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Flexible and High Performance Amorphous Indium Zinc Oxide Thin Film Transistor using Low Temperature Atomic Layer Deposition Jiazhen Sheng, Hwan-Jae Lee, Saeroonter Oh, and Jin-Seong Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11774 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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ACS Applied Materials & Interfaces

Flexible and High Performance Amorphous Indium Zinc Oxide Thin Film Transistor using Low Temperature Atomic Layer Deposition

Jiazhen Sheng1, Hwan-Jae Lee1, Saeroonter Oh2,*, Jin-Seong Park1,*

1

Divison of Materials Science and Engineering, Hanyang University, Seoul, 04763, Republic of Korea

2

Division of Electrical Engineering, Hanyang University, Ansan, Gyeonggi-do, 15588, Republic of Korea

Abstract Amorphous indium zinc oxide (IZO) thin films were deposited at different temperatures, by atomic layer deposition (ALD) using [1, 1, 1-trimethyl-N-(trimethylsilyl) silanaminato]-indium (INCA-1) as the indium precursor, diethlzinc (DEZ) as the zinc precursor, and hydrogen peroxide (H2O2) as the reactant. The ALD process of IZO deposition was carried by repeated supercycles, including one cycle of indium oxide (In2O3) and one cycle of zinc oxide (ZnO). The IZO growth rate deviates from the sum of the respective In2O3 and ZnO growth rates at ALD growth temperatures of 150, 175, and 200 °C. We propose growth temperature-dependent surface reactions during the In2O3 cycle that corresponds with the growth rate results. Thin film transistors (TFTs) were fabricated with the ALD-grown IZO thin films as the active layer. The amorphous IZO TFTs exhibited high mobility of 42.1 cm2 V-1 s-1 and good positive bias temperature stress stability. Finally, flexible IZO TFT was successfully fabricated on a polyimide substrate without performance degradation showing great potential of ALD-grown TFTs for flexible display applications.

Keywords: Atomic layer deposition, indium zinc oxide, oxide semiconductor, flexible TFT, low temperature, high mobility 1

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Introduction Oxide semiconductor thin film transistors (TFTs) are becoming the mainstream backplane technology in the display industry due to their attractive advantages over silicon-based TFTs, including good device performance, optical transparency, low process temperature, and low-cost fabrication process.1 Recently, oxide TFTs based on atomic layer deposition (ALD) are reported, showing comparable performance to conventional deposition methods, such as sputtering or solution methods.2-7 Carrier mobility values of 20, 13.4, and 6 cm2 V-1 s-1 have been reported for ZnO,2 ZnHfO,3 and AZO TFTs,4 respectively. Research groups commonly used ZnO-based TFTs with Al, Hf, or Mg doping,4,8,9 rather than incorporating indium oxide (In2O3), which is known to have a high electron mobility due to a free-electron-like band of In 5s states in the cubic bixbyite structure with linear conducting chains of edge-sharing octahedral.10-13 Indium zinc oxide (IZO) is considered as one of the potential candidates for the active layer for its high mobility, excellent optical transmission, chemical stability, thermal stability, and smooth surface.14 The ALD technique is based on self-limiting deposition realized by exposing the substrate to two or more gaseous reactant pulses in alternating cycles.15 ALD enables accurate thickness control, high conformity, and uniformity over large areas, due to its self-limiting deposition mechanism. Furthermore, since the processing temperature window varies widely among different materials, ALD provides great benefit for multicomponent growth of binary systems to form ternary materials, such as Al2O3/ZnO (aluminum-doped ZnO, AZO), ZnO/SnOx (zinc tin oxide, ZTO), and In2O3/ZnO (indium zinc oxide, IZO).16 However, the lack of understanding on ALD multicomponent growth for oxide semiconductor materials hinders the widespread application of ALD-grown oxides as the active layer of TFTs. ALD growth of bulk ZnO and In2O3 films may not directly correlate to In2O3-ZnO cycle-by-cycle growth of an IZO film. Also, the growth mechanisms may differ between the initial nucleation on the substrate and the steady-state ALD.17 Better understanding of the ALD growth of metal oxides, 2


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from growth rate control to desired thin film composition, and finally co-optimization with device characteristics, is required for its general use as the active layer in TFTs. In this paper, we first investigate the temperature-dependent growth mechanism of In2O3-ZnO cycle-by-cycle IZO thin films. Then, the ALD IZO deposited at different temperatures (< 200 °C) are used as the channel layer to study its effect on TFT device performance. To increase TFT performance, post annealing is often pursued, such as O2 annealing at temperatures exceeding 350 °C.10,18-25 However, such high process temperatures cannot be used for flexible TFTs, due to the temperature constraint of the plastic substrate. A low-temperature process is desired for usage in flexible displays, which are expected to play an essential role in next-generation displays. We fabricate high performance TFTs on a PI substrate to demonstrate the potential of TFTs with ALDgrown active layers for low-temperature flexible display applications.

Experiment We fabricated bottom gate, top contact structure TFTs on a glass substrate. First, a 100-nm ITO gate layer was deposited by sputtering at room temperature. Then, a 100-nm AlOx gate insulator layer was grown at 200 °C by ALD using trimethylaluminum (TMA) and H2O as the precursor and reactant, respectively. The IZO active layer with a thickness of 10 nm was deposited using 1,1,1trimethyl-N-(trimethylsilyl) silanaminato]-indium (INCA-1) as an indium precursor, diethlzinc (DEZ) as the zinc precursor, and hydrogen peroxide as the reactant. The indium precursor was maintained at 40 °C for sufficient pressure and dose. In2O3 and ZnO were grown one cycle at a time in alternating sequences throughout the entire IZO active layer. Growth temperatures of 150, 175, and 200 °C were selected to investigate its effect on the device performance of IZO TFTs. Then, the active region was patterned by a combination of photolithography and wet etching process steps. Ti/Al (5 nm/100 nm) source and drain electrodes were deposited by thermal evaporation and 3



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patterned by a lift-off method. The channel width (W) and length (L) dimensions are W/L = 40 µm / 20 µm. Devices were cured at 200 °C on a hotplate for 20 minutes. No further high temperature post-annealing treatments were done. To verify the electrical performance of a flexible ALD IZO TFT device, we fabricated the TFT on a 18-µm thick polyimide (PI) film coated onto glass. A multi-layered buffer consisted of SiO2/SiNx/SiO2 and a 30-nm thick Al2O3 layer is used. The IZO films deposited at different temperatures were investigated using Hall measurements (Ecopia; AH5TTT5), to obtain the majority carrier type, carrier concentration, and mobility. The chemical composition of the IZO thin film was obtained by Auger electron spectroscopy (AES). Analysis of the chemical binding states in the core-level region for the ALD IZO was conducted using X-ray photoelectron spectroscopy (XPS). Morphology of the film surface was observed by atomic force microscope (AFM; XE-100). The detailed electronic structures, related to changes in the band gap and band edge states below the conduction band, were analyzed by spectroscopic ellipsometry (SE). SE measurements were performed by a rotating analyzer system (RC2) with an auto retarder in the energy range of 1.25 eV ~ 5.05 eV. The semiconductor device properties were evaluated using a HP 4155A parameter analyzer in air at room temperature. The device stability under positive bias temperature stress (PBTS) was measured in vacuum at 60 °C, applying VGS = 20 V for a total stress time of 3600 s.

Results and Discussion We compare the steady-state growth rate of In2O3 and ZnO with the growth rate of IZO which is deposited by In2O3-ZnO cycle-by-cycle ALD, at different growth temperatures. In Figure 1, the steady-state growth rate of In2O3 thin films show little temperature dependence with 0.68 Å/cycle, while growth rate of ZnO increased from 0.7 Å/cycle at 150 °C to 1.7 Å/cycle at 200 °C. The growth rate of IZO with alternating cycles of In2O3/ZnO increased from 1.4 Å/cycle at 150 °C to 4


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1.8 Å/cycle at 200 °C, which deviates from the sum of the individual steady-state growth rates of In2O3 and ZnO as the growth temperature increases. Tanskanen et al.15,26 reported that mixing ALD cycles of SnOx and ZnO resulted in the growth rate of ZTO being less than the expected sum of the growth rates of the binary oxides. The discrepancy was attributed to the change in surface chemistry, particularly the reduction of OH groups caused by the ligand conversion during the SnOx cycle resulting in reduced reaction site density for the following ZnO cycle. The chemical composition of the IZO thin film is measured by AES and compared with the growth rate ratios of the individual binary oxides, to gain insight into the surface reactions during ALD. The chemical composition of the IZO thin films deposited at 150 to 200 °C is listed in Table 1 along with related individual growth rate values. Cation fraction of zinc (= Zn / (Zn + In) × 100) at 150 °C is 60 %, which is higher than the steady-state growth rate ratio of ZnO to the sum of both binary oxides ( ZnO / ( ZnO +

In2O3 ) = 53 %). When the deposition temperature increases to 200

°C, the steady-state growth rate increases by a factor of 2.4 while the composition of Zn merely increases from 60 to 64%. The results suggest different growth mechanisms are taking place between low and high temperatures. Thus, following previous research,15 we introduce two surface reaction scenarios dependent on the growth temperature of ALD IZO thin film. Figure 2 shows the 2 scenarios for the In2O3 half-reactions. Case I is where the INCA-1 releases the N[Si(CH3)3]2 ligand, and the number of DEZ reaction sites doubles for every OH* reaction with INCA-1. Case II is where INCA-1 releases an additional CH3 ligand, and the number of reactions sites for DEZ is halved for every two OH* reactions with INCA-1. The proposed mechanisms are summarized with the following half-reactions:

((CH3)3Si)2NIn(CH3)2 + OH* → O−In(CH3)2 + ((CH3)3Si)2NH↑

(1)

O−In(CH3)2 + 2H2O2 → O−In(OH*)2 + 2CH3OH↑

(2) 5



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((CH3)3Si)2NIn(CH3)2 + 2OH* → (O)2−InCH3 + ((CH3)3Si)2NH + CH4↑

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(3)

(O)2−InCH3 + H2O2 → (O)2−InOH* + CH3OH↑

(4)

OH* + Zn(CH2CH3)2 → O−Zn(CH2CH3)* + C2H6↑

(5)

O−Zn(CH2CH3)* + H2O2 → O−ZnOH* + C2H5OH↑

(6)

where “*” denotes the surface species, “↑” indicates the desorbed products. Half-reactions (1), (2) and (3), (4) refer to the two possible cases for the In2O3 cycle, and half-reactions (5), (6) refer to the ZnO cycle. It is well known that the reaction site density on the surface would affect the ALD growth rate.27 At low deposition temperatures, the dominant reaction of the In2O3 cycle occurs as Case I, where each In bonds to one OH* site and provide two OH* sites for the following ZnO cycle. Hence for Case I, increased OH site density on the surface enhances growth rate of ZnO resulting in relatively high composition ratio of Zn in IZO thin films, which corresponds to the 150 °C data. At higher growth temperatures, In bonds with two OH sites on the surface with first halfreaction (3), and provides only one OH site after the second half-reaction (4). The reduction of reaction site density on the surface during the In2O3 cycle largely suppresses the ZnO growth rate in the following ALD cycle. When deposition temperature increases, despite the expected growth rate increase due to enhanced reactivity, the number of surface sites drops as in Case II, resulting in IZO growth rate being less than the sum of that of the binary oxides as well as little increase in Zn composition (cation ratio of Zn from 60 to 64 %). Having seen the temperature-dependent ALD growth mechanism and the composition change of the deposited IZO thin films, we fabricated ALD IZO TFTs to study the influence of ALD growth temperature of the IZO active layer on the device performance. Figure 3 shows representative 6


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transfer characteristics of IZO TFTS with active layers grown at 150, 175, and 200 °C. Devices with IZO active layer grown at 200 °C exhibits increased on-current and a negative threshold voltage compared to other growth temperatures, which implies higher carrier conductivity. Device characteristics of ALD IZO TFTs as a function of growth temperature from 150 to 200 °C are summarized in Table 2. The threshold voltage (Vth) is determined as the gate voltage where ID = W/L × 10 nA. The field-effect mobility is extracted from the linear transfer characteristics as µFE = L ⋅ gm / (W Cox VDS) at VDS = 0.1 V, where gm is the transconductance and Cox is the oxide capacitance of the gate insulator. The Vth of the device is 14.3 V at 150 °C and decreases to -0.7 V at 200 °C. The field-effect mobility of TFTs with IZO grown at 150, 175, and 200 °C are 9.6, 12.2, and 42.1 cm2 V-1 s-1, respectively. The device incorporating IZO grown at 200 °C exhibits optimized electrical properties with the highest mobility, least hysteresis of 0.21 V, and steepest subthreshold slope (SS) with 0.29 V/decade. Table 3 lists the major parameters of Hall measurements that reflect the electrical properties of 40-nm IZO films. The carrier concentration rises by nearly two orders of magnitude, from 2.0×1018 to 1.3×1020 cm-3, when growth temperature increases from 150 to 200 °C. Hall measurement results suggest that the enhanced carrier conductivity at 200 °C originates from higher carrier concentration, along with higher mobility. The carrier concentration at 200 °C for the 40-nm IZO film would be too high to get a proper off-state current if fabricated into a device. IZO TFTs with different channel thicknesses were fabricated from 5 nm to 20 nm as shown in Figure S1. The IZO TFT with a 5-nm channel showed highly resistive behavior, while thicknesses of 15 and 20 nm exhibited conducting characteristics. Hence, using an optimum channel thickness is crucial in obtaining proper switching characteristics for transistor operation. The series resistances of IZO TFTs were extracted from devices of various channel lengths (10, 15, 20, 30 µm). The series resistances for growth temperatures 150, 175, and 200 °C were 160, 100, and 32 kΩ, respectively. Dependence of the 7



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series resistance (including the contact and bulk resistance) on temperature agrees with the temperature-dependency of resistivity obtained from Hall measurements (in Table 3). We carried out positive bias temperature stress (PBTS) measurements with VGS = +20 V for 3600 s at 60 °C. Figure 4 shows the PBTS stability characteristics for devices with the active layer grown at various temperatures. The Vth shift (∆Vth) under PBTS decreased from 4.5 V at 150 °C, to 2.2 V at 175 °C, to 1.0 V at 200 °C. As stress time progresses, the Vth shifts toward the positive direction without any significant changes of the SS value. Generally, in this case, the dominant mechanism for PBTS instability is attributed to charge trapping of electrons within the GI bulk or near the interface between the GI and active layer. The relationship between oxygen and metal bonding and electrical performance of the TFTs were analyzed by XPS, as shown in Figure 5. The O 1s peak was mainly investigated, and was decomposed into three sub-peaks, labeled as peak A, B, and C (as shown in Figure 4a). Peak A, located at 529.6 ± 0.05 eV, is generally attributed to oxygen anions forming metal-O bonds. Peak B, located at 531.3 ± 0.05 eV, is conventionally attributed to oxygen anions in oxygen-deficient environments, and is proportional to oxygen deficiencies. Peak C, located at 532.1 ± 0.05 eV, is assigned to O-impurities such as –OH groups.28 The relative fraction of peaks A, B, and C was compared in terms of their areal ratio with respect to growth temperature, as indicated in Figure 4b. As the growth temperature increases from 150 to 200 °C, the fraction of metal-O bonds slightly decreased while the amount of oxygen-deficiency-related carrier concentration appears to increase. The increase of oxygen-deficiency peaks with temperature obtained from XPS results agree with the decrease of oxygen composition in the IZO films obtained from AES measurements. Oxygen deficiency leads to higher carrier concentration, which moves the Fermi level upwards and shifts the Vth in the negative direction. 29-31 As the growth temperature increases, there is more oxygen deficiency which leads to a more negative Vth in the device characteristics as can be seen in Table 2. It should be noted oxygen-deficiencies can form both shallow donor states and deep electron trap 8


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states, which strongly correlate with the carrier concentration and the stability of the device.32,33 The shallow donor states can enhance carrier concentration, while deep trap states can cause charge trapping leading to degradation in carrier transport and device stability.33,34 In order to figure out the electronic structure of IZO films, especially the sub-gap states below the conduction band edge, SE measurements were carried out for the ALD IZO thin films. The imaginary dielectric function (ε2) spectra for the IZO thin films on Si substrates are shown in Figure 6. To quantify the band edge states and conduction band states, the SE spectra was fitted using a Gaussian model (band edge states) and a Tauc-Lorentz model (conduction band states). The Gaussian model fits were comprised of two distinct band edge states, denoted as D1 and D2, which are located at 0.02 eV and 0.33 eV away from the conduction band edge, respectively. The area ratios of D1 and D2 as a function of growth temperature are shown in Figure 6c, which exhibited increase of D1 states while D2 states decrease with rising growth temperature. D1 states can act as shallow donors that increase the carrier concentration, while D2 states are responsible for electron trap states during PBTS.33,34 Additionally, in order to investigate additional reasons for ALD IZO TFT device performance and stability improvement at high growth temperatures, we performed analysis of surface morphologies by atomic force microscopy (AFM) as shown in Figure 7. The IZO thin films showed smooth surfaces attributed to self-limiting ALD growth. Furthermore, the roughness of IZO films improved with increased growth temperature from RMS of 0.32 nm at 150 °C to 0.073 nm at 200 °C. This might have minor effect on ameliorating the stability by decreasing electrons trapped in the channel interface and electrode layers due to a smoothened surface of the IZO thin film. Figure 8 shows the elemental distribution of In, Zn, and O of ALD IZO thin film deposited at 200 °C, obtained from energy-dispersive X-ray spectroscopy (EDS) mapping. The microstructure of 40nm IZO thin films grown at 200 °C ALD can be observed by transmission electron microscopy (TEM) as shown in Figure 8e. No long-range ordering of the lattice is found in the cross-sectional 9



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TEM image of the IZO film on SiO2. Furthermore, selected area electron diffraction pattern (shown in the inset of Figure 8e) identifies the sample as amorphous phase, which differs from the recent demonstration of high performance ALD TFT where high mobility originates from crystallization of the active layer after thermal annealing at 350 °C.35 In our previous results,28,36 we found that both In2O3 (cubic) and ZnO (hexagonal) grown by atomic layer deposition using the same precursor and reactant as this work crystallizes at low growth temperatures (< 150 °C). However, crystalline growth is suppressed for IZO in this work due to the different surface reactions from the alternating binary oxides, resulting in an amorphous-phase IZO film even at growth temperatures of 200 °C. To verify the applicability of ALD IZO TFTs to flexible applications, the IZO TFTs were successfully fabricated on a PI film coated onto a glass substrate. Then, the PI film was delaminated from the glass substrate as shown in Figure 9. We investigated the device characteristics of the flexible TFTs with the optimized active layer ALD growth temperature of 200 °C. The flexible TFT exhibited good durability to bending with no performance degradation compared to TFTs fabricated on glass. The ALD IZO TFTs on freestanding PI films showed good device performance of µFE = 45 cm2 V-1 s-1, Vth = -2.2 V, and SS = 0.2 V/decade. The results reveal the potential of using TFTs with ALD-grown active layers for flexible display applications. To investigate the durability of electrical properties under mechanical stress, IZO TFTs (grown at 200 °C ALD) on PI substrates were tested under repetitive bending. Fig. 10 shows the changes in device parameters with increasing bending cycles. Electrical measurements were done after 100, 300, 1000, 2000, and 5000 bending cycles. The Vth shifts in the negative direction, and the saturation mobility decreases as the number of bending cycles increase. Different bending radii of 5, 2mm were used for different degrees of induced strain. Two plausible causes are suggested for the decrease in the Vth for tensile bending. One is the increase in the interatomic distances that cause an effective decrease in the energy level splitting (∆E) of the bonding and antibonding orbitals between 10


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the atoms in the semiconducting layer.37 More electrons are excited to the antibonding state, giving rise to an increase in electron concentration, which results in an increase in the channel conductivity, and hence, a negative shift in Vth. Secondly, the charge transport of oxide semiconductors is related to the generation of carriers caused by native defects such as oxygen vacancies.38-40 Thus, native defects may form inside the IZO film due to mechanical deformation under tensile strain, specifically oxygen vacancies that act as electron donors.41 The decrease of saturation mobility may be due to cumulative formation of micro-cracks and structural defects in the active bulk or interface during repeated cycling of mechanical stress.42

Conclusion We investigated the ALD growth of IZO thin films deposited at 150, 175, and 200 °C. By comparing the growth rate of IZO with the growth rates of In2O3 and ZnO, we found a discrepancy between the actual growth of IZO and the sum of the respective binary oxides, which increased with higher deposition temperature. We proposed two surface reaction cases for the In2O3 cycle, where higher temperature may cause the INCA-1 precursor to release an additional CH3 ligand so that In bonds to two OH sites. During the following ZnO cycle, OH sites on the surface are reduced, resulting in suppressed ZnO growth rate at high growth temperatures despite the increased reactivity. The Zn:In ratio increases merely from 1.52 to 1.75, despite the 2.4X increase of bulk ZnO ALD growth rate, when growth temperature increases from 150 °C to 200 °C. We fabricated thin film transistors with the ALD IZO thin films as the active layer. ALD growth temperature-dependent TFT performance was investigated via various thin film analysis techniques. IZO TFTs using 200 °C ALD for active deposition exhibited high mobility of 42.1 cm2 V-1 s-1 and good PBTS stability due to high carrier concentration and low electron trap states, confirmed by the amount of D1 and D2 states obtained via SE analysis. Finally, we successfully fabricated flexible IZO TFT on a PI 11



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substrate without performance degradation showing the potential of ALD-grown active layers for flexible display applications.

Supporting Information Transfer characteristics of IZO TFTs fabricated with different channel thicknesses from 5 nm to 20 nm (Figure S1).

AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] (Jin-Seong Park)

*

E-mail: [email protected] (Saeroonter Oh)

AUTHOR CONTRIBUTIONS J. S, H. –J. L, S. O, and J. –S. P. designed the InZnO ALD experiments and fabricated the thin film transistor on a flexible substrate. J. S, S. O, and J. –S. P. performed various film analyses (H. –J. L. did the TEM work). The manuscript was written by the contribution of all authors, who have approved the final version of the manuscript.

Acknowledgements This research was mainly supported by the MOTIE (Ministry of Trade, Industry & Energy (project number 10051403, 10052020, and 10052027) and KDRC (Korea Display Research Corporation). Also, this work was partially supported by the research fund of Samsung Display. In particular, the authors would like to thank UP Chemical Co. for supporting the INCA-1 precursor.

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[20] Illiberi, A.; Cobb, B.; Sharma, A.; Grehl, T.; Brongersma, H.; Roozeboom, F.; Gelinck, G.; Pood, P. Spatial Atmospheric Atomic Layer Deposition of InxGayZnzO for Thin Film Transistors, ACS Appl. Mater. Interfaces 2015, 7, 3671. [21] Lee, S. J.; Hwang, C. S.; Pi, J. E.; Yang, J. H.; Byun, C. W.; Chu, H. Y.; Cho, K. L.; Cho, S. H. High-Performance Amorphous Multilayered ZnO-SnO2 Heterostructure Thin-Film Transistors: Fabrication and Characteristics, ETRI Journal 2015, 37,1135. [22] Ahn, B. D.; Choi, D.W.; Choi, C. W.; Park, J. S. The Effect of the Annealing Temperature on the Transition from Conductor to Semiconductor Behavior in Zinc Tin Oxide Deposited Atomic Layer Deposition, Appl. Phys. Lett. 2014, 105, 092103. [23] Okyay, A. K.; Oruç, F. B.; Çimena, F.; Aygün, L. E. TiO2 Thin Film Transistor by Atomic Layer Deposition, Proc. of SPIE 2013, 8626, 862616-1. [24] Kim, S. J.; Heo, K. J.; Yoo, S. C.; Choi, S. G.; Rutile TiO2 Active-channel Thin-film Transistor Using Rapid Thermal, J. Korean Phys. Soc. 2014, 65, 1118. [25] Kim, S. H.; Jeong, K. S.; Lee, G. W.; Lee, H. D. Effects of the Al2O3 interlayer in ZnO thinfilm transistors fabricated via atomic layer deposition, J. Inf. Disp. 2013, 14, 61. [26] Hägglund, C.; Grehl, T.; Tanskanen, J. T.; Yee, Y. S.; Mullings, M. N.; Mackus, A. J. M.; MacIsaac, C.; Clemens, B. M.; Brongersma, H. H.; Bent, S. F. Growth, Intermixing, and Surface Phase Formation for Zinc Tin Oxide Nanolaminates Produced by Atomic Layer Deposition. J. Vac. Sci. Technol. 2016, 34, 021516. [27] Puurunen, R. L. Surface Chemistry of Atomic Layer Deposition: A Case Study for the Trimethylaluminum/Water Process. J. Appl. Phys. 2005, 97, 121301. [28] Wang, Y. H.; Ma, Q.; Zheng, L. L.; Liu, W. J.; Ding, S. J.; Lu, H. L.; Zhang D. W. Performance Improvement of Atomic Layer Deposited ZnO/Al2O3 Thin-Film Transistors by LowTemperature Annealing in Air. IEEE Trans. Electron Device. 2016, 63, 1893.

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[29] Naghavi, N.; Dupont, L.; Marcel, C.; Maugy, C., Laı¨k, B.; Rougier, A.; Gue´ry, C.; Tarascon, J. M. Systematic Study and Performance Optimization of Transparent Conducting Indium–Zinc Oxides Thin Films Electrochimica Acta 2001, 46, 2007-2013 [30] Jeong, S.; Ha, Y. G.; Moon, J. H.; Facchetti, A.; Marks, T. J. Role of Gallium Doping in Dramatically Lowering Amorphous-Oxide Processing Temperature for Solution-Derived Indium Zinc Oxide Thin-Film Transistors Adv. Mater. 2010, 22, 1346-1350. [31] Nayak, R. K.; Hedhili, M. N.; Cha, D.; Alshareef, H. N. High Performance Solution-Deposited Amorphous Indium Gallium Zinc Oxide Thin Film Transistors by Oxygen Plasma Treatment Appl. Phy. Lett. 2012, 100, 202106. [32] Kamiya, T.; Nomura, K.; Hosono, H. Origins of High Mobility and Low Operation Voltage of Amorphous Oxide TFTs: Electronic Structure, Electron Transport, Defects and Doping. J. Disp. Technol. 2009, 5, 7. [33] Park, H. W.; Chung, K. B.; Park, J. S. A Role of Oxygen Vacancy on Annealed ZnO Film in the Hydrogen Atmosphere. Current Applied Physics 2012, 12, 164. [34] Chung, K. B.; Long, J. P.; Seo, H.; Lucovsky, G.; Nordlund, D. Thermal Evolution and Electrical Correlation of Defect States in Hf-based High-κ Dielectrics on n-type Ge (100): Local Atomic Bonding Symmetry. J. Appl. Phys. 2009, 106, 074102. [35] Yeom, H. I.; Ko. J. B.; Mun, G.; Park, S. H. High Mobility Polycrystalline Indium Oxide ThinFilm Transistors by Means of Plasma-Enhanced Atomic Layer Deposition. J. Mater. Chem. C 2016, 4, 6873. [36] Park, J.; Jung, T. H.; Lee, J. H.; Kim, H. S.; Park, J. K. The Growth Behavior and Properties of Atomic Layer Deposited Zinc Oxide Films Using Hydrogen Peroxide (H2O2) and Ozone (O3) Oxidants. Ceram. Int. 2015, 41, 1839. [37] Rockett, A. Materials Science of Semiconductors SpringerVerlag, 2007, 195.

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[38] Kohan, A. F.; Ceder, G.; Morgan, D.; Van de Walle, C. G. First-Principles Study of Native Point Defects in ZnO Phys. Rev. B, 2000, 61, 15019. [39] Look, D. C.; Farlow, G. C.; Reunchan, P.; Limpijumnong, S.; Zhang, S. B.; Nordlund, K. Evidence for Native-Defects Donors for n-type ZnO Phys. Rev. Lett. 2005, 95, 225502. [40] Kim, Y. S.; Park, C. H. Rich Variety of Defects in ZnO via Attractive Interaction between O Vacancies and Zn Interstitials: Origin of n-Type Doping Phys. Rev. Lett. 2009, 102, 086403. [41] Sheng, J.; Choi, D. W.; Lee, S. H.; Park, J.; Park, J. S. Performance Modulation of Transparent ALD Indium Oxide Films on Flexible Substrates: Transition between Metal-Like Conductor and High Performance Semiconductor States J. Mater. Chem. C, 2016, 4, 7571 [42] Sheng, J.; Park, J.; Choi, D. W.; Lim, J.; Park, J. S. 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 ACS Appl. Mater. Interfaces 2016, DOI: 10.1021/acsami.6b11815

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List of Figure Captions Figure 1. Comparison of growth rates of In2O3 (solid triangle symbols), ZnO (open square symbols), and IZO deposition (red sphere symbols) at various growth temperatures. The arithmetic sum of In2O3 and ZnO growth rates are shown in black circle symbols.

Figure 2. Proposed growth mechanisms during In2O3 half-reactions. The rectangular slab represents an OH-terminated surface. For Case I (above), indium precursor (INCA-1) provides two OH reaction sites for DEZ, while for Case II (below), INCA-1 releases an additional CH3 ligand resulting in less OH reaction sites for DEZ. For higher ALD temperatures, the preference of Case II increases.

Figure 3. Transfer characteristics (ID vs. VGS) of the IZO TFTs with the ALD IZO active layer grown at (a) 150 oC, (b) 175 oC, and (c) 200 oC. Forward and reverse VGS sweeping shows the hysteresis characteristics. Devices have W/L = 40µm/20µm.

Figure 4. (a) Representative transfer curves for devices under PBTS (VGS = 20V at 60 oC for 1 h). (b) Change of device parameters after 1 h of PBTS under different growth temperatures; 150, 175, and 200 oC.

Figure 5. (a) XPS spectra of O 1s for IZO thin films under different growth temperatures 150, 175, and 200 oC. (b) Areal ratio of the three peaks (metal-O, oxygen-deficiency, -OH group) are compared between different growth temperatures. 18


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. (a) Comparison of ε2 spectra of IZO thin films deposited at 150-200 °C. (b) Schematic band diagram of IZO TFTs with defects levels (D1 and D2) denoted. (c) Comparison of area ratio of D1 and D2 as a function of ALD growth temperature.

Figure 7. AFM images of IZO thin film surfaces deposited at various temperatures, (a) 150 oC, (b) 175 oC, and (c) 200 oC.

Figure 8. EDS mapping of elemental distributions; (a) In, Zn, O combined, and indivisual (b) In, (c) Zn, and (d) O elements. (e) Transmission electron microscope (TEM) image of IZO films deposited at 200 oC. Selected-area diffraction pattern is shown in the inset.

Figure 9. (a) Schematic diagram of the IZO TFT (ALD temperature 200 °C) fabricated on a PI substrate. (b) Optimized transfer characteristics (ID vs. VGS) of the flexible IZO TFT. Inset: photograph of flexible TFT.

Figure 10. Variation in device parameters: (a) threshold voltage, (b) field-effect mobility, and (c) subthreshold slope as a function of bending cycles for flexible IZO TFTs fabricated on plastic substrates, for three different degrees of strain.

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List of Tables Table 1. Steady-state growth rate of binary oxides ZnO and In2O3, growth rate of ZnO/In2O3 layerby-layer IZO, and chemical composition of IZO thin films measured by AES at different ALD growth temperatures. Composition values are normalized to Zn. Growth rate (Å/cycle)

IZO composition

ALD temperature

ZnO

In2O3

IZO

Zn

In

O

150oC

0.72

0.64

1.36

1

0.66

1.69

175oC

1.41

0.69

1.75

1

0.57

1.59

200oC

1.72

0.71

1.82

1

0.57

1.55

Table 2. List of key device parameters of ALD IZO TFTs for active deposition temperature of 150, 175, and 200oC. Average and standard deviation values are included. ALD temperature 150 °C 175 °C 200 °C

Vth (V) 14.3±0.7 2.9±0.6 -0.7±0.4

µFE (cm2 V-1 s-1) 9.6±1.1 12.2±0.5 42.1±0.9

SS (V/decade) 0.33±0.01 0.34±0.05 0.29±0.04

Hysteresis (V) 1.2±0.1 0.79±0.2 0.21±0.1

ION/IOFF 1.69×109 3.75×109 5.03×109

Table 3. Hall measurement parameters of 40-nm IZO thin film with active layers grown at different ALD temperatures. Growth temperature 150 °C 175 °C 200 °C

Carrier concentration (1019 cm-3) 0.2±0.1 3.2±0.5 13.0±3.9

Hall mobility (cm2 V-1 s-1) 2.2±1.1 12.2±3.5 20.2±6.6

Resistivity (Ω⋅cm) 1.7±0.6 (1.7±10.1)×10-2 (2.6±10.1)×10-3

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Figure 1. Comparison of growth rates of In2O3 (solid triangle symbols), ZnO (open square symbols), and IZO deposition (red sphere symbols) at various growth temperatures. The arithmetic sum of In2O3 and ZnO growth rates are shown in black circle symbols.

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Figure 2. Proposed growth mechanisms during In2O3 half-reactions. The rectangular slab represents an OH-terminated surface. For Case I (above), indium precursor (INCA-1) provides two OH reaction sites for DEZ, while for Case II (below), INCA-1 releases an additional CH3 ligand resulting in less OH reaction sites for DEZ. For higher ALD temperatures, the preference of Case II increases.

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-2

10

-3

10

-4

10

-5

10

-6

10

ID (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Growth at 150oC

o

o

Growth at 175 C

Growth at 200 C

20.1V forward 20.1V reverse 0.1V forward 0.1V reverse

-7

10

-8

10

-9

10

-10

10

-11

10

-12

10

-13

10

-30 -20 -10

0

10

VGS (V)

20 -30 30 -20 -10

0

10

20 -30 30 -20 -10

VGS (V)

0

10

20

30

VGS (V)

Figure 3. Transfer characteristics (ID vs. VGS) of the IZO TFTs with the ALD IZO active layer grown at (a) 150 oC, (b) 175 oC, and (c) 200 oC. Forward and reverse VGS sweeping shows the hysteresis characteristics. Devices have W/L = 40µm/20µm.

23



-2 (a) 10

ID (A)

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-3 ALD temp. 175 ° C ALD temp. 200 ° C 10 ALD temp. 150 ° C -4 W/L = 40/20 10 -5 VDS = 20.1 V 10 -6 10 -7 10 0s -8 10 32s -9 100s 10 -10 300s 10 -11 1000s 10 3600s -12 10 -13 10 -30 -20 -10 0 10 20 -30 30 -20 -10 0 10 20 -30 30 -20 -10 0 10 20 30

VGS (V)

VGS (V)

VGS (V)

Figure 4. (a) Representative transfer curves for devices under PBTS (VGS = 20V at 60 oC for 1 h). (b) Change of device parameters after 1 h of PBTS under different growth temperatures; 150, 175, and 200 oC.

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(a)

Intensity (Arb. unit)

o

o

150 C

o

175 C

A

200 C

A

B

A

B

C

C

B C

534 531 528 534 531 528 534 531 528 525 Binding Energy (eV) Binding Energy (eV) Binding Energy (eV)

Binding Energy (eV)

(b) 100

Area ratio(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

150 C o 175 C o 200 C

80 60 40 20 0

O-M

O-deficient

O-H

O 1s peak components Figure 5. (a) XPS spectra of O 1s for IZO thin films under different growth temperatures 150, 175, and 200 oC. (b) Areal ratio of the three peaks (metal-O, oxygen-deficiency, -OH group) are compared between different growth temperatures.

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(a) 0.5

o

o

150 C

o

175 C

200 C

0.4 0.3 0.2 D2

0.1 0.0 1.5

2.0

2.5

D1

3.0

3.5

2.0

Photon Energy (eV) (b)

0.02eV

D2

0.33eV 3.24eV

Valence band

2.5

3.0

(c)

D1

D2 3.5

2.0

Photon Energy (eV)

Conduction band

D1

D1

D2

Peak Area (%)

ε2 Spectra

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2.5

3.0

3.5

Photon Energy (eV)

1.0 0.8

D1 D2

0.6 0.4 0.2 0.0

150

175

200

Growth Temperature(OC) Figure 6. (a) Comparison of ε2 spectra of IZO thin films deposited at 150-200 °C. (b) Schematic band diagram of IZO TFTs with defects levels (D1 and D2) denoted. (c) Comparison of area ratio of D1 and D2 as a function of ALD growth temperature.

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Figure 7. AFM images of IZO thin film surfaces deposited at various temperatures, (a) 150 oC, (b) 175 oC, and (c) 200 oC.

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Figure 8. EDS mapping of elemental distributions; (a) In, Zn, O combined, and indivisual (b) In, (c) Zn, and (d) O elements. (e) Transmission electron microscope (TEM) image of IZO films deposited at 200 oC. Selected-area diffraction pattern is shown in the inset.

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Figure 9. (a) Schematic diagram of the IZO TFT (ALD temperature 200 °C) fabricated on a PI substrate. (b) Optimized transfer characteristics (ID vs. VGS) of the flexible IZO TFT. Inset: photograph of flexible TFT.

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Figure 10. Variation in device parameters: (a) threshold voltage, (b) field-effect mobility, and (c) subthreshold slope, as a function of bending cycles for flexible IZO TFTs fabricated on plastic substrates, for three different degrees of strain.

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Flexible and High-Performance Amorphous Indium ... - ACS Publications

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