Low-Resistance Electrical Contact to Carbon Nanotubes with Graphitic

12 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 1, JANUARY 2012 Low-Resistance Electrical Contact to Carbon Nanotubes With Graphitic Interfacial Layer Yang Chai, Arash Hazeghi, Kuniharu Takei, Hong-Yu Chen, Student Member, IEEE, Philip C. H. Chan, Fellow, IEEE, Ali Javey, and H.-S. Philip Wong, Fellow, IEEE Abstract—Carbon nanotubes (CNTs) are promising candidates s-CNT and metal electrode [4]–[6]. However, experimental for transistors and interconnects for nanoelectronic circuits. Al- results have shown that the contact resistance is still large for though CNTs intrinsically have excellent electrical conductivity, the CNTs with metallic band structure [2], where the SB should the large contact resistance at the interface between CNT and metal hinders its practical application. Here, we show that electri- not exist at the interface between metallic CNT and metal. cal contact to the CNT is substantially improved using a graphitic This indicates that there is additional contact barrier or series interfacial layer catalyzed by a Ni layer. The p-type semiconduct- resistance for metallic CNT/metal. Recent experimental results ing CNT with graphitic contact exhibits high ON-state conduc- on both semiconducting and metallic CNT devices revealed that tance at room temperature and a steep subthreshold swing in a surface chemistry is very important for forming good electrical back-gate configuration. We also show contact improvement to the semiconducting CNTs with different capping metals. To study the contact between CNT and metal [7], [8]. The metal wetting role of the graphitic interfacial layer in the contact stack, the cap- to the tubular structure of the CNT is imperfect, where the ping metal and Ni catalyst were selectively removed and replaced metal atoms are not fully covered on the CNT surface. An with new metal pads deposited by evaporation and without further atomic-level physical gap exists between CNT and metal [9], annealing. Good electrical contact to the semiconducting CNTs [10]. The cohesion between metal and sp2 carbon is inversely was still preserved after the new metal replacement, indicating that the contact improvement is attributed to the presence of the correlated with the metal-carbon distance [11]. Researchers graphitic interfacial layer. also experimentally and theoretically demonstrated that the contact resistance to the CNT is dependent on the contact Index Terms—Amorphous carbon, carbon nanotube (CNT), contact, field-effect transistor, graphene, graphitic, interface. length, indicating that the contact resistance has a spreading resistance component [12]–[15]. I. INTRODUCTION To develop the CNT-based electronics to achieve its full performance potential, it is imperative to minimize the contact LECTRICAL contact is an indispensable part in inte- resistance to the CNT device. For Si CMOS technology, metal grated circuits. The small contact area between carbon E silicides have been widely used for ohmic contacts [1]. For nanotubes (CNTs) and metal electrode makes electrical cou- Si nanowire, doped epitaxial Si has been reported for the pling between them extremely difficult [1]–[3]. The large use of the contact material [16]. For carbon-based large-band- electrical contact resistance hinders the practical electronics gap semiconductors, such as silicon carbide and diamond, a applications of the CNTs, although it has high intrinsically elec- graphitic carbon layer has been used to form ohmic contact trical conductivity [2], [3]. The contact between semiconduct- [17]–[19]. Graphitic carbon, with metal-like resistivity, similar ing CNT (s-CNT) and metal is generally modeled as a Schottky chemical bonding to the CNT, and much better wettability to barrier (SB), resulting from the Fermi level mismatch between the CNT than other regular metal, is possibly a low-resistance contact material to the CNT. Recently, researchers have used Manuscript received May 25, 2011; revised July 25, 2011, August 15, 2011, electron-beam-induced carbon deposition to the CNT/metal September 15, 2011, and September 21, 2011; accepted September 25, 2011. Date of publication October 31, 2011; date of current version December 23, contact region inside a scanning electron microscope (SEM) 2011. This work was supported in part by the Focus Center for Functional or transmission electron microscope (TEM) and formed low- Engineered Nano Architectonics, which is one of the six research centers resistance electrical contact to multiwalled CNT [20]–[26]. The funded under the Focus Center Research Program, a Semiconductor Research Corporation subsidiary; by the Research Grant Council of Hong Kong Gov- graphitization of the carbon layer can be formed via dehydro- ernment under CERG Grant HKUST 611307; by the Berkeley Sensors and genation by electron beam irradiation. Kane et al. reported that Actuators Center; and by the World Class University program. The review of electrical contact to the single-walled metallic CNT device was this paper was arranged by Editor A. C. Seabaugh. Y. Chai, H.-Y. Chen, and H.-S. P. Wong are with the Department of Elec- greatly improved by a high-temperature annealing process in trical Engineering and the Center for Integrated Systems, Stanford University, vacuum [27], [28]. They showed that the chemisorbed carbona- Stanford, CA 94305 USA (e-mail: [email protected]). A. Hazeghi is with Quswami Inc., San Francisco, CA 94111 USA. ceous contamination to the CNT surface can be graphitized with K. Takei and A. Javey are with the Department of Electrical Engineering and the high-temperature anneal process [27], [28]. In an earlier Computer Sciences, University of California at Berkeley, CA 94720 USA. work, we have used amorphous carbon (a-C) as the interfacial P. C. H. Chan is with the Department of Electronic and Computer Engineer- ing, Hong Kong University of Science and Technology, Kowloon, Hong Kong, layer between the metal and the single-walled metallic CNT. and the Department of Electronic and Information Engineering, Hong Kong After the graphitization of the a-C assisted by Ni catalyst, the Polytechnic University, Kowloon, Hong Kong. electrical contact to the metallic CNT has been substantially Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. improved [3]. Theoretical calculations also suggested that good Digital Object Identifier 10.1109/TED.2011.2170216 electrical contact can be formed between the CNT and the 0018-9383/$26.00 © 2011 IEEE CHAI et al.: LOW-RESISTANCE ELECTRICAL CONTACT TO CNTs WITH GRAPHITIC INTERFACIAL LAYER 13 graphitic carbon [29], [30]. In this paper, we show that the use of a graphitic carbon (G-C) interfacial layer to s-CNT can improve the electrical contact to the s-CNT and reduce the subthreshold swing of transistors with these improved contacts. II. FABRICATION OF TEST STRUCTURE We used a 4-in Si wafer coated with 200-nm-thick thermal oxide as the substrate. We grew the horizontally aligned CNTs on quartz wafers and transferred the CNTs to the Si/SiO2 substrate. The detailed CNT growth and transfer process has been described elsewhere [31]. The average diameter of the CNTs is 1.2 nm in this work. The patterns of the metal elec- trodes were defined by a standard photolithography process. The metal was deposited by e-beam evaporation and then lifted off. The active region of the CNT device was defined by another photolithography process. The rest of the CNT outside the active region was etched away by oxygen plasma. The average Fig. 1. Electrical characterization of the s-CNT devices with different contact metals. (a) Schematic energy band diagram of the ON-state (p-channel only) site density of the CNTs is 2 CNT/µm. The number of CNTs for the SB-type CNT devices with different contact metals. The high-work- for devices with 1-µm channel width ranges from 1 to 3. In this function metal forms a small SB barrier to the s-CNT. (b) Representative paper, we select the devices with only a single CNT and report Id–Vgs transfer curves of single CNT devices with Pt, Pd, Au, Ti, and W contact metal. Vds is −1 V. The channel length is 1 µm. The ON current of their electrical characterization. the CNT devices shows no obvious dependence on the metal work function. results in a large series contact resistance or a physical barrier. III. RESULTS In contrast, Ti has a lower work function (4.4 eV) than Pt, but The CNT field-effect transistor (FET) with metal side- the CNT devices with Ti contact show better conductance than contacted configuration (metal/CNT sidewall) has been shown the Pt contact devices. These results clearly indicate that the to work as an SB transistor, where the SB height is mainly electrical contact resistance to s-CNT is not only determined determined by the metal work function and the band gap of by the SB height but also correlates the cohesive strength of the s-CNT [4]–[6]. For a typical p-type CNT transistor, the metal electrode-carbon interface [11]. Theoretical work has showed with high work function forms small SB height contact, as that the contact resistance to CNT also depends on the contact schematically shown in Fig. 1(a), where the Fermi level of the length, the coupling conductance, and the mean-free path of the metal aligns well with the valence band of the CNT. Palladium CNT [15]. To further understand the contact resistance to CNT, (Pd), which is a noble metal with high work function and we need to consider the factors previously mentioned. good wetting interactions with CNT, has been found with good Compared to regular metal contact, carbon itself is a material electrical contact to both semiconducting and metallic CNTs with the best wettability to the CNT surface. In this paper, [32], [33]. In this paper, we used five kinds of metal with we deposited a thin carbon film (“nominal” thickness ∼2nm) different work functions as the contact to the single CNT.

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