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Oct 31, 2016 - Department of Materials Science and Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea. §. Display Resea...
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A Study on the Electrical Properties of Atomic Layer Deposition Grown InOx on Flexible Substrates with Respect to N2O Plasma Treatment and the Associated Thin-Film Transistor Behavior under Repetitive Mechanical Stress Jiazhen Sheng,† Jozeph Park,‡,∥ Dong-won Choi,† Junhyung Lim,*,§ and Jin-Seong Park*,† †

Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro Seongdong-gu, Seoul, 04763, Republic of Korea ‡ Department of Materials Science and Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, 34141, Republic of Korea § Display Research and Development Center, Samsung Display Company, Limited, Yongin 446-711, Republic of Korea S Supporting Information *

ABSTRACT: Indium oxide (InOx) films were deposited at low processing temperature (150 °C) by atomic layer deposition (ALD) using [1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (InCA-1) as the metal precursor and hydrogen peroxide (H2O2) as the oxidant. As-deposited InOx exhibits a metallic conductor-like behavior owing to a relatively high free-carrier concentration. In order to control the electron density in InOx layers, N2O plasma treatment was carried out on the film surface. The exposure time to N2O plasma was varied (600−2400 s) to evaluate its effect on the electrical properties of InOx. In this regard, thin-film transistors (TFTs) utilizing this material as the active layer were fabricated on polyimide substrates, and transfer curves were measured. As the plasma treatment time increases, the TFTs exhibit a transition from metal-like conductor to a highperformance switching device. This clearly demonstrates that the N2O plasma has an effect of diminishing the carrier concentration in InOx. The combination of low-temperature ALD and N2O plasma process offers the possibility to achieve highperformance devices on polymer substrates. The electrical properties of InOx TFTs were further examined with respect to various radii of curvature and repetitive bending of the substrate. Not only does prolonged cyclic mechanical stress affect the device properties, but the bending direction is also found to be influential. Understanding such behavior of flexible InOx TFTs is anticipated to provide effective ways to design and achieve reliable electronic applications with various form factors. KEYWORDS: atomic layer deposition, indium oxide semiconductor, mechanical stress, flexible TFT, N2O plasma treatment

1. INTRODUCTION Recently, the fabrication of organic light-emitting displays (OLED) on mechanically flexible substrates has drawn a lot of attention. In order to realize flexible displays, thin-film transistor (TFT) devices must be fabricated on plastic substrates such as polyethylene naphthalate (PEN), polyethylene terephthalate (PET), or polyimide (PI).1 Such polymer-based materials are prone to degradation during the TFT backplane fabrication process that normally requires the growth of several layers at elevated temperatures and thermal annealing to activate the semiconductor properties. In this regard, PI substrates are promising for the realization of oxide semiconductor TFTs owing to their relatively high glass transition temperature, low coefficient of thermal expansion (CTE), and excellent mechanical properties compared to those of other polymers.2 However, process temperatures above 300 °C are necessary in order to realize sufficiently reliable oxide © 2016 American Chemical Society

TFTs with high performance. Many research groups have thus developed techniques such as ultraviolet radiation or microwave annealing in order to reduce the thermal budget while obtaining reasonable device properties.3−6 Considering the growth of high-quality inorganic films at temperatures that polymer substrates can withstand, atomic layer deposition (ALD) is at present most appropriate. The ALD method offers accurate control of the film thickness and composition and excellent uniformity over large areas, in comparison with the conventional deposition techniques, such as radio frequency (rf) magnetron sputtering, sol−gel process, and chemical vapor deposition (CVD).7 However, only few studies are available in the literature on ALD synthesis of oxide Received: September 18, 2016 Accepted: October 31, 2016 Published: October 31, 2016 31136

DOI: 10.1021/acsami.6b11815 ACS Appl. Mater. Interfaces 2016, 8, 31136−31143

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ACS Applied Materials & Interfaces semiconductors for mechanically flexible TFT applications.8,9 It is well-known that oxide films synthesized by the ALD process consist of binary systems with simple compositions, such as ZnO, Al2O3, ZrO, HfOx, and InOx. Among them, InOx was formerly reported to be a potential candidate for use as the active layer in TFT devices, because of its high-mobility n-type character originating from a single free electron-like band of In 5s states.10−12 As-deposited InOx generally exhibits metal-like conduction; therefore, in order to successfully implement this material as a semiconductor, the total carrier concentration must be reduced to convey switching properties to the resulting TFT. For example, the film thickness may be reduced to a few nanometers only, the film may be annealed in an oxygen-rich environment to reduce the source of free carriers (oxygen vacancies), or chemical doping may be done with high oxygen affinity metal cations. In this study, N2O plasma treatment (PT) is used as a lowtemperature post-treatment to effectively decrease the carrier concentration in ALD-grown InOx films. N2O PT was formerly reported to improve the transfer performance and reliability of semiconductor oxide TFTs.13,14 The exposure of the TFT channel surface to N2O plasma was found to induce chemical (oxidation) and/or physical (detachment of loosely bound oxygen) reactions.15 The present work involves film growth and post-annealing processes at temperatures below 150 °C, while the N2O treatment of InOx is carried out at room temperature without intentional heating. Successful fabrication of working TFT devices is demonstrated with appropriate combination of N2O PT, using flexible polyimide substrates. The TFTs are then subjected to repetitive mechanical bending, the effects of which on the transfer characteristics are studied in terms of radii of curvature and bending direction.

3. RESULTS AND DISCUSSION InOx layers were successfully synthesized by ALD growth using InCA-1 and H2O2 at a substrate temperature of 150 °C. In order to optimize the deposition conditions, the film thickness was monitored with respect to the number of ALD cycles, and the growth rate was evaluated as a function of the indium precursor dose. The film thickness increases linearly with the number of cycles, resulting in a net growth rate of ∼0.6 Å/ cycle. The latter is a saturated value at InCA-1 precursor doses exceeding 4 nmol/cm2, as shown in Figure 1. Such a growth behavior is attributed to the reaction of reactant molecules with a finite number of reactive sites on the surface during each ALD cycle.

Figure 1. Growth rate of ALD InOx layers with respect to (a) the number of cycles and (b) the precursor dose at a substrate temperature of 125 °C.

The thickness of the InOx active layers used in the bottom gate/top contact TFT structures was fixed at 18 nm. Asdeposited InOx contains a relatively high carrier concentration, which results in conductor-like TFT behavior. Various processes have been studied to reduce the carrier density in oxide films, of which N2O plasma treatment is reported to be effective.15,16 In the present work, InOx films have been exposed to N2O PT with various durations between 600 and 2400 s. The resulting electrical properties are indicated in Figure S2 of the Supporting Information (SI), which shows that the carrier concentration of InOx drops dramatically (from 2.3 × 1021 to 4.0 × 1015 cm−3) while the resistivity increases (from 2.2 × 10−3 to 2.7 × 102 Ω cm) with increasing N2O PT time, and the transfer curves are shown in Figure 2. The device fabricated without N2O PT exhibits a conductor-like behavior, and as the N2O PT time increases, the off-current level of the transfer curves decreases gradually while the threshold voltage (Vth) shifts toward positive values. Clear switching characteristics are obtained with PT time exceeding 1200 s, of which the representative parameters such as Vth, linear field effect mobility (μeff), saturation field effect mobility (μsat), subthreshold swing (SS), hysteresis, and ION/IOFF values are listed in Table 1. The μsat values of the TFTs exposed to N2O plasma for 1200, 1500, and 1800 s are 9.7 ± 0.1, 2.3 ± 0.3, and 0.5 ± 0.2 cm2/(V s), respectively. These results suggest that the N2O plasma treatment led to oxidation of the InOx active layer, reducing the concentration of oxygen vacancies therein. XPS composition analyses indicate that indeed the relative oxygen content in InOx increases with increasing PT time, as shown in Table S1 (SI).

2. METHODS TFTs with bottom gate and top contact structures were fabricated on 18-μm-thick polyimide (PI) substrates coated on glass. An initial SiO2/ SiNx/SiO2 stack was grown on the PI, followed by the synthesis of a 30-nm-thick AlOx buffer layer. The gate electrodes were formed by sputter-depositing a 100-nm-thick indium−tin oxide (ITO) layer at room temperature. A 100 nm AlOx layer was grown as the gate insulator at 150 °C by ALD using trimethylaluminum (TMA) and H2O as the precursor and reactant, respectively. The InOx active layer with a thickness of 18 nm was grown by thermal ALD at 150 °C using 1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]indium (InCA-1) as the indium precursor and hydrogen peroxide as the reactant. The active islands were patterned by a combination of photolithography and wet etching. In order to evaluate the effects of the N2O plasma treatment (PT) on the electrical properties, the PT time was varied from 0 to 2400 s. The source and drain electrodes were formed by thermally evaporating Ti/Al (5 nm/100 nm) metal stacks, which were patterned by the lift-off method. The channel width (W) and length (L) dimensions are W/L = 40/20 μm. The InOx films subjected to N2O PT were examined using Hall measurements (Ecopia, AH5TTT5) at room temperature to determine the carrier concentration and mobility. Chemical bonding states in the core-level region were analyzed using X-ray photoelectron spectroscopy (XPS), and the morphology of the film surface was observed using an atomic force microscope (AFM, XE-100). The semiconductor device properties were evaluated using a HP 4155A parameter analyzer in air at room temperature. The device stability under positive bias stress (PBS) was evaluated in air ambient, using a bias of VGS = 20 V for a total stress time of 3600 s. The bending tests were performed using a homemade bending machine with bending radii ranging from 5 to 15 mm. 31137

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the semiconductor/dielectric interface or defects in the oxide semiconductor is well-known to be the degradation mechanism.19 In the case of the N2O-plasma-treated devices, the PBTS stability decreases with increasing oxygen content in InOx; therefore, defects related to oxygen vacant sites are not expected to play the major role. Instead, oxygen interstitials (Oi) are theoretically predicted to act as electron trap centers and may be of importance here.20,21 The incorporation of excess oxygen during the N2O plasma treatment is highly likely to induce the presence of interstitial oxygen in InOx and consequently reduce the device reliability with respect to PBTS. In addition, the surface morphology of N2O-plasma-treated InOx films was examined by atomic force microscopy (AFM) as shown in Figure 5. The as-deposited layer exhibits a relatively smooth (RMS value of 0.103 nm) surface, while the roughness increases considerably with increasing N2O PT time, from 1200 s (RMS value of 0.321 nm) to 1800 s (RMS value of 0.755 nm). Such a structural modification may also have generated electron trap sites in the oxide semiconductor, thus affecting the device stability under PBTS. To fabricate mechanically flexible devices, the InOx TFTs were fabricated on PI-coated glass, and the PI substrates were delaminated from the underlying glass, as shown in Figure 6. The transfer characteristics of flexible TFTs incorporating InOx exposed to N2O PT for 1200 s as the active layer were studied, with respect to mechanical stress. Bending tests were performed with various radii of curvature ranging from 15 to 5 mm, as shown in Figure S2 (SI). Table 3 indicates the representative electrical parameters of the TFTs under different curvatures. As the mechanical strain induced in the TFT increases (i.e., decreasing bending radius), the Vth value becomes more negative. Such a phenomenon may be explained by two mechanisms. One involves the decrease in energy level splitting (ΔE) of the bonding and antibonding orbitals between the atoms in InOx under tensile strain.20 The additional electrons excited to the antibonding state are anticipated to induce negative shifts in Vth. Another reason may be attributed to the generation of oxygen vacancies under tensile strain.22−24 The slight decrease in saturation mobility may result from the increased interatomic spacing in InOx under tensile stain25 or the generation of defects that is manifested by increased SS and hysteresis values. For the successful implementation of flexible devices into bendable, foldable, and wearable products, the electronic devices should exhibit sufficient durability under prolonged and repetitive mechanical deformation. To study the reliability of InOx TFTs with respect to mechanical stress, repetitive bending tests were carried out with bending axes along the channel length (case I) and width (case II) as shown in Figure 7. Here, the bending radius was fixed at 5 mm (strain of 0.4%) and the transfer characteristics were recorded after 100, 300, 700, 1500, 3000, and 10 000 bending cycles. As shown in Figure 7, for both case I and II, the Vth shifts in the negative direction and the saturation mobility decreases as the number of bending cycles increases. It is generally reported in the

Figure 2. Transfer curves of TFTs incorporating InOx active layers, the surface of which has been subjected to different N2O PT durations.

In order to correlate the In−O bonding characteristics and the TFT performance, the O 1s peaks of the InOx XPS spectra were analyzed as shown in Figure 3. The O 1s peak was deconvoluted into three subpeaks, A, B, and C, based on Gaussian fitting. Subpeak A at a relatively low binding energy of 529.3 eV is generally attributed to the contribution of oxygen ions that form strong metal−oxygen bonds, and subpeak B, with a binding energy of 530.9 eV, is usually related to electrons emitted by oxygen ions near oxygen-deficient (or oxygen vacant, OV) regions. Subpeak C at a higher binding energy of 532.0 eV is typically associated with the presence of loosely bound oxygen on the surface or OH groups.17,18 The relative fraction of subpeaks A, B, and C may be compared in terms of their integrated intensity ratio with respect to plasma treatment time, as indicated in Table 2. The fraction of subpeak A increases from 78.03 to 83.52% as the N2O PT time increases from 0 to 2400 s, while that of subpeak B decreases from 20.57 to 14.97%. The XPS analyses indicate that the number of oxygen vacancies in InOx diminishes as a result of N2O plasma treatment, which is accompanied by a net decrease in free carrier density, while the formation of In−O bonds is promoted. Besides, nitrogen was not detected during XPS analyses for all N2O PT conditions, which is consistent with previous reports.15 Additionally, the device stability under positive bias temperature stress (PBTS) was evaluated. Under PBTS for 3600 s, the amount of Vth shift (ΔVth) increased with increasing PT time (Figure 4a). Figure 4b shows the transfer curves of a device incorporating InOx exposed to N2O PT for 1200 s, which exhibits the smallest ΔVth (1.03 V) in the positive direction, in comparison with the TFTs fabricated with InOx exposed to N2O plasma for 1500 s (ΔVth = 2.68 V) and 1800 s (ΔVth = 3.45 V). As the stress time increases, the Vth shifts toward positive values without significant changes in mobility and SS values. Under such circumstances, the trapping of electrons at

Table 1. Transfer Performance of TFT Exposed to N2O PT for Various Durations duration (s)

Vth (V)

μeff [cm2/(V s)]

μsat [cm2/(V s)]

SS (V/decade)

hysteresis (V)

ION/IOFF

1200 1500 1800

−0.2 ± 0.1 5.8 ± 0.1 14.5 ± 0.9

9.8 ± 0.3 3.4 ± 0.2 1.6 ± 0.5

9.7 ± 0.1 2.3 ± 0.3 0.5 ± 0.1

0.63 ± 0.12 0.38 ± 0.10 0.41 ± 0.09

0.9 ± 0.1 1.2 ± 0.2 11.0 ± 0.6

6.7 × 109 4.9 × 108 9.6 × 107

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Figure 3. XPS O 1s peaks for InOx films in (a) the as-deposited state and after (b) 1200 s and (c) 2400 s of N2O PT exposure.

Table 2. XPS O 1s Peak Intensity Ratios of InOx Films Exposed to N2O PT for Various Durations O−In Ov O−H

as is

600 s

1200 s

1800 s

2400 s

78.03 20.57 1.40

80.53 17.96 1.51

82.44 16.06 1.49

83.01 15.56 1.42

83.52 14.97 1.50

Figure 4. (a) Vth evolution under PBTS for TFTs fabricated with InOx active layers, the surface of which has been exposed to N2O PT for 1200, 1500, and 1800 s. (b) Evolution of the transfer curve under PBTS for the device fabricated with InOx exposed to N2O PT for 1200 s.

Figure 5. AFM surface images of (a) as-deposited and (b−e) N2O-PTtreated InOx films for various durations and (f) relation of surface roughness and O-deficiency of N2O-PT-treated InOx films.

literature than the degradation of electrical characteristics for amorphous oxide semiconductor is not significant when subjected to mechanical strain (bending) directions, owing to their amorphous microstructure. The InOx films exhibit a nondirectional amorphous phase (as confirmed by X-ray

diffraction analyses, not shown here). However, the changes in Vth, mobility, and SS values are found to be dependent on the bending direction. For case I, where the bending axis is perpendicular to the electrical current flow (Figure 7a), the Vth shifts slowly in the negative direction until 3000 bending cycles, after which a sudden, large shift occurs. On the other hand, the 31139

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Figure 6. (a) Cross-sectional diagram of a TFT device incorporating InOx as the active layer and (b) pictures of flexible TFT arrays on delaminated PI substrates.

Table 3. Transfer Performance of TFT with 1200 s N2O Plasma Treatment Bent with Jig Bars of 15/10/5 mm Radius bending radius (mm)

% strain

0 15 10 5

0 0.12 0.18 0.38

Vth (V) −0.2 −0.5 −0.7 −1.5

± ± ± ±

μsat [cm2/(V s)]

0.1 0.2 0.1 0.3

9.7 8.8 8.4 8.1

± ± ± ±

0.1 0.1 0.2 0.4

SS (V/decade) 0.63 0.96 0.98 0.97

± ± ± ±

0.12 0.07 0.13 0.21

hysteresis (V) 0.9 1.2 1.5 1.9

± ± ± ±

0.1 0.2 0.1 0.3

ION/IOFF 6.7 4.6 4.8 4.8

× × × ×

109 109 109 109

8b, and so the stress tolerance is higher than in case I, where the channel area of InOx is exposed (inset of Figure 8a). Therefore, at an initial stage of repetitive bending (≤700 cycles), it may be suggested that such geometric parameters determine the robustness of the devices with respect to mechanical stress. Note that the mobility decreases and the SS increases drastically after 700 cycles. This phenomenon may be interpreted in terms of stress concentration caused by geometric discontinuities in the strained TFT. In general, geometric discontinuities induce local concentrations of stress fields, as expressed in eqs 1 and 2 below29

mobility decreases gradually, while the SS value undergoes relatively minor change. For case II, where the bending axis is parallel to the current flow, the mobility decreases and the SS value increases dramatically after 700 cycles (Figure 7b). In addition, the hysteresis values increase with increasing bending cycles in comparison with case I. The increase in SS and hysteresis may be interpreted in terms of defect creation by the different bending geometry and the generation of trap sites at the semiconductor/gate insulator interface or the electrode/ semiconductor junctions.26 The decrease in mobility may be related to the formation of microcracks in the active layer. Oxide materials are brittle, and microcracks may easily form and extend in the direction parallel to the bending axis.27 For case I, microcracks are most likely to form in a direction perpendicular to the current flow in the channel layer; therefore, the initial mobility before the cyclic bending test is affected to a lesser extent than in the device in case II, as shown in Figure 8. In the latter, microcracks are expected to run in a direction parallel to the channel length, thereby forming additional current paths. Consequently, as the microcrack density increases after 700 bending cycles for case II, the threshold voltage shifts gradually toward negative values, allowing the device to turn on earlier than its original threshold point. The TFTs under consideration consist of source/drain electrodes in contact on top of the active island. At an early stage of cyclic bending, the induced stress may have a different influence on those interfaces. In general, multilayer stacks exhibit higher stiffness than single films; therefore, the regions where source/drain electrodes are present may be considered to be relatively more resistant to stress-induced deformation.28 For case II, the cross section of the bending axis consists mainly of the stiff InOx/metal stacks, as shown in the inset of Figure

σmax = Ktσ0

(1)

Kt ∝ D/r 2

(2)

where σmax is the maximum theoretical stress in the stress concentration region, Kt the theoretical stress-concentration factor, and σ0 the nominal normal stress. In addition, Kt is proportional to D/r2, where D and r are the thickness of the thicker and thinner layers, respectively. Therefore, while inducing bending stress, the source/drain edge may act as the stress concentration point (indicated by red dashed lines in Figure 8) near which electron trap sites and microcracks are likely to form along the InOx/metal boundary (indicated by red dashed lines in Figure 8). Such structural defects may also extend toward the center of the channel region (indicated by blue arrows in Figure 8). It should also be mentioned that after annealing at 200 °C for 20 min on a hot plate, the degraded TFTs recover most of their initial properties, which implies that mechanical stress may induce charge trapping and/or defect creation (Figure S3, SI). As the above results suggest, TFT arrays for flexible applications necessitate thorough design rules in order to 31140

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Figure 7. Evolution of the transfer performance of flexible InOx TFT fabricated on polyimide substrates as a function of bending cycles with respect to bending axis along (a) the channel width (case I) and (b) the channel length (case II), both with a bending radius of 5 mm.

Figure 8. Schematics of bent InOx TFTs for a bending axis along (a) the channel width (case I) and (b) the channel length (case II).

relatively high conductivity (∼10−4 Ω cm). In order to suppress the formation of excess carriers, N2O plasma treatment was carried out on the InOx surface, and high-mobility semiconducting properties [∼10 cm2/(V s)] were obtained when the treated material was utilized in TFT devices. The latter is attributed to the oxidation of InOx, thus diminishing the total concentration of oxygen vacancies that act as shallow electron donors at a relatively low temperature of 150 °C. TFTs were next fabricated on polyimide substrates using the N2O PT method, which allows the fabrication of working

minimize the effect of structural defects induced by mechanical stress. Geometrical factors are critical in determining the electrical properties of oxide semiconductor devices under various physical deformations.

4. CONCLUSION In this study, InOx films were grown by atomic layer deposition using InCA-1 as the metal precursor and H2O2 as the oxidant. As-deposited InOx layers exhibit conductor-like behavior with 31141

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(2) Liu, J.-g.; Ni, H.-j.; Wang, Z.-h.; Yang, S.-y.; Zhou, W.-f. OptoelectronicsMaterials and Devices; InTech, 2015; Chapter 3. (3) Teng, L. F.; Liu, P. T.; Lo, Y. J.; Lee, Y. J. Effects of Microwave Annealing on Electrical Enhancement of Amorphous Oxide Semiconductor Thin Film Transistor. Appl. Phys. Lett. 2012, 101, 132901. (4) Song, K. K.; Koo, C. Y.; Jun, T. H.; Lee, D. H.; Jeong, Y. M.; Moon, J. H. Low-temperature Soluble InZnO Thin Film Transistors by Microwave Annealing. J. Cryst. Growth 2011, 326, 23−27. (5) Seo, H. T.; Cho, Y. J.; Kim, J. W.; Bobade, S. M.; Park, K. Y.; Lee, J.; Choi, D.-K. Permanent Optical Doping of Amorphous Metal Oxide Semiconductors by Deep Ultraviolet Irradiation at Room Temperature. Appl. Phys. Lett. 2010, 96, 222101. (6) Tak, Y. J.; Ahn, B. D.; Park, S. P.; Kim, S. J.; Song, A. R.; Chung, K. B.; Kim, H. J. Activation of Sputter-Processed Indium-gallium-zinc Oxide Films by Simultaneous Ultraviolet and Thermal Treatments. Sci. Rep. 2016, 6, 21869. (7) Murata, K.; Fuyuki, T.; et al. Large Scaled ALD/PECVD Reactor for Flat Panel Display Application. ECS Trans. 2007, 11 (7), 31. (8) Sheng, J. Z.; Choi, D. W.; Lee, S. H.; Park, J.; Park, J. S. Performance Modulation of Transparent ALD Indium Oxide Films on Flexible Substrate: Transition between Metal-Like Conductor and High Performance Semiconductor State. J. Mater. Chem. C 2016, 4, 7571−7576. (9) Lin, Y. Y.; Hsu, C. C.; Tseng, M. H.; Shyue, J. J.; Tsai, F. Y. Stable and High-Performance Flexible ZnO Thin-Film Transistors by Atomic Layer Deposition. ACS Appl. Mater. Interfaces 2015, 7, 22610−22617. (10) Weiher, R. L.; Ley, R. P. Optical Properties of Indium Oxide. J. Appl. Phys. 1966, 37, 299−302. (11) Kawazoe, H.; Tanoue, H.; Ueda, N.; Un’no, H.; Omata, T.; Hosono, H. Generation of Electron Carriers in Insulating Thin Film of MgIn2O4 Spinelby Li+ Implantation. J. Appl. Phys. 1994, 76, 7935− 7941. (12) Weiher, R. L. Electrical Properties of Single Crystals of Indium Oxide. J. Appl. Phys. 1962, 33, 2834−2839. (13) Park, J. W.; Yoo, S. H.; Lee, D.; Kwon, H. Improvement of OnOff-Current Ratio in TiOx Active-Channel TFTs Using N2O plasma Treatment. IEEE Electron Device Lett. 2009, 30, 362−364. (14) Tsai, C- T.; Huang, C.-Y.; Chang, T.-C.; Chen, S.-C.; Lo, I.; Tsao, S.-W.; Hung, M.-C.; Chang, J.-J.; Wu, C.-Y. Influence of Positive Bias Stress on N2O Plasma Improved InGaZnO Thin Film Transistor. Appl. Phys. Lett. 2010, 96, 242105. (15) Remashan, K.; Hwang, D. K.; Park, S. D.; Bae, J. W.; Yeom, G. Y.; Park, S. J.; Jang, J. H. Effect of N2O Plasma Treatment on the Performance of ZnO TFTs. Electrochem. Solid-State Lett. 2008, 11, H55−H59. (16) Barnes, T. M.; Leaf, J.; Hand, S.; Fry, C.; Wolden, C. A. A Comparison of Plasma-Activated N2/O2 and N2O/O 2 Mixtures for Use in ZnO:N Synthesis by Chemical Vapor Deposition. J. Appl. Phys. 2004, 96, 7036. (17) Hosono, H. Ionic Amorphous Oxide Semiconductors: Material Design, Carrier Transport, and Device Application. J. Non-Cryst. Solids 2006, 352, 851−858. (18) Yao, J.; Xu, N.; Deng, S.; Chen, J.; She, J.; Shieh, H.; Liu, P. T.; Huang, Y. P. Electrical and Photosensitive Characteristics of a-IGZO TFTs Related to Oxygen Vacancy. IEEE Trans. Electron Devices 2011, 58, 1121−1126. (19) Park, J. S.; Maeng, W. J.; Kim, H. S.; Park, J. S. Review of Recent Developments in Amorphous Oxide Semiconductor Thin-film Transistor Devices. Thin Solid Films 2012, 520, 1679−1693. (20) Nahm, H. H.; Kim, Y. S.; Kim, D. H. Instability of Amorphous Oxide Semiconductors via Carrier-Mediated Structural Transition between Disorder and Peroxide State. Phys. Status Solidi B 2012, 249, 1277−1281. (21) Rockett, A. Materials Science of Semiconductors; Springer-Verlag: New York, 2007; p 195. (22) Kohan, A. F.; Ceder, G.; Morgan, D.; Van de Walle, C. G. FirstPrinciples Study of Native Point Defects in ZnO. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 15019.

devices at temperature that polymers can withstand. The flexible devices were subjected to repetitive bending tests, and it was found that the bending direction influences their electrical properties. Geometrical factors are suggested to affect the generation of charge trap sites or microcracks, which are believed to be responsible for the negative shift in Vth upon prolonged cyclic bending. It is therefore anticipated that understanding the behavior of oxide semiconductor devices with respect to various forms of mechanical stress will expedite the realization of flexible applications with sufficient durability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11815. Relative indium and oxygen content (atom %) in the plasma treated InOx films, based on XPS analyses (Table S1); variations in carrier concentration, Hall mobility, and resistivity of the InOx films as a function of exposure time to N2O plasma (Figure S1); photograph of a homemade bending machine with bending radii from 15 to 5 mm (Figure S2); representative transfer characteristics in the original state, after bending, 1 h after stress removal, and after thermal treatment on a hot plate (at 200 °C for 20 min), respectively, for bent TFT with axis along the (a) channel width (case I) and (b) channel length (case II) (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*J.L. e-mail: [email protected]. *J.-S.P. e-mail: [email protected]. Present Address ∥

R&D Center, Samsung Display, Yongin 17113, Republic of Korea. Author Contributions

J.S. and J.P. contributed equally to this work. J.S., J.P., D.-w.C., and J.-S.P. designed the InOx ALD experiments and fabricated the thin-film transistor on the flexible substrate. J.S., J.P., D.w.C., J.L., and J.-S.P. evaluated the mechanical bending stress with the associated InOx TFT and performed the film analyses. The manuscript was written by the contribution of all authors, who have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was mainly supported by the MOTIE (Ministry of Trade, Industry & Energy) (project numbers 10052027 and 10052020) and KDRC (Korea Display Research Corp.). Also, this work was partially supported by a research fund of Samsung Display and done by the Industry Technology R&D program of MOTIE/KEIT [10051080]. In particular, the authors thank UP Chemical Co. for supporting the INCA-1 precursor.

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REFERENCES

(1) Song, W. G.; Kwon, H. J.; Park, J.; Yeo, J. Y.; Kim, M. J.; Park, S. T.; Yun, S. R.; Kyung, K. U.; Grigoropoulos, C. P.; Kim, S. K.; Hong, Y. K. High-Performance Flexible Multilayer MoS2 Transistors on Solution-Based Polyimide Substrates. Adv. Funct. Mater. 2016, 26, 2426−2434. 31142

DOI: 10.1021/acsami.6b11815 ACS Appl. Mater. Interfaces 2016, 8, 31136−31143

Research Article

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DOI: 10.1021/acsami.6b11815 ACS Appl. Mater. Interfaces 2016, 8, 31136−31143