Fabrication and characterization of the performance of multi-channel carbon-nanotube field-effect transistors

Physics Letters A 366 (2007) 474–479 www.elsevier.com/locate/pla

Fabrication and characterization of the performance of multi-channel carbon-nanotube field-effect transistors Changxin Chen ∗ , Zhongyu Hou, Xuan Liu, Eric Siu-Wai Kong, Jiangping Miao, Yafei Zhang National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, and Institute of Micro/Nanometer Science & Technology, Shanghai Jiao Tong University, 1954 Hua Shan Road, Shanghai 200030, China Received 10 October 2006; received in revised form 13 February 2007; accepted 14 February 2007 Available online 1 March 2007 Communicated by R. Wu

Abstract A multi-channel carbon-nanotube field-effect transistor (CNTFET) with directed, controllable number of single-walled carbon nanotube (SWCNT) channels has been successfully fabricated by combining electric-field assisted alignment technique with atomic force microscopy (AFM) manipulation technology. Compared with the single-channel CNTFET, not only the ON current, transconductance and switch performance of this CNTFET are effectively improved but also the reliability and yield of the device are also enhanced with the multi-channel device structure. The transconductance of multi-channel CNTFETs has also been demonstrated to take on an approximately direct dependency on the SWCNT channel number. These merits make the multi-channel CNTFET promising to be applied in the domain of future nanoelectronic integrated circuits. © 2007 Elsevier B.V. All rights reserved. PACS: 85.35.Kt; 72.80.Rj; 85.30.Tv Keywords: Multi-channel carbon-nanotube field-effect transistors (CNTFET); Single-wall carbon nanotubes (SWCNTs); Atomic force microscopy (AFM) manipulation; Electric-field assisted alignment

1. Introduction Carbon nanotubes (CNTs) are good candidates for molecular devices because of their quasi one-dimensional nano-structure and unique electrical properties [1,2]. One of the important findings in this respect is that CNTs can be fabricated as channels of the field-effect transistors (FETs). Individual singlewall carbon nanotube (SWCNT) has been extensively used as conduction channels to assemble the carbon nanotube fieldeffect transistors (CNTFETs) [3–6]. However, the maximum endurable current of 25 µA per SWCNT limits the current output and transconductance of fabricated CNTFETs [7]. To acquire the higher transconductance or drive current, the parallel, multiple SWCNTs are needed to be applied as the channel of CNTFETs. Theoretical studies have also forecasted the CNT* Corresponding author.

E-mail addresses: [email protected], [email protected] (C. Chen), [email protected] (Y. Zhang). 0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.02.089

FETs with a periodic array of CNTs as channels will exhibit much better performance than the single-channel CNTFETs [8,9]. Besides, the multi-channel CNTFETs will also be expected to provide a higher device reliability and yield than the single-channel CNTFETs, which contributes to the practical implementation of CNTFETs in circuit applications, keeping in mind the device will fail if the only CNT channel breaks down in the single-channel CNTFETs. Recently, although there have been reports of high output current CNTFETs that use a large number of CNTs as the channel [10,11], their general device performance was not good enough due to the lack of techniques to produce well directed, controllable number of CNT channels, which limits their application range. In this work, a multi-channel CNTFET with directed, controllable number of SWCNT channels has been fabricated by combining the electric-field assisted alignment technique with atomic force microscopy (AFM) manipulation technology. Our technique provides a favorable way to construct and control the channels of CNTFETs. The performance

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of the multi-channel CNTFETs and the effects of the SWCNT channel number have been studied. Also, the device reliability and yield for this multi-channel CNTFET is discussed. 2. Experimental details 2.1. Deposition and breakdown of SWCNTs The SWCNTs used in this study were synthesized by the arc-discharge method and the average diameter of these singlewalled carbon nanotubes was 2.2 nm. The SWCNTs were pretreated to disperse them fully in the solvent. The synthesized SWCNTs were purified and subjected to chain-scission under the treatment of a 3:1 mixture of concentrate sulfuric acid and nitric acid at 80 ◦ C for 30 min in the reflux system. This acid treatment process decreases the length of the SWCNTs to about 2 µm, which allows the SWCNTs to be dispersed more readily. The undried SWCNTs were filtrated with isopropanol to remove the water in the sample (Drying SWCNTs in water can induce the agglomeration of the individual nanotubes into bundles). The as-filtered SWCNTs were then ultrasonicated in isopropanol and centrifuged at 15,000 rpm for 2.5 hours to further separate large SWCNT bundles from individual SWCNTs. The obtained supernatant was ultrasonically treated in isopropanol for about 20 hours in order to sufficiently disperse the SWCNTs. Using the standard UV lithography and lift-off technique, parallel source and drain electrodes (5 nm Cr/40 nm Au) were patterned on n-type silicon wafers which were thermally coated with a 100 nm thick silicon dioxide layer. Source and drain electrodes were 5 µm in length, 2 µm in width and had a gap of about 1 µm. After a 10V AC bias with a frequency of 5 MHz was applied between source and drain, a droplet of isopropanol (IPA) solution of SWCNT (SWCNTs concentration = 0.1 µg/ml) was introduced onto the structured wafer. The AC bias was switched off after the IPA solvent was evaporated. The wafer was then rinsed using IPA to clean the surface. Thus, SWCNTs were aligned between the source and drain electrodes. The more detailed description on the AC electric-field assisted alignment process can be found elsewhere [12]. After the deposition, with the heavily doped silicon (Si) as the gate, an electrical breakdown method [13,14] was used to selectively burn off the metallic SWCNTs and to keep the remaining semiconducting SWCNTs. The as-fabricated samples were observed under AFM, followed by the next processing step of AFM manipulation. 2.2. Construction of different numbers of SWCNT channels When the contact-mode AFM is used for imaging, the contact force between the AFM tip and sample surface will change whenever a change in the height of the sample surface is encountered. A feedback loop then changes the height of the cantilever over the surface to maintain a constant setpoint force. Therefore, when the applied vertical load of AFM tip to sample surface is small for imaging, the lateral forces acting on the sample will be too small to move SWCNTs. If, by contrast, the

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feedback was turned off, the cantilever was pressed down onto the substrate, and dragged along a predefined path over the surface, one would be able to shift the nanotubes laterally due to the large lateral forces [15]. This is the basis of our manipulation technique. In AFM manipulation, we used a commercial AFM mounted with the cantilevers with a force constant of about 0.58 N/m and a resonant frequency of about 30 kHz. The Si3 N4 AFM tip used is coated with gold and the nominal tip radius of curvature is about 30 nm. Fig. 1 shows a typical manipulation process of SWCNT channels. As shown in Fig. 1(a), three SWCNTs bridged the electrodes initially. The bridging SWCNTs were identified to be individual SWCNTs instead of SWCNT bundles by their height measuring with atomic force microscopy (AFM). It was considered as individual SWCNT when the measured AFM height was smaller than 2.4 nm in our experiment. Fig. 1(b) is the zoom-in view of the SWCNT on the top of Fig. 1(a). When imaging the SWCNT using the contact mode with a vertical load of 27.0 nN, no visible changes of the configuration or position were observed for the SWCNT. Increasing the vertical load up to 43.8 nN, some bends were observed on the SWCNT after scanning of AFM tip for imaging, as shown in Fig. 1(c). But the SWCNT still bridged the source and drain electrodes. It was found that it was difficult to disconnect the SWCNT with the electrodes. To shift the SWCNT more easily, the feedback was turned off and the AFM tip was made to move across the gap surface along the predefined path which is parallel to the electrodes. After this process, the AFM was returned to image mode to inspect the sample. It was found that the manipulated SWCNT was invisible at this time (see Fig. 1(d)). Moreover, the SWCNT could not be found near the original location. It is considered that the SWCNT was attached onto the AFM tip and taken away with the movement of the AFM tip. With the same operation, other SWCNTs in the electrode gap were also removed from the gap in turn (see Fig. 1(e) and (f)). Thus, the different numbers of SWCNT channels were achieved. It should be mentioned that some SWCNTs still remained in the gap and were not attached onto the AFM tip after the AFM manipulation in the experiment, which is similar to some other works on using AFM to manipulate CNTs [15,16]. As shown in Fig. 2, the SWCNT was moved and bended in the gap after the manipulation, which made it no longer connect both electrodes. Whether or not the SWCNTs would be attached onto the AFM tip depends on the magnitude of the van der Waals forces between the SWCNT and the substrate. When the van der Waals force between the SWCNT and the substrate is large enough, the SWCNT will remain on the substrate after the AFM manipulation. 2.3. Measurement of electrical characteristics Using the heavily doped Si as the gate, the electrical characteristics of the different channel numbers of CNTFETs were measured with the Agilent 4156C precision semiconductor parameter analyzer.

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Fig. 1. Three-dimensional AFM images of a typical manipulation process. (a) The initial channel configuration with multiple SWCNTs bridging the electrodes before AFM manipulation. (b)–(d) AFM images illustrating the removal of a SWCNT channel from the electrode gap. The arrow in (d) indicated the traveling path of the AFM tip. (e), (f) AFM images showing the results that the other SWCNTs in the electrode gap were removed from the gap in turn.

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Fig. 2. (a) Zoom-in AFM image of the SWCNT in Fig. 1(f). (b) AFM image of the SWCNT remaining in the gap after the AFM manipulation. The arrow in the image shows the traveling path of the AFM tip.

3. Results and discussion The transfer characteristics of CNTFETs with different channel numbers are shown in Fig. 3(a). The transistors exhibit the p-type characteristics. The drain current IDS exhibits a linear dependency on VG at first, and then tends to reach saturation with the decrease of VG due to the effect of the contact resistance between SWCNTs and electrodes [17,18]. Fig. 3(b) shows one of the typical output characteristic curves for the obtained multi-channel CNTFETs. Table 1 summarizes the key device parameters of three asfabricated CNTFETs with different channel numbers, which extracted from the current–voltage (I–V) characteristic curves. As show in Table 1, with the increase of SWCNT channel number, the ON current and transconductance are effectively enhanced. It is suggested that the transconductance is approximately direct proportional to the channel number. This is further verified by comparing a series of CNTFETs with different SWCNT channel numbers (Fig. 4). For a multi-channel CNTFETs with 17 SWCNT channels, a high transconductance of ∼7.3 µS and ON current of ∼34.5 µS is achieved, which greatly exceed those previously reported for solid-state back-gate individual nanotube FETs [16,17,19–23]. The linear dependency between the transconductance and SWCNT channel number can be understood by the relation between the gate–channel capacitance and channel number. When the space between neighboring nanotubes is larger than double diameter of SWCNTs, the screening of the gate charge between neighboring SWCNTs due to the charge spread on the gate plane can be ignored [8]. For this time, the gate–channel capacitance for the parallel SWCNTs on the plane is direct proportional to the SWCNT number, which can be expressed as Cg = nCg (n is the channel number). On the other hand, in the linear IDS –VDS region the transconductance of the CNTFET can be expressed as dIds /dVg = μh Cg Vds /L2 , where μh is the field-effect mobility of the device, and L is the length of the CNT between the source and drain electrodes [23]. Thus, it can be deduced that the transconductance is proportional to the SWCNT num-

Fig. 3. (a) Transfer characteristics of the CNTFETs with different SWCNT channel numbers. Inset: Logarithmic plot of transfer characteristic of the CNTFET with channel numbers N = 3. (b) Output characteristics of the CNTFET with 3 SWCNT channels.

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Table 1 Key device parameters of the CNTFETs with different SWCNT channel numbers at VDS = 1.0 V Channel number

VTH (V)

ON current (µA)

Transconductance (µS)

Inverse sub-threshold slope (mV/dec)

Current on-off ratio

Mobility (cm2 V−1 s−1 )

1 2 3

−2.37 −2.15 −1.50

1.7 3.0 5.5

0.46 0.87 1.40

589 254 171

3 × 105 7 × 105 1 × 106

244 232 248

Fig. 4. The dependency of the transconductance on the SWCNT channel number.

ber n for the fixed L and VDS . This direct dependency between the transconductance and SWCNT channel number opens up a promising way to linearly tune the transconductance of CNTFETs for meeting the different requirements in future nanoelectronic integrated circuits by varying the SWCNT channel number, which plays an analogous role of changing the gate width for traditional Si FETs. It was found that the VTH of the device increased with the increase of CNT channel numbers in CNTFETs (Fig. 3(a)). This can be attributed to the inconsistent turn-off performance for different SWCNT channels in the multi-channel FET, which makes a relatively larger gate voltage required to turn off all SWCNT channels. The switch performance of the device could also be improved by using the multi-channel device structure. As shown in Table 1, for a multi-channel FET with 3 SWCNT channels, an inverse subthreshold slope of S ∼ 171 mV/dec is obtained (see the inset in Fig. 3(a)), superior to the device with bottom gate and Al2 O3 dielectric (S ∼ 180 mV/dec) [19], while it is ∼589 mV/dec for single-channel CNTFETs. The obtained inverse subthreshold slope of multi-channel CNTFETs is inferior to recently reported values (63–70 mV/dec) [4,5] due to that the top-gate structure and thin high-κ gate insulator are used in those literatures. For the improvement of switch performance with the multi-channel structure, further studies are needed to elucidate its reason. We consider that the improvement of subthreshold slope could originate from a dispersion of subthreshold slopes of each channel because the threshold voltages are different in each channel. When multiple SWCNTs were applied to construct the CNTFETs, each SWCNT channel will may take on the inconsistent subthreshold switch performance due to the different interface trap density and SWCNT-electrode

contact characteristics. In the operation of the multi-channel CNTFETs, the SWCNT channels with the best subthreshold switch performance will play a dominant role for the switch performance of the device, which could cause a small subthreshold slope. The standard transistor model for the linear IDS –VDS region is used to analyze the carrier mobilities for the multichannel CNTFETs [24]. Analogous to the analysis for MOSFETs in the linear IDS –VDS regions, the hole conductive mobility μh for the device can be deduced from GDS = IDS /VDS = μh Cg /L2 (VGS –VTH ), where GDS is the zero bias conductance, Cg (= nCg ) is the gate–channel capacitance, and L is the gate length [25]. For per SWCNT channel in CNTFETs, the gate-channel capacitance can be expressed as Cg = (Cox Cg_Q )/(Cox + Cg_Q ), where Cox is the electrostatic gatecoupling capacitance of the gate oxide and Cg_Q is the quantum capacitance of the gated SWCNT. In the calculation, we take the approximation of ∼4 pF cm−1 for unit length quantum capacitance [24,26–28], which should be an upper bound for the true capacitance when only one subband is occupied in the tube. And the electrostatic gate-coupling capacitance of the gate oxide is given by   Cox = 2πεε0 L/ cosh−1 (h/r) ≈ 2πεε0 L/ ln(2h/r), where ε, h, and r are the dielectric constant and thickness of silicon dioxide, and the radius of SWCNTs, respectively [23, 24,29]. Using ε ∼ 3.9, h ∼ 100 nm, L ∼ 1 µm and r ∼ 1.1 nm, it can be deduced that the carrier effective mobility (conductive mobility) μh for our device is about 240 cm2 V−1 s−1 , which is much higher than those early reported for back-gate CNTFETs [23,30] but inferior to that recently reported for Pdcontacted CNTFETs [31,32]. Previous studies had reported the ballistic transport of CNTFETs [33] and the CNTFETs with very high mobility (∼79,000 cm2 V−1 s−1 ) which is attributed to the ohmically-contacted long CNT channel was used in the device and the resistance of the device is dominated by the intrinsic resistance of the nanotube channel [34]. For the relatively low mobility in our device, it can be attributed to the large contact resistance. A larger mobility can be obtained by further optimizing the metal-nanotube contacts, which is reported elsewhere [35,36]. The good ohmic CNT/metal contacts will decrease the carrier scatter at the CNT/metal interfaces so as to enhance the carrier mobility for the devices. The device reliability and yield of single- and multi-channel CNTFETs was also compared in order to evaluate their serviceability. In the experiment, 20 single- and multi-channel (channel number >3) CNTFETs are respectively measured in the laboratory atmosphere at intervals of a definite period. It has been

C. Chen et al. / Physics Letters A 366 (2007) 474–479

shown that the single-channel FETs are generally out of work after a period of about three weeks, while 95% multi-channel FETs can maintain stable performances after several months. This is because that the multi-channel CNTFETs can still keep operating when a portion of the channels break down, while the single-channel CNTFETs will fail if the only channel breaks down. The occurrence of the instability for few multi-channel CNTFETs may originate from that no firm contacts was formed between the SWCNTs and metal electrodes. When the devices are run and placed for a long time, some SWCNTs will not keep reliable electrical contacts with the electrodes. The acquirement of the firm contacts by contact optimizations [37] and the use of the surface passivation film can be expected to enhance the stability of our devices further. On the other hand, in the case of only considering the influence of the channel to the device yield, if the required effective SWCNT channel number for the practical application is m, we deduce that the device yield can be expressed as y=

n−m 

Cnl k l (1 − k)n−l ,

l=0

where k is the failure rate of per SWCNT channel and n is the total fabricated channel number. It indicates that the device yield can also be greatly enhanced by increasing the fabricated channel number n. 4. Conclusions Multi-channel CNTFETs with directed, controllable number of SWCNT channels have been successfully fabricated by using an electric-field alignment technique in combination with an AFM manipulation technology. The multi-channel device structure can effectively improve the performance of CNTFETs, which makes the device exhibit not only larger transconductance and ON current but also higher device reliability and yield. The transconductance of multi-channel CNTFETs has been demonstrated to have an approximately direct dependency on the SWCNT channel number. Our results show this MCCNTFET very promising to be applied in the domain of future nanoelectronic integrated circuits. Acknowledgements This work is supported by National Basic Research Program of China No. 2006CB300406; Shanghai Science and Technology Grant No. 05nm0533; National Natural Science Foundation of China No. 60576064; and Shanghai Municipal Commission for Science and Technology Grant No. 03DZ14025. References [1] W. Hoenlein, F. Kreupl, G.S. Duesberg, A.P. Graham, M. Liebau, R.V. Seidel, E. Unger, IEEE Trans. Comp. Pack. Technol. 27 (2004) 629.

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