Journal of the Korean Physical Society, Vol. 57, No. 6, December 2010, pp. 1836∼1841
Effect of Sintering Time at Low Temperature on the Properties of IGZO TFTs Fabricated by Using the Sol-gel Process Jun Hyuk Choi, Jong Hyun Shim, Soo Min Hwang, Jinho Joo,∗ Kyung Park, Hyoungsub Kim and Hoo-Jeong Lee School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746
Jun Hyung Lim† Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
Mi Ran Moon and Donggeun Jung Department of Physics, Institute of Basic Science and Brain Korea 21 Physics Research Division, Sungkyunkwan University, Suwon 440-746 (Received 12 January 2010, in final form 11 April 2010) We explored the application of the sol-gel process technique to the fabrication of InGaZnO (IGZO) thin film transistors (TFTs). We fabricated IGZO TFTs by using the sol-gel method and evaluated the effect of the sintering time on the electrical properties of the IGZO system with an atomic ratio of In:Ga:Zn = 2:1:1. In the process, IGZO precursor solutions were prepared by mixing In nitrate, Ga nitrate, and Zn acetate and were then deposited on a p-type Si-wafer covered with a thermallygrown SiO2 layer by spin-coating. The sintering process was performed for 3 h, 6 h or 12 h at 300 ◦ C in the ambient atmosphere. The source/drain electrodes of the TFT devices were fabricated using Al thermal evaporation. For all of the samples, a low off current (∼10−11 A) and on-to-off current ratio (∼5 × 104 ) were obtained in their transfer curves. The saturation mobility increased with increasing sintering time: for the samples sintered for 3 h, 6 h and 12 h, the saturation mobilities were calculated to be 0.825 cm2 /Vs, 1.65 cm2 /Vs, and 2.06 cm2 /Vs, respectively. Based on the XPS and TEM analyses, the enhancement of the mobility was attributed to the increase in the number of oxygen vacancies and the nanocrystalline structure in the amorphous matrix with increasing sintering time. These results demonstrate for the potential application of sol-gel processed IGZO devices on flexible polymer substrates. PACS numbers: 73.61.-r, 73.61.Jc, 73.90.+f Keywords: Flexible polymer substrates, IGZO, Sol-gel, Thin film transistor DOI: 10.3938/jkps.57.1836
I. INTRODUCTION Zinc-oxide thin film transistors (TFTs) with heavy metal cations such as indium zinc oxide (IZO) [1–3], zinc tin oxide (ZTO) [4–6], and indium gallium zinc oxide (IGZO) [7] offer an alternative to amorphous Si TFTs, due to their high mobility, high stability, and transparency. Among them, amorphous IGZO has been investigated for use as an active-channel layer since the first report on its use by Nomura et al. [8] . Recently, flexible and transparent TFTs were fabricated by depositing the IGZO channel layer on a polymer substrate by using pulsed laser deposition (PLD) and were found to exhibit moderate mobility [9]; however, their electrical properties need to be improved for practical applications. In ad∗ E-mail: † E-mail:
[email protected]; Fax: +82-31-299-4749
[email protected]; Fax: +82-31-299-4749
dition, because polymer films such as polyimide (PI) and polyethylene-terephthalate (PET) are used as substrates for flexible devices, the processing temperature should be lowered to prevent their degradation. For the application of IGZO, therefore, it is necessary to improve the electrical properties and to develop a processing route that operates at low temperature. There are many deposition techniques for ZnO-based TFTs, as reported in previous studies, and in most cases, the active layers are fabricated using conventional vacuum processes such as radio- frequency-dc magnetron sputtering [10], pulsed laser deposition [11], and atomic layer deposition [12] due to their high reliability and reproducibility. In contrast, solution-based processes, which do not require a vacuum, are attractive because they provide for precise controllability of the composition, as well as cost-effectiveness and high throughput. In our previous study [13], we fabricated amorphous
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Effect of Sintering Time at Low Temperature on the Properties · · · – Jun Hyuk Choi et al.
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IGZO TFTs by using the sol-gel process at a sintering temperature of 400 ◦ C and evaluated the effect of the Ga content on their electrical properties and microstructure. We also found that it was possible to sinter the IGZO devices at a lower temperature of 300 ◦ C. In this study, therefore, we fabricated IGZO TFTs with an atomic ratio of In:Ga:Zn = 2:1:1 by using the sol-gel method and optimized the sintering time at 300 ◦ C for the purpose of improving their properties. The effects of increasing the sintering time on the microstructure, the chemical binding energy (metal-oxygen bonding), and the resultant electrical properties were also evaluated.
II. EXPERIMENTS In order to fabricate InGaZnO thin film transistors using the solution process, we selected In nitrate hydrate [In(NO3 )3 ·xH2 O, Aldrich], Ga nitrate hydrate [Ga(NO3 )3 ·xH2 O, Aldrich], and Zn acetate dehydrate [Zn(CH3 COO)2 ·2H2 O, Aldrich] as the precursors and 2-methoxyethanol (CH3 OCH2 CH2 OH) and monoethanolamin (MEA) as the solvent and the stabilizer, respectively. We controlled the mixing ratio of the precursors to obtain InGaZnO films with an atomic ratio of In:Ga:Zn = 2:1:1. The precursors were mixed with 2methoxyethanol and MEA to obtain a concentration of 0.5 M. To make a homogeneous solution, we stirred the precursor solution at 60 ◦ C for two hours. The solution was deposited onto a highly-doped p-type Si-wafer covered with a thermally-grown 100-nm thick SiO2 layer by spin coating at 3000 rpm and then dried at 150 ◦ C for 10 min in air to form a gel-film. In order to evaluate the effect of the sintering time at a low sintering temperature on the microstructure and electrical properties, we annealed the gel-coated films at 300 ◦ C for 3 h, 6 h or 12 h in air, and hereafter, these samples are denoted as the 3-h sample, 6-h sample, and 12-h sample, respectively. We fabricated a TFT structure with a bottom gate and top contact by utilizing a highly-doped p-type Si substrate as the gate electrode and a 100 nm-thick SiO2 layer as the gate dielectric. The patterned Al layer used as the source and the drain electrodes was deposited by using the thermal evaporation technique via a typical lift-off process. The channel width and length of the fabricated TFTs were 100 µm and 50 µm, respectively. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed using a Seiko Exstar 6000 (TG/DTA 6100) system at temperatures ranging from room temperature to 1050 ◦ C with a heating rate of 10 ◦ C/min in air. We observed the crystallinity and the surface composition of the films by using high-resolution transmission electron microscopy (HRTEM, JEOL, JEM 2100F) and X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA2000), respectively. The electrical behaviors (ID - VD and ID - VG )
Fig. 1. Thermo gravimetric analysis and differential thermal analysis curves for the IGZO solution.
were measured using a semiconductor parameter analyzer (HP4145B) at room temperature in the ambient atmosphere and the field effect electron mobility (µ) and threshold voltage (Vth ) were calculated from the ID - VG plot.
III. RESULTS AND DISCUSSION The thermal behaviors of the precursor solutions were analyzed by using TGA and DTA as shown in Fig. 1. The weight losses during heating to ∼110 ◦ C in the TG curve signaled the evaporation of the solvents (2methoxyethanol and MEA). The DTA curve shows endothermic and exothermic peaks at 106.7 ◦ C and 216.8 ◦ C, respectively. The endothermic peak resulted from the detachment of H2 O from the metal acetate and nitrate, which usually occurs at 100 - 110 ◦ C [14], and the exothermic peak corresponded to the formation of the IGZO phase. This result suggests that the IGZO compound formed at a temperature below 235 ◦ C. The overall reaction can occur through four unit reactions, viz. the decomposition reactions of the metal precursors and the formation of an IGZO, as follows: In(NO3 )3 · 3H2 O → In(OH)3 + 3HNO3 , Ga(NO3 )3 · 3H2 O → Ga(OH)3 + 3HNO3 , Zn(CH3 COO)2 · 2H2 O → Zn(OH)3 +2CH3 COOH, xIn(OH)3 + yGa(OH)3 + zZn(OH)2 → Inx Gay Znz O4−δ +H2 O.
(1) (2) (3)
(4)
Figures 2(a) - (c) show high-resolution electron micrographs of the films sintered at 300 ◦ C for (a) 3 hr, (b) 6 hr, and (c) 12 hr. For all of the films, their microstructure had a densely packed region, including many small pores,
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Journal of the Korean Physical Society, Vol. 57, No. 6, December 2010
Table 1. Measured atomic ratios of In, Ga, and Zn and binding energy of the O 1s level from the XPS spectra. Sample 3-h 6-h 12-h
Atomic ratio In:Ga:Zn 1.68:0.64:1 1.81:0.66:1 1.80:0.68:1
In 3d (eV) 445.0 445.1 444.9
Ga 2p (eV) 1119.5 1118.7 1117.9
Zn 2p (eV) 1022.9 1022.6 1022.0
Table 2. Threshold voltages, off currents, on-to-off current ratios, and electron saturation mobilities of IGZO devices.
Otot
Sample
Vth (V)
Ioff (A)
Ion /Ioff
531.5 531.0 530.6
3-h 6-h 12-h
33.1 32.0 31.0
4.2 × 10−11 3.8 × 10−11 4.5 × 10−11
1.7 × 104 3.9 × 104 5.4 × 104
Fig. 2. TEM images of IGZO films sintered for (a) 3 h, (b) 6 h, and (c) 12 h.
and the density appeared to be relatively low. The 3-h sample consisted of an amorphous phase whereas the 6-h sample and the 12-h sample exhibited lattice fringes in an amorphous matrix, indicating that the crystalline structure formed as the sintering time was increased. The observation of the microstructure indicates that sinter-
Mobility (cm2 /Vs) 0.83 1.65 2.06
ing for 3 - 12 hrs at 300 ◦ C did not change the density, but affected the crystallization of IGZO. We quantitatively characterized the chemical properties and metal-oxygen bonding state of the IGZO films by using X-ray photoelectron spectroscopy (XPS). The measured atomic ratios of In:Ga:Zn were 1.68:0.64:1, 1.81:0.66:1, and 1.80:0.68:1 for the 3-h sample, the 6h sample, and the 12-h sample, respectively, as given in Table 1, which deviate from the nominal mole ratio of 2:1:1. Figure 3 shows the (a) In 3d, (b) Ga 2p, (c) Zn 2p, and (d) O 1s peaks of the XPS spectra of the IGZO films. The In 3d5/2 peak at 444.6 eV, the Ga 2p3/2 peak at 1118.9 eV, and the Zn 2p3/2 peak at 1021.8 eV are known to correspond to the In-O, the Ga-O, and the Zn-O bonds, respectively [15]. For the In 3d spectra, there was no difference between the samples. On the other hand, the Ga 2p3/2 and the Zn 2p3/2 spectra were shifted toward lower binding energies with increasing. The Ga 2p3/2 peaks of the 3-h, 6-h, and 12-h samples were centered at 1119.1, 1118.7, and 1117.9 eV, respectively, and the corresponding Zn 2p3/2 peaks were centered at 1022.9, 1022.6, and 1022.0 eV. These peak shifts toward a lower bonding energy are thought to be related to either the different crystallographic structures or different oxygen bondings between the metal atoms. The peak for the lower binding energy of the O 1s component (OI ) centered at 530.35 eV is known to be due to O2− ions surrounded by Zn, Ga, and In atoms in the IGZO compound system, and that the higher binding energy component (OII ) at 531.85 eV is known to be associated with O2− ions, which are in oxygen-deficient regions in the IGZO structure [16,17]. In general, the variation in the intensity of this component (OII ) is attributed to the change in the concentration of oxygen vacancies. The O 1s peak was fitted by using two nearly Gaussian distributions to evaluate their relative contributions to the O state, i.e., the oxygen binding with the metal or the formation of oxygen vacancies. The OII /Otot ratios increased as the sintering time was increased. The ratios of the 3-h, 6-h, and 12-h samples were estimated to be 47.8%, 48.0%, and 55.8%, respectively, as shown in Table 1. Because OII -related oxygen vacancies supply free electron carriers to the IGZO film, we expected the electron concentration to increase, and hence, the saturation mobility, to increase with increasing sintering time.
Effect of Sintering Time at Low Temperature on the Properties · · · – Jun Hyuk Choi et al.
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Fig. 4. Device with a bottom-gate TFT structure with an IGZO active layer.
Fig. 3. (Color online) XPS spectra of the (a) In 3d, (b) Ga 2p, (c) Zn 2p, and (d) O 1s levels of the IGZO film.
Fig. 5. (Color online) Drain current-drain voltage output characteristics and drain current-gate voltage transfer curve.
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Journal of the Korean Physical Society, Vol. 57, No. 6, December 2010
Figure 4 shows the device with a bottom-gate and topcontact TFT structure. We used the SiO2 layer as the gate dielectric and highly doped-Si as the gate electrode. The source and drain electrodes consisted of Al deposited by the thermal evaporation technique. Figure 5 shows the drain current-drain voltage (I DS - V DS ) output characteristics and I DS - V GS transfer curve. All of the devices behaved as n-channel transistors and hard drain current saturation was observed in the output curves. Table 2 summarizes the electrical properties. For all of the samples, a low off current (∼10−11 A) and on-to-off current ratio (∼104 ) curves were observed in the transfer. The threshold voltage was obtained by using a linear extrapolation of the drain current-gate voltage (I DS - V GS ) curve at V DS = 20 V. The threshold voltages of the 3-h, 6-h and 12-h samples were estimated to be 33.1 V, 32.0 V, and 31.0 V, respectively, showing similar positive values with a slightly negative shift. With a positive threshold voltage, this device behaved as an enhancement-mode device that is initially off and requires a positive gate voltage to be fully turned on. The electron saturation mobility (µsat ) was calculated from the slope by fitting a straight line to the plot of √ IDS vs. V GS in the saturation region: Ci µsat W (VGS − Vth )2 (5) IDS = 2L where W and L are the width and the length of the source and the drain electrodes, respectively, and Ci is the capacitance per unit area of the gate insulator. With increasing sintering time, the saturation mobility was increased for the 3-h, 6-h, and 12-h samples, the saturation mobilities were calculated to be 0.83 cm2 /Vs, 1.65 cm2 /Vs, and 2.06 cm2 /Vs, respectively. For the 3-h sample with an amorphous structure, its transistor characteristic is probably due to overlapping of the spherical s-orbitals of the heavy post-transition metal cations [18]. The improvement in the mobility for the 12-h sample is thought to result from the enhanced controllability of the oxygen vacancies because the ratio of the nanocrystalline component to the amorphous matrix increased as the sintering time was increased, as supported by the TEM and the XPS analyses.
IV. CONCLUSIONS We successfully synthesized IGZO TFTs with an atomic ratio of In:Ga:Zn = 2:1:1 by using the sol-gel process. The sintering temperature of the IGZO film was established by thermal analysis, revealing that an exothermic reaction occurred in the range of 201 - 237 ◦ C. Therefore, the gel-films were sintered at 300 ◦ C for 3 h, 6 h or 12 h, and the effects of the sintering time on the microstructure, chemical binding energy (metaloxygen bonding), and resultant electrical properties were evaluated.
According to the XPS and TEM studies, as the sintering time were increased, the number of oxygen vacancies and the proportion of the nanocrystalline structure in the amorphous matrix increase, respectively. For all of the samples, a low off current (∼10−11 A) and on-to-off current ratio (∼5 × 104 ) were obtained in the transfer curves. The saturation mobility increased with increasing sintering time: for the 3-h, 6-h, and 12-h samples, the saturation mobilities were calculated to be 0.83 cm2 /Vs, 1.65 cm2 /Vs, and 2.06 cm2 /Vs, respectively. Consequently, the results demonstrate the potential for application of these sol-gel processed IGZO devices on flexible polymer substrates.
ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0413).
REFERENCES [1] N. L. Dehuff, E. S. Kettenring, D. Hong, H. Q. Chiang, J. F. Wager, R. L. Hoffman, C.-H. Park and D.A. Keszler, J. Appl. Phys. 97, 064505 (2005). [2] P. Barquinha, A. Pimentel, A. Marques, L. Pereira, R. Martins and E. Fortunato, J. Non-Cryst. Solids 352, 1749 (2006). [3] P. Barquinha, A. Pimentel, A. Marques, L. Pereira, R. Martins and E. Fortunato, J. Non-Cryst. Solids 352, 1756 (2006). [4] H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong and D. A. Keszler, Appl. Phys. Lett. 86, 013503 (2005). [5] W. B. Jackson, R. L. Hoffman and G. S. Herman, Appl. Phys. Lett. 87, 193503 (2005). [6] P. G¨ orrn, M. Sander, J. Meyer, M. Kr¨ oger, E. Becker, H.-H. Johannes, W. Kowalsky and T. Riedl, Adv. Mater. 18, 738 (2006). [7] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature 432, 488 (2004). [8] K. Nomura, A. Takagi, T. Kamiya, H. Ohta, M. Hirano and H. Hosono, Jpn. J. Appl. Phys. 45, 4303 (2006). [9] S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata and T. Kawai, J. Appl. Phys. 93, 1624 (2003). [10] A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano and H. Hosono, Thin Solid Films 486, 38 (2005). [11] E. M. C. Fortunato, P. M. C. Barquinha, A. C. M. B. G. Pimentel, A. M. F. Gon¸calves, A. J. S. Marques, R. F. P. Martins and L. M. N. Pereira, Appl. Phys. Lett. 85, 2541 (2004). [12] P. F. Carcia, R. S. McLean and M. H. Reilly, Appl. Phys. Lett. 88, 123509 (2006). [13] J. H. Lim, J. H. Shim, J. H. Choi, J. Joo, K. Park, H. Jeon, M. R. Moon, H. Kim and H.-J. Lee, Appl. Phys. Lett. 95, 012108 (2009).
Effect of Sintering Time at Low Temperature on the Properties · · · – Jun Hyuk Choi et al. [14] L. Znaidi, G. J. A. A. Soler Illia, S. Benyahia, C. Sanchez and A. V. Kanaev, Thin Solid Films 428, 257 (2003). [15] T. Sz¨ or´enyi, L. D. Laude, I. Bert´ oti, Zs. Geretovszky and Z. K´ antor, Appl. Surf. Sci. 96, 363 (1996). [16] J. C. C. Fan and J. B. Goodenough, J. Appl. Phys. 48, 3524 (1977).
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[17] B. Kumar, H. Gong and R. Akkipeddi, J. Appl. Phys. 97, 0637061 (2005). [18] J. H. Na, M. Kiramura and Y. Arakawa, Appl. Phys. Lett. 93, 063501 (2008).
