Fabrication of a solution-processed thin-film transistor ...

Superlattices and Microstructures 42 (2007) 361–368 www.elsevier.com/locate/superlattices

Fabrication of a solution-processed thin-film transistor using zinc oxide nanoparticles and zinc acetate Sul Lee, Sunho Jeong, Dongjo Kim, Bong Kyun Park, Jooho Moon ∗ Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea Available online 25 May 2007

Abstract We have fabricated a solution-processed ZnO thin-film transistor without vacuum deposition. ZnO nanoparticles were prepared by the polyol method from zinc acetate, polyvinyl pyrrolidone, and diethyleneglycol. The solution-processable semiconductor ink was prepared by dispersing the synthesized ZnO in a solvent. Inverted stagger type thin-film transistors were fabricated by spin casting the ZnO ink on the heavily doped Si wafer with 200 nm thick SiO2 , followed by evaporation of Cr/Au source and drain electrodes. After the drying and heat treatment at 600 ◦ C, a relatively dense ZnO film was obtained. The film characteristics were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). In order to obtain the electrical properties of the solution-derived transistor, the on–off ratio, threshold voltage, and mobility were measured. c 2007 Elsevier Ltd. All rights reserved.

Keywords: ZnO; Nanoparticles; Transistor; Carrier mobility

1. Introduction There is great demand for large-area thin-film transistor liquid crystal displays (TFT-LCDs) which have the advantages of high resolution, light weight, slim size, and low power consumption compared to other flat panel displays (FPDs). Because of its high mobility, polycrystalline silicon has been used as an active material in TFTs, but it requires expensive and complex processing. Recent years have seen a growing interest in realizing thin-film transistors based

∗ Corresponding author. Tel.: +82 2 2123 2855; fax: +82 2 365 5882.

E-mail address: [email protected] (J. Moon). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.036

362

S. Lee et al. / Superlattices and Microstructures 42 (2007) 361–368

on solution-processable semiconducting materials for the applications in which low-cost, lowtemperature manufacturing is demanded [1–3]. For this reason, organic materials have been studied as an alternative to polycrystalline silicon and in particular pentacene has received a significant research focus. Although it exhibits the largest channel mobility among the organic semiconductors, the theoretical limit of channel mobility is only 10 cm2 V−1 s−1 and it is generally inapplicable to the low-cost solution process [4,5,7]. Solution-processable inorganic materials which are stable in air and suitable for solution process have been researched. Zinc oxide (ZnO), a wide band gap semiconductor with various applications such as varistors and light-emitting diodes, has been considered as a solution-processable inorganic materials [6]. Fortunato et al. fabricated a ZnO TFT at room temperature by RF sputtering from which the mobility of 70 cm2 V−1 s−1 was observed, indicating there is much room for further improvement in the device performance [7]. Solution-processing techniques have also been utilized to fabricate ZnO devices but they have had poor device performance or needed to use a high-temperature annealing (over 500 ◦ C) and an additional hydrothermal growth [8–11]. We report here the synthesis of ZnO nanoparticles and fabrication of TFT devices using ZnO particles by a solution process. The particles were characterized by SEM and XRD. The saturation mobilities and threshold voltages of solution-processed TFTs were characterized by current–voltage (I –V ) measurement. 2. Experimental We synthesized ZnO nanoparticles by the polyol method, which consists of the hydrolysis of an ionic salt in an organic solvent [12]. To prevent agglomeration of the synthesized particles, 10 g of polyvinylpyrrolidone (PVP, MW = 10 000, Aldrich) was dissolved in 250 mL diethyleneglycol (DEG, 99.9% Aldrich), and zinc acetate dihydrate [Zn(CH3 COO)2 · 2H2 O] (Aldrich) was then added with a concentration of 0.1 M. The suspension was stirred during heating up to 180 ◦ C and 1.8 g of deionized (DI) water was injected into the reactor, which was maintained at 180 ◦ C. Upon injection, the nucleation started, and the solution turned opaque. The reaction was continued for 30 min and the particles were washed and collected by centrifugation. The shape and size of the synthesized particles were observed by scanning electron microscopy (SEM) (JEOL, JSM 6700F). The XRD patterns of ZnO particles were recorded by employing an X-ray diffractometer (Rigaku, D/max-Rint 2000) with Cu Kα radiation. Weight loss during heating was monitored by a thermogravimetric (TG) analyzer (SETARAM 90) with the heating rate of 5 ◦ C min−1 . For the preparation of the spin-castable ink, a mixed solvent of ethylene glycol and 2methoxyethanol was used to disperse 0.5 g of ZnO nanoparticles (denoted as ink 1). To enhance the connectivity between particles, 0.41 g of zinc acetate was also dissolved into ink 1 (denoted as ink 2). The rheological behavior of these inks was monitored by a cone and plate type viscometer (DV-III, Brookfield). Inverted staggered type devices were fabricated on heavily doped silicon substrate with 200 nm thick silicon dioxide dielectric. Before spin coating, the substrates were cleaned by piranha solution (sulfuric acid:hydrogen peroxide = 4:1) and rinsed with DI water twice. Ink 1 and ink 2, respectively, were spin coated on the SiO2 surface. After spin coating at 2500 rpm for 30 s, the films were annealed at 600 ◦ C for 5 h in either air or oxygen atmosphere. Finally Cr (1 nm)/Au (49 nm) was evaporated through a shadow mask to define the source and drain electrode patterns. Three types of the TFT films were prepared for the measurement of I –V

363


Fig. 1. (a) SEM image of the synthesized ZnO particles and (b) particle size distribution obtained by image analysis.

Table 1 Preparation conditions for ZnO thin-film transistors Sample

Ink composition

Annealing temperature (◦ C)

Annealing condition

Mobility (cm2 V−1 s−1 )

Vth (V)

On–off current ratio

1 2 3

ZnO ZnO ZnO + Zn(Ac)2

600 600 600

Air O2 O2

2 × 10−5 2 × 10−3 3 × 10−3

7.5 31 −1

5 × 10 3 × 103 5 × 102

characteristics, as summarized in Table 1. The I –V characteristics were measured in air by an Agilent 5263A semiconductor parameter analyzer. 3. Results and discussion Fig. 1 shows the SEM image and particle size distribution determined by image analysis. The synthesized ZnO nanoparticles have relatively spherical shape with a size of 33 ± 7 nm, but they exhibit some edges rather than perfect curved surfaces. This reflects the difference in surface energy in the ZnO along different crystallographic directions. In the wurtzite structure, the relative growth rate of each crystallographic plane differs somewhat according to the crystal orientation, so that it is difficult for crystalline ZnO to grow symmetrically into spherical particles. It is known that the [100] plane is preferred as a dominant growth direction [13]. Fig. 2(a) shows the thermal behavior of the synthesized ZnO. Physically absorbed ethylene glycol evaporates first at 150–200 ◦ C. The PVP adsorbed on the surface of ZnO, which was added as a stabilizer, starts to decompose over 200 ◦ C and burns out to great extent between 200 to 500 ◦ C. The total weight loss was determined to be ∼1.2 wt%. Thermogravimetric analysis

364


Fig. 2. (a) TGA graph and (b) XRD diffraction patterns for the ZnO particles.

(TGA) reveals that ZnO thin films prepared from the polyol process-derived ZnO nanoparticles require heat-treatment at least over 300 ◦ C to mostly eliminate the organic phases. The X-ray diffraction pattern is also shown in Fig. 2(b). All the diffraction peaks well match the hexagonal wurtzite-structure ZnO peaks, indicating phase-pure crystalline nanoparticles. ZnO particles can be synthesized at relatively low temperature by the polyol synthesis method without further heattreatment for crystallization. In order to obtain a dense and uniform semiconducting layer from spin coating, the solutionprocessable ZnO particle suspension (i.e., ink) needs to be well-controlled. The ZnO suspension should be dilute and well-dispersed to have a low viscosity. The rheological measurement reveals that the ZnO ink has the viscosity of 6.7 mPa s at 90 s−1 , showing nearly Newtonian fluid behavior. In addition, our ZnO ink does not show any phase separation, at least for two days. Crack-free uniform ZnO films were obtained from the ink, followed by annealing at 200, 400, and 600 ◦ C. The ZnO particles are densely packed in the film, but not connected to each other when fired at 200 ◦ C as shown in Fig. 3(a). The particles start to be sintered at 400 ◦ C (Fig. 3(b)), whereas the particles are well-connected three dimensionally at 600 ◦ C (Fig. 3(c)). To develop a continuous transport pathway in the granular film, its microstructure is critical. Based on these


365

Fig. 3. SEM images of spin-cast ZnO films annealed at various temperatures: (a) 200 ◦ C, (b) 400 ◦ C, and (c) 600 ◦ C, respectively.

microstructural observations, we select an optimum heat-treatment temperature as 600 ◦ C for the measurements of device performance since the samples fired at both 200 and 400 ◦ C lack sufficient particle connectivity. Fig. 4(a) and (b) show the drain current–drain voltage (I D –VD ) characteristics and the drain current–gate voltage (I D –VG ) characteristic of a solution-processed ZnO-based TFT fabricated

366


Fig. 4. (a) Output behavior and (b) transfer characteristics of the sample 1 device after annealing in air; (c) output behavior and (d) transfer characteristics of the sample 2 device after annealing in O2 .

using ink 1. The sample was annealed in air at 600 ◦ C prior to I –V measurement. The electrical performance of the device annealed in air is significantly poor. The on–off ratio is 5 × 10 and the saturated mobility is 2 × 10−5 cm2 V−1 s−1 when the drain voltage is 60 V. For the device annealed in oxygen, however, the on–off ratio and mobility are enhanced to 3 × 103 and 2 × 10−3 cm2 V−1 s−1 , respectively, as shown in Fig. 4(c) and (d). The improved electrical characteristics are likely attributed to the reduction of defects in the ZnO thin film. Empty oxygen sites (i.e., oxygen vacancies, VO.. ) are potential wells that can trap either one (VO. ) or two (VOx ) electrons and these oxygen vacancies create deep levels in the band gap structure [14– 19]. In addition, it is known that interstitial zinc atoms, acting as shallow donors, can be produced together with the oxygen vacancies [20]. Annealing the zinc oxide film in oxygen atmosphere could decrease the number of shallow donors, lowering the off-current. Besides, oxygen vacancies which hinder the free movement of the carriers could disappear during the heat treatment in oxygen. As a result, the mobility and on-current of the device become larger than those of the device annealed in air. It is confirmed that the resistivity of the device treated in oxygen is ten times larger than that of the one treated in air. Fig. 5 shows the drain current–drain voltage (I D –VD ) characteristics and the drain current–gate voltage (I D –VG ) characteristic of sample 3, which was spin coated with ink 2. The device was annealed in oxygen at 600 ◦ C prior to the I –V measurement. The mobility was 3 × 10−3 cm2 V−1 s−1 and the on–off ratio was 5 × 102 . It is observed by SEM that zinc acetate added in the ZnO ink assists in developing a denser and smoother surface structure of the ZnO film in which the ZnO nanoparticles are well connected (Fig. 6). For TFTs fabricated with ZnO nanoparticles regardless of annealing atmosphere, the devices have quite high threshold voltage,


367

Fig. 5. (a) Output behavior and (d) transfer of the sample 3 device after annealing in O2 .

Fig. 6. SEM image of the ZnO film annealed in air at 600 ◦ C fabricated from ink 2 in which the zinc acetate was added together with ZnO particles.

7.5 V in Fig. 4(b) and 31 V in Fig. 4(d). However, the threshold voltage reduced to −1 V when the device was made from ink 2 containing zinc acetate.

368


During the film drying, zinc acetate recrystallizes mostly at the areas of the particle contacts due to capillary pressure, and in turn it transforms to ZnO when annealed in air at 600 ◦ C, as confirmed by X-ray diffraction. Such an improvement in the microstructure of ZnO film could play a role in lowering the threshold voltage. Grain boundaries are known to be an obstacle for carrier transport, significantly affecting the conduction behavior of polycrystalline thinfilm devices. Hossaina et al. have stated that a double Schottky barrier is formed in the grain boundary, which causes a nonlinear increase in the drain current with gate voltage. Consequently, a wide sub-threshold slope and gradual change in the mobility were observed [21]. Based on our experimental results, it is concluded that the addition of zinc acetate is effective in reducing the threshold voltage of nanoparticle-based ZnO TFTs. 4. Conclusions N-type TFTs have been fabricated from ZnO nanoparticles by spin coating a solutionprocessable semiconducting ink. Synthesized ZnO nanoparticles were used to prepare the welldispersed stable ink. Control of the defect structure and connection of the ZnO nanoparticles are essential for obtaining higher field-effect mobility and lowering the threshold voltage in ZnO particle-based solution-processed TFTs. By annealing in oxygen atmosphere, we could achieve a field-effect mobility of 2 × 10−3 cm2 V−1 s−1 and a threshold voltage of 31 V. When zinc acetate was added to the ZnO ink to assist the particle interconnection, an improved mobility of 3 × 10−3 cm2 V−1 s−1 and lower threshold voltage of −1 V were observed. Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Lab. Program funded by the Ministry of Science and Technology (No. R0A-2005-000-10011-0). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

M. Mushrush, A. Facchetti, M. Lefenfeld, H.E. Katz, T.J. Marks, J. Amer. Chem. Soc. 125 (2003) 9414–9423. A. Afzali, C.D. Dimitrakopoulos, T.L. Breen, J. Amer. Chem. Soc. 124 (2002) 8812. H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123. C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. H.E. Katz, A.J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.Y. Lin, A. Dodabalapur, Nature 404 (2000) 478. J.H. Lee, P. Lin, J.C. Ho, C.C. Lee, Electrochemical Solid-State Lett. 9 (2006) G117. E. Fortunato, A. Pimentel, L. Pereira, A. Gonc¸alves, G. Lavareda, H. Aguas, I. Ferreira, C.N. Carvalho, R. Martins, J. Non-Cryst. Sol. 338–340 (2004) 806. B.J. Norris, J. Anderson, J.F. Wager, D.A. Keszler, J. Phys. D: Appl. Phys. 36 (2003) L105. J. Weng, Y. Zhang, G. Han, Y. Zhang, L. Xu, J. Xu, X. Huang, K. Chen, Thin Solid Films 478 (2005) 25. Y. Ohya, T. Niwa, T. Ban, Y. Takahashi, Japan J. Appl. Phys. Part 1 40 (2001) 297. B. Sun, H. Sirringhaus, Nano Lett. 5 (2005) 2408. L. Poul, S. Ammar, N. Jouini, F. Fi´evet, F. Villain, Solid State Sci. 3 (2001) 31. X.M. Liu, Y.C. Zhou, J. Cryst. Growth 270 (2004) 527. J.M. Smith, W.E. Vehse, Phys. Lett. A 31 (1970) 147. K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. W.E. Carlos, E.R. Glaser, D.C. Look, Physica B 308 (2001) 976. B.X. Lin, Z.X. Fu, Y.B. Jia, Appl. Phys. Lett. 79 (2001) 943. F. Oba, S.R. Nishitani, S. Isotani, H. Adachi, I. Tanaka, J. Appl. Phys. 90 (2001) 824. X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285. Vladislav Ischenko, Adv. Funct. Mater. 15 (2005) 1945. Faruque M. Hossaina, Physica E 21 (2004) 911–915.