Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic

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Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited YO with Ozone 2

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Hanearl Jung, Woo-Hee Kim, Bo-Eun Park, Whang Je Woo, Il-Kwon Oh, Su Jeong Lee, Yun Cheol Kim, Jae-Min Myoung, Satoko Gatineau, Christian Dussarrat, and Hyungjun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14260 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic-Layer-Deposited Y2O3 with Ozone Hanearl Junga§, Woo-Hee Kimb§, Bo-Eun Parka, Whang Je Wooa, Il-Kwon Oha, Su Jeong Leec, Yun Cheol Kimc, Jae-Min Myoungc, Satoko Gatineaud, Christian Dussarrate, and Hyungjun Kima* a

School of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-Ro, Seodaemun-Gu,

Seoul 03722, Republic of Korea

b

Division of Advanced Materials Engineering, Chonbuk National University, 567 Baekje-daero,

deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea

c

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-Ro,

Seodaemun-Gu, Seoul 03722, Republic of Korea

d

e

Air Liquide Korea Co., LTD. 50 Yonsei-ro, Seodaemun-Gu, Seoul 120-749, Republic of Korea

Air Liquide Laboratories, 28 Wadai Tsukuba Ibaraki 300-4247, Japan

*Corresponding author: Tel: +82-2-2123-5773, Fax: +82-2-313-2879 E-mail address: [email protected]

KEYWORDS : IGZO TFT, passivation, Y2O3, ozone, atomic layer deposition, NBIS.

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ABSTRACT

We report the effect of Y2O3 passivation by atomic layer deposition (ALD) using various oxidants, such as H2O, O2 plasma, and O3, on In–Ga–Zn–O thin-film transistors (IGZO TFTs). A large negative shift in the threshold voltage (Vth) was observed in the case of the TFT subjected to the H2O-ALD Y2O3 process; this shift was caused by a donor effect of negatively charged chemisorbed H2O molecules. In addition, degradation of IGZO TFT device performance after the O2 plasma-ALD Y2O3 process (field-effect mobility (µ) = 8.7 cm2/(V·s), subthreshold swing (SS) = 0.77 V/dec, and Vth = 3.7 V) was observed, which was attributed to plasma damage on the IGZO surface adversely affecting the stability of the TFT under light illumination. In contrast, the O3-ALD Y2O3 process led to enhanced device stability under light illumination (∆Vth = −1 V after 3 h of illumination) by passivating the subgap defect states in the IGZO surface region. In addition, TFTs with a thicker IGZO film (55 nm, which was the optimum thickness under the current investigation) showed more stable device performance than TFTs with thinner IGZO film (30 nm) (∆Vth = −0.4 V after 3 h of light illumination) by triggering the recombination of holes diffusing from the IGZO surface to the insulator–channel interface. Therefore, we envisioned that the O3-ALD Y2O3 passivation layer suggested in this paper can improve the photostability of TFTs under light illumination.


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Introduction

Transparent oxide semiconductors represented by amorphous In–Ga–Zn–O (IGZO) have been widely adopted as a promising channel material for thin-film transistors (TFTs) by virtue of their large field-effect mobility (µ) of >10 cm2/(V·s), small subthreshold swing (SS), superior uniformity, and good low-temperature processability1. Recently, several prototype displays, such as high-definition liquid crystal displays, active-matrix flexible displays, and active-matrix organic light-emitting diode displays, with IGZO TFTs used as driving circuits have been demonstrated2,3. However, unsolved issues need to be addressed for IGZO TFTs to be implemented into practical products, which include bias stability issues such as the threshold voltage (Vth) shift caused by negative-bias stress, positive bias stress, and positive bias constant current stress4. Thus far, extensive efforts have been devoted to improving the device stability and reliability of IGZO TFTs. Post-deposition annealing is widely known to have reduced trap states in the IGZO bulk region; in particular, a wet oxygen annealing process at ≥300 °C can reduce the bulk traps more effectively compared with a dry oxygen method5. Although the bulk defects are reduced in the annealed TFTs, a non-negligible threshold voltage shift (∆Vth), which originates mainly from back-channel surfaces, remains. Because of the instability induced by ambient effects, especially water and oxygen, for TFTs with an exposed back channel, some researchers have enhanced the device stability of IGZO TFT devices by altering the surface properties of the IGZO channel via a water-mediated photochemical treatment6 or by changing the Ga mole ratio of IGZO



near the back channel7; however, a passivation layer has been more likely to improve the stability of oxide semiconductor TFTs8–10. In many previous passivation-layer studies, SiOx11–15, SiNx13,16, Al2O314,17–20, and various organic materials21–24 have been widely used in conjunction with various deposition methods such as chemical vapor deposition (CVD)12,13, sputtering11,15,16,18,19, spin coating21–24, and atomic layer deposition (ALD)17,20. These materials can successfully passivate the IGZO surface against various gases, leading to improved device stability and reliability; however, the instability of IGZO TFTs induced by light illumination remains to be solved. In this regard, Nomura et al. found that a large density of defect states immediately above the valence band maximum (VBM) at the back-channel surface resulted in instability under exposure to light with a photon energy (~2.3 eV) smaller than the band-gap energy of amorphous IGZO (~3.15 eV)25. Therefore, improving device photostability under light illumination necessitates the development of a proper passivation layer that effectively passivates defect states on the IGZO surface. In this context, formation of a passivation layer with ALD is considered an excellent choice because it enables the deposition of large-area, uniform, pinhole-free thin films at low deposition temperatures17,26,27. The resultant films in practice have demonstrated outstanding barrier properties against the permeation of ambient water and oxygen28,29. Furthermore, processes of the ALD passivation layers are expected to affect properties of the underlying IGZO channel layer, mainly depending on what reactant is employed. In spite of its technical importance, there has


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been only a previous study on the reactant effects of ALD oxide materials using H2O or O3 for encapsulation layers of TFTs20. To date, however, no report is available for improvement of photostability with encapsulation layers from ALD processes using various reactants. Motivated by aforementioned outstanding encapsulation properties with the ALD method, it prompts a systematic investigation for various reactant effects of ALD oxide passivation layers. In this work, therefore, we comprehensively investigate performance and reliability characteristics of IGZO TFTs, depending on the use of water vapor (H2O), oxygen plasma (O2 plasma), and ozone (O3) as oxidants in ALD Y2O3 passivation layers. Herein, the Y2O3 is chosen as a model encapsulation oxide material since it demonstrated the efficacy in eliminating deep subgap defects causing photoinstability of TFTs30. The photostability of IGZO TFTs depending on ALD Y2O3 encapsulation processes performed using these three oxidants were comparatively examined on the basis of negative-bias light-illumination stability (NBIS) tests.

Experimental Section

1. Preparation of ALD Y2O3 passivation layers For this study, we used a commercial ALD chamber (Lucida M100-PL, NCD Co.) with a double showerhead for good uniformity. A precursor of Y(iPrCp)2(N-iPr-amd), supplied by Air Liquide, was evaporated at 130 °C in a stainless-steel bubbler to attain a sufficiently high vapor pressure. Ar gas, which was controlled using a mass flow controller (MFC), was used as both a



precursor carrier and a purge gas. Three types of counter oxidants were used: H2O for thermal ALD (H2O-ALD), O2 plasma for plasma-enhanced ALD (O2 plasma-ALD), and ozone for ozone ALD (O3-ALD). The flow rates of water vapor were controlled by a leak valve, and the flow rates of O2 and O3 were controlled by an MFC. For O3-ALD, O2 gas and N2 gas were flowed into an ozone generator that was set to generate ozone at rates greater than 250 g/m3. For O2 plasma-ALD, O2 plasma was generated between the substrate heater and the showerhead connected to a radio-frequency plasma generator. During the ALD processes, the substrate temperature was maintained at 200 °C. The Y2O3 ALD processes were based on the process conditions developed in our previous work with H2O31, O2 plasma32, and O333. Growth characteristics and film properties on ALD Y2O3 processes are available in Figures S1–3 of Supporting Information.

2. Fabrication of back-gated IGZO TFTs Before deposition of the IGZO layer, a degenerately doped p-type Si substrate coated with a thermally oxidized 100-nm-thick SiO2 dielectric layer was used as a back gate. This substrate was treated using a conventional piranha cleaning process (H2O2:H2SO4 = 1:3) for 15 min and dried by flowing N2 gas. To minimize the contamination from the surroundings, this substrate was loaded inside a loading chamber equipped with a transfer arm. After the base pressure of the loading chamber reached 7.5 × 10−6 Torr, the substrate was transferred into the main UHV chamber, which was maintained at 1 × 10−8 Torr. Inside the main chamber, the 30- and


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55-nm-thick IGZO channel layers were deposited onto the SiO2/Si substrate at room temperature using an IGZO (In2Ga2ZnO7) target with a sputtering power of 150 W. Before the deposition of the IGZO film, Ar gas containing 2 vol% of O2 was supplied to the chamber to initiate the sputtering at a working pressure of 5 × 10−3 Torr. The IGZO channel length and width were patterned to 100 and 1000 µm, respectively, through the shadow mask. Source and drain electrodes were deposited by thermal evaporation of Al through a shadow mask to a thickness of 100 nm. During the thermal evaporation of Al, the working pressure was maintained at 5 × 10−5 Torr and the deposition rate was kept constant. The IGZO TFTs were thermally annealed at 250 °C for 1 h to improve and stabilize their device performance34. Finally, 100 nm of Y2O3 passivation layer was deposited onto each of the IGZO TFTs using the H2O-, O2 plasma-, or O3-ALD process. The cross-sectional IGZO TFT structure is schematically depicted in Figure S4.

3. Electrical characterizations of IGZO TFTs Device characteristics for the TFTs were evaluated at room temperature under atmospheric conditions using a Keithley 236 semiconductor parameter analyzer equipped with a 6-W light lamp (480 nm in wavelength, 2.6 eV in photon energy). To characterize transfer curves, we used 20 V of drain bias (VD) to operate the TFT in the saturation region. To evaluate the reliabilities of the IGZO TFTs, NBIS tests of the IGZO TFTs were performed. For the NBIS measurements, the



gate bias (VG) was maintained at −20 V under light illumination while the source and drain were grounded. From the transfer curves, key electrical parameters of field-effect mobility (µ), threshold voltage (Vth), and subthreshold swing (SS), were estimated. The µ was calculated in the saturation region with the following equation: ID = (µ Cox W/2L) (VG-Vth)2 where Cox is the capacitance per unit area of the insulator, W and L are the width and length of IGZO channel, respectively. The Vth was determined from the intercept of the extrapolated curve with the voltage axis, and the SS was calculated by the equation: SS = dVG/d(logID).

Results and Discussion

Figure 1 shows the effect of the ALD Y2O3 passivation layers on the device performance of IGZO TFTs (30-nm-thick IGZO) wherein the passivation layers were deposited under three different oxidants (i.e., H2O, O2 plasma, and O3). The as-fabricated device exhibits excellent electrical properties of µ = 10.3 cm2/(V·s), SS = 0.19 V/dec, and Vth = 0.9 V, with on/off current ratio (Ion/Ioff) > 108. However, a substantial reduction of the Vth was observed for TFT devices with a H2O-ALD Y2O3 passivation layer, resulting in very high turn-off voltages to deplete the IGZO channel, which we will discuss later. In contrast, only a slight degradation in performance was observed for the devices with a Y2O3 passivation layer formed via the O2 plasma-ALD (µ = 8.7 cm2/(V·s), SS = 0.73 V/dec, Vth = 3.7 V) or O3-ALD Y2O3 (µ = 7.6 cm2/(V·s), SS = 0.35 V/dec, Vth


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= 3.9 V) process. The slightly diminished electrical properties of the TFTs in these cases might be attributable to the generation of extra interface states during the O2 plasma-ALD process35 and to the reduced free-electron carrier density1,36 during the O3-ALD process, respectively, which will be explained later. All TFTs showed very low gate leakage currents (< 10-11 A) as shown in Figure S5. Figure 2 shows the device reliabilities of the IGZO TFTs (a) without and (b, c) with passivation layers under light illumination. First, the performance of the unpassivated TFTs was substantially degraded by the light-illumination stress, as shown in Figure 2(a). More specifically, no notable performance change was observed upon illumination with light (Light (initial) in Figure 2(a)), whereas a large negative Vth shift (∆Vth) greater than −5 V was observed after 1 h of illumination; the ∆Vth reached −14.8 V after 3 h, with a slightly decreased µ. To evaluate the Y2O3 passivation effect under light illumination, we conducted NBIS tests with IGZO TFTs passivated with O2 plasma-ALD and O3-ALD Y2O3 layers. Notably, the IGZO TFT device with a H2O-ALD Y2O3 film was excluded because of its large negative shift of Vth upon deposition of the Y2O3 layer. Figure 2(b) shows the NBIS results for the IGZO TFT with an O2 plasma-ALD Y2O3 passivation layer. Unlike the unpassivated IGZO TFT, degradation of device performance is observed immediately after the light illumination, with a stretched transfer curve (Light (initial) in Figure 2(b)), with µ = 8.1 cm2/(V·s), SS = 0.62 V/dec, and Vth = 1.5 V. Furthermore, under the



NBIS test, a much greater degradation in device performance was observed for IGZO TFT devices passivated using O2 plasma-ALD compared with the performance of an unpassivated IGZO TFT, which we will discuss in more detail later. The passivation effect of O3-ALD Y2O3 is shown in Figure 2(c). Interestingly, the IGZO TFT with an O3-ALD Y2O3 passivation layer remained stable under light illumination, with negligible ∆µ, ∆Vth, and ∆SS values. The changes of the IGZO TFT device performance (i.e., ∆µ, ∆Vth, and ∆SS) as a function of stress time are summarized in Figure 2(d). Specifically, the photoresponse of the TFTs is effectively improved with the O3-ALD Y2O3 passivation layer. Figure 3 illustrates the possible degradation mechanism of photo-instability in the TFTs (a) without and (b–d) with each passivation layer under the NBIS condition. First, the negative ∆Vth in the unpassivated TFT (Figure 3(a)) is explained by the accumulation of positive charges in the channel, wherein holes originate from the photoresponse of subgap states. More specifically, these subgap states are concentrated in the back-channel surface region (black-empty circles in Figure 3(a)); thus, the photogenerated holes can be diffused to the insulator–channel interface from the negative electrical field during NBIS tests. The diffused holes are then captured by donor-like states at the interface, thereby forming fixed positive charges, incurring the negative ∆Vth30. Furthermore, the generated holes tend to accumulate near the source side to lower the potential barrier under measurement (VG = −30 to 30 V, VD = 20 V), resulting in aggravation by the carrier injection from the source to the channel region, which in turn results in the generation of


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photoleakage currents37. In addition, the ∆Vth can be explained by the adsorption or desorption of O2 gas in the back channel during the stress test because of the gas-sensitive IGZO surface38,39. Figure 3(b) shows the degradation mechanism for a TFT passivated with H2O-ALD Y2O3. In previous studies on ALD metal oxide passivation layers using H2O20,40, this degradation was attributed to chemisorbed H2O molecules on the IGZO surface39,41. More specifically, adsorbed H2O in molecular form provides a partial negative charge to the IGZO surface. The donation of electrons (referred to as the “donor effect”) from chemically absorbed H2O molecules to the metal oxide surface has also been reported in previous studies on oxide-based TFTs42–44. In addition, hydrogen penetration inside the IGZO films can take place through dissociative adsorption, when exposed to water vapors during the H2O-ALD process45. In regard to this effect, Van der walle reported that the hydrogen can behave as a shallow donor, i.e. a source of conductivity, in ZnO46, and the similar hydrogen effect can also be observed in previous reports47,48. Accordingly, these extra electrons induced by H2O adsorption or hydrogen ingress can bring high electron concentration to the IGZO surface (back-channel layer), resulting in such the large current density and unmeasurable Vth. However, the stretch-out phenomenon of the TFT passivated with O2 plasma-ALD Y2O3 observed in Figure 2(b) illustrates that the mechanism of ∆Vth differs from that of the unpassivated device. As previously described, the ∆Vth of the unpassivated device under light conditions is caused by both holes trapping in the insulator–channel interface and O2 gas adsorption and desorption in the back channel during the stress tests. However, the



Y2O3-passivated TFT devices should exhibit a negligible effect of O2 gas on the back channel and only the holes-trapping effect will be considered with respect to the degradation of device performance. In this regard, the plasma-induced damage during the O2 plasma-ALD process can generate additional subgap states on the IGZO surface (red-empty circles in Figure 3(c)). These plasma-induced damages can also be found in previous literatures10,16,49. As a result, holes-trapping events can occur more dominantly under the NBIS stress35. The results imply that the O2 plasma-ALD Y2O3 passivation layer could not effectively enhance the stability of the TFT device under light-illumination conditions. However, the O3-ALD Y2O3-passivated TFT showed enhanced device stability after the NBIS test, as shown in Figure 2(c). As previously mentioned, the instability under the light exposure is explained by a large density of defect states immediately above the VBM at the back-channel surface. The origin of the subgap states has not yet been clarified; however, first-principles calculations have revealed that oxygen deficiencies (i.e., VO) form deep occupied states closer to the VBM if a defect structure forms a large free space50. For the O3-ALD Y2O3 process, these VO-related subgap states on the IGZO surface region were significantly reduced by virtue of the strong oxidation power of O3, similar to the O3 annealing36 or treatment effect51,52, leading to enhanced photostability of the O3-ALD Y2O3-passivated IGZO TFT under light illumination. In Figure 3(d), the subgap states on the IGZO surface region can be reduced accordingly after the O3-ALD process (gray-empty circles in Figure 3(d)) and fewer holes are photogenerated in the IGZO surface region and trapped in the


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insulator–channel interface; a much retarded negative ∆Vth (ca. −1 V) is thereby observed even after 3 h of the NBIS test. Therefore, the primary reason for the improved TFT stability is attributable to a decrease in the number of oxygen-deficiency defects at the back channel during the formation of a passivation layer. A similar passivating effect was observed for Y2O3 in a previous study30. However, Kamiya et al. reported that oxygen vacancies generate free electrons and affect the electrical conductivity of IGZO films53. Thus, the slight reduction of device performance but much improved photostability for the O3-ALD Y2O3-passivated IGZO TFTs can be explained by the decrease in the number of oxygen deficiencies and the carrier concentration during the passivation process. Nomura et al. have also reported that device photostability during NBIS tests is affected by the thickness of an IGZO film30. They demonstrated that TFT with a thicker IGZO layer exhibited a smaller ∆Vth after the NBIS test because of a decreased number of trapped holes at the insulator– channel interface. Accordingly, to further enhance device stability under light-illumination conditions, we fabricated TFTs with thicker IGZO films (55 nm) as well as the O3-ALD Y2O3 passivation layer. Notably, we limited the thickness of the IGZO films to 55 nm because much thicker IGZO films additionally lead to a large negative Vth and full depletion. The TFTs with 30-nm- and 55-nm-thick IGZO films show similar transfer curves, as shown in Figure S6. Figure 4(a) shows the time dependence of the transfer curves under NBIS for the thicker IGZO TFTs with the O3-ALD Y2O3 layers. As the IGZO thickness was increased to 55 nm, enhanced photostability



was achieved, as shown in Figure 4(b), with a minimal ∆Vth of −0.4 V after 3 h of NBIS. The enhanced device stability can be explained by the reduced amount of trapped holes at the insulator–channel interface, the mechanism of which is schematically shown in Figure 4(c). With increasing thickness of the IGZO film, photogenerated holes encounter difficulty reaching the gate insulator–channel interface because the photogenerated holes are prone to recombine halfway through the diffusion near the back channel. Moreover, the device performance of the O3-ALD Y2O3-passivated TFT with a 55-nm-thick IGZO film (µ = 11.5 cm2/(V·s), SS = 0.21 V/dec, with Vth = −0.5 V) is superior to that with a 30-nm-thick IGZO film, except for the slightly negative 1.5-V shift of Vth caused by the thickness effect54. Kim et al. reported the effect of thickness on the density of states (DOS) from bulk traps, which degrade device performance by capturing charges, in IGZO films55. The enhanced device performance of a TFT with thicker IGZO film can also be explained by the reduced DOS in the IGZO film. It is worth noting that device performances of passivated IGZO TFTs with O3-ALD Y2O3 in the current study are comparable with previous reports18,47,56–58, together with the effective passivation of the defect states at the back-channel surface as compared to other passivation layers reported elsewhere18,47,56–63. Recently, Xie et al. reported almost negligible Vth shift after 2,500 s of NBIS test with 380 nm wavelength light with molybdenum-doped ZnO (MZO) as a UV shield layer and nitrogen-doped IGZO channel (IGZO:N). However, the degradation of device performance was observed with MZO passivated IGZO:N TFTs as compared to unpassivated IGZO:N TFTs62. On the other hand, according to


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Furuta et al., passivation of fluorinated silicon nitride (SiNx:F) can also suppress the NBIS degradation of IGZO TFTs63. The SiNx:F-passivated TFTs showed slight Vth shift (~0.15 V) after 10,000s of NBIS test with wavelength of 460 nm, but the device performance remained unchanged. On the basis of this efficacy, we successfully demonstrated not only significantly enhanced photostability under subgap-energy light illumination (~2.6 eV) but also relatively enhanced device performance through a combination of O3-ALD Y2O3 passivation and thicker IGZO films. It is worthy to note that the improved photostability through O3-based ALD process can also be applied to other metal oxides, such as Al2O3 or ZrO2, for formation of the passivation layer with low water vapor transmission rate64–67. Therefore, we expect that the current experimental results provide useful information for the practical implementation of TFT passivation layers.

Conclusion

We confirmed that the device performance and photostability of IGZO TFTs were critically dependent upon the ALD process of Y2O3 passivation using different oxidation sources (i.e., H2O, O2 plasma, and O3). The H2O-ALD Y2O3 process was found to result in unmeasurable TFT device characteristics because of the large negative ∆Vth resulting from the donor effect of negatively charged chemisorbed H2O molecules. In the case of the O2 plasma-ALD Y2O3 process, extra interface states were created by O2 plasma exposure during the ALD process, which significantly degraded the device performance of the IGZO TFTs. In contrast, the O3-ALD Y2O3



process led to passivation of deep subgap defects such as oxygen vacancies at the back-channel surface, thereby improving the photostability under the NBIS test conditions. To further optimize the device performance, we fabricated TFTs with thicker IGZO films (55 nm, as optimized under the current experimental conditions) and subsequently applied the same O3-ALD Y2O3 passivation process. As a result of additional recombination effects of photoexcited holes diffusing through the channel, a greater improvement in photostability (∆Vth = −0.4 V after 3 h) was achieved under the NBIS tests.

ASSOCIATED CONTENT Supporting Information. Growth characteristics and film properties on ALD Y2O3 processes. Schematic illustration of IGZO TFT structure. Gate leakage currents of TFTs without passivation layer and with various passivation layers. Transfer curves of TFTs with 30-nm- and 55-nm-thick IGZO films. NBIS results with different channel length of IGZO TFTs (W/L = 1000 µm/50 µm).

AUTHOR INFORMATION Corresponding author *E-mail address: [email protected]

Author Contributions


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§

Authors Hanearl Jung and Woo-Hee Kim contributed equally.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Air Liquide Laboratories Korea, Seoul, South Korea, as a precursor supplier. This work was also supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. NRF-2017R1C1B5076821) and the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project. (CISS-2016M3A6A6930869)



Figures

Figure 1. Transfer curves of IGZO TFTs without passivation layer and with various passivation layers.


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Figure 2. Transfer curves after NBIS test of IGZO TFTs (a) without passivation layer, (b) with O2 Plasma-ALD Y2O3, and (c) with O3-ALD Y2O3. (d) ∆µ, ∆Vth, ∆SS of IGZO TFTs after NBIS test.



Figure 3. Possible degradation mechanism of photo instability in the TFTs (a) without and (b-d) with each passivation layers under the NBIS condition.


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Figure 4. (a) Transfer curves after NBIS test of IGZO TFTs with 55-nm thick IGZO TFT. (b) ∆Vth of 30- and 55-nm thick IGZO TFT after NBIS test. (c) Possible mechanism of the enhanced device stability with 55-nm-thick IGZO TFT.



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Enhanced Light Stability of InGaZnO Thin-Film Transistors by Atomic

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