carbon nanotubes blend

Thin Solid Films 517 (2009) 4011–4014

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Thin film transistors by solution-based indium gallium zinc oxide/carbon nanotubes blend Keun Woo Lee, Kon Yi Heo, Sang Hoon Oh, Abderrafia Moujoud, Gun Hee Kim, Hyun Jae Kim ⁎ School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea

a r t i c l e

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Available online 7 February 2009 Keywords: Semiconducting II–III materials Single-walled carbon nanotubes (SWNTs) Solution-based thin film transistors IGZO/SWNTs blend Carrier transport rod

a b s t r a c t Solution-based indium gallium zinc oxide (IGZO)/single-walled carbon nanotubes (SWNTs) blend have been used to fabricate the channel of thin film transistors (TFTs). The electrical characteristics of the fabricated devices were examined. We found a low leakage current and a higher on/off currents ratio for TFT with SWNTs compared to solution-based TFTs made without SWNTs. The saturation field effect mobility (μsat) of about 0.22 cm2/Vs, the current on/off ratio is ~ 105, the subthreshod swing is ~ 2.58 V/decade and the threshold voltage (Vth) is less than − 2.3 V. We demonstrated that the solution-based blend active layer provides the possibility of producing higher performance TFTs for low-cost large area electronic and flexible devices. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental details

ZnO-based transparent electronic devices have been extensively studied. High mobility Indium gallium zinc oxide (IGZO) for transparent active-channel TFTs have been demonstrated [1]. A number of flexible electronics by sol–gel process have been already reported using ZnO [2,3], ZTO [4], IZO [5] and IGZO [6]. These TFT fabricated by sol–gel process showed high off currents and low on/off current ratio compared to TFT made by vacuum process. Recently carbon nanotubes (CNTs) have been used in blends with other conjugated polymers to improve electrical properties of devices. BO et al. reported organic-TFT with active channel based on a blend of pentacene and SWNTs [7], E. Kymakisa et al. demonstrated that the injection of SWNTs into conjugated polymer improve the properties of organic photovoltaic cells [8]. CNTs have a mixture of metallic and semiconducting tubes which would show a various TFT's characteristics [9–11]. In this paper, SWNTs are used as a carrier transport rod to improve the electrical performance of TFT's channel. The active layer of TFT was formed by solution-process. The solution was prepared by dispersing SWNTs in ethanol solvent and blending with IGZO solution. Two sets of devices were fabricated and tested: TFT channels with solution-based IGZO/SWNTs and TFT channels without SWNTs (solution-based IGZO). We demonstrated that a suitable amount of SWNTs in the solution play an important role in enhancing the electrical properties of TFTs.

Fig. 1 shows the structure of IGZO/SWNTs TFT. A top-view SEM image of IGZO/SWNTs channel thin film is also shown in Fig. 1. A 200nm-thick MoW as a gate metal was sputtered on a SiO2 buffered glass substrate with a surface area of 70 × 70 mm2. Then, a 200-nm-thick SiNx film was grown on the substrate by plasma enhanced chemical vapor deposition to act as a gate dielectric. In this experiment, we used SWNTs powder with the outer diameter and length of 1.0–1.2 nm and 5–20 μm, respectively. The SWNTs powder was dissolved in ethanol solvent and stirred by ultrasonication for 24 h at room temperature. The SWNTs concentration ratio was varied between 0.02 wt.% and 0.08 wt.% which is calculated by the ratio of mass of SWNTs and precursor of In, Ga and Zn. The precursor of IGZO was prepared by dissolving 1.0 M of zinc acetate dihydrate (Zn(CH3COO)22H2O), 0.5 M of gallium nitrate hydrate (Ga(NO3)33H2O), and 0.5 M of indium nitrate hydrate (In(NO3)3xH2O) in 20 mL of 2-methoxyethanol solvent. Monoethanolamine was added as a sol stabilizer and acetic acid (CH3COOH) was dropped into the solution to make a homogenous solution while stirring. After vigorously stirring for 1 h at 60 °C, the IGZO sol was aged for 72 h. The blended solution was prepared by mixing the dispersed SWNTs in ethanol solvent and IGZO solution. This solution was spin-coated on glass/buffer substrate (corning 1737), then it was annealed at 450 °C for 1 h to give a 100-nm-thick active layer as shown in Fig. 1a. Finally the metal electrode was deposited by rf magnetron sputtering of indium zinc oxide (IZO) material. The performances of the solution-processed IGZO TFTs with length (L) of 150 μm and width (W) of 1000 μm were measured in the dark at room temperature with a semiconductor parameter analyzer 4156C. The structural properties of solution processed IGZO/SWNTs

⁎ Corresponding author. E-mail address: [email protected] (H.J. Kim). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.145

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K.W. Lee et al. / Thin Solid Films 517 (2009) 4011–4014

Fig. 1. (a) The solution-based IGZO/SWNTs blend TFT fabrication process (b) Schematic structure of a IGZO/SWNTs TFT device and top-view SEM image of a representative TFT test device.

film were investigated by scanning electron microscopy (SEM) measurements. The electrical properties of solution-based IGZO/ SWNTs films were investigated by Hall measurements in the van der Pauw configuration in a magnetic field of B = 0.5 T at room temperature. The optical transmission measurements were performed using a UV-near IR grating spectrometer. 3. Results and discussion The transmission of a spin-coated IGZO and IGZO/SWNTs thin films deposited on glass/SiO2/SiNx substrate in the wavelength range from 200 to 900 nm is shown in Fig. 2. The spectrum shows that the IGZO/SWNTs thin film is highly transparent with above 80% transparency in the visible range (350–900 nm).The optical absorp-

tion coefficient (α) calculated from the transmittance was used to determine the optical bandgap (Eg) and is given by the relation 2

ðαhvÞ = hv −Eg




ð1Þ

where hν is the photon energy. The optical bandgap can be determined by the extrapolation of the linear region from a plot of (α)2 vs. photon energy (hν) near the onset of the absorption edge to the photon energy axis. The optical bandgap of the solution-based IGZO thin film was measured to be 3.86 eV. The solution-based IGZO/ SWNTs (0.02 wt.%) thin film was measured to have a bandgap of 3.94 eV, which is higher than the value reported for the optical bandgap of a-IGZO (3.1–3.3 eV [12]). An increase of 0.08 eV is due to the metallic nature of the SWNTs, that contributes to increase in absorption for photon energy [13] and the increase of carrier concentration (see Table 1). Even though the concentration of SWNTs increases in our experiment, the optical bandgap was saturated. The solution-based IGZO/SWNTs (0.04 wt.%) thin film and IGZO thin film were investigated by Hall measurements. Table 1 summarizes the measured data. We found that for the active layer of IGZO/ SWNTs the free carrier concentration, hall mobility, and electrical resistivity are 2 times of magnitude higher, 3 times higher and 3-order lower, respectively, than for the active layer of IGZO. It is clear that adding SWNTs have a positive effect on device performance. Fig. 3b shows the output curves of the IGZO/SWNTs (0.04 wt.%) at various gate voltages. Fig. 3a shows the transfer curve of the IGZO/

Table 1 Electrical properties of TFTs with different active layers using Hall measurement.

Fig. 2. UV–vis spectrum of spin-coated IGZO/SWNTs thin films on a glass/SiO2/SiNx substrate as a function of the concentration of SWNTs.

Active layer

Free carrier concentration [cm−3]

Hall mobility [cm2/V s]

Resistivity [Ω cm]

IGZO IGZO/SWNTs (0.04 wt.%)

9.55 × 1012 1.63 × 1013

6.9 22.6

1.95 × 107 1.93 × 104

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Fig. 4. Hysteresis characteristics of a IGZO/SWNTs (0.04 wt.%) TFT.

Fig. 3. (a) Drain current–gate voltage Ids–Vg and log (Ids)–Vg transfer characteristics of the IGZO/SWNTs (0.04 wt.%) with Vds of 10.1 V, (b) Drain current–drain voltage (Ids– Vds) output characteristics.

SWNTs (0.04 wt.%) TFT with a drain-source voltage Vds of 10.1 V. The IGZO/SWNTs (0.04 wt.%) TFT behaves as an n-channel transistor and exhibits good linear/saturation behavior. The electrical parameters were determined from a plot of I1/2 ds vs. Vg on the basis of the following relationship in the saturation regime Ids =


2 W  Ci μ sat Vg −Vth L

ð2Þ

where Ids is the drain current, W of 1000 µm and L of 150 µm are the channel width and length, respectively, Ci is the capacitance per unit area, μsat is the field-effect mobility, and Vth is the threshold voltage. Table 2 shows that with a saturation field effect mobility (μsat) of about 0.22 cm2/Vs at a Vds of 10.1 V, the current on/off ratio is ~ 105, the subthreshold swing is ~2.58 V/decade and the threshold voltage (Vth) is less than −2.3 V. The leakage current (about 1.33 fA/μm2) was low. It is believed that SWNTs play an important role as a carrier transport rod in TFTs along the channel length and its concentration in the solution has to be optimized. Fig. 4 shows the hysteresis characteristics of a IGZO/SWNTs (0.04 wt.%) TFT while sweeping gate voltage (Vg) between −30 V

Table 2 Summary of the electrical characteristics of IGZO/SWNTs TFT. Active layer

Threshold voltage [V]

On/off current ratio

Subthreshold swing [V/decade]

Mobility [cm2/V s]

IGZO/SWNTs (0.04 wt.%)

− 2.3

~105

2.58

0.22

Fig. 5. Electrical stability of a IGZO/SWNTs (0.04 wt.%) TFT.

and 30 V which has ΔVg of about 5.0 V at Ids of 100 nA, which is larger than the value (about 2.0 V) reported for ΔVg of amorphous IGZO TFT with SiNx dielectric [14]. This large value suggests that the active layer/dielectric interface and dielectric materials play a very important role in determining the device characteristics [15]. The electrical stability of the IGZO/SWNTs (0.04 wt.%) TFT was evaluated by repeating transfer characteristics. Fig. 5 shows 7-times repeating transfer characteristics of the same TFT device at Vds of 5.0 V while sweeping gate voltage (Vg) between −30 V and 30 V. During repetition, threshold voltage (Vth) shift to more negative Vg values with repeating gate voltage sweep indicating that the holes are trapped (positive space charge) within the channel or at the interface. After the fourth gate voltage sweep, threshold voltage (Vth) shift was saturated. 4. Conclusion In conclusion, to improve carrier transport in the channel region of TFT, we blended suitable amounts of SWNTs with IGZO in solution. We found that the electrical properties and optical bandgap are changed by the addition of the SWNTs when compared to solution-based IGZO thin film. The TFT with channel fabricated using solution-based indium gallium zinc oxide (IGZO)/SWNTs show good performance. The saturation field effect mobility (μsat) of about 0.22 cm2/Vs, the current on/off ratio is ~ 105, the subthreshod swing is ~ 2.58 V/decade and the threshold voltage (Vth) is less than −2.3 V. We demonstrated that the solution-based blend active layer provides the possibility of producing higher performance TFTs for low-cost large area electronic and flexible devices.

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Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOST) (No. R0A-2007-000-10044-0 (2007)). References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature 488 (2004) 432. [2] C.S. Li, Y.N. Li, Y.L. Wu, B.S. Ong, R.O. Loutfy, J. Phys. D: Appl. Phys. 41 (2008) 125102. [3] B.S. Ong, C. Li, Y. Li, Y. Wig, R. Loutfy, J. Am. Chem. Soc. 129 (2007) 2750. [4] Y.J. Chang, D.H. Lee, G.S. Herman, C.H. Chang, Electrochem. Solid-State Lett. 10 (2007) H135. [5] C.G. Choi, S.J. Seo, B.S. Bae, Electrochem. Solid-State Lett. 11 (2008) H7.

[6] G.H. Kim, H.S. Shin, B.D. Ahn, K.H. Kim, W.J. Park, H.J. Kim, J. Electrochem. Soc. 156 (1) (2009) H7. [7] X.Z. Bo, N.G. Tassi, C.Y. Lee, M.S. Strano, C. Nuckolls, Graciela B. Blancheta, Appl. Phys. Lett. 87 (2005) 203510. [8] E. Kymakisa, G.A.J. Amaratunga, Appl. Phys. Lett. 80 (2002) 112. [9] S. Lastella, G. Mallick, R. Woo, S.P. Karna, D.A. Rider, I. Manners, Y.J. Jung, C.Y. Ryu, P.M. Ajayanc, J. Appl. Phys. 99 (2006) 024302. [10] A. Javey1, J. Guo, Q. Wang, M. Lundstrom, H. Dai1, Nature 424 (2003) 654. [11] R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E. Scuseria, D. Tománek, J.E. Fischer, R.E. Smalley, Science 273 (1996) 483. [12] A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 486 (2005) 38. [13] E. Kymakisa, I. Alexandou, G.A.J. Amaratunga, Synth. Met. 127 (2002) 59. [14] A. Suresh, P. Wellenius, A. Dhawn, J. Muth, Appl. Phys. Lett. 90 (2007) 123512. [15] S. Yaginuma, J. Yamaguchi, K. Itaka, H. Koinuma, Thin Solid Films 486 (2005) 218.