Nitroacetylactone as a (co)Fuel for the Combustion Synthesis of High

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Nitroacetylactone as a (co)Fuel for the Combustion Synthesis of High-Performance Indium-Gallium-Zinc Oxide Transistors Yao Chen, Binghao Wang, Wei Huang, Xinan Zhang, Gang Wang, Matthew J. Leonardi, Yan Huang, Zhiyun Lu, Tobin J. Marks, and Antonio Facchetti Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00663 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Chemistry of Materials

Nitroacetylactone as a (co)Fuel (co)Fuel for the Combustion Synthesis of HighighPerformance IndiumIndium-GalliumGallium-Zinc Oxide Transistors Yao Chen, †, ‡ Binghao Wang, † Wei Huang, † Xinan Zhang, † Gang Wang, † Matthew J. Leonardi, † Yan Huang, ‡ Zhiyun Lu, ‡ Tobin J. Marks†, ∥∗ and Antonio Facchetti†, ⊥∗ †

Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. ‡

Key Laboratory of Green Chemistry and Technology (Ministry of Education), College of Chemistry, Sichuan University, Chengdu 610064, P. R. China. ∥

Department of Materials Science and Engineering and the Argonne Northwestern Solar Energy Research Center (ANSER), Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.

⊥Flexterra

Inc., 8025 Lamon Avenue, Skokie, Illinois 60077, USA.

ABSTRACT: Thin-film combustion synthesis has been utilized for the fabrication of solution processed and high-performance metal-oxide thin-film transistors (MOTFTs) at lower temperatures than conventional sol-gel processes. The fuel-oxidizer ensemble in the MO precursor solution/film plays an important role in achieving high-efficiency and low-residual combustion byproducts. However, unlike conventional bulk combustion, only a very limited number of thin-film fuels have been investigated. Here we report the use of an efficient new (co)fuel, 3-nitroacetylactone (NAcAcH), incorporating a -NO2 group, for the combustion synthesis of display-relevant indium-gallium-zinc-oxide (IGZO) thin films. Compared to the traditional acetylacetone (AcAcH) fuel, a higher enthalpy of combustion (988.6 vs 784.4 J/g) and a lower ignition temperature (107.8 vs 166.5 °C) are achieved for NAcAcH-based formulations. The resulting NAcAcH-derived IGZO TFTs exhibit far higher average electron mobili2 -1 -1 2 -1 -1 ties (5.7 cm V s ) than AcAcH-derived TFTs (2.7 cm V s ). More importantly, when combining AcAcH with NAcAcH as co2 -1 -1 fuels in an optimal molar ratio of 1.5:0.5, an even larger TFT electron mobility (7.5 cm V s ) and more stable devices were achieved. Comprehensive IGZO precursor/film analysis and characterization by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and X-ray reflectivity (XRR) explain the basis of the film microstructure and TFT performance trends.

INTRODUCTION The past two decades have witnessed great advances in solu1-3 tion-processed organic and inorganic semiconductors. Among these materials, thin-film transistor (TFT)-based metal oxide (MO) semiconductors have attracted much attention due to their high electron mobilities, excellent optical transparency, good environmental/thermal stability, and superior mechanical flexibility compared to conventional 1, hydrogenated amorphous silicon (a-Si:H) and organic TFTs. 4-10 Furthermore, the commercialization of amorphous IGZObased TFTs, as an alternative to a-Si:H and poly-Si for activematrix displays, demonstrates that IGZO TFTs successfully balance electrical performance, thin-film/electrical uniformi5, 11-14 ty, and stress stability. Additionally promising, IGZO films can be fabricated by low-cost, solution-based deposition techniques (e. g. spin-coating, spray coating, printing) in contrast to conventional capital-intensive vapor-deposition techniques (e. g., thermal evaporation, sputtering, pulsed1, 15-16 laser deposition, atomic layer deposition, etc.). In pioneering studies, several groups have exploited several methods to realize high performance solution-processed IGZO TFTs, such as fabrication with ultraviolet irradiation, sol-gel

on a chip, spray pyrolysis, and high-pressure thermal anneal13, 17-20 However, some of these approaches suffer from ing. FAB-incompatible fabrication conditions, requirement of specialized equipment, and/or expensive/sensitive chemicals.

In 2011, we first integrated the combustion synthesis process with the solution phase growth of oxide-based electronic films.19 The unique self-generating energy characteristics of this process enables a significant reduction in the processing temperature versus conventional so-gel thermal condensation syntheses (Figure 1).19 Subsequently, several groups reported the use of combustion synthesis to fabricate oxide-based electronics, such as transistors, organic/perovskite solar cells, and memory devices, to cite a few.21-30 Furthermore, we expanded the scope of combustion to fabricate not only semiconducting MOs, but also conductors and dielectrics,25 as well as advancing oxide thin film processing from spin-coating to printing to spray coating.18 However, up to now, unlike conventional bulk combustion processes, only three types of fuels (acetylactone, benzoylacetone, urea) have been utilized for thin-film combustion, which poses severe limitations on future development (Figure 1).19, 31

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Figure 1. (a) Depiction of the two-different oxide film synthetic approaches with, at the bottom, the chemical structure of three fuels. (b) Energetics of combustion synthesis-based process versus the conventional processes. In this contribution, we design and synthesize a new fuel, 3-nitroacetylacetone (NAcAcH, Figure 2), for the combustion synthesis of IGZO transistors. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) results show that NAcAcH-based combustion precursors exhibit, compared to AcAcH, a significantly greater combustion enthalpy (988.6 vs 784.4 J/g) and a lower ignition temperature (107.8 vs 166.5 °C). XRR results show that the IGZO films based on NAcAcH as the fuel exhibit a higher electron density (1.55 vs 1.52 e/Å3) and XPS indicates a higher M-O-M content (75% vs 72%). Interestingly, when combining NAcAcH with AcAcH as mixed fuels (1.5:0.5 molar ratio), the combustion enthalpy (832.9 J/g) and ignition temperature (142.4 °C) are in between those of the single-fuel systems, however, the corresponding IGZO films exhibit the highest electron density (1.65 e/Å3) and M-O-M content (76%). IGZO TFTs fabricated with combustion synthesis using NAcAcH as a fuel show a higher mobility (5.69 cm2 V-1 s-1) than those based on AcAcH (2.67 cm2 V-1 s-1) with SiO2 as the gate dielectric. More importantly, we find that IGZO TFTs fabricated using the optimal AcAcH:NAcAcH fuel mixture achieve a further enhanced mobility of 7.53 cm2 V-1 s-1.

Figure 2. (a) Top-contact, bottom-gate MO TFT geometry and chemical structures of fuel used in this study. (b) Example of balanced equations for combustion synthesis using AcACH and NAcAcH. EXPERIMENTAL SECTION

Synthesis. NAcAcH was synthesized and purified according 32 to the literature. Detailed synthetic procedures can be found in the Supporting Information. Combustion Precursor Preparation. In(NO3)3 (99.999%), Zn(NO3)2 (99.999%), and Ga(NO3)3 (99.999%), acetylacetone, and ammonium hydroxide solutions (28% NH3 in H2O) were purchased from Sigma-Aldrich. The metal salts were stored in a vacuum desiccator. For IGZO TFT precursors, appropriate amounts of the metal salts In(NO3)3, Ga(NO3)2, and Zn(NO3)3 in a molar ratio of 1:0.11:0.29, were dissolved in 2-methoxyethanol (2ME) to achieve a 0.05 M total metal concentration. Acetylacetone and ammonium hydroxide solutions (28% NH3 in H2O) were then added, and the mixture was allowed to stir for 12 h. The first group of n+ devices had M :(AcAcH+NAcAcH = 2) molar ratios of 1:2 and these TFTs are indicated in the manuscript as IGZO-I n+ n+ (M : AcAcH: NAcAcH = 1: 2: 0), IGZO-II ( M : AcAcH: n+ NAcAcH = 1: 1.5: 0.5), IGZO-III (M : AcAcH: NAcAcH =1: 1: n+ 1), IGZO-IV (M : AcAcH: NAcAcH = 1: 0.5: 1.5), IGZO-V n+ (M : AcAcH: NAcAcH = 1: 0: 2). In order to exclude film/device property variations due to different amount of the base (NH4OH), all precursor solutions have the same amount of base. The volumes of NH4OH were half the volume of AcAcH in IGZO-I to decrease the effect from the NH4OH. For the second group of devices the molar ratio of n+ M :AcAcH was fixed at 0.5 and varying amounts of NAcAcH n+ were used (NAcAcH : M = 0.25, 0.5, 0.75, 1) and the resultn+ ing devices are indicated as IGZO-VI (M : AcAcH: NAcAcH n+ = 1: 2: 0.25), IGZO-VII (M : AcAcH: NAcAcH = 1: 2: 0.5), n+ IGZO-VIII (M : AcAcH: NAcAcH = 1: 2: 0.75), and IGZO-IX n+ (M : AcAcH: NAcAcH =1: 2: 1). DSC and TGA Sample Preparation and Measurements. After aging 12 h, 5 mL aliquots of the IGZO precursor solutions were dried for 48 h under 100 mTorr. Experiments were carried out on 4-5 mg IGZO samples using a SDT Q600 (TA Instruments Inc.) instrument under N2. The heating rate was -1 -1 10 °C min under a 70 mL min N2 flow. Thin-Film and TFT Fabrication and Characterization. All solutions were filtered through a 0.2 μm PFET syringe filter ++ before use. n silicon wafers with a 300 nm thermally grown SiO2 (WRS Materials) were used as the gate electrodes and dielectric layers, respectively (device structure shown in Figure 2a). The substrates were ultrasonically solvent-cleaned (acetone and isopropyl alcohol) and then cleaned with an O2 plasma for 5 min before use. The IGZO precursor solutions were spin-coated on the substrates at 4000 rpm for 30 s in dry box (relative humidity ∼15%) and subsequently annealed


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Chemistry of Materials for 20 min at 300 °C in ambient (relative humidity ∼30%).

This

Figure 3. (a) DSC-derived combustion enthalpy and exotherm temperature for different precursors during combustion synthesis. (b) Energetics of combustion-based processes for different (co)fuels versus the conventional route. process was repeated another three times to obtain the desired IGZO film thickness (7-8 nm). Al source and drain (S/D) electrodes (thickness = 40 nm) were then deposited by thermal evaporation through metal shadow masks. Two batch of devices were fabricated, and more than 15 devices were measured for each batch. The channel width and length for all devices were 1000 and 100 μm, respectively. TFT characterization was performed under ambient conditions using an Agilent B1500A semiconductor parameter analyzer. The carrier mobility (μ) was evaluated in the saturation region with the conventional metal-oxide-semiconductor field10 effect transistor model using eq 1, ‫ܫ‬஽ௌ ൌ

µ஼೔ ௐ ଶ௅

ሺܸீௌ െ ்ܸ௛ ሻଶ

(1)

where IDS is the drain-source current, Ci is the dielectric ca-2 pacitance per unit area (the Ci of 300 nm SiO2 is 11 nF cm without frequency dependence), W and L are the channel width and length, respectively, VGS is the gate-source voltage, and VTh is the threshold voltage.

Atom force microscopy (AFM) and scan electron microscopy (SEM) images were recorded on a Bruker Dimensional Icon atomic force microscopy system in the tapping mode and a Hitachi SU8030 FE-SEM, respectively. Grazing incidence X-ray diffraction (GIXRD) data and X-ray reflectivity(XRR) measurements were acquired with a Rigaku ATXG thin-film diffraction workstation using Cu Kα (1.54 Å) radiation. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250 Xi spectrometer. Note, the film surface was etched about 2 nm before collection of the XPS spectra. RESULTS AND DISCUSSION

Initial selection of NAcAcH was dictated by the question as whether the incorporation of an oxidizing unit (NO2) within the coordinating AcAc motif would produce metal chelates having a single oxidizer-fuel-metal combination and whether these systems would combust upon thermal annealing. Furthermore, we were encouraged by recent pioneering studies investigating the use of dimethyl-2-nitromalonate for the fabrication of MOTFTs.33-34 In these studies, the authors designed and synthesized multimetallic Zn and In complexes of 2-nitromalonate, which exhibit strongly exothermic decomposition in an O2 atmosphere. The authors also fabricated IZO and ZnO

TFTs at 300 °C, which exhibited electron mobilities of 2.11 cm2 V-1 s-1 and ˂ 5×10-2 cm2 V-1 s-1, respectively.33-34 Unfortunately these figures of merit are low and the authors did not address performance difference vis-à-vis ethylmalonate. IGZO film growth was not reported. We first attempted the synthesis of pure metal 3nitroacetylacetonate complexes, however, we were unable to isolate well-defined In, Zn, and Ga NAcAcˉ complexes (see SI for details). This result probably reflects the poor coordinating ability of the NAcAcˉ anion, due to reduced electron density on the oxygens due to the strong electron-withdrawing character of the NO2 group, as well as the expected tridentate nature compared to bidentate acetylacetonate. These results are in agreement with the work of Schneider et. al., where a marked difference in the coordination chemistry of dimethylmalonate and 2nitromalonate was observed.33-34 Thus, we isolated metalNAcAcH mixtures after solvent removal having the formula metal acetate + NAcAcH + NH4OH + 2ME. When subjected to DSC analysis, no heat evolution is observed, indicating that NAcAcH does not combust under these conditions, and thus does not act as a fuel-oxidizer couple (see SI). For the above reasons, we studied IGZO precursor compositions based on metal nitrates as the In/Ga/Zn sourceoxidizer with NAcAcH as the ancillary fuel, and additionally explored mixed AcAcH/NAcAcH fuels where the molar ratio was varied. All of the dried precursors from these formulations evidence combustion processes (vide infra). Specifically, we investigated IGZO compositions (In:Ga:Zn = 1:0.11:0.29 molar ratio) where first the total metal:fuel molar ratio is 1:2 and where the fuel AcAcH: NAcAcH = n: m molar ratio was incrementally varied from n = 2, m = 0 (denoted IGZO-I -- the conventional AcAcH combustion synthesis formulation,9 here used as a reference), n = 1.5, m = 0.5 (IGZO-II), n = 1.0, m = 1.0 (IGZO-III), n = 0.5, m = 1.5 (IGZO-IV), n = 0, m = 2 (IGZO-V -- the formulation having only NAcAcH as fuel). All of these compositions were characterized, after solvent evaporation or spincoating, by thermal analysis, charge transport, and morphological measurements. Next, to further understand metal-fuel-performance correlations, other IGZO compositions having a Mn+: AcAcH = 1:2 molar ratio and using different amounts of NAcAcH (NAcAcH mol = 0.25, 0.5,



0.75, 1) and the devices are indicated as IGZO-VI (Mn+: AcAcH: NAcAcH = 1: 2: 0.25), IGZO-VII (Mn+: AcAcH: NAcAcH = 1: 2: 0.5), IGZO-VIII (Mn+: AcAcH: NAcAcH =1: 2: 0.75), and IGZO-IX (Mn+: AcAcH: NAcAcH =1: 2: 1). Thermal Analysis. The combustion enthalpy (ΔHexo) and ignition temperature (Texo) of the dried IGZO-n precursors (n = I to V, Mn+: fuel = 1: 2) were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Figures 3 and S1 and Table S1 summarize the relevant data. The results show that both Texo and ΔHexo are strongly dependent on the fuel composition. Specifically, ΔHexo monotonically increases [IGZO-I (784.4 J/g), IGZO-II (832.9 J/g), IGZO-III (922.0 J/g), IGZO-IV (955.7 J/g), IGZO-V (988.6 J/g)] while Texo monotonically declines [IGZO-I (166.5 °C), IGZO-II (142.4 °C), IGZO-III (130.6 °C), IGZO-IV (121.1 °C), IGZO-V (107.8 °C)] on increasing the NAcAcH content. This result indicates that NAcAcH can decrease the combustion initiation temperature, which may play an important role in accelerating the overall fuel combustion process, and greatly enhances the heat of combustion. Based on previously established correlations between ΔHexo and IGZO TFT

performance,9 the tactic of using NAcAcH in combustion fuels should be beneficial to oxide TFT charge transport.9 Fuel-Dependent Transistor Response. To elucidate the mechanism by which combustion fuel composition affects charge transport properties, IGZO TFTs were fabricated using IGZO-n (n = I-V) formulations for the channel layers. TFTs were fabricated on Si-SiOx substrates on which the precursor solutions were spin-coated, annealed at 300 °C for 20 min, and the process repeated another 3 times before device completion by thermally evaporating Al source/drain electrodes (W/L =1000/100 μm). As shown in the Figures 4, S2 and Table 1, IGZO-I formulations using only AcAcH as a fuel afford TFTs with carrier mobilities of 2.7 cm2 V-1 s-1, similar to previous results.9 However, formulations comprising only NAcAcH as the fuel (IGZO-V) afford devices with a significantly higher mobility of 5.7 cm2 V-1 s-1. The threshold voltage (VTh) of the latter devices also falls from 4.7 V (IGZO-I) to 2.8 V (IGZO-V). Additionally, all of these IGZO TFTs exhibit on/off current ratios (Ion/Ioff) in the range of 106-107, which is large considering that these devices do not have accurately patterned gate or semiconductor layers. These exceptional data must reflect a dramatically intensified combustion process and denser M-O-M lattice formation (vide infra).

Figure 4. Transport characteristics of IGZO-I-V devices. (a) Representative transfer plots and (b) electron mobility and threshold voltage statistics for the indicated devices. (c) TFT mobility and threshold voltage variation as a function of time under positive gate-bias stress (Vgs = 20 V) for the indicated devices. Table 1. TFT Performance Metrics for Combustion Synthesized of IGZO TFTs on 300 nm Si/SiO2 Substrates with Al Sourcea Drain Electrodes. Composition Mobility Vth Von SS (V/dec) Sample Ion/Ioff (molar ratio) (cm2 V-1 s-1) (V) (V) IGZO-I Mn+:AcAcH:NAcAcH = 1:2:0 2.67±0.45 4.76±1.01 -7.95±1.45 ∼106-7 0.57±0.19 IGZO-II Mn+:AcAcH:NAcAcH = 1:1.5:0.5 7.53±0.20 3.57±1.45 -10.95±5.08 ∼106 0.71±0.16 IGZO-III Mn+:AcAcH:NAcAcH: = 1:1:1 5.10±0.68 4.21±3.27 -4.23±2.25 ∼107 0.61±0.20 IGZO-IV Mn+:AcAcH:NAcAcH: = 1:0.5:1.5 4.27±0.82 4.45±1.77 -3.51±4.27 ∼107 0.51±0.24 IGZO-V Mn+:AcAcH:NAcAcH = 1:0:2 5.69±0.27 2.77±0.87 -11.87±1.98 ∼107 0.57±0.15 IGZO-VI Mn+:AcAcH:NAcAcH: = 1:2:0.25 3.07±0.78 2.84±1.71 -4.94±1.18 ∼107 0.72±0.16 IGZO-VII Mn+:AcAcH:NAcAcH = 1:2:0.5 3.94±1.01 0.97±2.57 -4.67±0.71 ∼107 0.72±0.18 IGZO-VIII Mn+:AcAcH:NAcAcH = 1:2:0.75 3.27±0.29 3.73±1.90 -2.11±1.08 ∼107 0.59±0.12


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Chemistry of Materials IGZO-IX a

Mn+:AcAcH:NAcAcH = 1:2:1

5.35±0.47

4.43±1.07

-0.93±1.02

∼107

0.63±0.17

Average of ≥ 20 devices.

Figure 5. Transport characteristics of IGZO-I and IGZO-VI-IX devices. (a) Representative transfer plots and (b) electron mobility and threshold voltage statistics for the indicated devices. (c) TFT mobility and threshold voltage variation as a function of time under positive gate-bias stress (Vgs = 20 V) for the indicated devices.

Figure 6. (a) O1s XPS spectra and (b) M-O-M ratios calculated from the O1s spectra of the indicated IGZO films. (c) XRR plots and (d) corresponding electron density and thickness profiles. Interestingly, IGZO-n devices comprising both fuels also exhibit statistically higher mobilities than that of the IGZO-I control (Table 1). Specifically, IGZO-III and IGZOIV average mobilities are slightly lower but close (4.3-5.1 cm2 V-1 s-1) to that of IGZO-V, however, that of IGZO-II is statistically much larger (7.5 cm2 V-1 s-1). Note that all IGZO devices were fabricated on 300 nm SiO2 gate dielectrics and lower operation voltages and higher electron moieties are known to be achieved using other highcapacitance gate dielectrics (e.g., Al2O3, ZrO2, SAND).13 These results are in agreement with the thermal data,

where the IGZO-I heat of combustion is considerably lower than those of the other systems, however, they differ from our previous study where a linear correlation was established between ΔHexo and the carrier mobility for carbohydrate-assisted combustion synthesis.9 The operational stability of the present IGZO-n TFTs was next assessed by positive-gate bias-stress measurements. The devices were subjected to a constant gate bias of +20V (VDS = 0 V) for 200 s intervals for 1200 s durations in ambient without any encapsulation/protective layer. As



shown in Figures 4 and S3, in comparison to the IGZO-I device, the IGZO-V device exhibits more stable mobility and VTh values, with a variation of ~5% and ~10% after 1200 s stress time, respectively. In contrast, the IGZO-I device µ and VTh vary by ~22% and ~60%, respectively. All the remaining devices (IGZO-II-IV), derived from NAcAcH-containing formulations, also exhibit more stable operation than those without NAcAcH. The effects of adding different amounts of NAcAcH to the IGZO precursor were next investigated while fixing the AcAcH:metal molar ratio at 2:1. These experiments provide more insights on the role of NAcAcH as co-fuel. Note that this molar ratio was found to be optimal for carbohydrate-assisted combustion synthesis.9 Thus, IGZO TFTs were also fabricated using the procedure discussed previously but with formulations having the following compositions: Mn+: AcAcH: NAcAcH = 1: 2: 0.25 (IGZO-VI), 1: 2: 0.5 (IGZO-VII), 1: 2: 0.75 (IGZO-VIII), 1: 2: 1 (IGZO-IX). As shown in Table 1, Figures 5 and S4, similar mobility trends are found when adding NAcAcH, with all the NAcAcHbased devices showing higher electron mobilities (3.075.35 cm2 V-1 s-1) than that of the IGZO-I (2.67 cm2 V-1 s-1) control, with IGZO-IX outperforming the others (5.35 cm2 V-1 s-1). Furthermore, all IGZO TFTs exhibit Ion/Ioff in the range of 106 - 107. Finally, bias-stress measurements (Figures 5 and S5), indicate that all IGZO-VI-IX are more stable than IGZO-I device, again with IGZO-IX being the most stable of the series (µ and VTh variations of ~8% and ~40% after 1200 s, respectively). Since the first cohort of devices (IGZO-II-V) exhibit the best overall performance, the morphology and microstructure of these IGZO films was fully investigated as discussed in the following sections. Thin-Film Morphology and Microstructure. XPS measurements were performed on the IGZO-n films to better understand the origin of the TFT performance differences with combustion fuel composition. The strength and type of the metal-oxygen bond can strongly affect electron transport in these materials since weakly bound oxygen or oxygen not fully coordinated by a metal ion can introduce trap states, reduce transport efficiency, and decrease bias-stress stability in the corresponding TFTs.3536 The O1s spectra were deconvoluted here into three individual components: M-O-M (529.8 ± 0.1 eV), M-OH (531.1 ± 0.1 eV), and weakly bound M-O-R (532.2 ± 0.1 eV).37 The M-O-M O1s (ηM‑O‑M) content strongly correlates with the oxide TFT performance and stability.9, 38 From the XPS data it is clear that the films prepared using NAcAcH as the fuel consistently exhibit larger M-O-M content (74-76%) versus that of IGZO-I (72%, Figure 6). In particular, IGZO-II has the largest M-O-M content of 76%. This result is only partially consistent with the thermal analysis, considering that ∆Hexo increases monotonically as the NAcAcH content is increased. However, all of the XPS data are fully consistent with the TFT performance trends, where greater charge transport is meas-

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ured in films having larger M-O-M lattice contents. Additionally, the C1s spectra (Figure S6) show a very low carbon content, indicating negligible contamination in these films. Additional film analysis by AFM, SEM, and GIXRD evidence no substantial differences in film morphologies, however, XRR measurements shed light on the variations in electrical performance. Thus, film surface morphology by AFM (Figure S7) and SEM (Figure S8) indicate that all of the films are very smooth with an RMS roughness of 0.13-0.17 nm. GIXRD plots (Figure S9) also demonstrate that all films are amorphous as seen previously for IGZO AcAcH-based combustion formulations.9 Finally, XRR measurements were carried out to accurately assess film thickness and electron density (Figures 6 and S10, Table S2). The average electron density of each film is calculated by integrating the electron density profile over the film region and extracting the slope of the integrated profile.37 Thus, the mass densities of the IGZO-II, IGZO-III, IGZOIV, IGZO-V films are 6.16, 6.05, 5.94 , 5.79 g/cm3, respectively, much higher than that of IGZO-I (5.68 g/cm3). Furthermore, the film thickness of IGZO-I (7.6 nm) is lower than those of the others (7.9-8.4 nm) and that of IGZO-II is the thinnest of the series (6.8 nm) (Figure 6). These results indicate that the high combustion enthalpy of the NAcAcH vs. AcAcH precursor formulations can enhance film density and lattice content, which improves the device performance.18 However, mixing the two fuels in optimal quantities as in IGZO-II strongly enhances the performance, probably dictated by a delicate balance between heat generation and total gas emission during the combustion reaction. Note that for the same fuel molar content, NAcAcH evolves more gaseous byproducts than AcAcH as clearly indicated in Figure 2b. These results demonstrate that NAcAcH is an efficient (co)fuel for combustion synthesis to realize high-performance IGZO TFTs. CONCLUSIONS This work demonstrates the use of a new (co)fuel, NAcAcH, for the combustion processing of IGZO thin films. The combination of NAcAcH with AcAcH in a molar ration of 1.5 to 0.5 results in optimal combustion reaction leading to IGZO films (IGZO-II) with the highest microstructural quality. The corresponding IGZO-IIbased TFTs exhibit the largest carrier mobilities and significantly enhanced bias stress stability. We believe that this work provides insights into the chemical design of combustion synthesis reagents and that the NAcAcH (co)fuel can be extended to the synthesis of other metal oxide films to fabricate high performance electronic devices.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

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Chemistry of Materials Synthesis, DSC-TGA curves, Output curves, Transfer curves, Bias-stress test of IGZO TFTs, GIXRD, Electron density profiles, AFM and SEM images.

AUTHOR INFORMATION Corresponding Author

∗[email protected]; [email protected] ACKNOWLEDGMENT This work made use of the J.B. Cohen X-Ray Diffraction Facility, TPIC Keck-II facility, and SPID facility of the NUANCE Center at Northwestern U., which received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Y.C. thank the joint-Ph.D. program supported by China Scholarship Council for fellowships.

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