Expeditious, scalable solution growth of metaloxide films by combustion blade coating forflexible electronicsBinghao Wanga, Peijun Guob, Li Zengc,d,e, Xia Yuf, Aritra Sila, Wei Huanga, Matthew J. Leonardia, Xinan Zhanga,g,Gang Wanga, Shaofeng Luf, Zhihua Chenf, Michael J. Bedzykc,d,e, Richard D. Schallera,b, Tobin J. Marksa,c,d,e,1,and Antonio Facchettia,f,1

aDepartment of Chemistry and the Materials Research Center, Northwestern University, Evanston, IL 60208; bCenter for Nanoscale Materials, ArgonneNational Laboratory, Lemont, IL 60439; cApplied Physics Program, Northwestern University, Evanston, IL 60208; dDepartment of Materials Science andEngineering, Northwestern University, Evanston, IL 60208; eThe Materials Research Center, Northwestern University, Evanston, IL 60208; fFlexterraCorporation, Skokie, IL 60077; and gSchool of Physics and Electronics, Henan University, 475004 Kaifeng, China

Contributed by Tobin J. Marks, March 12, 2019 (sent for review January 28, 2019; reviewed by Roy G. Gordon, Hagen Klauk, and Tse Nga Ng)

Metal oxide (MO) semiconductor thin films prepared from solutiontypically require multiple hours of thermal annealing to achieveoptimal lattice densification, efficient charge transport, and stabledevice operation, presenting a major barrier to roll-to-rollmanufacturing. Here, we report a highly efficient, cofuel-assisted scal-able combustion blade-coating (CBC) process for MO film growth,which involves introducing both a fluorinated fuel and a preanneal-ing step to remove deleterious organic contaminants and promotecomplete combustion. Ultrafast reaction and metal–oxygen–metal(M-O-M) lattice condensation then occur within 10–60 s at 200–350 °Cfor representative MO semiconductor [indium oxide (In2O3), indium-zincoxide (IZO), indium-gallium-zinc oxide (IGZO)] and dielectric [alumi-num oxide (Al2O3)] films. Thus, wafer-scale CBC fabrication of IGZO-Al2O3 thin-film transistors (TFTs) (60-s annealing) with field-effect mo-bilities as high as ∼25 cm2 V−1 s−1 and negligible threshold voltagedeterioration in a demanding 4,000-s bias stress test are realized.Combined with polymer dielectrics, the CBC-derived IGZO TFTs onpolyimide substrates exhibit high flexibility when bent to a 3-mmradius, with performance bending stability over 1,000 cycles.

thin-film transistor | solution process | ultrashort annealing time |combustion synthesis | blade coating

The recent commercialization of metal oxide (MO) thin-filmtransistors (TFTs) highlights the many attractions of amor-

phous MOs (a-MOs) over competing semiconductor technolo-gies (1–9). Furthermore, and in contrast to current capital-intensive vapor deposition growth, solution-processed a-MOfilms promise to reduce manufacturing costs (10–14) as well asenable flexible optoelectronics on inexpensive plastic substrates(15–21). This possibility reflects the substantial lowering of MOfilm-processing temperatures from >450–600 °C to recently aslow as 150–300 °C (11, 22–29). However, this dramatic temper-ature reduction has not been accompanied by a correspondingreduction in film-processing time, which remains far greater(presently 1–2 h minimum) than acceptable for efficient, con-tinuous additive manufacture, as in typical FAB lines. A no-ticeable exception is capital-intensive flash lamp annealing(FLA) for millisecond fabrication of silicon shallow junctions,which has recently been adapted to MO film growth (30–32).However, the high energy-density radiation pulses instantlygenerate temperatures >1,000 °C at the film surface with verylarge temperature gradients downward. These can produce ex-treme nonuniformity in FLA films and very large thermalstresses that permanently damage underlying functional layersand plastic substrates (32–34).In the past decade, several laboratories including this one have

shown that incorporating combustive fuel-oxidizer redox couplesin MO-precursor solutions yields dense, electronic-quality MOnanometer-thick films at far lower temperatures than conven-

tional sol–gel processes (15, 35, 36). Nevertheless, the sub-sequent post–film-deposition annealing step in combustion canrequire up to 2 h and is essential for lattice densification andeliminating organic contaminants (37, 38). Here, we report that aliquid MO precursor combining a coordinating fuel, AcAcH, afluorinated cofuel, FAcAcH (Fig. 1A), and the corresponding metalnitrates and, equally important, removing extraneous solvent fromthe precursor films before combustion onset, dramatically shortenthe processing/annealing time to as little as 10–60 s. The scope ofthis approach is demonstrated by fabricating MO TFTs with thetechnologically important oxide semiconductors indium oxide(In2O3), indium-zinc oxide (IZO), and indium-gallium-zinc oxide(IGZO). Maximum electron mobilities (μmax) for spin-coated300 °C/10 s-annealed MO film on 300-nm SiO2/Si substrates are7.2 (In2O3), 4.7 (IZO), and 2.6 (IGZO) cm2 V−1 s−1. Furthermore,we demonstrate expeditious wafer-scale fabrication of MO TFTs bycombustion blade-coating (CBC) both an IGZO channel layer andan alumina (Al2O3) gate dielectric layer. The CBC Al2O3 films ex-hibit a large areal capacitance of 386 nF cm−1 at 10 kHz (k = 6.8), anextremely low leakage current of 8.1 × 10−8 A cm−2 at 2 MV cm−1,

Significance

Solution-processing metal oxide (MO) films at low temperatureenables their integration in mechanically flexible and inex-pensive substrates for unconventional optoelectronics. How-ever, temperature reduction has not been accompanied by acorresponding reduction in film-processing time, which remainsfar greater than acceptable for efficient/continuous additivemanufacturing as in typical fabrication lines. Here, we report ahighly efficient cofuel-assisted combustion process, which in-volves introducing both a fluorinated fuel and a preannealingstep, that achieves ultrafast reaction and metal–oxygen–metal(M-O-M) lattice condensation within 10–60 s for several MOsemiconductors and aluminum oxide dielectric. The resultingMO transistors exhibit high carrier mobility, excellent bias sta-bility, and good flexibility.

Author contributions: B.W., L.Z., R.D.S., T.J.M., and A.F. designed research; B.W., P.G.,X.Y., A.S., M.J.L., S.L., and Z.C. performed research; B.W., W.H., X.Z., G.W., and M.J.B.analyzed data; and B.W., T.J.M., and A.F. wrote the paper.

Reviewers: R.G.G., Harvard University; H.K., Max Planck Institute for Solid State Research;and T.N.N., University of California, San Diego.

The authors declare no conflict of interest.

Published under the PNAS license.1To whom correspondence may be addressed. Email: t-marks@northwestern.edu ora-facchetti@northwestern.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1901492116/-/DCSupplemental.

Published online April 19, 2019.

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and a high breakdown field (BF) of 7.2 MV cm−1. The field-effectmobilities of these CBC IGZO-Al2O3 TFTs (300 °C/60 s) are ashigh as 25.2 cm2 V−1 s−1 with negligible threshold voltage de-terioration in a 4,000-s bias stress test. Finally, highly flexiblesolution-processed CBC IGZO TFTs with a top-gated polymerdielectric on polyimide (PI) films are reported.

ResultsMO-Precursor Powder Analysis and Thin-Film Growth. To probe theimportance of solvent removal from the MO-precursor powders/films before combustion, and that of the cofuel, we first in-vestigated wet and dry IGZO powders as well as wet, dry, andcombustion-derived IGZO films prepared with AcAcH and/orFAcAcH (see Fig. 1 and SI Appendix for details). The IGZOpowder/film precursor solutions were prepared by 14 h of aging amixture of In/Ga/Zn nitrates (1.4 mol, 1:0.1:0.3 atomic ratio) asthe metal source/oxidizer, AcAcH as the fuel (1.4 mol), and

FAcAcH as the cofuel (0.0 or 0.32 mol) in 2-methoxyethanol (2-ME) (SI Appendix, Fig. S1 and Table S1). Proton NMR (1H NMR)spectroscopy (SI Appendix, Figs. S2–S9) indicates that duringthe IGZO solution aging, the AcAcˉ coordinates to the metalions, while less basic FAcAcˉ does not but nevertheless influ-ences the metal–AcAc coordination (11). For powder prepara-tion, the precursor solutions were dried in a vacuum for 5 and∼30 h at 25 °C to obtain “wet” and “dry” samples, respectively(Fig. 1B). For film growth (Fig. 1C), the same solutions werespin-coated to afford wet films, while “preannealing” at 120 °C/60 s yielded dry MO-precursor films. Subsequent combustion ofthe wet or dry films by annealing at 200–300 °C for 10 s to 20 minafforded the corresponding MO films. Fig 1 B and C shows theprocess details and nomenclature.Simultaneous thermogravimetry and differential scanning

calorimetry (TGA/DSC) measurements were next used to in-vestigate residual solvent and cofuel effects on the MO thermal

Fig. 1. Chemical structures and synthetic pathways to MO films. (A) Chemical structures of acetylacetone and 1,1,1-trifluoroacetylacetone. (B) Syntheticpathway for wet and dry MO-precursor powders. (C) Film-fabrication processes with the schematic evolution of MO-precursor coordination chemistry andfilm thickness. TFT structures and materials used are also shown. Combustion synthesis (CS), fuel-assisted CS (FA-CS), preannealed CS (P-CS), and preannealedFA-CS (P-FA-CS) processes are done by spin-coating. BC/CS, blade coating/combustion synthesis; BC/FA-CS, blade coating/fuel-assisted combustion synthesis;CBC, combustion blade coating.

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and compositional evolution during combustion. TGA/DSC data(SI Appendix, Fig. S10) show that the combustion ignition tem-perature (Tc) for dry combustion synthesis (CS) IGZO and fuel-assisted CS (FA-CS) IGZO powders were 149.5 and 144.7 °C,respectively, versus 147.9 and 137.9 °C for the corresponding wetsamples (SI Appendix, Table S2). The combustion heats (ΔHexo)/residual weights (RW) for the wet CS IGZO and FA-CS IGZOpowder samples were 366.0 kJ g−1/33.4% and 429.6 kJ g−1

/25.0%, respectively, compared with 640.6 kJ g−1/42.2% and662.8 kJ g−1/37.1% for the corresponding dry powders. Theseresults indicate that residual solvent significantly depresses theheat generated during combustion, a key factor in film densifi-cation and ultimate film charge-transport capacity. Furthermore,cofuel addition significantly increases the heat generated andaccelerates precursor decomposition to volatile components.Independent of the precursor composition and cofuel present,

the wet films resulting from a single spin-coating step are ∼20 nmthick, as determined by ellipsometry. These films contain sig-nificant quantities of solvent, metal acetylacetonate complexes,

and other organic species, as determined by 1H NMR spectros-copy (SI Appendix, Figs. S2–S9). Preannealing the wet films at120 °C for 60 s removes solvent and significantly densifies them,with the film thickness reduced to ∼8 nm. Note that this stepdoes not result in combustion or formation of significant metal–oxygen–metal (M-O-M) lattice condensation, as assessed byTGA/DSC and X-ray photoelectron spectroscopy (XPS) (videinfra). Subsequent CS of both wet and dry (prebaked) films at200–300 °C for 10 s to 20 min then results in ∼3-nm-thick films.For MO film analysis and TFT fabrication, the spin-coating/annealing process was repeated four times (vide infra).

Oxide Thin-Film Characterization. XPS was next applied to char-acterize the oxygen-bonding states and their evolution with filmprocessing and fuel/cofuel composition (Fig. 2A). The O(1s)ionization can be deconvoluted into three peaks at 529.9, 531.2,and 532.3 eV, assigned to oxygen anions in an M-O-M lattice, M-OR bonds, and M-OH species, respectively (39). Data for allsamples are summarized in SI Appendix, Table S3. From the XPSdata, it is clear that both the wet and dry IGZO films (Fig. 2A,

Fig. 2. Structural/morphological properties and device performance of IGZO films/TFTs. (A) O(1s) X-ray photoelectron spectra and their deconvolutions in spin-coated IGZO films subjected to the indicated processing stages: As-spun (wet), preannealed (dry), CS, P-CS, and P-FA-CS. (B and C) Cross-section TEM images of four-layer spin-coated IGZO films processed by CS (B) and P-CS (C) methods. (D) Transfer plots of 300 °C-processed IGZO TFTs on 300-nm SiO2/Si substrates processed byCS, P-CS, and P-FA-CS methods. (E) 3D plot of mobility-total annealing time-deposition process for IGZO TFTs on 300-nm SiO2/Si substrates.

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Left) do not contain significant densities of M-O-M species;however, the latter films have higher M-OR content (40.5% vs.30.2%), consistent with more extensive acetylacetonate coordinationand film densification on solvent removal. These data also confirmthat negligible combustion occurs during the spin-coating/preannealing steps. Remarkably, comparing the XPS data for thewet film without cofuel heated at 300 °C for 20 min, which corre-sponds to the conventional combustion process for CS IGZO films(11, 15), to that obtained by annealing the dry film for 10 s in thepresent process to yield preannealing CS (P-CS) IGZO films, re-veals that substantial/comparable M-O-M lattice formation occurs(74.5% vs. 75.0%, respectively) despite a >100× reduction inannealing time. Furthermore, using the fluorinated cofuel withpreannealing (samples P-FA-CS IGZO) additionally densifies theIGZO films to a 75.9% M-O-M content. Top-view and cross-section transmission electron microscopy (TEM) of the IGZOfilms (Fig. 2 B and C and SI Appendix, Figs. S11–S12) fabricated byCS and P-CS further confirm the superior quality of the latter,which are denser and more uniform, in contrast to the multilayermorphology of the conventional CS sample. Finally, grazing in-cidence X-ray diffraction (GIXRD) film scans (SI Appendix, Fig.S13) indicate that the CS, P-CS, and P-FA-CS derived IGZO(300 °C/60 s) films remain amorphous.

IGZO TFTs. Next, the performance of IGZO TFTs on 300-nmSiO2/Si dielectric/substrates [Al source-drain contacts; width/length (W/L) = 1,000/100 μm] fabricated using the differentprocessing methods (CS, FA-CS, P-CS, P-FA-CS) and combus-tion temperatures/times are compared and contrasted. Fig. 2Dand SI Appendix, Fig. S14 show representative TFT transfer andoutput characteristics. The device statistics are summarized inFig. 2E and SI Appendix, Tables S1 and S4. The P-CS and P-FA-CS IGZO TFTs begin to function at 200 °C/20 min processingwith average mobilities (μ) of 0.08 and 0.1 cm2 V−1 s−1, re-spectively, while all TFTs fabricated by the other methods areinactive. After 225 °C/20-min annealing, the μ of the IGZO TFTsincreases from 0.17 cm2 V−1 s−1 (CS) to 0.78/0.92 cm2 V−1 s−1

(P-CS/P-FA-CS). These values are unprecedented for solution-processed IGZO TFTs at such a low temperature. As shown inTable 1, when the annealing temperature is further increased to250 °C and then to 300 °C, the mobility increases dramatically evenfor the rather short annealing times (10–60 s). For example, themobilities of the P-CS and P-FA-CS IGZO TFTs annealed at300 °C for 10 s are 1.60 and 2.05 cm2 V−1 s−1, compared with only0.19 cm2 V−1 s−1 for CS-derived IGZO TFTs. Annealing at thesame temperature for 60 s further increases μ of the P-FA-CS de-vices to 5.44 cm2 V−1 s−1, even larger than that of CS IGZO filmannealed for 20 min (4.2 cm2 V−1 s−1) and >10× greater than that ofCS IGZO annealed for 60 s (0.41 cm2 V−1 s−1). Note that the

control IGZO TFTs fabricated by sol–gel exhibit far lower per-formance, demonstrating that the present strategy is only appli-cable to combustion-derived formulations (SI Appendix, Fig.S14 and Table S5). Having the mobility data for IGZO TFTsfabricated at different temperatures and annealing times showsthat there is a tradeoff between the annealing temperature andannealing time to achieve a specific mobility. For instance, SIAppendix, Fig. S14I shows the P-FA-CS IGZO TFTs with elec-tron mobilities of 1 cm2 V−1 s−1 fabricated at different temper-atures and annealing times for each layer, which follows anexponential relationship.

Scope: Other MO Thin-Film Materials and Transistor-Structure PerformanceCorrelations. Considering the excellent results achieved for the P-FA-CS IGZO films discussed above, the scope of this study was expandedto other important MO semiconductors such as binary In2O3 andternary IZO films. Comparative XPS analysis (Fig. 3A and SI Ap-pendix, Fig. S15) indicates that, in all cases, using the P-CS and P-FA-CS processing enhances M-O-M content by ∼2–3%. For instance,300 °C/10 s-processed In2O3 and IZOM-O-M content increases goingfrom CS (71.5 and 71.9%, respectively) to P-CS (74.1 and 73.4%,respectively) to P-FA-CS (74.5 and 74.0%, respectively) processedfilms. Interestingly, although the carrier mobilities of the In2O3 andIZO TFTs are greater than those of corresponding IGZO TFTs (videinfra), the M-O-M ratios are smaller than those of the IGZO films,reflecting strong Ga oxygen “getter” effects (37). Next, the perfor-mance of In2O3 and IZO TFTs are compared in Fig. 3B, Tables 1,and SI Appendix, Figs. S16–S17 and Table S4. No TFT response isobserved for 200 °C/20 min-processed CS In2O3 TFTs, while μ of the225 °C/20 min-processed CS-In2O3 TFTs is 0.21 cm2 V−1 s−1. Incontrast, P-CS In2O3 TFTs begin to function on annealing at 200 °C/20 min with μ = 0.075 cm2 V−1 s−1, increasing to ∼3 cm2 V−1 s−1 at225 °C/20 min. On annealing at 250 °C for 60 s, μ of the In2O3 TFTsincreases from CS (0.50 cm2 V−1 s−1) to P-CS (2.94 cm2 V−1 s−1). Theμ of the P-CS In2O3 TFTs further increases to 9.60 cm2 V−1 s−1 after250 °C/20-min annealing, vs. 2.06 cm2 V−1 s−1 for CS In2O3 TFTs.When the In2O3 films are annealed at 300 °C for only 10 s, highmobilities of 5.20 cm2 V−1 s−1 (P-CS) and 6.52 cm2 V−1 s−1 (P-FA-CS)are achieved, vs. 1.25 cm2 V−1 s−1 for CS In2O3 TFTs. For IZO TFTs,CS films and P-CS films annealed at 225 °C/20 min exhibit μ =0.04 and 0.5 cm2 V−1 s−1, respectively. When processed at 250 °C/60 s,the P-CS films exhibit a μ = 2.34 cm2 V−1 s−1, 8.5× higher than that ofthe CS-IZO TFTs (0.33 cm2 V−1 s−1). For 300 °C/10-s annealing, theCS-IZO TFTs exhibit a μ = 0.31 cm2 V−1 s−1, considerably lower thanthose of P-CS, 2.37 cm2 V−1 s−1 and P-FA-CS, 3.78 cm2 V−1 s−1

processed devices.To better understand the origin of the impressive P-FA-CS

IGZO TFT performance, X-ray reflectivity (XRR) measure-ments were performed on the IGZO channel layer (SI Appendix,

Table 1. Average electron mobilities of IGZO, In2O3, and IZO TFTs on 300 nm SiO2/Si substrates processed by the indicated methods

MethodPerformanceparameters

IGZO In2O3 IZO

225 °C/20 min 250 °C/60 s 300 °C/10 s 300 °C/60 s 225 °C/20 min 250 °C/60 s 300 °C/10 s 250 °C/60 s 300 °C/10 s

CS μ (cm2 V−1 s−1) 0.17 ± 0.08 0.10 ± 0.05 0.19 ± 0.10 0.41 ± 0.10 0.21 ± 0.13 0.50 ± 0.06 1.25 ± 0.21 0.33 ± 0.05 0.31 ± 0.05VT (V) 27.96 ± 10.20 31.37 ± 16.16 45.24 ± 10.16 10.50 ± 1.89 28.85 ± 5.32 −18.5 ± 4.68 −2.27 ± 1.82 −20.75 ± 3.22 −13.25 ± 2.70Ion/Ioff 106 105 105 106 104 103 103 105 105

FA-CS μ (cm2 V−1 s−1) 0.19 ± 0.11 0.08 ± 0.03 0.35 ± 0.14 1.62 ± 0.21 0.24 ± 0.13 0.32 ± 0.08 2.16 ± 0.33 0.28 ± 0.07 0.83 ± 0.24VT (V) 26.70 ± 10.13 33.24 ± 17.25 21.17 ± 7.63 3.34 ± 1.75 26.70 ± 5.08 9.23 ± 2.85 −12.08 ± 2.85 −2.29 ± 1.72 12.4 ± 1.68Ion/Ioff 106 105 106 106 105 104 103 106 105

P-CS μ (cm2 V−1 s−1) 0.78 ± 0.20 0.29 ± 0.08 1.60 ± 0.22 3.67 ± 0.72 2.96 ± 0.50 2.94 ± 0.45 5.20 ± 0.62 2.32 ± 0.38 2.37 ± 0.45VT (V) 21.59 ± 6.54 29.15 ± 8.86 11.12 ± 4.74 4.28 ± 1.02 −19.90 ± 4.56 10.49 ± 3.41 −23.98 ± 4.55 −3.13 ± 2.50 12.01 ± 1.24Ion/Ioff 107 106 106 107 104 104 104 106 105

P-FA-CS μ (cm2 V−1 s−1) 0.92 ± 0.25 0.20 ± 0.09 2.05 ± 0.52 5.44 ± 1.22 3.82 ± 0.77 2.28 ± 0.33 6.52 ± 0.74 2.11 ± 0.32 3.78 ± 0.59VT (V) 16.49 ± 3.56 20.8 ± 5.49 7.56 ± 2.43 3.61 ± 0.61 −24.87 ± 3.85 −26.97 ± 6.30 −45.86 ± 8.85 −22.15 ± 2.23 9.20 ± 2.58Ion/Ioff 107 106 106 107 103 104 103 106 105

The results were measured from 3 batches having more than 30 devices.

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Fig. S18) which, recall, is fabricated by four spin/preannealing/annealing steps. These measurements assess the film electrondensity (ρe) profiles, film thickness, and average electron density(ρavg) profiles (Fig. 3 C and D). The conventional CS IGZO filmwith 300 °C/20-min annealing of each layer yields ρavg = 1.5 e Å−3,similar to previous reports (10, 40). While the ρavg decreases to agreater extent for 300 °C/60 s-annealed CS IGZO films (1.42 eÅ−3), it increases to 1.49 and 1.53 e Å−3 for FA-CS and P-FA-CSprocessed IGZO films. This result is unprecedented for 60 s-annealed films and is comparable to that of sputtered films(10). Interestingly, the XRR-derived film thicknesses also de-pend on the growth method. The 60 s-annealed P-FA-CS IGZOfilms exhibit a thickness (10.5 nm) that is slightly less than the10.7 nm of the conventional CS IGZO films with 20-min annealing,supporting the XPS and TFT results that the preannealing step andcofuel-assisted combustion significantly enhance film densificationand charge-transport capacity.

Extension to Oxide Dielectric Films. The morphological and mi-crostructural requirements for TFT dielectric layers are evenmore stringent than for semiconducting layers, to ensure lowleakage currents, high breakdown voltages, high capacitances,and minimal bulk/interface trap densities. Thus, harsh processingconditions (>450 °C/1 h) and/or significant film thicknesses(≥100 nm) are typically required for high-performance solution-processed MO dielectric films (10, 39, 41–43). Here, we showthat the present processing method yields high-quality Al2O3dielectric films for low-voltage MO TFT operation. Thus, theAl2O3 precursors Al(NO3)3 and AcAcH/NH4OH in 2-ME withor without FAcAcH as a cofuel were spin-coated onto n++ Sisubstrates and subsequently preannealed at 120 °C for 60 s, thenimmediately annealed at 350 °C for 60 s. This process was re-peated four times to achieve the desired film thickness. Fig. 3Edetails the insulating properties of the CS, FA-CS, P-CS, and P-FA-CS processed Al2O3 dielectrics (∼20 nm thick), indicating

that the CS Al2O3 film leakage current density (4.1 × 10−6 A cm−1)at 1 MV cm−1 and the BF (1.9 MV cm−1) are inferior to thoseof P-CS Al2O3 (1.1 × 10−7 A cm−1 and 2.8 MV cm−1, respectively)and especially P-FA-CS Al2O3 (7.5 × 10−8 A cm−1 and 3.2 MV cm−1)films. SI Appendix, Fig. S19 provides the frequency- and voltage-dependent capacitance per unit area of the aforementioned dielec-trics, and the corresponding dielectric constants are shown in Fig. 3F.The CS-Al2O3 films exhibit the largest dielectric constant (k ∼ 9.4),likely reflecting impurities and large densities of polar hydroxylgroups (39), as well as a pronounced fall in capacitance forfrequencies >7 × 104 Hz. In contrast, the P-CS and P-FA-CS Al2O3films exhibit frequency-stable capacitances of 317 nF cm−2 for P-CSand 286 nF cm−2 for P-FA-CS up to 4 × 105 and 5 × 105 Hz, re-spectively, and dielectric constants of 7.9 and 7.3, respectively. Thatthe lower dielectric constant of these films is due to lowered impu-rities and fluoride incorporation (AlF3, k ∼ 2.2) is supported by XPSdata, which indicate Al-F features at 685.6 eV (SI Appendix, Fig. S20)(44). From deconvolution, the estimated atomic F:Al ratios for theFA-CS and P-FA-CS Al2O3 films are 1:13.9 and 1:10.8, respectively.Note that no F incorporation is observed by XPS in the FAcAcH-assisted semiconductor MO films, even though the annealing tem-perature is 50–100 °C lower than that for the Al2O3 films (SI Ap-pendix, Fig. S20). This result likely reflects the very strong Al-F bondenergy (675 kJ mol−1) versus that of the other metal fluorides: In-F,516 kJ mol−1; Ga-F, 584 kJ mol−1; and Zn-F, 364 kJ mol−1 (45).Regarding the oxygen environment (SI Appendix, Fig. S21), thedeconvoluted XPS O(1s) spectra assign Al-O-Al at 531.1 ± 0.1 eVand O-associated hydroxyl groups at 532.3 ± 0.1 eV (46, 47). TheAl-O-Al content increases from 59.5% (CS) to 77.5% (FA-CS) to80.0% (P-CS) to 82.6% (P-FA-CS), again demonstrating that thecofuel/prebaking approach accelerates MO dielectric oxidelattice densification.

Printed MO Films and Devices. Blade coating (BC) was next appliedto MO film growth using the present processing methodology as

Fig. 3. Structural and electrical characterization of combustion-processed MO films. (A) Ratio of O(1s) X-ray photoelectron spectroscopic M-O-M peak area tototal peak area for In2O3, IZO, and IGZO films subjected to the indicated processing. (B) Distribution of saturation mobilities of In2O3 and IZO TFTs fabricatedby the indicated deposition processes. (C) Fitted XRR electron-density profiles of the indicated IGZO films. (D) Film thickness and average electron density ofIGZO films processed by the indicated methods. (E) Leakage current-density versus electrical field (J-E) of Al2O3 dielectrics fabricated by the indicatedmethods. (F) Calculated dielectric constant (103 Hz) and BF for 350 °C-annealed CS, FA-CS, P-CS, and P-FA-CS Al2O3 dielectric films.

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CBC. CBC has advantages over spin-coating, including concur-rent preannealing during film deposition, minimal precursorsolution wastage (reduced by ∼10×), and scalability (48). Criticaldeposition parameters including coating speed (5–20 mm s−1),gap height (100/300 μm), and substrate temperature (roomtemperature/70 °C) were optimized to balance metal ion-AcAcˉ/FAcAcˉ coordination, solvent evaporation, solution flow ther-modynamics, and desired final film thickness (SI Appendix, Fig.S22 and Table S6). CBC of the IGZO semiconductor layer on Si/SiO2 substrates was first optimized, yielding the parameters:10 mm s−1 coating speed, 100-μm gap height, 70 °C substratetemperature, and four successive coatings for ∼6-nm-thick films.Fig. 4A, Table 2, and SI Appendix, Fig. S23 summarize devicemetrics for the IGZO films processed by different methods.Thus, while the μ of spin-coated CS IGZO TFTs annealed at250 °C/60 s is only 0.10 cm2 V−1 s−1 (vide supra), the BC/CS andBC/FA-CS devices exhibit mobilities of 0.26 and 0.96 cm2 V−1 s−1,respectively. By raising the temperature to 300 °C, the BC/FA-CS

IGZO TFT mobilities reach 3.74 cm2 V−1 s−1 (10-s annealing)and 7.66 cm2 V−1 s−1 (60-s annealing), which are significantlylarger than obtainable by spin-coating the IGZO layer (2.05 and5.44 cm2 V−1 s−1, respectively). This result may reflect more effi-cient preannealing, which occurs immediately after dispensing theIGZO-precursor solution.Fig. 4C and SI Appendix, Table S6 summarize IGZO TFT

performance on 300-nm SiO2/Si substrates (source/drain = Al;W/L = 1,000/100 μm) versus that of the recent literature re-port, clearly demonstrating the technological attractions ofBC/FA-CS methodology. Positive gate-bias stress (PTBS)measurements were also carried out on the 60 s-annealed P-FA-CS and BC/FA-CS IGZO TFTs (Fig. 4D and SI Appendix,Fig. S24). These devices were subjected to a constant VGS of+20 V for up to 2,000 s (400-s intervals) in ambient, withoutencapsulation or back channel protection. The P-FA-CSIGZO devices exhibit ΔVT of ∼+9 V after 2,000 s, implyinga large density of acceptor-like electron-trapping states (11, 17, 49).

Table 2. Performance metrics of IGZO TFTs on 300 nm SiO2/Si substrates processed by variouscombustion methods

Processing method Performance parameters 250 °C/60 s 300 °C/10 s 300 °C/60 s

BC/CS μ (cm2 V−1 s−1) 0.26 ± 0.05 0.62 ± 0.05 4.53 ± 0.67VT (V) 24.1 ± 4.79 4.55 ± 1.59 9.78 ± 0.99Ion/Ioff ∼106 ∼106 ∼106

BC/FA-CS μ (cm2 V−1 s−1) 0.96 ± 0.12 3.74 ± 0.23 7.66 ± 0.60VT (V) 20.1 ± 6.71 22.54 ± 0.97 15.82 ± 0.42Ion/Ioff ∼107 ∼107 ∼106

Fig. 4. Comparison of IGZO TFT performance and stability. (A) IGZO TFT mobilities on 300-nm SiO2/Si with IGZO films processed by the indicated methods. (B)J-E curves of 350 °C/60 s Al2O3 dielectrics processed by the indicated methods. (C) Comparison with literature data for solution-processed IGZO TFT mobilitieson 300-nm SiO2/Si vs. annealing time; references are provided in SI Appendix, Table S7. DUV, deep UV; HPA, high-pressure annealing. (D) Threshold voltageshift, ΔVT, of the indicated IGZO TFTs under PTBS (VGS = +20 V). Note “BC” indicates “blade-coating,” while all other devices were fabricated by spin-coatingunless otherwise noted.

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However, no obvious shift is observed for BC/FA-CS IGZO TFTseven after a gate-bias stress time of 2,000 s with ΔVT ∼+2 V.Next, BC Al2O3 dielectric films and BC IGZO/Al2O3 TFTs

were fabricated to further demonstrate the technological ad-vances provided here. BC/FA-CS Al2O3 fabrication process/pa-rameters are identical to those of IGZO films, except that thegap height is 300 μm. Fig. 4B shows the variations in leakagecurrent densities for 350 °C/60 s-annealed Al2O3 films at dif-ferent electric field strengths, and SI Appendix, Fig. S25 showsthe electrical and dielectric properties. Remarkably, the BC/

FA-CS Al2O3 films, ∼15 nm thick after four sequential coat-ings, exhibit excellent dielectric characteristics, again with theperformance of the processed films (J = 8.1 × 10−8 A cm−2 at 2.0MV; BF = 7.2 MV cm−1) outperforming the BC/CS ones (J =4.3 × 10−7 A cm−2 and BF = 6.5 MV cm−1). Both films exhibitfrequency-stable capacitance up to 5 × 105 Hz, and their di-electric constants are 6.8 and 6.6, respectively. Fig. 5 A and B andMovie S1 show the BC/FA-CS IGZO/Al2O3 device fabricationprocess on a 4-inch-diameter Si substrate and the optical imageof a low-voltage IGZO-Al2O3 TFT with patterned IGZO and S/D

Fig. 5. Scalable CBC fabrication of high-performance IGZO TFTs on both high-k Al2O3 dielectrics and low-k polymer dielectrics. (A) Photo of the BC process;Inset shows blade-coated devices on a 4-inch Si wafer. (Scale bar, 1 cm.) (B) Optical image of a patterned low-voltage IGZO TFT. (Scale bar, 100 μm.) (C)Transfer (Left) and output (Right) characteristics of low-voltage CBC IGZO TFT on high-k Al2O3 dielectrics. The annealing time for both IGZO and Al2O3 layers is60 s. (D and E) Distribution of saturation mobilities (D) and ΔVT vs. time (E) of CBC/FA-CS-derived IGZO TFTs on P-FA-CS Al2O3/Si substrates (VGS = +1.0 V). (Fand G) Transfer curves (F) and mobility variation (G) of top-gate flexible CBC/IGZO TFTs on a PI substrate as a function of bending radius. (Inset) Photo ofdevice under bending. (Scale bar, 1 cm.) (H) Mobility and threshold voltage stability for bending test cycles with a radius of 6 mm.

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layers. These devices exhibit excellent TFT characteristics atlow voltage (<2 V), negligible I-V hysteresis (Fig. 5C), andfield-effect mobilities centered at ∼18.0 cm2 V−1 s−1 (maximumvalue = 25.2 cm2 V−1 s−1) with an SD of ±2.5 cm2 V−1 s−1 over100 devices (Fig. 5D and SI Appendix, Fig. S26). These TFTsalso exhibit excellent current on/off modulation, subthresholdswing, and threshold voltage (VT) values of >4 × 104, 0.15 ±0.03 V per decade, and 0.55 ± 0.27 V, respectively, as well as anegligible ΔVT of +0.06 V after 4,000 s stress time (Fig. 5E andSI Appendix, Fig. S27).Finally, top-gate, top-contact IGZO TFTs were fabricated on

a ∼25-cm2, 35-μm-thick PI substrate with a polymer dielectric(Activink D3300; Flexterra Corporation) and a CBC IGZO layerannealed for only 60 s at 300 °C (Fig. 5F). Fig. 5G shows thetransfer plots of these devices at various bending radii about acurvature parallel to the channel length. These devices exhibitonly a slight turn-on voltage (Von)/VT shift from +0.1/20.1 to 0.0/21.9 V as the bending radius is decreased from ∞ to 3 mm, alongwith a negligible mobility decline from 0.45 to 0.43 cm2 V−1 s−1.The lower mobility of these devices compared with that achievedwith SiO2 reflects the difference in device architecture and thelower dielectric constant of polymer dielectric (50). Importantly,these TFTs exhibit excellent mechanical flexibility with negligibleperformance loss on bending at 6 mm for 1,000 times (Fig. 5Hand SI Appendix, Fig. S28).

ConclusionsLow-temperature, wafer-scale fabrication of MO films via ul-trafast combustion BC is reported. Using a fluorinated cofuel topromote metal coordination and impurity removal, combinedwith a precursor preannealing step to eliminate solvent, greatlyenhances the efficiency of CS, which can be reduced to times asshort as 10 s at temperatures as low as 200–300 °C. Thus, func-tional (μ > 1 cm2 V−1 s−1) MO TFTs are obtained at tempera-tures/times as low as 250 °C/60 s and/or 300 °C/10 s for In2O3,IZO, and IGZO. This approach is also extendable to a MO di-electric such as Al2O3. Additionally, ultrafast annealing of blade-coated IGZO and Al2O3 films yields excellent semiconductorand insulating characteristics and produces low-voltage wafer-scale MO TFTs with highly stable characteristics and ΔVT <0.06 V under 4,000 s bias stress. Using a polymeric top-gateddielectric on PI film then yields highly flexible, solution-processed CBC/IGZO TFTs. We anticipate that these resultswill help advance low-cost, high-performance, roll-to-roll com-patible large-area printed electronics as well as other technolo-gies where high-quality MO films are essential components.

MethodsPrecursor Preparation and Characterization. All combustion precursor mate-rials were purchased from Sigma-Aldrich and stored in a vacuum desiccator.First, measured quantities of the metal salts were dissolved in 5.0 mL of 2-ME.Thus, 75.2 mg of In(NO3)3 was used for the In2O3 precursor; 60.16 mg of In(NO3)3 and 9.47 mg of Zn(NO3)2 were used for the IZO precursor; 63.36 mg ofIn(NO3)3, 10.74 mg of Zn(NO3)2, and 5.38 mg of Ga(NO3)3 were used for theIGZO precursor; and 93.78 mg of Al(NO3)3 was used for the Al2O3 precursor.Next, 25 μL of acetylacetone and 11.25 μL of 14.5 M NH3(aq) were added insequence to each metal nitrate precursor, except that 75 μL of acetylacetoneand 33.75 μL of 14.5 M NH3(aq) were added to the In(NO3)3 precursor. Thesolutions were then stirred overnight (∼14 h) before film fabrication. Forfuel-assisted combustion precursors, 1,1,1-trifluoro-2,4-pentanedione (10 wt% to the total weight of metal nitrates) was added to the above precursorsolutions 1 h before spin-coating/BC. Wet and dried precursors wereobtained by vacuum evaporation using Schlenk line for 5 and 30 h, re-spectively. Both DSC and TGA measurements were performed on a SDTQ60 instrument (TA Instruments, Inc.). The experiments were carried out on∼1 mg of wet or dry samples with a heating rate of 10 °C min−1 under a70 mL min−1 N2 flow. Vacuum-dried precursors in anhydrous dimethylsulfoxide-d6 were used for 1H NMR spectroscopic analysis. Spectra wererecorded on a Bruker Avance III 500-MHz spectrometer.

Metal Oxide Thin-Film and Device Fabrication. All of the solutions were filteredthrough 0.2-μm syringe filters before fabrication. n++ silicon wafers with/without 300-nm SiO2 (WRS Materials) used as substrates were solvent-cleaned and then cleaned with an oxygen plasma for 5 min before use.For spin-coating fabrication, the In2O3, IZO, IGZO, and Al2O3 precursors werespin-coated on the substrates (n++ Si, 300 nm SiO2/Si) at 3,500 rpm for 30 s ina controlled atmosphere box [relative humidity (RH) ∼ 20%] and optionallypreannealed at 120 °C for 60 s (RH ∼ 35%). Then, the resulting films wereimmediately placed on a 200–350 °C hotplate and annealed for 10 s to20 min (RH ∼ 35%). This process was repeated four times to obtain thedesired film thickness. For BC film fabrication, first, the substrate (n++ Si or300 nm SiO2/n

++ Si) was placed on the blade-coater (Erichsen Coatmaster510) setup on a surface maintained at a temperature of 25–70 °C. Next, theblade was approached to the substrates maintaining a gap of 100–300 μm.After that, the precursor solution (10 μL for 1 inch × 1 inch square substrate,80 μL for a 4-inch-diameter substrate) was injected at the interface betweenthe substrate and the blade until a meniscus formed. The blade was hori-zontally transported at a constant velocity of 5–20 mm s−1, and the carriedsolution dried at a rate depending on the substrate temperature. Finally, thefilm was annealed at 250–350 °C for 10–60 s. This process was repeated fourtimes to obtain the desired film thickness. Semiconductor layer patterningwas achieved by spin-coating one layer of S1813 photoresist (5,000 rpm for30 s, 1.2 μm) on the surface, which was thermally annealed at 115 °C for1 min and then exposed to UV light (Heidelberg μPG 501 mask writer)for 26 ms. Next, the S1813 film was removed by soaking in MF319 for 30 s,and the films were cleaned with deionized (DI) water and dried with an N2

flow. The exposed oxide films were next removed in aqueous oxalic acid(10% wt/vol) for 30 s and rinsed with DI water. The remaining S1813 film wasthen removed with acetone. Source/drain electrode patterning was achievedby spin-coating/annealing/exposing/rinsing the S1813 photoresist on theIGZO layer. Al source and drain (S/D) electrodes (thickness = 40 nm) werethen deposited by thermal evaporation, followed by soaking in acetone for1 min. For flexible TFTs, PI film with thickness of 35 μmwas used as substrate,after blade-coating the IGZO layer using the above-described procedure, AlS/D was patterned on it, and then polymer solution (Activink D3300) wasspin-coated as dielectric. After UV curing under 1 J cm−2 and 120 °C/10-minannealing, the resulting polymer film (e = 3.3) has a thickness 650 nm and anareal capacitance of 4.5 nF cm−2. The flexible devices were finished byevaporating 50 nm of Au as gate electrodes. The channel width and lengthfor devices on the SiO2/Si substrate were defined as 1,000 and 100 μm, re-spectively, while those for wafer-scale IGZO-Al2O3 devices and flexiblepolymer-gated IGZO devices were 100 and 10 μm, respectively.

MO Thin-Film Characterization. The surface characteristics of the resultingfilms were measured with a Bruker Dimensional Icon AFM system in thetapping mode. XPS was performed on a Thermo Scientific ESCALAB 250 Xispectrometer, and the sample surfaces were etched before testing. O(1s)spectra were fitted using three Gaussian−Lorentzian convolution functionsafter subtracting a linear baseline. GIXRD and XRR were conducted with aRigaku Smartlab workstation using CuKα (1.54-Å) radiation. Top-view TEMand cross-section TEM (CS-TEM) were collected using a JEOL JEM-ARM300Ftransmission electron microscope. The samples for top-view TEM imageswere obtained by spin-coating/annealing MO films on single-crystal NaCl,which was then dissolved in water and transferred onto Cu grids. The CS-TEM samples were prepared directly from actual TFT devices with standardfocused ion-beam milling techniques (FEI Helios NanoLab 600). A ∼2-μm-thick platinum layer was deposited before the ion milling to protect sam-ples from ion-beam damage. Electrical characterization of semiconductorswas performed using MO TFT devices. Electrical characterization of dielec-trics was performed using metal–insulator–metal devices. n++ Si wafers with1.5-nm natural SiO2 layer were used as substrates, and then MO dielectricswere deposited by spin-coating or blade-coating on them. After that, topelectrodes (Al, 40 nm) were thermally evaporated with size of 200 μm ×200 μm. Both the dielectrics and MO TFT measurements were performedunder ambient conditions using an Agilent B1500A semiconductor param-eter analyzer. The carrier mobility (μ) was evaluated in the saturation region.The areal capacitance for 300 nm SiO2/Si is taken as 11 nF cm−2 here. Theareal capacitances of high-k Al2O3 and low-k polymer dielectrics (Cdiel)were calculated based on the equation 1/Ci = 1/Cdiel+1/CSiO2, assuming thatthe areal capacitance of native silicon oxide (1.5 nm) is 2,303.6 nF cm−2 (39).The Ci is the measured areal capacitance for dielectric films on 1.5 nmSiO2/Si wafer.

ACKNOWLEDGMENTS. We thank the US–Israel Binational Science Foundation(Grant AGMT-2012250///02), the Northwestern University Materials Research

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Science and Engineering Center (MRSEC) (NSF Grant DMR-1720139), the AirForce Office of Scientific Research (Grant FA9550-18-1-0320), and FlexterraCorporation for support of this research. A.F. thanks the Shenzhen PeacockPlan Project (Grant KQTD20140630110339343) for support. This work wasperformed, in part, at the Center for Nanoscale Materials, a US Departmentof Energy Office of Science User Facility and supported by the US Depart-ment of Energy, Office of Science, under Contract DE-AC02-06CH11357. Thiswork made use of the J. B. Cohen X-Ray Diffraction Facility, Northwestern

University Micro/Nano Fabrication Facility, Electron Probe InstrumentationCenter Facility, Keck Interdisciplinary Surface Science Facility, and ScannedProbe Imaging and Development Facility of the Northwestern UniversityAtomic and Nanoscale Characterization Experimental Center, which receivedsupport from the Soft and Hybrid Nanotechnology Experimental Resource(NSF Grant NNCI-1542205); the MRSEC Program (NSF Grant DMR-1720139);the International Institute for Nanotechnology (IIN); the Keck Foundation;and the State of Illinois, through the IIN.

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