowler-Nordheim (F-N) equation
(
I ∼ V 2 exp -
)
4dx2m (qΦ)1.5 3qλV
and is independent of temperature. The F-N equation essentially describes the tunneling of carriers close to the Fermi energy through a narrow surface potential barrier. When a high electric field, which is very typical of the 1D structure, is present at the metal/CNT interface a triangularshaped surface potential barrier is formed. As the width of the barrier at the Fermi energy approaches a few nanometers, the charge carriers have a non-negligible probability of tunneling.28 No significant temperature dependence, along with a strong voltage dependence, of the junction current ratifies the dominance of F-N tunneling, which is the deterministic mechanism when the barrier height is smaller than the applied bias (i.e., Φ < 1.0 eV). In conclusion, the present study reveals the underlying conduction mechanisms of two predominant nanotube junctions, Y junction and crossed junction, which govern the electrical properties of a CNT network. Ambipolar characteristics were observed over a wide temperature range at the three terminal junctions of Y-SWNTs. This ambipolar behavior is attributed to the nanoscale junction formed between the metallic and semiconducting branches of the Y-SWNT, in which the Fermi level of the metallic CNT is supposedly located in the middle of semiconducting CNT band gap. In contrary, rectification characteristics were apparent at the crossed junctions of SWNTs because of the formation of a p-p isotype heterojunction at the crossover. Low-temperature I-V measurements further unveiled the conduction mechanism pertinent to individual junctions in a CNT network. In the case of Y-SWNTs, thermionic emission barriers of nearly identical values (∼110 meV) for both the forward and reverse bias modes were obtained, supporting the ambipolar characteristics of the junction. The corresponding values for the crossed junctions are ∼150 and ∼230 meV, respectively, which reiterate the rectification behavior of the heterojunction. No significant temperature dependence of the electrical characteristics could be observed below about 100 K and thus indicating the F-N tunneling as the prevailing conduction mechanism. The present study
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highlighting the transport mechanisms at various nanotube junctions may provide valuable insight into the electrical characteristics of CNT-mat, the more realistic configuration for future nanoelectronic device implementation. Acknowledgment. This project was partially supported by AFOSR grant (FA 9550-05-1-0232), SAMSUNG (SAIT) and SRC grants. References (1) Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. Science 2002, 297, 787. (2) Terrones, M.; Terrones, H.; Banhart, F.; Charlier, J. C.; Ajayan, P. M. Science 2000, 288, 1227. (3) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R. Phys. ReV. B 1992, 45, 6234. (4) Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Nature (London) 1999, 402, 273. (5) Odintsov, A. A. Phys. ReV. Lett. 2000, 85, 150. (6) Le´onard, F.; Tersoff, J. Phys. ReV. Lett. 2000, 84, 4693. (7) Rochefort, A.; Avouris, Ph. Nano Lett. 2002, 2, 253. (8) Choi, W. B.; Cheong, B. H.; Chae, S. D.; Bae, E. J.; Lee, J. W.; Kim, J. R.; Kim, J. J. Appl. Phys. Lett. 2003, 82, 275. (9) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (10) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G. Nano Lett. 2005, 5, 757. (11) Vedala, H.; Huang, J.; Yang Zhou, X.; Kim, G.; Roy, S.; Choi, W. B. Appl. Surf. Sci., in press. (12) Martel, R.; Derycke, V.; Lavoie, C.; Appenzeller, J.; Chan, K. K.; Tersoff, J.; Avouris, Ph. Phys. ReV. Lett. 2001, 87, 256805. (13) Heinze, S.; Tersoff, J.; Martel, R.; Derycke, V.; Appenzeller, J.; Avouris, Ph. Phys. ReV. Lett. 2002, 89, 106801. (14) Khahani, M. A.; Yi, J.-H.; Ayssa, B. IEEE Trans. Nanotechnol. 2006, 5, 237. (15) Choi, Y.; Choi, W. Carbon 2005, 43, 2737. (16) Ratkin, A.; Papadopoulos, C.; Xu, J. M. Phys. ReV. B. 2000, 61, 5793. (17) Kim, D. H.; Huang, J.; Rao, B. K.; Choi, W. B. J. Appl. Phys. 2006, 99, 056106. (18) Tans, S. J.; Verschueren, A. R.; Dekker, C. Nature (London) 1998, 393, 49. (19) Lee, J. O.; Oh, H.; Kim, J. R.; Kang, K.; Kim, J. J.; Kim, J. H.; Yoo, K. H. Appl. Phys. Lett. 2001, 79, 1351. (20) Treboux, G.; Lapstun, P.; Silverbrook, K. J. Phys. Chem. B. 1999, 103, 1871. (21) Kim, H.; Lee, J.; Lee, S.; Song, Y. J.; Choi, B. Y.; Kuk, Y.; Kahng, S.-J. Surf. Sci. 2005, 581, 241. (22) Milnes, A. G.; Feucht, D. L. Heterojunctions and Metal-Semiconductor Junctions; Academic Press: New York, 1972. (23) Papadopoulos, C.; Ratkin, A.; Li, J.; Vedeneev, A. S.; Xu, J. M. Phys. ReV. Lett. 2000, 85, 3476. (24) Liu, L. W.; Fang, J. H.; Lu, L.; Zhou, F.; Yang, H. F.; Jin, A. Z.; Gu, C. Z. Phys. ReV. B. 2005, 71, 155424. (25) Sze, S. M. Physics or Semiconductor DeVices, 2nd ed.; Wiley: New York, 1981. (26) Choi, W. B.; Chu, J. U.; Seok, K. S.; Ju, J. E.; Lee, J. W.; Kim, J. J.; Lee, J. O. Appl. Phys. Lett. 2001, 79, 3696. (27) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Appl. Phys. Lett. 2002, 80, 2773. (28) Gro¨ning, O.; Nilsson, L.-O.; Gro¨ning, P.; Schlapbach, L. Mat. Res. Soc. Symp. Proc. 2001, 675, W6.1.1.
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