pubs.acs.org/Langmuir © 2009 American Chemical Society
Carbon Nanotube Schottky Diode via Selective Electrochemical Metal Deposition Hyunseob Lim,† Hyun Jae Song,† Yoonmi Lee,† Hyun-Joon Shin,‡ and Hee Cheul Choi*,† †
Department of Chemistry and Division of Advanced Materials Science and ‡Pohang Accelerator Laboratory and Department of Physics, Pohang University of Science and Technology, San 31, Hyoja-Dong, Nam-Gu, Pohang 790-786, Korea Received September 21, 2009. Revised Manuscript Received November 26, 2009
A single walled carbon nanotube (SWNT) Schottky diode was fabricated via selective electrochemical metal deposition on a prefabricated SWNT field effect transistor device. By electrochemically depositing Pd on only one of the prepatterned Ti electrodes, asymmetric Ohmic (at Pd-SWNT) and Schottky (at SWNT-Ti) contacts were resolved, resulting in efficient current rectification. The selective electrochemical deposition was performed by electrically isolating two Ti electrodes connected through a SWNT by depleting hole carriers in the SWNT upon the simultaneous application of high positive gate voltage during the deposition process. The successful selective deposition of Pd metals was confirmed by X-ray photoelectron spectroscopy.
Introduction The contact formed at semiconductor and metal interface is one of the most important subjects in micro- and nanoelectronics because the overall performance of the device is significantly affected by the degree of charges transported through the contact. Consequently, there have been many efforts to understand the fundamentals of semiconductor-metal contacts, from which new functional electronic devices have become available.1 For example, the Schottky contact formed at the junction between semiconducting carbon nanotubes and electrode metal in single walled carbon nanotube field effect transistor (SWNT-FET) devices has been successfully developed as a signal inducer for the highly sensitive detections of biomolecules.2 A ballistic transport through a semiconducting SWNT channel has been also demonstrated by fabricating FET devices of which metal electrodes are switched from Ti or Cr to Pd, through which both Fermi energy levels of the SWNT and electrode are well aligned to form Ohmic contacts.3 The carbon nanotube Schottky diode also has been realized by addressing two different types of carbon-nanotube-metal-electrode contacts using conventional lithographic tools.4,5 Recently, *To whom correspondence should be addressed. E-mail: choihc@postech. edu. (1) (a) Fuhrer, M. S.; Nygard, J.; Shih, L.; Forero, M.; Yoon, Y.-G.; Mazzoni, M. S. C.; Choi, H. J.; Ihm, J.; Louie, S. G.; Zettl, A.; McEuen, P. L. Science 2000, 288, 494–497. (b) Deshmukh, M. M.; Prieto, A. L.; Gu, Q.; Park, H. Nano Lett. 2003, 3, 1383–1385. (c) Majewski, L. A.; Schroeder, R.; Grell, M. Appl. Phys. Lett. 2004, 85, 3620–3622. (d) Tang, Q.; Li, H.; Liu, Y.; Hu, W. J. Am. Chem. Soc. 2006, 128, 14634– 14639. (e) Di, C.; Wei, D.; Yu, G.; Liu, Y.; Guo, Y.; Zhu, D. Adv. Mater. 2008, 20, 3289–3293. (2) (a) Chen, R. J.; Choi, H. C.; Bangsaruntip, S.; Yenilmez, E.; Tang, X.; Wang, Q.; Chang, Y. L.; Dai, H. J. Am. Chem. Soc. 2004, 126, 1563–1564. (b) Byon, H. R.; Choi, H. C. J. Am. Chem. Soc. 2006, 128, 2188–2189. (3) (a) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 6949–6952. (b) Javey, A.; Qi, P.; Wang, Q.; Dai, H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 13408–13410. (4) (a) Manohara, H. M.; Wong, E. W.; Schlecht, E.; Hunt, B. D.; Siegel, P. H. Nano Lett. 2005, 5, 1469–1474. (b) Yang, M. H.; Teo, K. B. K.; Milne, M. I. Appl. Phys. Lett. 2005, 87, 253116. (c) Lu, Q.; An, L.; Fu, Q.; Liu, J.; Zhang, H.; Murduck, J. Appl. Phys. Lett. 2006, 88, 133501. (d) Chen, C.; Lu, Y.; Kong, E. S.; Zhang, Y.; Lee, S.-T. Small 2008, 4, 1313–1318. (5) (a) Esfarjani, K.; Farajian, A. A.; Hashi, Y.; Kawazoe, Y. Appl. Phys. Lett. 1999, 74, 79–81. (b) Kong, J.; Cao, J.; Dai, H. Appl. Phys. Lett. 2002, 80, 73–75. (c) Zhou, Y.; Gaur, A.; Hur, S. H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4, 2031–2035. (d) Li, H.; Zhang, Z.; Marzari, N. Nano Lett. 2008, 8, 64–68.
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we have demonstrated that such asymmetric metal electrodes can be readily obtained via a simple mass transport and reduction of Li ions that are preintercalated between the SWNT surface and noncovalently functionalized pyrene molecules.6 Although this method has a key advantage over the conventional lithographic approaches in terms of the reduced number of required lithography steps, it still suffers from a major drawback: the requirement of graphitic layers for the Li ion intercalation, which makes it difficult to be widely employed to one-dimensional nanomaterials other than carbon nanotubes. Herein, we demonstrate a selective electrochemical metal deposition approach to address asymmetric metal electrodes to fabricate SWNT Schottky diodes. Such a selective electrochemical metal deposition approach is advantageous over the previous Li ion mass transport method because it just does not require graphitic substrate layers, but more importantly because it allows one to choose various metal ions beyond Li, through which the metal work function can be unreservedly tuned.
Experimental Section Fabrication of Individual SWNT-FET. The growth of SWNTs on the SiO2/Si substrate containing iron oxide nanoparticles was conducted by chemical vapor deposition (CVD) method at 900 °C for 5 min under the controlled gas flow rate of 1000/500/ 10 sccm of CH4/H2/C2H4, respectively. For the preparation of iron oxide catalyst nanoparticles, polyamidoamine (PAMAM) dendrimers were used as a Fe3þ ion carrier.7 The SWNT-FET devices were fabricated by conventional photolithography: Az5214 (Clariant Industry Ltd.) and 365 nm UV source were used as photoresist and exposure light, respectively. Titanium was thermally evaporated. Cyclovoltammetry and Selective Electrochemical Metal Deposition. All the electrochemical measurements and treatments were carried out using a VersaSTAT 3 potentiostat/galvanostat (Princeton Applied Research). (6) Lim, H.; Shin, H. S.; Shin, H.-J.; Choi, H. C. J. Am. Chem. Soc. 2008, 130, 2160–2161. (7) Choi, H. C.; Kim, W.; Wang, D. W.; Dai, H. J. Phys. Chem. B 2002, 106, 12361–12365.
Published on Web 12/08/2009
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Figure 1. (a) Ids-Vds curve (Vgs = 0 V) and (b) Ids-Vgs curve (Vds=100 mV) of SWNT transistor before electrochemical deposition.
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Figure 3. (a) Linear scale and (b) logarithmic scale Ids-Vds curves of the SWNT-FET device after selective electrochemical Pd deposition (Vgs = 0 V).
Figure 2. Cyclovoltammogram upon the reduction and oxidation of Pd2þ ions on prepatterned Ti electrodes. Arrows indicate the direction of the cyclic voltage sweep.
Characterization. Ids-Vds and Ids-Vgs curves were measured using a semiconductor analyzer (Keithley, 4200SCS). Scanning photoelectron microscopy (SPEM) and space-resolved X-ray photoelectron spectroscopy (XPS) were carried out at the 8A1 (undulator U7) beamline of the synchrotron facility at Pohang Accelerator Laboratory, POSTECH. In SPEM, the X-ray was focused by a diffractive X-ray lens (Fresnel zone plate), and the photoelectrons from the sample produced by the focused X-rays were analyzed by an electron energy analyzer. The energy of X-rays was 563 eV, and the focused X-ray size was below 1 μm.
Results and Discussion To fabricate carbon nanotube Schottky diodes via the selective electrochemical metal electrode deposition method, SWNT-FET devices composed of single semiconducting SWNT channels were first fabricated following the previously reported method.6,8 Briefly, individual SWNTs were grown at low yield directly on SiO2/Si (500 nm thick SiO2 layer on highly doped p-type Si(100)) substrates by the CVD method, and then the fabrication of FET devices was finalized by thermally evaporating Ti (30 nm) for both source (S) and drain (D) electrodes by conventional photolithography. Figure 1 shows representative (a) Ids-Vds and (b) Ids-Vgs characteristic curves of the as-fabricated SWNT-FET device. The Ids-Vds curve is not perfectly linear because Ti source and drain electrodes form symmetric Schottky contacts to SWNTs (Figure 1a, Figure 4a and b). A slight asymmetry in the Ids-Vds curve (Figure 1a) seems to be induced by the different contact resistances that are caused during the fabrication process. The Ids-Vgs curve shows a clear p-type semiconducting behavior (8) Tang, Q.; Moon, H. K.; Lee, Y.; Yoon, S. M.; Song, H. J.; Lim, H.; Choi, H. C. J. Am. Chem. Soc. 2007, 129, 11018–11019.
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Figure 4. Energy diagrams of an as-prepared SWNT-FET device (a and b) and after electrochemical deposition of Pd on one of the Ti electrodes (c and d) upon forward and reverse bias voltage applications. While the as-prepared devices show symmetric Schottky contacts (a and b), the device having Pd deposits show Ohmic-Schottky asymmetric contacts (c and d).
with a threshold voltage of 3 V9 and a complete carrier depletion beyond Vgs = 5 V (Figure 1b). For the fabrication of a Schottky diode through addressing asymmetric metal electrodes, Pd was chosen to be electrochemically deposited on top of only one of the Ti electrodes because Pd-carbon nanotube contact is known to form an Ohmic contact, while the Ti-carbon nanotube makes Schottky contact. In contrast to the symmetric Ti-Ti electrode system that displays a similar current level upon both forward and reverse bias applications, the asymmetric Pd-Ti electrode system is expected to block the current flow when a reverse bias is applied, and high current will flow when a forward bias is applied, resulting in the rectification of current flow. The selective electrochemical metal deposition of Pd was performed in a homemade Teflon electrochemical cell as shown in Scheme 1. The Ti(D) was employed as a working electrode, while Pt and Ag/AgCl (KCl saturated in water) were used as counter and reference electrodes, respectively. Aqueous mixture solutions of PdCl2 (50 μM) and NH4Cl (50 μM) were used as precursor and electrolyte, respectively.10 During the electrochemical deposition process, electrodeposition or electroless deposition of Pd on the SWNT sidewall as well as Ti(S) electrode may also occur nonspecifically because Ti(D) is electrically connected (9) Note that the threshold voltage was determined from the Ids-Vgs curve drawn in a linear scale. (10) (a) Favier, F.; Walter, E. C.; Zach, M. P. Science 2001, 293, 2227–2231. (b) Chiba, A.; Kabe, T.; Wu, W.-C. Mater. Sci. Forum 2003, 437, 65–68.
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Scheme 1. Schematic View of the Selective Electrochemical Pd Deposition Process on One of the Ti Electrodes of a SWNT-FET Device (WE, Working Electrode; CE, Counter Electrode; and RE, Reference Electrode)
Figure 5. Ids-Vgs curves of SWNT-FET after selective electrochemical metal deposition (a) when a forward bias voltage is applied to the Pd/Ti (D) electrode (Vds = 100 mV) and (b) when a reverse bias voltage is applied to the Pd/Ti (D) electrode (Vds = -100 mV).
to the Ti(S) electrode through the semiconducting SWNT.11 To avoid such nonspecific Pd deposition, a positive gate voltage (Vgs = 8 V) was applied during the process, by which hole carriers are completely depleted from the SWNT channel, converting it into an insulator. To determine the specific electrochemical deposition condition, a cyclovoltammetric curve for the reduction of Pd2þ ions was separately obtained using a prepatterned titanium electrode in the absence of SWNTs (Figure 2). Since the Pd2þ reduction peaks were observed at -0.75 V under the potentiostatic mode, the selective electrochemical deposition of Pd only on the Ti(D) electrode was conducted by supplying -0.75 V to the working electrode for 5 s to deposit 5 nm thick palladium. After the deposition, the Ids-Vds curve was measured again, and the diodelike current rectification was clearly observed with a significantly increased on-state current level (Figure 3). This diodelike behavior is clearly attributed to the formation of asymmetric work function levels from the source and drain electrodes and can be rationalized as follows: As depicted in Figure 4c, there should be high current under forward bias (i.e., positive bias voltage applied to Pd/Ti(D) electrode) because the (11) (a) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 124, 9058–9059. (b) Fan, Y.; Goldsmith, B. R.; Collins, P. G. Nat. Mater. 2005, 4, 906–911. (c) Quinn, B. M.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2005, 127, 6146–6147. (d) Kauffman, D. R.; Star, A. Nano Lett. 2007, 7, 1863–1868.
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majority charge carriers (holes) in the SWNT can be transported well through the Ohmic contact between palladium and SWNT. On the other hand, the current flow should be largely prohibited under reverse bias (i.e., negative bias voltage applied to Pd/Ti(D) electrode) because holes have to overcome the Schottky barrier formed at the titanium-SWNT contact (Figure 4d). The aforementioned explanation was experimentally proved by monitoring distinctively different ON state resistance (RON) values observed when the Ids-Vgs characteristic curves were measured under forward and reverse bias voltage applications (Figure 5). When the Pd/Ti(D) electrode is positively biased (forward bias), the RON is ∼ 200 kΩ (quantum conductance, GON ≈ 0.03(4e2/h)) due to the efficient hole injection into the valence band of SWNT (Figures 5a and 4c). On the other hand, when the Pd/Ti(D) is negatively biased (reverse bias), the RON is substantially increased up to ∼10 MΩ (GON ≈ 0.0006(4e2/h)) because holes are injected through the Schottky barrier at the Ti(S)-SWNT junction; hence, holes have to be injected by nonefficient tunneling and thermionic emission processes (Figures 5b and 4d). These results not only clarify the formation of asymmetric Ohmic-Schottky contacts, but also the measured RON values also agree well with previously known ones obtained from individual Ohmic and Schottky SWNT-FET devices. For example, the RON of SWNT-FET (when the length of SWNT = 3 μm, diameter = 3 nm) having Ohmic contacts (Pd-SWNT) is 60 kΩ (GON ≈ 0.1(4e2/h)),3a while the RON of SWNT-FET with Schottky barriers (Ti-SWNT) is substantially increased up to 6 MΩ (GON ≈ 0.001(4e2/h)).12 Note that the RON depends on the length and diameter of the SWNT. We should also note that such selective electrochemical deposition of Pd also could be achieved under the galvanostatic mode. When a constant current of 0.5 μA was applied for the galvanostatic deposition of Pd, similar Schottky diode properties were observed from a separate SWNT-FET device (Supporting Information Figure S1). Especially, the AFM images in Figure 6 demonstrate that the nonspecific deposition of Pd nanoparticles (12) (a) Heinze, S.; Tersoff, J.; Martel, R.; Derycke, V.; Appenzeller, J.; Avouris, Ph. Phys. Rev. Lett. 2002, 89, 106801. (b) Appenzeller, J.; Knoch, J.; Derycke, V.; Martel, R.; Wind, S.; Avouris, Ph. Phys. Rev. Lett. 2002, 89, 126801.
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Pd 3d3/2 (left, 341 eV) and 3d5/2 (right, 336 eV) were clearly detected with much higher intensities from A than from B. Note that the small Pd peaks from B originate from physically adsorbed PdCl2 salts of which positions are shifted toward higher binding energy (342.2 and 336.8 eV).
Figure 6. AFM images of a SWNT-FET device (a) before and (b) after electrochemical metal deposition. Scale bars are 1 μm. After deposition, no Pd nanoparticle was observed on the sidewall of the SWNT.
Figure 7. (a) SPEM image collected based on the areal Si 2p photoelectron intensities from a SWNT-FET device after electrochemical deposition. (b) Space-resolved XPS spectra obtained at A and B positions. Left electrode (A) is deposited with Pd.
on the sidewall of the SWNT is well prohibited during the electrochemical deposition. The selective deposition of Pd on only one of the Ti electrodes was also characterized by SPEM and space-resolved XPS. An SPEM image of the whole device was first obtained by mapping the Si 2p photoelectron (103 eV) data (Figure 7a), and then XPS spectra were obtained at the positions marked with dots of A and B (Figure 6b). As shown in Figure 7b, two peaks corresponding to
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Conclusion We demonstrated a convenient electrochemical method to address asymmetric metal electrodes on SWNT-FET devices, resulting in SWNT Schottky diodes. Under both potentiostatic and galvanostatic modes, only one of the preaddressed Ti electrodes was electrochemically deposited with Pd. Nonspecific deposition of Pd on either the SWNT surface or the other side of Ti electrodes was prohibited by applying a positive gate voltage during the deposition process. The successful selective deposition of Pd induced the rectifying current (Ids) flow as measured from the Ids-Vds curves and asymmetric ON state resistances (RON) under forward and reverse bias voltage applications during the Ids-Vgs scan. Considering the popularity of the electrochemical deposition method and the diversity in the selection of metals, this approach is a promising facile process that can be applied to various one- or two-dimensional nanostructures to fabricate Schottky diodes, and may allow further detailed studies about the charge carrier transport at metal-SWNT junctions. Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by MEST (2008-04306, 2007-8-1158, 2005-01325), KOSEF through EPB center (R11-2008-052-02000), Korean Research Foundation (MOEHRD, KRF-2005-005-J13103). H.C.C. thanks the World Class University (WCU) program (R31-2008-000-10059-0). SPEM and XPS: 8A1 beamline of the Pohang Accelerator Laboratory (PAL) at POSTECH. Supporting Information Available: Experimental results (Ids-Vds and Ids-Vgs characteristics) for galvanostatic mode. This material is available free of charge via the Internet at http://pubs.acs.org.
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