High-quality indium–gallium–zinc oxide films synthesized by atomic layer deposition using a single cocktail precursor based on a liquid-delivery system and their application in transistors and inverters

Abstract

High-quality indium–gallium–zinc oxide (IGZO) films synthesized by atomic layer deposition (ALD) using a single cocktail precursor based on a liquid-delivery system are demonstrated for the first time. A bottom-gate-top-contact thin-film transistor (TFT) consisting of an ALD-derived IGZO channel exhibited outstanding device performance, including a high linear field-effect mobility (∼20 cm² V⁻¹ s⁻¹), a substantial on/off ratio (∼5 × 10⁹), and a low subthreshold swing (∼0.07 V dec⁻¹) at an operating voltage of 5 V. High reliability and reproducibility of the proposed ALD-based IGZO TFT are confirmed from the statistics of device key parameters. The operational stability of the IGZO TFT subjected to electrical bias stresses for 20 000s is investigated, which is attributed to the oxygen-related defect states of the ALD-derived IGZO film and the field-induced interaction with oxygen on the surface. In addition, full recovery of V_th without additional processes was achieved, indicating that the instability mechanism can be explained by shallow charge trapping at the interface. A simple and effective method of oxygen annealing is induced to improve the operational stability of the IGZO TFT by suppressing the oxygen-related shallow trap states. Finally, an enhancement-load-type n-channel metal-oxide semiconductor inverter consisting of two IGZO TFTs proposed in this study is developed.

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Indium–gallium–zinc oxide (IGZO) films have garnered substantial attention as active channel layers in thin-film transistors (TFTs), owing to their high carrier mobility even in the amorphous phase, low leakage current density, and favorable environmental stability.¹ Various methods for forming IGZO channel films, such as sputtering, sol–gel processes, and atomic layer deposition (ALD), have been demonstrated for TFT-based applications.² Sputtering has been widely used to achieve IGZO films with good uniformity and reproducibility.³ However, limited controllability of thickness and cation compositions is challenging to achieve high-quality IGZO films for emerging TFT applications such as display backplanes and logic circuits.³

The ALD method, characterized by excellent thickness/composition controllability, low defect states, and high step coverage due to self-limited and plasma-free gas-phase chemical reactions, has been extensively explored to achieve high-quality IGZO films using metal precursors.³ However, issues such as high process complexities, low growth rates, and extended processing times, particularly in the formation of quaternary oxide films, must be addressed to align with industrial requirements.⁴ The uniform and reliable composition of IGZO through ALD presents a challenge, as the different vapor pressures induced by three different metals can significantly affect the process time and batch-to-batch variation.⁵ Furthermore, the growth rate per cycle (GPC) can change significantly, complicating the control of ALD-derived IGZO film thickness.⁵

In this study, we demonstrate, for the first time, high-quality IGZO films synthesized through a practical and reliable ALD process using a single cocktail precursor based on a liquid-delivery system and their applications in TFTs and inverters. Material characteristics, including morphology, crystallinity, and oxygen-related bonding states, were analyzed to assess the quality of the ALD-derived IGZO films based on process optimization. We also demonstrate bottom-gate top-contact TFTs based on ALD-derived IGZO films with conventional silicon dioxide (SiO₂) dielectrics and evaluate their device performance. Additionally, we investigate the electrical stability of the IGZO TFTs without passivation against prolonged positive and negative bias stresses in atmospheric and vacuum environments as well as their recovery characteristics. Furthermore, we investigate the effect of oxygen annealing on the operational stability of the IGZO TFT by performing comparative analyses of oxygen-related defect states and shallow charge trapping. Finally, we demonstrate an enhancement-load-type n-channel metal-oxide semiconductor (NMOS) inverter consisting of two TFTs based on the optimized ALD-derived IGZO channel layers.

Experimental

Materials and methods

Deposition of IGZO films through ALD using a single cocktail precursor. Fig. 1a illustrates the proposed ALD process using a single cocktail precursor based on a liquid-delivery system. Triethylindium (TEI, 99.65%), trimethylgallium (TMG, 99.5%), and diethylzinc (DEZ, 99.5%) served as the In, Ga, and Zn sources with a molar ratio of 1 : 1 : 1, respectively. Fig. S1 (ESI†) shows the statistic plot of each atomic concentration obtained from X-ray photoelectron spectroscopy (XPS) spectra of 20 ALD-derived IGZO films. The extracted atomic concentrations of In, Ga, Zn, and O are 17.15 ± 3.6 at%, 15.12 ± 4.5 at%, 15.01 ± 5.4 at%, and 50.74 ± 2.2 at%, respectively, which indicate high repeatability. A single cocktail IGZO precursor was synthesized at 400 rpm for 20 min and then delivered to a liquid mass-flow controller (Horiba, LM-F250-M2) and vaporizer (Horiba, MV-2000) through the He carrier gas flow. Subsequently, the In–Ga–Zn cocktail precursor and Ar gas were mixed and vaporized at 50 °C under vacuum of approximately 8 × 10⁻² Torr where the cocktail precursor and Ar carrier gas were supplied at rates of 0.1 g min⁻¹ and 50 sccm, respectively. The vaporized In–Ga–Zn cocktail precursor was delivered to a chamber for 1 s through a pipeline heated at 80 °C to avoid liquefaction of the vaporized precursor during the delivery. The process temperature and pressure in the chamber were optimized at 200 °C and 1 Torr, respectively, which are essential for film formation between condensation and decomposition while avoiding incomplete reaction and re-evaporation. The O₂ reactant gas was delivered for 5 s, and a plasma power of 200 W was applied to decompose the reactant gas. Finally, it was purged with Ar gas for 30 s. Through the proposed ALD using a single cocktail precursor based on a liquid-delivery system, the procedure and time for the deposition of quaternary oxide films were significantly reduced (up to one third). The saturation of the GPC was confirmed by carefully controlling the dose time (1 s) and flow rate (0.1 g min⁻¹) owing to the self-limiting growth which is a hallmark of the ALD process.⁶ Fig. S2 (ESI†) shows the GPC as a function of dose time and flow rate. The GPC was saturated at ∼1.25 Å per cycle as a sufficient precursor dose was provided, which indicates that all reactive surface sites were consumed and no other unintended reactions occurred.⁶ To confirm the GPC, the thickness of all the ALD-derived IGZO films synthesized with different deposition cycles was determined through scanning electron microscopy (SEM) measurements. Fig. S3a–c (ESI†) show the cross-sectional SEM images of the ALD-derived IGZO films synthesized with different deposition cycles. The IGZO films synthesized with 100, 160, and 200 deposition cycles exhibited film thicknesses of 12.7, 19.8, and 25.6 nm, respectively, which indicates that the corresponding GPC was ∼1.25 Å per cycle. It is therefore concluded that the GPC of the proposed ALD process using a single cocktail precursor based on a liquid-delivery system is ∼1.25 Å per cycle.

Fig. 1 (a) Illustration of the proposed ALD process using a single cocktail precursor based on a liquid-delivery system. (b) Schematic of the fabrication process flow for a bottom-gate top-contact IGZO TFT with a conventional SiO₂ dielectric film. (c) Optical image of an IGZO TFT fabricated in this study.

Fabrication process of TFTs based on ALD-derived IGZO films. Fig. 1b shows a schematic of the fabrication process flow for a bottom-gate top-contact IGZO TFT with a conventional SiO₂ dielectric film. Highly doped silicon (Si) and thermally grown 100 nm thick SiO₂ films were used for the gate electrode and insulator, respectively. Subsequently, an IGZO channel film was formed through ALD and patterned using photolithography and wet etching. 75 nm-thick aluminum (Al) source/drain electrodes were deposited by electron-beam evaporation at a pressure of approximately 10⁻⁶ Pa after patterning to complete the fabrication of the IGZO TFT with a channel width of 600 μm and a length of 40 μm (Fig. 1c). A thermal-annealing process under an oxygen-enriched (99%) environment at 300 °C was employed to improve the metal–oxygen (M–O) bonding states of the ALD-derived IGZO film.⁶

Material and electrical characterization. Nuclear magnetic resonance (NMR; Varian 400-MR, Varian, USA) measurements were conducted to investigate the chemical structure of the single cocktail IGZO precursor and its purities. Atomic force microscopy (AFM; XE-150, Park Systems, South Korea) and SEM (S-4800, Hitachi, Japan) were utilized to characterize the morphological characteristics and thickness profiles of the IGZO films, respectively. Structural properties and chemical bonding states were investigated using X-ray diffraction (XRD; SmartLab, Rigaku, Japan) and XPS (K-Alpha +, Thermo Fisher Scientific, USA), respectively. The Hall mobility (μ_Hall) of the synthesized IGZO film was determined via a Hall effect measurement system (HL5500C, Nanometrics, Canada) with a van der Pauw method. The electrical characteristics of the IGZO TFT were measured using a semiconductor parameter analyzer in ambient air. A positive bias stress (PBS) of 5 V and a negative bias stress (NBS) of −5 V were applied.

Results and discussion

Material characteristics of precursors for ALD-derived IGZO films

Fig. 2a shows an optical image of the synthesized single cocktail IGZO precursor for the ALD-derived IGZO film. All ligands for precursors exhibiting high thermal stability are composed of alkyl groups, where the ligands for TEI and DEZ are based on ethyl groups and the ligand for TMG is based on the methyl group.^7,8 Notably, the chemical reaction between precursors does not occur when synthesizing the single cocktail IGZO precursor. This is because the ligands for all precursors are composed of same alkyl groups and their ligand bonding is completed. To determine the purity of each precursor, ¹H NMR spectra were analyzed. As shown in Fig. 2b, the ¹H spectra of TEI exhibited triplet and quartet peaks, corresponding to methylene protons of ethyl group attached with In.⁹Fig. 2c shows that the singlet ¹H peak in the NMR spectra of TMG is assigned to the protons of the methyl groups attached to Ga.¹⁰ As shown in Fig. 2d, the triplet and quartet peaks in the ¹H spectra of DEZ correspond to methylene protons of ethyl group attached with Zn.⁹ No other peak in the NMR spectra was observed in each precursor, indicating high purity of TEI, DEZ, and TMG precursors used for the ALD-derived IGZO film.

Fig. 2 (a) Optical image of the synthesized single cocktail IGZO precursor, ¹H NMR spectra of (b) TEI, (c) TMG, and (d) DEZ precursors.

Material characteristics of ALD-derived IGZO films

Fig. 3a shows XPS spectra of the ALD-derived IGZO film synthesized using a single cocktail precursor. The existence of In, Ga, and Zn peaks in the XPS profile provides clear evidence for the formation of the IGZO film through ALD based on a single cocktail precursor. To determine the morphological characteristics of the ALD-derived IGZO film, which can affect the charge transport characteristics, AFM measurements were conducted. As shown in Fig. 3b, a surface roughness of 0.106 nm was achieved in the ALD-derived IGZO film synthesized using a single cocktail precursor, which indicates the smooth morphological characteristics comparable to those of high-quality IGZO films reported in the literature.¹¹ We investigated the effect of deposition cycle on the surface roughness of the ALD-derived IGZO films. As shown in Fig. 3c, a relatively smooth surface roughness was obtained from the ALD-derived IGZO film synthesized with 160 deposition cycles, compared to those synthesized with 100 and 200 deposition cycles. In addition, the chemical bonding states of the ALD-derived IGZO films were investigated by analyzing the O_1s spectra. Fig. 3d shows the O_1s spectra of ALD-derived IGZO films synthesized with 160 deposition cycles, deconvoluted into three peaks through Gaussian fitting distribution, assigned to M–O bonding, oxygen vacancies (V₀) in the lattices, and metal–hydroxide (M–OH) bonding.¹¹ High proportional ratio of M–O bonding states (81.66%) was achieved in the ALD-derived IGZO film synthesized using a single cocktail precursor, which can be observed in high-quality IGZO films reported in the literature.¹² To optimize the deposition cycle of the ALD process, we analyzed the chemical bonding states of the ALD-derived IGZO films synthesized with different deposition cycles. As shown in Fig. 3e, a relatively high proportional ratio of M–O bonding states was observed in the ALD-derived IGZO film synthesized with 160 deposition cycles, compared to those synthesized with 100 and 200 deposition cycles.¹² Next, the structural properties of the optimized ALD-derived IGZO film were investigated by analyzing XRD patterns. As shown in Fig. 3f, only one broad peak at 32° corresponding to the amorphous phase was observed, indicating the formation of an amorphous IGZO film through ALD. The inset in Fig. 3f shows the cross-sectional SEM image of the optimized ALD-derived IGZO film, confirming a uniform and smooth surface. A pin-hole-free and dense IGZO film with a thickness of 19.8 nm was achieved through the optimization of the deposition cycle, indicating the realization of high-quality IGZO films through the proposed ALD using a single cocktail precursor based on a liquid-delivery system.

Fig. 3 Material characteristics of the ALD-derived IGZO film synthesized using a single cocktail precursor: (a) XPS spectra of the ALD-derived IGZO film. (b) AFM image of the ALD-derived IGZO films synthesized with 160 deposition cycles. (c) Surface roughness as a function of deposition cycle obtained from AFM measurements. (d) Deconvoluted O_1s spectra of the ALD-derived IGZO film synthesized with 160 deposition cycles. (e) Proportional ratio of V₀ as a function of deposition cycle obtained from XPS analyses through Gaussian fitting distribution. (f) XRD pattern of the optimized ALD-derived IGZO film; the inset illustrates the cross-sectional SEM image.

Electrical characteristics of TFTs based on ALD-derived IGZO films

To analyze the electrical characteristics of the ALD-derived IGZO film synthesized using a single cocktail precursor, μ_Hall was determined using a Hall effect measurement system. The μ_Hall of 19.2 cm² V⁻¹ s⁻¹ was obtained from the ALD-derived IGZO film through a van der Pauw method, which is comparable to that reported in the literature.¹³Fig. 4a shows the transfer characteristics of the TFT based on the ALD-derived IGZO channel layer when the applied gate–source voltage (V_GS) and drain–source voltage (V_DS) were set at 5 V and 1 V, respectively. A high field-effect mobility (μ) of 18 cm² V⁻¹ s⁻¹ in the linear region, a high on/off current ratio of 10⁹, a threshold voltage (V_th) of 0.50 V, and a low subthreshold swing (SS) of 0.10 V dec⁻¹ were achieved. Typically, high-k gate dielectrics like aluminum oxide have been used for low-voltage operating TFTs.¹⁴ This is because charge carriers induced in the IGZO TFT with high-k dielectrics can increase the effective mobility with an increase in the applied gate bias.¹⁵ However, in this study, high mobility IGZO TFTs with conventional SiO₂ gate dielectrics operating at 5 V were realized. As shown in Fig. 4b, the output characteristics exhibited good linear and saturation behaviors at low (near 0 V) and high V_DS, respectively, which can be attributed to the suppressed electric contact at the interface and the relatively low leakage current of the ALD-derived IGZO channel layer.¹⁶ To further investigate the reliability of the proposed ALD, key device parameters such as V_th, μ, on/off ratio, and SS were analyzed for 20 IGZO TFTs fabricated in different batches at different times, as shown in Fig. 4c–f. On average, a V_th of 0.54 V, a μ of 17.65 cm² V⁻¹ s⁻¹, a log (on/off ratio) of 9.02, and an SS of 0.10 V dec⁻¹ were achieved. Furthermore, the IGZO TFTs exhibited the standard errors of 0.054, 0.255, 0.067, and 0.006 for V_th, μ, log (on/off ratio), and SS with a device yield of >90%, respectively. The excellent device performance with a small standard error and high device yield is presumed to result from high-quality IGZO films synthesized through ALD using a single cocktail precursor based on a liquid-delivery system. High reliability and reproducibility of the proposed ALD-based IGZO films make them practical for TFT-based applications such as display backplanes and logic circuits.¹⁷

Fig. 4 Representative (a) transfer and (b) output curves of TFTs based on ALD-derived IGZO channel layers using a single cocktail precursor. Statistical plots of (c) V_th, (d) field-effect mobility in a linear region, (e) log (on/off current ratio), and (f) SS obtained from 20 IGZO-TFT samples.

Operational stability of TFTs based on ALD-derived IGZO films

Next, the electrical stability of the TFT based on the ALD-derived IGZO channel layer was investigated by conducting PBS and NBS tests. Fig. 5a and b show the transfer characteristics of the IGZO TFTs as a function of stress time during the PBS and NBS tests for 20 000 s, respectively. V_th was positively shifted in the PBS test, while in the NBS test, it was only slightly shifted in the negative direction. Typically, the positive shift in V_th under PBS in the TFT is ascribed to charge trapping at the interface between the channel and dielectric layers or charge injection into the gate dielectric layer.¹⁸ No significant change in SS was observed during the PBS test, indicating that acceptor-like defect states were not further created.¹⁹ The negative shift of V_th under NBS in the TFT is attributed to charge trapping at the interface or V₀ acting as a shallow donor state.¹⁸ The V_th shift (ΔV_th) under NBS is smaller than that under PBS in the IGZO TFT because the electron transport is dominant.²⁰Fig. 5c summarizes ΔV_th as a function of stress time in NBS and PBS tests. Although the ΔV_th under PBS is larger than that under NBS, a relatively small ΔV_th of 1.3 V after PBS for 20 000s was observed in the IGZO TFT without passivation. These advances can be attributed to the suppressed oxygen-related defect states of the high-quality ALD-derived IGZO channel film, which is in good agreement with the results obtained from the XPS analysis.²¹ Furthermore, V_th fully recovered without any additional process when kept in ambient air (Fig. 5d). Notably, a significant restoration in V_th of the IGZO TFT to the initial was observed in the beginning, indicating that the instability mechanism under prolonged bias stress is presumed to shallow charge trapping at the interface between the channel and dielectric layers.^22,23 Next, the PBS test on the IGZO TFT was conducted in a vacuum to investigate the effect of interactions with oxygen/water from the outside environment. Typically, O₂⁻ can act as an electron acceptor based on field-induced adsorption, which leads to the positive shift in V_th of the IGZO TFT where the IGZO channel layer is exposed to oxygen.^24,25 As shown in Fig. 5e, a smaller ΔV_th was observed after the PBS test in a vacuum than that in ambient air, indicating that the ΔV_th can be significantly affected by field-induced interactions with oxygen on the surface of the IGZO channel.^24,25 Furthermore, a full recovery of V_th within a shorter time and a similar tendency on a large portion of the recovery in the beginning were observed (Fig. 5f). These results indicate that the operational stability induced by the electrical bias stress can be enhanced by suppressing the field-induced interactions with oxygen in ambient air.

Fig. 5 Transfer characteristics of the TFT based on the ALD-derived IGZO film using a single cocktail precursor as a function of stress time during (a) PBS and (b) NBS tests in ambient air and (c) ΔV_th as a function of stress time upon PBS and NBS. (d) Recovery characteristics as a function of time when kept in ambient air. (e) Electrical instability of the TFT based on the ALD-derived IGZO film using a single cocktail precursor under prolonged PBS in a vacuum and (f) comparison of recovery characteristics as a function of time.

Effects of O₂ annealing on device performance of TFTs based on ALD-derived IGZO films

To further improve the device performance of the TFT based on the ALD-derived IGZO channel layer, we performed oxygen-enriched (O₂) annealing at 300 °C. Fig. 6a shows the transfer characteristics of the IGZO TFT before and after O₂ annealing. Improved device performance including a steeper SS (0.06 V dec⁻¹) was achieved, which can be attributed to the suppressed oxygen-related shallow trap states.²⁶ For further investigation on the effect of O₂ annealing, we determined the interface trap density in the TFT based on the ALD-derived IGZO channel layer using the following equation based on the SS²⁷where D_it is the interface trap density, C_i is the capacitance of the gate dielectric, q is the elementary charge, k is the Boltzmann constant, and T is the temperature. D_it in the IGZO TFT treated through O₂ annealing was reduced from 7.11 × 10¹¹ to 4.26 × 10¹¹ cm⁻² eV. Next, we analyzed the O_1s spectra of the ALD-derived IGZO film after O₂ annealing. As shown in Fig. 6b, a decrease in the proportional ratio of V₀ was observed in the IGZO TFT after O₂ annealing (14.52%), compared to the initial (16.22%). These results indicate that the post thermal-annealing under an oxygen-enriched environment is presumed to result in the improved device performance of the TFT based on the ALD-derived IGZO film with suppressed oxygen-related defect states. Next, we investigated the effect of O₂ annealing on the operational stability of the TFT based on the ALD-derived IGZO film by conducting the PBS test. Fig. 6c shows the transfer characteristics of the IGZO TFT treated through O₂ annealing as a function of stress time during the PBS test for 20 000 s in ambient air. As observed above, V_th was positively shifted and SS was maintained during the PBS test. However, the instability of the V_th shift under prolonged electrical bias stress was significantly suppressed by employing O₂ annealing. As shown in Fig. 6d, a relatively small ΔV_th was achieved in the IGZO TFT treated through O₂ annealing compared to the initial, which indicates that oxygen-related shallow trap states are essential for high device performance including operational stability in the TFT based on the ALD-derived IGZO film.^24,25

Fig. 6 (a) Representative transfer characteristics of the TFT based on the ALD-derived IGZO film using a single cocktail precursor before and after O₂ annealing. (b) Deconvoluted O_1s spectra of the ALD-derived IGZO film after O₂ annealing. (c) Transfer characteristics of the IGZO TFT treated through O₂ annealing as a function of stress time during PBS in ambient air. (d) ΔV_th in the IGZO TFT before and after O₂ annealing as a function of stress time upon PBS.

NMOS inverter consisting of two IGZO TFTs

We developed a NMOS inverter consisting of the optimized IGZO TFTs fabricated via ALD using a single cocktail precursor for application in logic circuits. Fig. 7a and b show the representative voltage transfer characteristics and extracted direct-current (DC) gain of the NMOS inverter. The inset in Fig. 7a depicts the equivalent circuit diagram of the enhancement-load-type NMOS inverter with two IGZO TFTs proposed in this study. One n-type TFT as a drive-TFT is controlled by the applied input voltage (V_IN) and the other n-type TFT as a load-TFT is in the ON-state by connecting the gate and drain electrodes to operate in a saturation region, where the output voltage (V_OUT) becomes the drain voltage (V_DD).²⁸ Notably, V_OUT, determined by V_DD − V_th was difficult to reach V_DD (5 V) because load-TFT normally turns on in the enhancement mode NMOS inverter.²⁸ As shown in Fig. 7a, the developed NMOS inverter exhibited a full-voltage swing with a rail-to-rail output. Furthermore, a maximum DC gain of 31.48 and a noise margin of 0.2 V were achieved (Fig. 7b). We note that this is the first demonstration of the NMOS inverter consisting of two IGZO TFTs fabricated through ALD using a single cocktail precursor based on a liquid-delivery system.

Fig. 7 (a) Representative voltage transfer characteristics of the enhancement-load-type NMOS inverter consisting of two IGZO TFTs fabricated through ALD using a single cocktail precursor; the inset illustrates a schematic of the equivalent circuit diagram of the enhancement-load-type NMOS inverter. (b) DC gain of the developed NMOS inverter.

Conclusions

We demonstrated the synthesis of high-quality amorphous IGZO films through a process design for ALD using a single cocktail precursor based on a liquid-delivery system. The processing procedure and time can be significantly reduced while maintaining the metal composition and GPC. We also demonstrated their applications for transistors and inverters through optimization. 5 V-operating TFTs with the ALD-derived IGZO channel layers achieved excellent device performance and operational stability against prolonged NBS and PBS. Moreover, the developed enhancement-load-type NMOS inverter based on two IGZO TFTs exhibited outstanding voltage transfer characteristics with a high DC gain and noise margin. We believe that the high-quality quaternary oxide films synthesized through ALD using a single cocktail precursor based on a liquid-delivery system can be a promising candidate for various TFT-based applications in display backplanes and logic circuits.

Author contributions

S. P. conducted the synthesis, characterization, and electrical measurements as well as wrote the original article. S. P., W. J., J. N., J. L., and S. H. supported the experiments and investigated the analysis as well as reviewed the article. T. H. and Y. J. directed and supervised entire experiments and contributed to the analysis of data while developing and revising the manuscript. All authors read and approved the final manuscript.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare no competing interest.

Acknowledgements

This work was supported by the Technology Development Program of Ministry of SMEs and Startups (MSS) (project no. S3205546), the Technology Innovation Program of Ministry of Trade, Industry and Energy (MOTIE) (project no. RS-2023-00266568), and by the Excellent researcher support project of Kwangwoon University in 2024.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01843e

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