Journal ofMaterials Chemistry C

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Division of Advanced Materials, Korea Resea

Gajeong-ro, Yuseong-gu, Daejeon 305-600, R

Fax: +82 42-860-7200; Tel: +82 42-760-7260

† Electronic supplementary informationSEM images, XRD patterns, thermalcharacteristics, Fig. S1–S15. See DOI: 10.1

‡ Young Hun Kang and Sunho Jeong con

Cite this: J. Mater. Chem. C, 2014, 2,4247

Received 21st January 2014Accepted 17th March 2014

DOI: 10.1039/c4tc00139g

www.rsc.org/MaterialsC

This journal is © The Royal Society of C

Two-component solution processing of oxidesemiconductors for thin-film transistors viaself-combustion reaction†

Young Hun Kang,‡ Sunho Jeong,‡ Jung Min Ko, Ji-Yoon Lee, Youngmin Choi,Changjin Lee* and Song Yun Cho*

Indium zinc oxide (IZO) thin films were fabricated via self-combustion of In and Zn salts coordinated with

fuel and oxidizer ligands. The intense heat generated from the exothermic reaction compensated for the

energy required for oxide formation and reduced the temperature required to anneal the oxide films.

Thermal analysis of the fuel and oxidizer precursors confirmed the generation of exothermic heat at a

relatively low annealing temperature. With the aid of the internal energy that evolved as heat from the

combustion reaction, the formation of the metal–oxygen–metal lattice and the removal of organic

ligands could be easily accomplished with lower amounts of externally supplied energy. IZO thin-film

transistors (TFTs), obtained from this combustive In–Zn pair at a low annealing temperature of 350 �C,showed a significantly enhanced field-effect mobility of 13.8 cm2 V�1 s�1 and a high on/off current ratio

of 1.06 � 108. Inkjet printing of the combustive precursors yielded TFTs with a high field-effect mobility

of 5.3 cm2 V�1 s�1 and an on/off ratio of 106. The high performance, good device uniformity, and high

yield of TFTs fabricated by our self-combustion method demonstrate the potential of the proposed

system to facilitate the processing of flexible printed electronics.

1. Introduction

Amorphous metal oxide thin-lm transistors (TFTs), widelyused in electronics, have several advantages such as high elec-trical performance and good device uniformity. Their disad-vantages include high cost of processing equipment andfacilities, which is typical of vacuum-based techniques such aspulsed laser deposition, atomic layer deposition, chemicalvapor deposition, and radio-frequency magnetron sputtering.1–4

Solution-phase techniques, on the other hand, feature easycontrol of material composition, large-scale deposition, highthroughput, and low cost.5–9 Challenges in developing solution-processed metal oxide TFTs include nding easily processablematerials, and effecting the hydrolysis, condensation, densi-cation, and complete decomposition of organic ligands at lowannealing temperatures. In the absence of high-temperatureannealing, organic ligands (e.g., nitrides, halides, and acetates)can remain in the metal oxide thin lm, and the conversion ofthe oxide precursor can be incomplete, which might result in

rch Institute of Chemical Technology, 141

epublic of Korea. E-mail: scho@krict.re.kr;

(ESI) available: XPS spectra, AFM andanalysis, and transfer and output039/c4tc00139g

tributed equally to this work.

hemistry 2014

lower carrier mobility and adversely affect the TFT subthresholdslope as well as the on/off current and switching voltage.

Recently, new approaches have been investigated to producelow-temperature, solution-processed metal oxide TFTs fromindium oxide, indium zinc oxide (IZO), indium gallium zincoxide (IGZO), yttrium indium oxide (YIO), and indium zinc tinoxide (IZTO).10–15 Chang and co-workers reported a highmobility of 11.8 cm2 V�1 s�1 in indium oxide TFTs annealed at250 �C via an O2/O3 atmospheric process.16 Sirringhaus and co-workers accomplished a mobility of�10 cm2 V�1 s�1 in IZO andIGZO TFTs at 275 �C using moisture-sensitive alkoxide precur-sors; the lms had to be prepared in a glove box for reproduc-ibility and annealed under a humid atmosphere to activate themetal oxide conversion reaction within the precursor lms.17

Park and co-workers fabricated IZO and IGZO TFTs with amobility of�10 cm2 V�1 s�1 on aluminum oxide gate insulators,via photochemical reactions activated by deep ultraviolet irra-diation in a nitrogen atmosphere.18 However, to achieve gooddevice performance, these approaches required additionalphoto/chemical post-treatment to produce the desired oxidesemiconductor skeletons for TFT applications. In contrast,recently developed chemical methodologies based on combus-tion reactions have facilitated the fabrication of high perfor-mance soluble oxide semiconductors simply by using theconventional annealing process.19,20 The combustion reactionsupplies heat to the precursor lms, dramatically lowering theannealing temperature down to 300 �C while achieving mobility

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as high as 6.5 cm2 V�1 s�1. A drawback to this novel method-ology is the effort involved in nely adjusting the chemicalcomposition of the metal precursors, fuel, oxidizers, and addi-tives to induce an exothermic reaction that generates sufficientheat.21–27

In the present study, a chemical combustion pathway usingself-combustion is described, and the obtained results arecompared with those of previously discussed chemical meth-odologies for the low-temperature synthesis of metal oxideTFTs. The conventional combustion system, which includesaddition of external fuels and oxidizers, is an incredibly sensi-tive process. Even minor deviation from the exact amount offuel, oxidizer, or pH controller required can result in malfunc-tion of the TFTs. The fundamental issue of our study is thepreparation of a stable and reproducible precursor solution fora TFT by means of “self”-combustion chemistry. Our facile,efficient self-combustion process overcomes the shortcomingsof conventional combustive approaches by simply blending twometal precursors that contain coordinated fuel and oxidizerligands. In the fabrication of soluble oxide TFTs for optoelec-tronic devices, the pivotal issue is the preparation of a stable andreproducible precursor solution. We demonstrate the combus-tion of the precursors without additional fuels, oxidizers, orother components. This simple precursor solution results inmore stable and uniform TFT devices owing to its easilycontrollable composition and facile activation, for the produc-tion of high performance soluble oxide semiconductors. Wereport here our initial results regarding the role of theexothermic self-combustion process and the chemistry of metalprecursor (fuel–oxidizer) pairings in enhancing the character-istics of TFTs. The performance of TFTs is discussed as afunction of the precursor composition and the annealingtemperature. Finally, the fabrication of oxide TFTs via inkjetprinting of the self-combustive precursor solution will beaddressed.

2. Results and discussionTerminology

The following terminology is used in this publication todescribe the pair of In–Zn precursors. The precursor solutionconsisting of Zn(C5H7O2)2$xH2O (ZnAC) and In(NO3)3$xH2O(InNO) is denoted as INOZACO. The precursor solution ofZn(C5H7O2)2$xH2O and In(C5H7O2)3$xH2O (InAC) is IACZACO.The precursor solution of Zn(NO3)2$6H2O (ZnNO) andIn(NO3)3$xH2O and that of ZnCl2 (ZnCl) and InCl3 (InCl) aredenoted as INOZNOO and IClZClO, respectively.

The combustion reaction between organic fuels (e.g., acety-lacetone, urea, carbohydrazide, and citric acid) and oxidizers(e.g., nitrate) can lower the effective annealing temperature forthe formation of metal oxide compounds from oxide precur-sors.23,24 When a fuel–oxidizer pair is moderately heated, ahighly intense exothermic reaction occurs. The evolved heat canaccelerate the transformation of metal hydroxides or alkoxidesinto the corresponding metal oxide frameworks at sufficientlylow external temperatures. Previous studies have demonstratedthat the combination of metal nitrates (as both a metal source

4248 | J. Mater. Chem. C, 2014, 2, 4247–4256

and an oxidizer) and acetylacetone (AcAc, as both a chelatingagent and fuel) is most effective, among the various oxidizersand fuels, in generating high performance oxidesemiconductors.23,24

As a representative material system for the chemicalcombustion-based synthesis, we studied indium gallium oxide(IGO), for which amixture of indium nitrate hydrate and galliumnitrate hydrate was used as an oxidizer, and AcAc was used as afuel. First, the electrical characteristics of fuel-free IGO TFTswere investigated in order to determine the most adequatechemical composition of In and Ga (ESI Fig. S1†). Taking intoconsideration all device parameters, including eld-effectmobility, threshold voltage, off current, and subthreshold swing,an IGO composition with an In/Ga molar ratio of 7 : 3 waschosen. The mobility was 1.0 cm2 V�1 s�1 when the IGO channellayer was annealed at 300 �C. As expected, when the AcAc fuelwas incorporated, with NH4OH to control the pH, the deviceperformance dramatically improved. The eld-effect mobilitiesof IGO TFTs depended on the AcAc composition (R1 value of 1 to3) and NH4OH composition (R2 value of 1 to 4), and aresummarized in ESI Table S1.† R1 indicates the molar ratio ofAcAc to the metal salts and R2 indicates the molar ratio of watermolecules in an ammonium hydroxide (NH4OH) solution tometal salts, respectively. When the IGO channel layers wereannealed at 300 �C, their highest average eld-effect mobilitywas 12.5 cm2 V�1 s�1, which is higher than those of the previ-ously reported, combustion-derived In2O3, IZO, ZTO, and YIOTFTs. Interestingly, even with an annealing temperature as lowas 250 �C, the device was still active with a eld-effect mobility of2.45 cm2 V�1 s�1. This indicates that the IGO framework canindeed be formed via combustion chemistry, and the IGOprecursor solutions were of the appropriate composition.However, as shown in Fig. 1(a) and (b), among the 11 syntheticconditions, high performance was only achieved when AcAc andNH4OH were incorporated at R1 ¼ 1 and R2 ¼ 3, respectively. Formost synthetic conditions, the eld-effect mobilities were below5 cm2 V�1 s�1 and 1 cm2 V�1 s�1 for IGO TFTs annealed at 300and 250 �C, respectively. Of greater concern was the bimodaldistribution of eld-effect mobility, shown in Fig. 1(c), observedeven under the optimized synthetic conditions, upon testing 33devices. Fig. 1(d) shows the transfer characteristics of a repre-sentative TFT. This result implies that it is nearly impossible toobtain stable and reproducible TFTs through a multiple-component-based combustion reaction. In this study, the prep-aration of the IGO precursor solution was carried out in an Ar-lled glove box to avoid the undesirable incorporation of water,which can limit the fabrication of large area TFTs.

These statistical data established that the formation of oxideframeworks by chemical combustion is signicantly inuencedby the chemical composition of the precursor mixture andcomplicated chemical reactions of each component, whichshould be well-controlled to ensure reproducibility and highperformance in the resulting devices. To ensure sufficientexothermic heat, the chemical environment around the metalprecursors should be adjusted by adding the appropriateamount of acid or base, in addition to optimizing the relativecomposition of the oxidizer and fuel. In metal-salt-based

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Fig. 1 Electrical performance of IGO TFTs, prepared via conventional chemical combustion with added fuel (AcAc) and NH4OH. The TFTperformance depends on the ratio of AcAc and NH4OH at annealing temperatures of (a) 250 and (b) 300 �C. R1 indicates the molar ratio of AcActo metal salts. R2 indicates the molar ratio of water molecules in an ammonium hydroxide solution to metal salts, respectively. (c) Deviceuniformity of IGO TFTs prepared from the IGO precursor with a composition ratio of AcAc/NH4OH ¼ 1/3 at 300 �C. (d) A representative transfercurve for an IGO TFT.

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chemistries, metal cations are solvated by water molecules,which are supplied in the form of hydrates or by the addition ofexcess water, and then undergo hydrolysis through the loss of aproton by one or more water molecules. Subsequently,condensation takes place, generating metal–oxygen–metal (M–

O–M) bridges for metal-oxide frameworks. When AcAc isinvolved as a fuel, it should be present in the vicinity of metalnitrates in the dried lms to induce vigorous combustionbetween the oxidizer (metal nitrate) and the fuel during thesubsequent annealing. AcAc also strongly tends to form achemical complex with metal cations. Free AcAcs, which do notchemically interact with the metal cations, do not take part inthe combustion reaction, instead act as impurities that have anadverse inuence on device performance. In those reactions,the coordination number of the metal cations and thecompetitive formation of complexes by water molecules andAcAc are critically dependent on the environmental pH aroundthe metal cations. Therefore, for multi-component combustionreactions, it is critical to control the chemical composition ofthe precursor solution: metal nitrate, AcAc, water, and pH-controlling acid or base. In addition, both the acid and base canstimulate the hydrolysis and condensation reaction; thus, thepresence of these reactants makes the chemical reaction muchmore complicated.

This journal is © The Royal Society of Chemistry 2014

In contrast, a strong exothermic reaction can also be initi-ated via combustion of the acetylacetonate complex, triggeredby nitrate ions (Scheme 1). That is, acetylacetonate (fuel) andnitrate (oxidizer) ligands can serve as reactants in thecombustion reaction of the IZO precursors. The exothermicheat generated by self-combustion of a fuel–oxidizer precursorpair can be used to form the corresponding oxides withoutapplying a high annealing temperature. The use of fuel andoxidizer ligands eliminates the need for other chemical addi-tives, which greatly simplies the combustion-derived synthesisand improves its reproducibility. The ligands for eitheroxidizers or fuels are incorporated in stoichiometric ratios withregard to the metal precursors, so that their number is notaltered during the combustion reaction. Water molecules maybe associated with the precursors in the form of hydrates; theamount of associated water molecules can be easily and accu-rately detected by thermogravimetric analysis. Therefore, theonly variable that must be controlled is merely the relativecomposition of metal precursors, the inuence of which can bemonitored systematically through chemical analysis of thecorresponding metal oxide skeletons and in-depth investigationof device performance.

To investigate the spontaneous exothermic reaction of thefuel–oxidizer pair, thermogravimetric analysis (TGA) and

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Scheme 1 A schematic diagram of the metal oxide film synthesis via self-combustion of metal precursors bearing coordinated fuel and oxidizerligands.

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differential thermal analysis (DTA) were conducted on variousIZO precursors that were prepared using different combinationsof In and Zn precursors (Fig. 2). A pair of In and Zn precursorscontaining acetylacetonate (fuel) and nitrate (oxidizer) ligandsyielded lms that clearly exhibited different thermal behaviorcompared to those from other pairs of In and Zn precursors,showing abrupt thermal decomposition around 200 �C. IACZACOand IClZClO, which were not fuel–oxidizer combinations,underwent conventional condensation and showed small andbroad endothermic peaks in the DTA thermogram in thetemperature range of 100–300 �C, possibly due to the decom-position of organic ligands and the hydrolysis of the precursors.In addition, small exothermic peaks involving signicant massloss in the range of 250–500 �C could be explained by dehy-droxylation, polycondensation, and pyrolysis of the organiccomponents.28,29 These results meant that the conventional IZOprecursors required a high temperature of over 400 �C forcomplete conversion into the M–O–M lattice. The nitrate-basedprecursor of INOZNOO, another conventional precursor, showedtwo remarkable exothermic peaks at 120 and 350 �C with thecorresponding mass loss in two steps. The initial �30% massloss at the lower temperature could have originated fromdehydration of water coordinated with the metal nitrate.22 Thesecond mass loss may have resulted from decomposition ofanhydrous nitrate and the conversion of metal hydroxide tometal oxide via condensation. Therefore, nitrate-based precur-sors can be regarded as suitable candidates for TFT materials

Fig. 2 Thermal behavior of IZO precursors prepared from various In and

4250 | J. Mater. Chem. C, 2014, 2, 4247–4256

requiring a moderately high annealing temperature of �400 �Cdespite their conventional processability.

On the other hand, an extremely intense exothermic peak at200 �C was observed in the INOZACO precursor and was attrib-uted to the combustion process, with a correspondingtremendous loss in mass in the TGA thermogram, as shown inFig. 2(a). The very fast and vigorous chemical reaction, indi-cated by a single narrow exothermic peak and abrupt mass loss,may provide supplemental energy to drive the formation of theM–O–M lattice and the removal of the organic ligands. From thethermal behavior of the In–Zn precursors, the decompositionand condensation temperature of the INOZACO precursor wasfound to be lower than those of other precursors, due to the heatsupplied by the exothermic reaction. The precursors are listedin the decreasing order of decomposition temperature asfollows: INOZACO > INOZNOO > IACZACO > IClZClO. Such anexothermic reaction is expected to have a signicant inuenceon the performance of IZO TFTs fabricated with low-tempera-ture annealing.

To verify the chemical and structural composition of the IZOthin lms, X-ray photoelectron spectroscopy (XPS) was per-formed. Fig. 3 and ESI Fig. S2† show the O 1s XPS spectra of theIZO thin lms prepared with different combinations andcomposition ratios of In and Zn precursors. An asymmetric O 1speak was consistently tted to three different componentscentered at 529 � 0.3 eV, 530.6 � 0.3 eV, and 531.7 � 0.3 eV,respectively, using Gaussian and Lorentzian functions. It was

Zn precursors: (a) INOZACO, (b) IACZACO, (c) INOZNOO, and (d) IClZClO.

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Fig. 3 XPS spectra of IZO thin films prepared from various In–Zn precursor pairs after annealing at 350 �C: (a) INOZACO, (b) IACZACO, (c) INOZNOO,and (d) IClZClO.

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clear that the low binding energy at 529 � 0.3 eV was caused bythe O2� ions, which were surrounded by In and Zn atoms. Themiddle binding energy at 530.6 � 0.3 eV was closely related tothe O2� ions in the oxygen-decient region. The high bindingenergy at 531.7 � 0.3 eV was attributed to loosely boundhydroxyl groups (M–OH), which were caused by the absorbedH2O, O2, or OH species on the surface of the IZO thinlms.7,11,30–32 The IZO lms from INOZACO prepared by thecombustion process and those from IACZACO, INOZNOO, andIClZClO prepared by the conventional process showed signi-cant differences in the relative intensity of the three differentcomponents in the O 1s peak. In particular, a large amount ofM–O–M oxygen was observed in the INOZACO thin lm incontrast to IACZACO, INOZNOO, and IClZClO. From a comparisonwith the other two peaks that appeared at 530–532 eV, we alsoestimated that INOZACO possessed a larger concentration ofoxygen vacancies and a minimal hydroxyl group content. Adispersed vacant state with short interaction distances shouldinduce a minimum conduction band for efficient carriertransport, which can be achieved in metal oxide lattices but notin hydroxide lattices. Thus, M–O–M formation is an essentialrequirement for achieving high electrical performance from theamorphous metal oxide lms. Generally, the annealingtemperature is critical to the TFT performance of solution-processed metal oxide semiconductors because the formationof the M–O–M lattice is driven by thermal condensation anddehydroxylation. Therefore, the degree of oxide lattice forma-tion and oxygen vacancy generation primarily depends on theexternal thermal energy. From the XPS data, it can be inferredthat the internal energy generated from the combustion ofINOZACO largely contributed to the formation of M–O–M bondsand the suppression of M–OH bonds. Moreover, it is worthnoting that the INOZACO thin lms prepared at an annealingtemperature of 300 �C showed a larger amount of M–O–Moxygen than the thin lms from other conventional IZOprecursors that were annealed at 350 �C, as shown in ESIFig. S2.†

The composition ratio of the combustive InNO and ZnAC

precursors in the INOZACO thin lms also affected the relativeintensity of three different components in the O 1s peak, asshown in ESI Fig. S3.† The dependence of the O 1s peakintensity on the composition of InNO and ZnAC can be explainedin terms of the relative ratio between the fuel and the oxidizer.

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For example, when the content of fuel (ZnAC) is higher than thatof the oxidizer (InNO) or vice versa, the combustion process maynot proceed to completion, resulting in a reduced M–O–M andan increased M–OH composition. In other words, an equivalentamount of fuel and oxidizer is required to create a suitablecombustion reaction. This explanation is also supported by thethermal behavior of INOZACO prepared from InNO/ZnAC ¼ 1 : 5,which exhibits relatively small exothermic peaks at a muchhigher temperature than where the peaks of InNO/ZnAC ¼ 3 : 3appear, as shown in ESI Fig. S5.† To verify further the inuenceof the fuel/oxidizer composition ratio on the electrical proper-ties of IZO TFTs, IACZNOO TFTs were fabricated from differentratios of InAC and ZnNO precursors. The electrical performanceof IACZNOO TFTs does in fact depend on the ratio of InAC toZnNO, as shown in ESI Fig. S9.† Interestingly, in the case wherethe amount of In precursor is much higher than that of the Znprecursor, namely IAC/ZNO ¼ 5 : 1, the electrical performance ofthe fabricated TFT was poor, with a low on/off current ratio andlow mobility. This result can be attributed to the imbalancebetween the content of the fuel (coordinated with In) and thecontent of the oxidizer (coordinated with Zn). These resultsindicate that an equivalent amount of fuel and oxidizer isnecessary to realize high eld effect mobilities and high on/offcurrent ratios from INOZACO and IACZNOO TFTs fabricated withthis process. The IZO thin lms' chemical composition andstructural characteristics, dependent on the ratio of In and Znprecursors, determined the electrical performance of the cor-responding IZO TFTs, which will be discussed below.

The surface morphology and roughness of the IZO thin lmsprepared from different In and Zn precursors were investigatedby atomic force microscopy (AFM) and eld emission scanningelectron microscopy (FE-SEM), as shown in ESI Fig. S6.† TheIZO thin lms from InNO and ZnAC precursors showed a nano-crystalline structure whose features were uniformly distributedand densely packed, compared to the relatively small nano-crystallites on the InNO–ZnNO thin lms observed in the AFMsurface images. Films prepared from InAC–ZnAC did not formnanocrystals, while those prepared from InCl–ZnCl yielded largerough agglomerates with a size of around 200 nm. Although itwas nearly impossible to obtain the exact structural informationon these nanocrystallites, we estimated that they developedduring precursor condensation and became embedded in theoverall amorphous IZO lm structure, which was characterized

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using X-ray diffraction (XRD) as shown in ESI Fig. S8.† SEMimaging of the InAC–ZnAC thin lms also revealed the existenceof numerous pores on the surface, even though the surfaceappeared quite smooth when examined by AFM. The lmthickness was derived from cross-sectional SEM images of theIZO thin lms, which were sliced via ion milling. All IZO lmshad a thickness of approximately 8 nm.

IZO TFTs with a bottom-gate, top-contact structure werefabricated from spin-coated IZO channels on SiO2 dielectricsaer annealing at 350 �C in air. Al source–drain electrodes of100 nm were subsequently deposited on IZO channel layers. Allof the IZO TFTs operated properly as n-channel enhancement-mode devices and exhibited clear pinch-off and good currentsaturation, as shown in Fig. 4 and 5. All of the transistorparameters, such as mobility, threshold voltage, and on/offcurrent ratios, are summarized in Table 1.

The electrical performance of the INOZACO TFTs wascompared with other conventional IZO TFTs to investigate theinuence of combustion chemistry on TFT properties, as shownin Fig. 4. The electrical performance of the INOZACO TFTsfabricated via combustion was signicantly better than that ofthe other IZO TFTs, showing a much higher average eld-effectmobility of 8.70 cm2 V�1 s�1 and an enhanced on/off currentratio of 1.06 � 108. The electrical performance of the INOZACOTFTs was also signicantly better than that of a-Si:H TFTs. Theinuence of the annealing temperature on the electricalperformance of the INOZACO TFTs is shown in Table 1 and ESIFig. S10.† Increasing the annealing temperature from 250 to 350�C raises the carrier mobility for INOZACO TFTs above that oftypical solution-processed IZO TFTs (Table 1). The INOZACOTFTs fabricated at a low temperature of 250 �C exhibitedreasonable electrical performance, even though the combustion

Fig. 4 The transfer and output characteristics of IZO TFTs fabricated fromand (d) IClZClO.

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reaction did take place at a low temperature of around 200 �C.For complete formation of the M–O–M bonds, we believe that aminimum of 300 �C of external thermal energy is required, eventhough the heat generated from the combustion reaction canprovide additional energy for the metal oxide formation. Thisenergy requirement can be veried by the additional increase inthe M–O–M XPS peak intensity as the annealing temperature isincreased from 250 to 350 �C (ESI Fig. S2†). An annealingtemperature of 300 �C is compatible with exible substrates,such as polyimide or PAR lms. The INOZACO TFTs prepared at300 �C exhibited a good average eld-effect mobility of 3.67 cm2

V�1 s�1 and an on/off current ratio of 2.57 � 107, with a slightdisplay of hysteresis.

The transfer and output characteristics of the INOZACO TFTsprepared from various composition ratios of InNO to ZnAC

precursors were investigated to establish the optimal compo-sition for high TFT electrical performance (Fig. 5). The IZO TFTsfabricated via the self-combustion of coordinated fuel andoxidizer ligands at an annealing temperature of 350 �C showedexcellent TFT properties with negligible electrical hysteresis.The average saturation mobilities corresponding to variousInNO and ZnAC composition ratios were in the range of 0.31–8.70 cm2 V�1 s�1 with an on/off current ratio of 104 to 108.Interestingly, when the Zn/In ratio was 2 : 1, the electricalperformance of the IZO TFTs was particularly poor, which wasascribed to a reduction in charge carriers due to the decrease inthe In content. Generally, the addition of In3+ can create moreoxygen vacancies, which can accelerate electron mobilitythroughout the amorphous structure of the IZO thin lm, wherethe In–O bond strength is weaker than that of the Zn–O bondowing to the difference in electronegativity between In andZn.33–35 It has also been reported that extensive overlap between

different In and Zn precursors: (a) INOZACO, (b) IACZACO, (c) INOZNOO,

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Fig. 5 The transfer and output characteristics of INOZACO TFTs fabricated from precursors with different composition ratios of InNO to ZnAC: (a)InNO/ZnAC¼ 5/1, (b) InNO/ZnAC¼ 4/2, (c) InNO/ZnAC¼ 3/3, (d) InNO/ZnAC¼ 2/4, and (e) InNO/ZnAC¼ 1/5. (f) The averagemobility of the INOZACO TFTs.

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neighboring s orbitals of In3+, which are larger than those ofZn2+, may improve the electrical conductivity of IZO thin lms.36

Therefore, it is natural that the INOZACO TFTs with higher In

Table 1 The characteristics of the IZO TFTs according to the different cotemperatures

IZO precursorAverage mobility(cm2 V�1 s�1)

Maximum mobility(cm2 V�1 s�1)

Thvol

InNO(3)–ZnAC(3) 8.70 � 1.75 13.80 12InAC(3)–ZnAC(3) 0.72 � 0.09 0.73 21InNO(3)–ZnNO(3) 6.20 � 0.29 6.53 17InCl(3)–ZnCl(3) 1.61 � 0.02 1.63 16InNO(5)–ZnAC(1) 7.79 � 1.22 9.70 9InNO(4)–ZnAC(2) 8.49 � 1.29 9.09 10InNO(2)–ZnAC(4) 1.78 � 0.95 2.80 22InNO(1)–ZnAC(5) 0.31 � 0.07 0.37 22InNO(3)–ZnAC(3) 1.17 � 0.28 1.49 17InNO(3)–ZnAC(3) 3.67 � 1.08 4.91 16

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content exhibited better electrical properties than INOZACO TFTscontaining higher Zn content. Nevertheless, the INOZACO TFTswith a stoichiometric ratio of In and Zn exhibited the best eld

mbinations and compositions of In–Zn precursors at various annealing

resholdtage (V)

Sub-thresholdslope (V) On/off ratio

Annealingtemperature (�C)

.47 � 2.68 7.2 1.06 � 108 350

.99 � 4.18 9.6 1.18 � 106 350

.24 � 3.65 10.8 3.55 � 107 350

.55 � 0.37 9.6 3.93 � 107 350

.01 � 1.18 8.4 4.48 � 104 350

.05 � 1.93 8.4 2.31 � 108 350

.56 � 1.09 9.6 3.31 � 108 350

.25 � 5.93 16.8 2.62 � 107 350

.49 � 4.93 13.2 2.62 � 106 250

.72 � 3.85 12.2 2.57 � 107 300

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Fig. 6 Device uniformity of the INOZACO TFTs prepared from aprecursor with a composition ratio of InNO/ZnAC ¼ 3/3.

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effect mobility with the highest on/off current ratio. Thisparticular electrical performance could be ascribed to the highcondensation efficiency from the effective combustion reactionwhere the amounts of fuel and oxidizer were equivalent. Theseresults also correspond well to the XPS data that show anextremely large M–O–M peak in the IZO thin lm with acomposition ratio of InNO/ZnAC ¼ 3 : 3.

Device uniformity is of substantial importance for the prac-tical use of TFTs. To measure the uniformity of the INOZACOTFTs, 30 TFT units were fabricated at an annealing temperatureof 350 �C. The performance of TFT units was measured, andstandard deviations were obtained for the mobility (m ¼ 8.70 �1.75 cm2 V�1 s�1) and the turn-on voltage (VON ¼ 12.47� 2.68 V)(Fig. 6). This result demonstrates that the INOZACO TFTs exhibitgood device uniformity and reproducible yield. A very highmobility (13.8 cm2 V�1 s�1) was observed in some of the devicessuggesting that the device performance can be further improvedby optimizing the processing conditions.

The precursor solutions prepared in our study were alsoapplied towards the inkjet printing of TFTs. A solution ofINOZACO with an identical composition ratio of In and Zn wasinkjet-printed and annealed at 350 �C; the resulting lm wasthen incorporated into a TFT. The transfer and output char-acteristics of the inkjet-printed INOZACO TFT are shown in ESIFig. S11.† The saturation mobility was 5.3 cm2 V�1 s�1 and theon/off current ratio was 106. This result demonstrates that ahigh performance IZO TFT can be fabricated by inkjetprinting without photolithography or vacuum-basedtechniques.

3. Conclusions

We have demonstrated that IZO thin lms suitable for TFTscan be fabricated by the self-combustion of a two-componentmixture of metal salts whose ligands function both as a fueland as an oxidizer. The properties of the IZO TFTs depended onthe precursor composition and annealing temperature. TFTsfabricated using the self-combustion system demonstratedhigh reproducibility and device uniformity, compared to thosefabricated via previously reported general combustion of multi-component systems. The combustion of In and Zn precursorscontaining acetylacetonate (fuel) and nitrate (oxidizer) can

4254 | J. Mater. Chem. C, 2014, 2, 4247–4256

release extremely intense exothermic heat, and this evolutionof internal energy can be utilized to lower the externally appliedtemperature for annealing of the oxide precursors. Our resultsconrmed that the energy from the combustion reaction canfacilitate the formation of the M–O–M lattice and the removalof organic ligands, inducing higher carrier mobility in IZOTFTs. Furthermore, the precursor composition had a remark-able effect on the chemical and electrical characteristics of theIZO thin lms. The IZO TFTs prepared by using an equivalentstoichiometric ratio of fuel and oxidizer at an annealingtemperature of 350 �C showed a superior eld-effect mobility of13.8 cm2 V�1 s�1 and an on/off current ratio of 1.06 � 108. IZOTFTs fabricated by inkjet printing of the precursor mixture,followed by annealing at 350 �C, exhibited a high mobility of5.3 cm2 V�1 s�1 with an on/off ratio of 106. Such high TFTperformance at low processing temperature, with good deviceuniformity and high yield, demonstrates the potential forapplication of the proposed system in exible printed elec-tronics. An additional advantage that should be emphasized isthat our system allows preparation of high-quality solution-processed semiconductor lms from a simple mixture of twooxide precursors without any additives. Application of our self-combustion method to the synthesis of ternary oxide lms isunder investigation.

4. Experimental sectionPreparation of the IGO precursor solutions (generalcombustion with additives)

All reagents, indium nitrate hydrate (In(NO3)3$H2O, 99.99%),gallium nitrate hydrate (Ga(NO3)3$H2O, 99.9%), mono-ethanolamine (NH2CH2CH2OH, 99%), acetylacetone(CH3COCH2COCH3, 99%), ammonium hydroxide solution(28% in water), and 2-methoxyethanol (CH3OCH2CH2OH,anhydrous 99.8%) were purchased from Aldrich and usedwithout additional purication. General precursor solutions,without the fuel and oxidizer for the combustion reaction,were rst prepared in order to optimize the ratio of metal ions.0.2 M metal salt precursor solutions were prepared in2-methoxyethanol with monoethanolamine as a stabilizer. Themolar ratios of In : Ga in the precursor solutions were 100 : 0,94 : 6, 88 : 12, and 76 : 24. The prepared clear solutions werestirred for 3 h at room temperature prior to spin-coating. Theprecursor solutions for inducing the combustion reactionwere prepared as follows. Metal salt precursors with aconcentration of 0.2 M (In nitrate : Ga nitrate ¼ 94 : 6 molarratio) were dissolved in 2-methoxyethanol with the appropriateamount of AcAc, and an ammonium hydroxide solution wassubsequently added gently with stirring. The composition ofAcAc was varied over a range of R1 values from 1 to 3, where R1

indicates the molar ratio of AcAc to metal salts. The compo-sition of ammonium hydroxide solution was varied over arange of R2 values from 1 to 4, where R2 indicates the molarratio of water molecules in an ammonium hydroxide solutionto metal salts. The clear solutions were stirred for 12 h at roomtemperature prior to spin coating.

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Preparation of the IZO precursor solutions (additive-free self-combustion)

The IZO precursor solutions were prepared using zinc acetyla-cetonate hydrate (Zn(C5H7O2)2$xH2O), zinc nitrate hexahydrate(Zn(NO3)2$6H2O), zinc chloride (ZnCl2), indium acetylacetonate(In(C5H7O2)3$xH2O), indium nitrate hydrate (In(NO3)3$xH2O),and indium chloride (InCl3), which were all obtained fromSigma-Aldrich. Aer the metal salts were dissolved in anhy-drous 2-methoxyethanol, the solutions were thoroughly stirredfor more than 6 h at room temperature to prepare the homo-geneous precursor solution. The total concentration of the IZOprecursor was xed at 0.1 M, and the molar ratio of indiumnitrate hydrate to zinc acetylacetonate hydrate was varied in therange of 1 : 5–5 : 1 to determine the optimal composition.Various In and Zn precursors were combined at a xed molarconcentration of 0.05 M : 0.05 M to compare the thermalbehavior and TFT properties of lms prepared from thecombustive IZO precursor solutions with those of the conven-tional IZO. Thermal behavior was observed in an ambientatmosphere via TGA and DTA (SDT 2060, TA Instruments, USA).The heating rate of TGA and DTA was 10 �Cmin�1. The samplesfor thermal analysis were prepared by evaporating the INOZACO,IACZACO, INOZNOO, and IClZClO precursor solutions at 40 �C in avacuum oven for 1 day.

Characterization of the IZO thin lms

The microstructure of the IZO thin lms was investigated byusing an ultra high resolution (UHR) FE-SEM (S-5500, Hitachi,Japan). The thickness of the IZO thin lms was measured withthe UHR FE-SEM aer the cross-sectional samples wereprepared in the following manner. Platinum thin lms with athickness of approximately 100 nm were deposited on theprepared IZO thin lms with a coating machine (E-1030, Hita-chi, Japan) to distinguish each layer. The IZO thin lms werethen cut with an ion milling system (E-3500, Hitachi, Japan) at apower level of 6 keV and a constant Ar ow rate of 50 sccm. Thene surface roughness of the IZO thin lms was explored byAFM (NanoScope, Veeco, USA) in an area of 3 mm2 via noncon-tact mode. The IZO thin lms produced aer annealing at350 �C were analyzed via XRD (D-MAX-RC, Rigaku, Japan). XRDmeasurements were performed using a thin lm diffractometerin which samples were xed at an angle of 3� to the X-ray beamwith 2q scan of the detector. The source power was 60 mA/40 kV,which was stronger than that for normal powder XRD. An X-rayphotoelectron spectrometer (K-Alpha, Thermo Fisher Scientic,USA), with an Al Ka excitation source, was used to characterizethe surface chemical composition and the chemical state of theIZO thin lms. All XPS spectra were calibrated against the C 1selectron peak at 286.5 eV.

Fabrication and electrical measurement of IZO TFTs

A bottom-gate, top-contact design was adopted for the IZO TFTsas a typical device structure. The IZO precursor solutions weredeposited on a 300 nm thick SiO2 lm on a Si wafer (heavily n-type doped) using a simple spin-coating method. Prior to

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deposition, the substrate was consecutively cleaned by consec-utive treatments of ultrasonication in a detergent solution,acetone, and hot isopropyl alcohol. The SiO2 substrate was thendried and treated with an ozone plasma for 20 min. The IZOprecursor solutions were spin-coated at 4000 rpm for 30 s andpre-baked on a hot plate at 100 �C for 10 min to remove theresidual solvent and organic materials. Aer the pre-baking, theIZO thin lms were annealed at 350 �C for 1 h on a hot plate inambient air. The top-contact source and drain electrodes(100 nm thickness, Al) were thermally evaporated through apatternedmetal shadowmask. The ratio of the channel width tochannel length was 60 (width ¼ 3000 mm; length ¼ 50 mm). Theinkjet printing was also carried out using a piezoelectric inkjetnozzle with an orice size of 30 mm (Microfab, USA).

The IZO ink for inkjet printing consisted of the In and Znprecursors in a ratio identical to the optimal composition forthe spin-coated lms. The inkjet printing was carried out at25 �C using a piezoelectric inkjet nozzle with an orice size of30 mm (Microfab, USA) operated at a head frequency of 200 Hzand a resolution of 500 DPI. Pre-baking and annealing wasconducted in the manner described above.

The electrical characteristics of the IZO TFTs were measuredin air using a semiconductor parameter analyzer (4155C, Agi-lent, USA). The measurements were typically taken in contin-uous mode, and the transfer curve was recorded before theoutput curve. The eld-effect mobility (msat) and the thresholdvoltage (Vth) were calculated for the saturation region viaeqn (1),

ID ¼ msatCi

W

2LðVG � VthÞ2; (1)

where ID is the saturation current, Ci is the capacitance per unitarea of the dielectric, VG is the source-gate voltage, andW and Lare the TFT channel width and length, respectively.

Acknowledgements

This work was supported by a grant from the R&D ConvergenceProgram of Ministry of Science, ICT, and Future Planning andKorea Research Council for Industrial Science and Technologyof Republic of Korea (Grant B551179-09-06-00). We would like tothank Dr T.-S. Bae (UHR FE-SEM group, Korea Basic ScienceInstitute, Jeonju) for assistance with the SEM imaging.

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