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Electrical characteristics of bendable a-IGZO thin-film transistors with split channels and top-gate structure
Microelectronic Engineering 159 (2016) 179–183
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrical characteristics of bendable a-IGZO thin-film transistors with split channels and top-gate structure Hyungon Oh, Kyoungah Cho ⁎, Sukhyung Park, Sangsig Kim ⁎ Department of Electrical Engineering, Korea University, Anam-ro 145, Sungbuk-gu, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 19 March 2016 Accepted 22 March 2016 Available online 24 March 2016 Keywords: Thin-film transistor a-IGZO Bending curvature radius Plastic substrate Split channel
a b s t r a c t In this study, we fabricate top-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with five split channels on a bendable plastic substrate and investigate the electrical characteristics as a function of bending curvature radius. Owing to the channel width splitting effect, our TFTs have outstanding characteristics including a high mobility of 71.8 cm2/V·s and an on/off ratio of 108. Our bending study reveals that the operation regions of our TFT are categorized into safe, transition, and definitive mechanical failure regions as the value of the bending curvature radius decreases. In the transition region, the threshold voltage is shifted from 1.0 to 2.1 V, and the mobility is decreased from 71.8 to 25.9 cm2/V·s. The electrical failure of bendable TFTs results from microcracks induced by mechanical strain. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, bendable thin-film transistors (TFTs) have received much attention as components for flexible displays [1–3]. In particular, a-Indium Gallium Zinc Oxide (a-IGZO) TFTs as switching devices for operating displays are extremely promising owing to their higher mobility than a-Si TFTs. In addition, a-IGZO TFTs have the leading role in the development of next-generation transparent flexible displays [4–7]. Most of the research work on bendable a-IGZO TFTs has focused on bottomgate structures used in liquid crystal displays rather than top-gate structures used as driving devices for active-matrix organic light-emitting diodes. And bendable a-IGZO TFTs with top-gate structure have been studied mainly for not a coplanar type but a staggered type. Hence, in this study, we fabricate a-IGZO TFTs with top-gate structures on plastic substrates and examine their electrical characteristics as functions of the bending curvature radius. Additionally, channel width splitting in poly-Si TFTs is an effective way to improve driving characteristics such as driving current, mobility, and threshold voltage [8]. The channel width splitting method has not been introduced in the fabrication of a-IGZO TFTs yet. Thus, in this study, we introduce this method for fabricating a-IGZO TFTs to improve their electrical characteristics. 2. Experiment details In this study, an a-IGZO TFT with five split channels was fabricated on a 200-μm-thick polyethersulfone substrate. The a-IGZO channels ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Cho), [email protected] (S. Kim).
http://dx.doi.org/10.1016/j.mee.2016.03.044 0167-9317/© 2016 Elsevier B.V. All rights reserved.
having a thickness of 150-nm were deposited by sputtering using a target of IGZO (In2O3:Ga2O3:ZnO = 1:1:1 mol%) in pure Ar gas at a pressure of 1.0 mTorr with a power of 120 W for 40 min. The length and width of the channel were 20 μm and 2 μm, respectively. The source and drain electrodes of Al were deposited by thermal evaporation, and the Al2O3 gate dielectric was deposited by atomic layer deposition using trimethylaluminum and H2O at 150 °C. Then an 80-nm-thick Al layer used as a gate electrode was deposited by thermal evaporation. Side- and top-view schematic diagrams of a top-gate a-IGZO TFT are shown in Fig. 1(a) and (b), respectively; and a-IGZO TFTs on homemade bending stages with curvature radii of 18, 15, 12, and 9 mm are shown in Fig. 1(c). Mechanical stress was applied parallel to the channels, and the electrical characteristics were measured with an HP 4155C semiconductor analyzer at room temperature. In addition, we examined the mechanical stability of our TFT through the continuous bending test using a home-made bending machine. 3. Results and discussion Fig. 2 shows (a) the output curves and (b) the transfer characteristics obtained at a drain-source voltage (VDS) of 3 V for a representative a-IGZO TFT without any mechanical stress. These characteristics reveal n-type behavior. The field-effect mobility in the linear regime and the subthreshold swing (S.S) value are determined by the following equations:
IDS ¼
i C OX μ W h 2ðV GS −V TH ÞV DS −V 2DS 2 L
ð1Þ
180
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 1. (a) Side- and (b) top-view schematic diagrams of the top-gate a-IGZO TFT, and (c) optical images of homemade bending stages.
SS ¼
∂V GS ∂ð logIDS Þ
ð2Þ
below [15]: εsurface ð%Þ ¼
where μ is the field effect mobility, Cox (280 nF/cm2) is the gate dielectric capacitance per unit area, VTH is the threshold voltage, and W/L (10 μm/20 μm) is the ratio of the channel width and length. Herein, the entire width of the channel is the sum of the five split channels of 2 μm each. The field effect mobility, the S.S, and the on/off ratio are 71.8 cm2/V∙s, 216 mV/dec, and 108, respectively. These electrical characteristics are comparable or superior to those of a-IGZO TFTs previously reported in refs. 9–14; their field-effect mobilities, S.Ss, and on/off ratios are listed in Table 1. It is worth noting that our TFT exhibits the highest field-effect mobility although the process temperature is lowest. The split channel in our TFT accounts for the good characteristics. The channel splitting lowers the series resistance [8] since the multipaths in the split-channel TFTs are connected in parallel. The lowering of the series resistance causes a drain voltage drop in the conduction channel of the TFTs, and consequently the field-effect mobility is increased by the channel-splitting effect. The electrical characteristics of our TFT are examined as a function of bending curvature radius. Our TFT on homemade bending stages with tensile curvature radii of 18, 15, 12, and 9 mm experiences mechanical strains of 0.5, 0.6, 0.8, and 1.1%. The strain is calculated by the equation
d f þ ds 1 þ 2η þ χη2 100 ð1 þ ηÞð1 þ χηÞ 2R
ð3Þ
where df and ds are the thicknesses of the film and the substrate, respectively, η is df/ds, χ is the Young's modulus ratio of the gate dielectric layer to the substrate (χ = Yf/Ys), and R is the bending curvature radius; and df, ds, Yf, and Ys are 260 nm, 200 μm, 182 GPa, and 2.7 GPa, respectively [16,17]. The thickness of the film, df, is the sum of thicknesses of all layers in our TFT, but the Young's modulus ratio, χ, is calculated with the Young's modulus of the gate dielectric layer, Al2O3, because the oxide layer is the part that is most vulnerable to mechanical stress in our TFT. Fig. 3(a) shows the transfer characteristics of VDS of 3 V as a function of bending curvature radius, demonstrating an electrical failure with a bending curvature radius of 9 mm corresponding to a strain of 1.1%. Existing bendable inorganic TFTs cannot endure a mechanical strain of greater than 1% because oxides used as gate dielectric layers or channel layers are inherently vulnerable to harsh mechanical strain compared with organic materials. Fig. 3(b), (c), and (d) demonstrate the variations in on-current, threshold voltage (VTH), and mobility of the bent TFT with curvature radii of 18, 15, and 12 mm. Herein the maximum, the minimum, and the average values are obtained from the measured values of eleven different TFTs. In this study, the operation regions of our TFT are divided into three categories, as represented in Fig. 3(b), (c) and
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 2. (a) IDS-VDS curves and (b) IDS-VGS curves of the a-IGZO TFT.
(d): safe region, transition region, and definitive mechanical failure region. With the bending, the on-current and mobility are decreased from 7.5 to 2.7 μA and from 71.8 to 25.9 cm2/V·s, respectively, and VTH is shifted toward positive voltages up to 2.1 V. In addition, a severe degradation in the electrical characteristics is observed at a bending curvature radius of 12 mm. In spite of such a severe degradation, the mobility is still better than those of other IGZO TFTs. This supports the advantage of our TFT being constructed with split channels. In addition, using Al as an electrode material in this study allows for TFTs that are more durable against mechanical strain than those using Cr electrodes because the Young's modulus of Al (68 GPa) is smaller than that of Cr (117 GPa). The criteria for the safe and transition regions are determined here to be a VTH shift of 0.5 V and a change of 10% in the mobility.
181
According to the classification, our TFT operates safely under bending with a tensile curvature radius of 15 mm corresponding to a strain of 0.6% since the shift of VTH is 0.3 V and the change in mobility is less than 5%. However, the characteristics of our TFT deteriorate under bending with a tensile curvature radius of 12 mm corresponding to a strain of 0.8%, which appears at a VTH shift of 1.2 V and a change of 39% in the mobility. The deterioration of the characteristics are attributed to the generation of defects caused by mechanical deformation in transition regions [18]. In particular, the defects generated in a channel layer can reduce the on-current and shift VTH in the positive direction, which are in accordance with our results shown in Fig. 3. Generally, microcracks induced by mechanical strain are responsible for the electrical failure of bendable TFTs. Fig. 4 shows the optical images of a-IGZO TFTs (a) before and after the bending with tensile curvature radii of (b) 15, (c) 12, and (d) 9 mm. The five split channels are depicted by faint lines in all of the images, while microcracks appear after the bending with a tensile curvature radius of 12 mm and are clearly visible after the bending with a tensile curvature radius of 9 mm. The presence of the microcracks causes the electrical failure of the TFTs. As shown in Fig. 4(d), microcracks occur over the source and drain electrodes rather than in the channels, indicating that the contacts between the electrodes and channels are broken but that the channels with narrow widths endure the mechanical strain. Fig. 5 represents the VTH shift (△VTH) and the change of mobility (μ/ μ0) of the a-IGZO TFT as a function of the number of bending cycles with a tensile strain of 0.6%; the insets show photographs of the devices in flat and bent state inside a home-made machine. Herein, μ and μ0 are the mobility measured in flat state after the bending and the initial mobility, respectively. The changes in the electrical characteristics are negligible even after the TFT undergoes the continuous bending of 2000 cycles, which demonstrates the great potential of the a-IGZO TFTs for flexible electronic systems.
4. Conclusion In this study, we fabricate a top-gate a-IGZO TFT with five split channels and investigate the variation in the electrical characteristics as a function of bending curvature radius. Our TFT exhibits a mobility of 71.8 cm2/V ∙ s and an on/off ratio of 108. The excellent characteristics are a result of the channel width splitting effect. Based on mechanical strain, the operation regions of our TFT are categorized into safe, transition, and definitive mechanical failure regions. The microcracks observed after the bending with a tensile curvature radius of 9 mm are responsible for the failure in the electrical characteristics of the a-IGZO TFT.
Table 1 Electrical characteristics of a-IGZO TFTs. Ref.
Device type
Channel size (μm)
S.S (mV/dec)
μ (cm2/V·s)
Ion/Ioff
Max. process temperature (°C)
[9]
Bottom-gate inverted staggered TFT (bendable)
150
14.5
107
150
[10]
Bottom-gate inverted staggered TFT (rigid)
W = 280 L = 35 W = 60 L=2 W = 20 L = 20 W = 24 L = 16 W = 24 L = 16 W = 20 L = 10 W = 10 L = 20
430
6.3
108
350
7
200
[11]
Bottom-gate inverted staggered TFT (rigid)
[12]
Top-gate staggered TFT (rigid)
[13]
Top-gate staggered TFT (bendable)
[14] This work
Top-gate staggered TFT (bendable) Top-gate coplanar TFT (bendable)
240
19
10
580
5.6
108
200
750
10.9
107
200
48.5
10
5
220
10
8
150
100 216
71.8
182
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 3. (a) Transfer characteristics, (b) ION_Bending/ION_Flat, and (c) △VTH and μBending/μFlat of the a-IGZO TFT as a function of curvature radius.
Acknowledgments This work was supported in part by the Mid-career Researcher Program (no. NRF-2013R1A2A1A03070750, NRF-2015R1A2A1A15055437), grant funded by the Korean Government (MSIP) (no. NRF-
2015R1A5A7037674) and Basic Science Research Program (no. NRF2015R101A1A01057641) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology, the Brain Korea 21 Plus Project in 2015, and a grant from Samsung Display Co. Ltd.
Fig. 4. Optical images of a-IGZO TFTs (a) before bending and after bending with curvature radii of (b) 15, (c) 12, and (d) 9 mm.
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 5. △VTH and μ/μ0 of the a-IGZO TFT as a function of the number of bending cycle. Insets show the devices in flat and bent states inside a home-made bending machine.
References [1] J. Liang, L. Li, X. Niu, Z. Yu, Q. Pei, Nat. Photonics 7 (2013) 817. [2] M. Jong Han, D.Y. Khang, Adv. Mater. 27 (2015) 4969. [3] S. Das, R. Gulotty, A.V. Sumant, A. Roelofs, Nano Lett. 14 (2014) 2861.
183
[4] S.H. Jin, S.K. Kang, I.T. Cho, S.Y. Han, H.U. Chung, D.J. Lee, J. Shin, G.W. Baek, T.I. Kim, J.H. Lee, J.A. Rogers, ACS Appl. Mater. Interfaces 7 (2015) 8268. [5] J.S. Seo, J.H. Jeon, Y.H. Hwang, H. Park, M. Ryu, S.H.K. Park, B.S. Bae, Sci. Report. 3 (2013) 2085. [6] G.J. Lee, J. Kim, J.H. Kim, S.M. Jeong, J.E. Jang, J. Jeong, Semicond. Sci. Technol. 29 (2014) 035003. [7] H.C. Lai, Z. Pei, J.R. Jian, B.J. Tzeng, Appl. Phys. Lett. 105 (2014) 033510. [8] C.W. Byun, A.M. Reddy, S.W. Son, Y.W. Lee, S.K. Joo, IEEE Trans. Electron Devices 60 (2013) 1390. [9] N. Münzenrieder, C. Zysset, T. Kinkeldei, G. Tröster, IEEE Trans. Electron Devices 59 (2012) 2153. [10] J. Jeong, G.J. Lee, J. Kim, B. Choi, Appl. Phys. Lett. 100 (2012) 112109. [11] M. Mativenga, J.K. Um, D.H. Kang, R.K. Mruthyunjaya, J.H. Chang, G.N. Heiler, T.J. Tredwell, J. Jang, IEEE Trans. Electron Devices 59 (2012) 2501. [12] C.Y. Lin, C.W. Chien, C.H. Wu, H.H. Hsieh, C.C. Wu, Y.H. Yeh, C.C. Cheng, C.M. Lai, M.J. Yu, IEEE Trans. Electron Devices 59 (2012) 1701. [13] C.Y. Lin, C.W. Chien, C.C. Wu, Y.H. Yeh, C.C. Cheng, C.M. Lai, M.J. Yu, C.M. Leu, T.M. Lee, IEEE Trans. Electron Devices 59 (2012) 1956. [14] M.J. Yu, Y.H. Yeh, C.C. Cheng, C.Y. Lin, G.T. Ho, B.C.M. Lai, C.M. Leu, T.H. Hou, Y.J. Chan, IEEE Electron Device Lett. 33 (2012) 47. [15] K. Fukuda, K. Hikichi, T. Sekine, Y. Takeda, T. Minamiki, D. Kumaki, S. Tokito, Sci. Report. 3 (2013) 2048. [16] M.K. Tripp, C. Stampfer, D.C. Miller, T. Helbling, C.F. Herrmann, C. Hierold, K. Gall, S.M. George, V.M. Bright, Sensors Actuators A 130 (2006) 419. [17] T.J. Young, M.A. Monclus, T.L. Burnett, W.R. Broughton, S.L. Ogin, P.A. Smith, Meas. Sci. Technol. 22 (2011) 125703. [18] B.U. Hwang, D.I. Kim, S.W. Cho, M.G. Yun, H.J. Kim, Y.J. Kim, H.K. Cho, N.E. Lee, Org. Electron. 15 (2014) 1458.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Electrical characteristics of bendable a-IGZO thin-film transistors with split channels and top-gate structure Hyungon Oh, Kyoungah Cho ⁎, Sukhyung Park, Sangsig Kim ⁎ Department of Electrical Engineering, Korea University, Anam-ro 145, Sungbuk-gu, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 October 2015 Received in revised form 19 March 2016 Accepted 22 March 2016 Available online 24 March 2016 Keywords: Thin-film transistor a-IGZO Bending curvature radius Plastic substrate Split channel
a b s t r a c t In this study, we fabricate top-gate amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with five split channels on a bendable plastic substrate and investigate the electrical characteristics as a function of bending curvature radius. Owing to the channel width splitting effect, our TFTs have outstanding characteristics including a high mobility of 71.8 cm2/V·s and an on/off ratio of 108. Our bending study reveals that the operation regions of our TFT are categorized into safe, transition, and definitive mechanical failure regions as the value of the bending curvature radius decreases. In the transition region, the threshold voltage is shifted from 1.0 to 2.1 V, and the mobility is decreased from 71.8 to 25.9 cm2/V·s. The electrical failure of bendable TFTs results from microcracks induced by mechanical strain. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, bendable thin-film transistors (TFTs) have received much attention as components for flexible displays [1–3]. In particular, a-Indium Gallium Zinc Oxide (a-IGZO) TFTs as switching devices for operating displays are extremely promising owing to their higher mobility than a-Si TFTs. In addition, a-IGZO TFTs have the leading role in the development of next-generation transparent flexible displays [4–7]. Most of the research work on bendable a-IGZO TFTs has focused on bottomgate structures used in liquid crystal displays rather than top-gate structures used as driving devices for active-matrix organic light-emitting diodes. And bendable a-IGZO TFTs with top-gate structure have been studied mainly for not a coplanar type but a staggered type. Hence, in this study, we fabricate a-IGZO TFTs with top-gate structures on plastic substrates and examine their electrical characteristics as functions of the bending curvature radius. Additionally, channel width splitting in poly-Si TFTs is an effective way to improve driving characteristics such as driving current, mobility, and threshold voltage [8]. The channel width splitting method has not been introduced in the fabrication of a-IGZO TFTs yet. Thus, in this study, we introduce this method for fabricating a-IGZO TFTs to improve their electrical characteristics. 2. Experiment details In this study, an a-IGZO TFT with five split channels was fabricated on a 200-μm-thick polyethersulfone substrate. The a-IGZO channels ⁎ Corresponding authors. E-mail addresses: [email protected] (K. Cho), [email protected] (S. Kim).
http://dx.doi.org/10.1016/j.mee.2016.03.044 0167-9317/© 2016 Elsevier B.V. All rights reserved.
having a thickness of 150-nm were deposited by sputtering using a target of IGZO (In2O3:Ga2O3:ZnO = 1:1:1 mol%) in pure Ar gas at a pressure of 1.0 mTorr with a power of 120 W for 40 min. The length and width of the channel were 20 μm and 2 μm, respectively. The source and drain electrodes of Al were deposited by thermal evaporation, and the Al2O3 gate dielectric was deposited by atomic layer deposition using trimethylaluminum and H2O at 150 °C. Then an 80-nm-thick Al layer used as a gate electrode was deposited by thermal evaporation. Side- and top-view schematic diagrams of a top-gate a-IGZO TFT are shown in Fig. 1(a) and (b), respectively; and a-IGZO TFTs on homemade bending stages with curvature radii of 18, 15, 12, and 9 mm are shown in Fig. 1(c). Mechanical stress was applied parallel to the channels, and the electrical characteristics were measured with an HP 4155C semiconductor analyzer at room temperature. In addition, we examined the mechanical stability of our TFT through the continuous bending test using a home-made bending machine. 3. Results and discussion Fig. 2 shows (a) the output curves and (b) the transfer characteristics obtained at a drain-source voltage (VDS) of 3 V for a representative a-IGZO TFT without any mechanical stress. These characteristics reveal n-type behavior. The field-effect mobility in the linear regime and the subthreshold swing (S.S) value are determined by the following equations:
IDS ¼
i C OX μ W h 2ðV GS −V TH ÞV DS −V 2DS 2 L
ð1Þ
180
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 1. (a) Side- and (b) top-view schematic diagrams of the top-gate a-IGZO TFT, and (c) optical images of homemade bending stages.
SS ¼
∂V GS ∂ð logIDS Þ
ð2Þ
below [15]: εsurface ð%Þ ¼
where μ is the field effect mobility, Cox (280 nF/cm2) is the gate dielectric capacitance per unit area, VTH is the threshold voltage, and W/L (10 μm/20 μm) is the ratio of the channel width and length. Herein, the entire width of the channel is the sum of the five split channels of 2 μm each. The field effect mobility, the S.S, and the on/off ratio are 71.8 cm2/V∙s, 216 mV/dec, and 108, respectively. These electrical characteristics are comparable or superior to those of a-IGZO TFTs previously reported in refs. 9–14; their field-effect mobilities, S.Ss, and on/off ratios are listed in Table 1. It is worth noting that our TFT exhibits the highest field-effect mobility although the process temperature is lowest. The split channel in our TFT accounts for the good characteristics. The channel splitting lowers the series resistance [8] since the multipaths in the split-channel TFTs are connected in parallel. The lowering of the series resistance causes a drain voltage drop in the conduction channel of the TFTs, and consequently the field-effect mobility is increased by the channel-splitting effect. The electrical characteristics of our TFT are examined as a function of bending curvature radius. Our TFT on homemade bending stages with tensile curvature radii of 18, 15, 12, and 9 mm experiences mechanical strains of 0.5, 0.6, 0.8, and 1.1%. The strain is calculated by the equation
d f þ ds 1 þ 2η þ χη2 100 ð1 þ ηÞð1 þ χηÞ 2R
ð3Þ
where df and ds are the thicknesses of the film and the substrate, respectively, η is df/ds, χ is the Young's modulus ratio of the gate dielectric layer to the substrate (χ = Yf/Ys), and R is the bending curvature radius; and df, ds, Yf, and Ys are 260 nm, 200 μm, 182 GPa, and 2.7 GPa, respectively [16,17]. The thickness of the film, df, is the sum of thicknesses of all layers in our TFT, but the Young's modulus ratio, χ, is calculated with the Young's modulus of the gate dielectric layer, Al2O3, because the oxide layer is the part that is most vulnerable to mechanical stress in our TFT. Fig. 3(a) shows the transfer characteristics of VDS of 3 V as a function of bending curvature radius, demonstrating an electrical failure with a bending curvature radius of 9 mm corresponding to a strain of 1.1%. Existing bendable inorganic TFTs cannot endure a mechanical strain of greater than 1% because oxides used as gate dielectric layers or channel layers are inherently vulnerable to harsh mechanical strain compared with organic materials. Fig. 3(b), (c), and (d) demonstrate the variations in on-current, threshold voltage (VTH), and mobility of the bent TFT with curvature radii of 18, 15, and 12 mm. Herein the maximum, the minimum, and the average values are obtained from the measured values of eleven different TFTs. In this study, the operation regions of our TFT are divided into three categories, as represented in Fig. 3(b), (c) and
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 2. (a) IDS-VDS curves and (b) IDS-VGS curves of the a-IGZO TFT.
(d): safe region, transition region, and definitive mechanical failure region. With the bending, the on-current and mobility are decreased from 7.5 to 2.7 μA and from 71.8 to 25.9 cm2/V·s, respectively, and VTH is shifted toward positive voltages up to 2.1 V. In addition, a severe degradation in the electrical characteristics is observed at a bending curvature radius of 12 mm. In spite of such a severe degradation, the mobility is still better than those of other IGZO TFTs. This supports the advantage of our TFT being constructed with split channels. In addition, using Al as an electrode material in this study allows for TFTs that are more durable against mechanical strain than those using Cr electrodes because the Young's modulus of Al (68 GPa) is smaller than that of Cr (117 GPa). The criteria for the safe and transition regions are determined here to be a VTH shift of 0.5 V and a change of 10% in the mobility.
181
According to the classification, our TFT operates safely under bending with a tensile curvature radius of 15 mm corresponding to a strain of 0.6% since the shift of VTH is 0.3 V and the change in mobility is less than 5%. However, the characteristics of our TFT deteriorate under bending with a tensile curvature radius of 12 mm corresponding to a strain of 0.8%, which appears at a VTH shift of 1.2 V and a change of 39% in the mobility. The deterioration of the characteristics are attributed to the generation of defects caused by mechanical deformation in transition regions [18]. In particular, the defects generated in a channel layer can reduce the on-current and shift VTH in the positive direction, which are in accordance with our results shown in Fig. 3. Generally, microcracks induced by mechanical strain are responsible for the electrical failure of bendable TFTs. Fig. 4 shows the optical images of a-IGZO TFTs (a) before and after the bending with tensile curvature radii of (b) 15, (c) 12, and (d) 9 mm. The five split channels are depicted by faint lines in all of the images, while microcracks appear after the bending with a tensile curvature radius of 12 mm and are clearly visible after the bending with a tensile curvature radius of 9 mm. The presence of the microcracks causes the electrical failure of the TFTs. As shown in Fig. 4(d), microcracks occur over the source and drain electrodes rather than in the channels, indicating that the contacts between the electrodes and channels are broken but that the channels with narrow widths endure the mechanical strain. Fig. 5 represents the VTH shift (△VTH) and the change of mobility (μ/ μ0) of the a-IGZO TFT as a function of the number of bending cycles with a tensile strain of 0.6%; the insets show photographs of the devices in flat and bent state inside a home-made machine. Herein, μ and μ0 are the mobility measured in flat state after the bending and the initial mobility, respectively. The changes in the electrical characteristics are negligible even after the TFT undergoes the continuous bending of 2000 cycles, which demonstrates the great potential of the a-IGZO TFTs for flexible electronic systems.
4. Conclusion In this study, we fabricate a top-gate a-IGZO TFT with five split channels and investigate the variation in the electrical characteristics as a function of bending curvature radius. Our TFT exhibits a mobility of 71.8 cm2/V ∙ s and an on/off ratio of 108. The excellent characteristics are a result of the channel width splitting effect. Based on mechanical strain, the operation regions of our TFT are categorized into safe, transition, and definitive mechanical failure regions. The microcracks observed after the bending with a tensile curvature radius of 9 mm are responsible for the failure in the electrical characteristics of the a-IGZO TFT.
Table 1 Electrical characteristics of a-IGZO TFTs. Ref.
Device type
Channel size (μm)
S.S (mV/dec)
μ (cm2/V·s)
Ion/Ioff
Max. process temperature (°C)
[9]
Bottom-gate inverted staggered TFT (bendable)
150
14.5
107
150
[10]
Bottom-gate inverted staggered TFT (rigid)
W = 280 L = 35 W = 60 L=2 W = 20 L = 20 W = 24 L = 16 W = 24 L = 16 W = 20 L = 10 W = 10 L = 20
430
6.3
108
350
7
200
[11]
Bottom-gate inverted staggered TFT (rigid)
[12]
Top-gate staggered TFT (rigid)
[13]
Top-gate staggered TFT (bendable)
[14] This work
Top-gate staggered TFT (bendable) Top-gate coplanar TFT (bendable)
240
19
10
580
5.6
108
200
750
10.9
107
200
48.5
10
5
220
10
8
150
100 216
71.8
182
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 3. (a) Transfer characteristics, (b) ION_Bending/ION_Flat, and (c) △VTH and μBending/μFlat of the a-IGZO TFT as a function of curvature radius.
Acknowledgments This work was supported in part by the Mid-career Researcher Program (no. NRF-2013R1A2A1A03070750, NRF-2015R1A2A1A15055437), grant funded by the Korean Government (MSIP) (no. NRF-
2015R1A5A7037674) and Basic Science Research Program (no. NRF2015R101A1A01057641) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology, the Brain Korea 21 Plus Project in 2015, and a grant from Samsung Display Co. Ltd.
Fig. 4. Optical images of a-IGZO TFTs (a) before bending and after bending with curvature radii of (b) 15, (c) 12, and (d) 9 mm.
H. Oh et al. / Microelectronic Engineering 159 (2016) 179–183
Fig. 5. △VTH and μ/μ0 of the a-IGZO TFT as a function of the number of bending cycle. Insets show the devices in flat and bent states inside a home-made bending machine.
References [1] J. Liang, L. Li, X. Niu, Z. Yu, Q. Pei, Nat. Photonics 7 (2013) 817. [2] M. Jong Han, D.Y. Khang, Adv. Mater. 27 (2015) 4969. [3] S. Das, R. Gulotty, A.V. Sumant, A. Roelofs, Nano Lett. 14 (2014) 2861.
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