Effects of Composition Ratio on Solution-processed ...

ECS Transactions, 53 (2) 197-202 (2013) 10.1149/05302.0197ecst ©The Electrochemical Society

Effects of Composition Ratio on Solution-processed InGaZnO Thin-Film Transistors Jeong-Soo Leea, Seung-Min Songa, Soo-Yeon Leea, Yong-Hoon Kimb, Jang-Yeon Kwonc and Min-Koo Hana a

School of Electrical Engineering and Computer Science, Seoul National University, Seoul, 151-742, Korea b Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi, 463-070, Korea c School of Integrated Technology, Yonsei University, Incheon, 406-840, Korea

We fabricated solution-processed IGZO TFTs with various composition ratio of precursors according to the contents of In, Ga, and Zn. Threshold voltage of solution-processed IGZO TFTs was -0.43 V, 3.86 V, and 11.12 V when the composition ratio was varied by 7:1:2, 6:3:1, and 5:1:4, respectively. Saturation mobility of solution-processed IGZO TFTs was 1.4 cm2/V·sec, 0.84 cm2/V·sec, and 0.3 cm2/V·sec with the composition ratio of 7:1:2, 6:3:1, and 5:1:4 respectively. When In composition ratio increased, the threshold voltage decreased and saturation mobility increased because of the increase of electron concentration. When Ga composition ratio increased, the off-current was decreased because of the suppression of formation of oxygen vacancies. When Zn composition ratio increased, the hysteresis was decreased because of the reduction of interstitial states between channel and insulator.

Introduction Solution-processed oxide thin film transistors (TFTs) employing spin-coating, dipping, and ink-jet printing have attracted considerable attention for active matrix display because of its high mobility, high throughput, good uniformity for large area, and suitability for achieving low cost fabrication contrary to vacuum processes, such as rf magnetron sputtering and pulsed laser deposition, which require complicated and high manufacturing cost processes (1, 2). Among various ZnO-based oxide semiconductors, indume galium zinc oxide (IGZO) TFTs maybe promising candidates for achieving high performance such as high mobility as well as good reliability (3, 4). In solution-processed oxide TFTs, the composition ratio of precursors should be controlled carefully because the electrical characteristics of TFTs are sensitive to the composition ratio of precursors (5). However, the effects of composition ratio on solution-processed IGZO TFTs are scarcely reported. The purpose of this paper is to investigate the electrical characteristics such as threshold voltage and saturation mobility of solution-processed IGZO TFTs with various composition ratio of precursors according to the contents of In, Ga, and Zn and to analyze the effects of composition ratio on solution-processed IGZO TFTs.

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ECS Transactions, 53 (2) 197-202 (2013)

Experimental We fabricated solution-processed IGZO TFTs with annealing temperature of 350 °C on active layer. IGZO TFTs with inverted staggered structure were fabricated on wafer substrates as Figure 1. Heavily boron doped p-type silicon wafer substrate of 760 μm was used as the gate and gate insulator was fabricated with Silicon dioxide (SiO2) of 2000 Å using thermal oxidation. The precursor-based solution of IGZO for active layer was prepared with various composition ratio in order to observe the effects of composition ratio of precursors. The precursor-based solution of IGZO was dissolved with In nitrate hydrate (In(NO3)3·xH2O, FW of 390.91, Aldrich), Ga nitrate hydrate (Ga(NO3)3·xH2O, FW of 255.74, Aldrich), and Zn acetate dihydrate (Zn(C2H3O2)2·2H2O, FW of 219.5, Aldrich) powders in 2methoxyethanol (C3H8O2) of 3mL as 7:1:2, 6:3:1, and 5:1:4 of composition ratio. The mixed IGZO solution was stirred at 75 °C for 1 h to promote the dissolving process. After spin-coating, IGZO films were annealed at 350 °C for 1 hour by hot-plate annealing process and cooled down to the room temperature. The thicknesses of IGZO films decreased from 400 Å to 250 Å during annealing due to the solvent and halide residue were evaporated. Finally, 300 nm thick indium tin oxide (ITO) film as source and drain electrodes was deposited by dc sputtering.

250 Å

1000 Å 2000 Å

~ ~ 760 탆 Figure 1. Structure of solution-processed IGZO TFTs

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Electrical characteristics of IGZO TFTs according to composition ratio Transfer characteristics of solution-processed IGZO TFTs according to composition ratio are shown in Figure 2. Figure 2 (a), (b), and (c) show the transfer characteristics of IGZO TFTs with composition ratio of 7:1:2, 6:3:1, and 5:1:4, respectively and each device was fabricated three times to insure the reproductivity. Solution-processed In2O3 TFT was also fabricated to verify the electrical characteristics of extreme case of high composition ratio of In and to confirm the role of In shown as Figure 2 (d). 1E-4

1E-4 1E-5 1E-6

1E-5 1E-6

Hot plate Annealing = 350 °C No Patterning

1E-7

1E-8

1E-8

IDS [A]

IDS [A]

1E-7

Solution-processed IGZO TFTs on Wafer W/L = 3000/500 μm VD S = 10 V

1E-9 1E-10


1E-9 1E-10

IGZO_7:1:2 IGZO_7:1:2' IGZO_7:1:2''

1E-11

Solution-processed IGZO TFTs on Wafer W/L = 3000/500 μm VDS = 10 V

IGZO_5:1:4 IGZO_5:1:4' IGZO_5:1:4''

1E-11 1E-12

1E-12

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1E-13 -20

-10

0

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(a)

IDS [A]

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Solution-processed IGZO TFTs on Wafer W/L = 3000/500 μm VDS = 10 V

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IDS [A]

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10

(b)

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0

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1E-9 1E-10

IGZO_6:3:1 IGZO_6:3:1; IGZO_6:3:1''

1E-11

1E-8

Solution-processed IGZO TFTs on Wafer W/L = 3000/500 μm VD S = 10 V Hot plate Annealing = 350 °C No Patterning

In2 O3

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Figure 2. Transfer characteristics of solution-processed IGZO TFTs with composition ratio of (a) 7:1:2, (b) 6:3:1, (c) 5:1:4, and (d) In2O3 TFT IGZO TFT with low In composition ratio as 5:1:4 showed low on-current as Figure 2 (b) and In2O3 TFT with high In composition ratio showed high conductivity as Figure 2 (d). In recent reports of sputter-processed oxide TFTs, In is known as In 5s orbitals mainly form the oxygen vacancies at conduction band bottom so that In has a role in the increase of the electron concentration. IGZO TFT with high Ga composition ratio as 6:3:1 had high threshold voltage and low off-current as Figure 2 (c). Ga compensates carriers generated by suppression of formation of oxygen vacancies with In inclusion so

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ultimately reduces the election concentration. When Zn composition ratio decreased from 7:1:2 of Figure 2 (a) to 6:3:1 of Figure 2 (c), the hysteresis was increased because Zn modulates the shallow tail state below conduction band and reduces the interstitial states between channel and insulator (6). Figure 3 shows the transfer characteristics of solution-processed IGZO TFTs according to composition ratio and In2O3 TFT, simultaneously, and Figure 4 (a) and (b) show the threshold voltage and saturation mobility, respectively, according to composition ratio. Threshold voltage of solution-processed IGZO TFTs was -0.43 V, 3.86 V, and 11.12 V with the composition ratio of 7:1:2, 6:3:1, and 5:1:4, respectively. Saturation mobility of solution-processed IGZO TFTs was 1.4 cm2/V·sec, 0.84 cm2/V·sec, and 0.3 cm2/V·sec with the composition ratio of 7:1:2, 6:3:1, and 5:1:4 respectively. The threshold voltage of solution-processed IGZO TFTs increased with decreasing In composition ratio and with increasing Ga composition ratio as shown in Figure 4 (a) because of the decrease of electron concentration of IGZO active layer (7-10). In increases the electron concentration with forming the oxygen vacancies at conduction band bottom and Ga decreases the electron concentration with compensating carriers generated by suppression of formation of oxygen vacancies with In inclusion. The saturation mobility of solution-processed IGZO TFTs decreased with decreasing In composition ratio and with increasing Ga composition ratio as shown in Figure 4 (b) because of the decrease of electron concentration. The hall mobility is proportional to the carrier concentration in oxide semiconductors (11).

1E-4 1E-5 1E-6

IDS [A]

1E-7 1E-8

Solution-processed IGZO TFTs VDS = 10 V Hot plate Annealing = 350 °C

1E-9

IGZO (7:1:2) IGZO (6:3:1) IGZO (5:1:4) In2O3

1E-10 1E-11 1E-12 1E-13 -20

-10

0

10

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VGS [V]

Figure 3. Transfer characteristics of solution-processed IGZO TFTs according to composition ratio and In2O3 TFT

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1.6

12

1.4

Saturation Mobility [cm2/Vsec]

14

Threshold Voltage [V]

10 8 6 4 2 0 -2

1.2 1.0 0.8 0.6 0.4 0.2 0.0

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7:1:2

6:3:1

5:1:4

7:1:2

(a)

6:3:1

5:1:4

(b)

Figure 4. (a) Threshold voltage and (b) Saturation mobility of solution-processed IGZO TFTs according to composition ratio

Conclusion We fabricated solution-processed IGZO TFTs with various composition ratio of precursors according to the contents of In, Ga, and Zn. Our experiment results showed that the composition ratio of precursors should be controlled carefully because the electrical characteristics of TFTs are sensitive to the composition ratio of precursors. The effects of composition ratio of precursors on solution-processed IGZO TFTs at low annealing temperature of 350 °C were successfully analyzed. Threshold voltage of solution-processed IGZO TFTs was -0.43 V, 3.86 V, and 11.12 V with the composition ratio of 7:1:2, 6:3:1, and 5:1:4, respectively. Saturation mobility of solution-processed IGZO TFTs was 1.4 cm2/V·sec, 0.84 cm2/V·sec, and 0.3 cm2/V·sec with the composition ratio of 7:1:2, 6:3:1, and 5:1:4 respectively. When In composition ratio increased, the threshold voltage decreased and saturation mobility increased because of the increase of electron concentration. When Ga composition ratio increased, the off-current was decreased because of the suppression of formation of oxygen vacancies. When Zn composition ratio increased, the hysteresis was decreased because of the reduction of interstitial states between channel and insulator.

References 1. E. M. C. Fortunato, P. M. C. Barquinha, A. C. M. B. G. Pimentel, A. M. F. Goncalves, A. J. S. Marques, R. F. P. Martins, and L. M. N. Pereira, Appl. Phys. Lett., 85, 2541 (2004). 2. H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong, and D. A. Keszler, Appl. Phys. Lett., 86, 013503 (2005). 3. S. J. Seo, C. G. Choi, Y. H. Hwang, and B. S. Bae, J. Phys. D: Appl. Phys., 42, 035106 (2009). 4. Y. J. Chang, D. H. Lee, G. S. Herman, and C. H. Chang, Electrochem, Solid-State Lett., 10, H135 (2007).

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5. K. Nomura, T. Kamiya, H. Ohta, T. Uruga, M. Hirano, and H. Hosono, Phys. Rev. B, 75, 035212 (2007). 6. T. Iwasaki, N. Itagaki, T. Den, H. Kumomi, K. Nomura, T. Kamiya, and H. Hosono, Appl. Phys. Lett., 90, 242114 (2007). 7. S. W. Tsao, T. C. Chang, S. Y. Huang, M. C. Chen, S. C. Chen, C. T. Tsai, Y. J. Kuo, Y. C. Chen, W. C. Wu, Solid-State Electronics, 54, 1497 (2010). 8. S. H. Jeong, Y. M. Jeong, and J. H. Moon, J.PCL, 112(30), 11082 (2008). 9. K. Nomura, A. Takagi, T. Kamiya, H. Ohta, M. Hirano, and H. Hosono, Jpn. J. Appl. Phys., 45(5B), 4303 (2006). 10. J. S. Park, J. K. Jeong, Y. G. Mo, H. D. Kim, and C. J. Kim, Appl. Phys. Lett., 93, 033513 (2008). 11. K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, and H. Hosono, Appl. Phys. Lett., 85, 1993 (2004).

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