Journal ofMaterials Chemistry C

PAPER

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aDivision of Advanced Materials, Korea Resea

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

Fax: +82-42-860-7200; Tel: +82-42-860-7260bDepartment of Chemical Convergence Ma

Technology, 217 Gajeong-ro, Yuseong-gu, De

† Electronic supplementary informationcharacteristics with various GPTMS ratiodielectric properties, XPS spectra of Aimages, TG-DTA curves of AlOx precursand XRD patterns are available. Fig. S1–S

Cite this: J. Mater. Chem. C, 2014, 2,5695

Received 29th April 2014Accepted 12th May 2014

DOI: 10.1039/c4tc00874j

www.rsc.org/MaterialsC

This journal is © The Royal Society of C

Soluble oxide gate dielectrics prepared using theself-combustion reaction for high-performancethin-film transistors†

Eun Jin Bae,a Young Hun Kang,a Mijeong Han,a Changjin Lee*ab and Song Yun Cho*a

We report the fabrication of high-performance metal oxide thin-film transistors (TFTs) with AlOx gate

dielectrics using combustion chemistry in a solution process to provide energy to convert oxide

precursors into oxides at low temperatures. Our self-combustion system utilizing two Al precursors as a

fuel and an oxidizer is systematically compared with conventional combustive Al precursors with urea in

terms of combustion efficiency and dielectric properties. AlOx gate dielectric layers are spin-coated from

a solution of combustive AlOx precursors in 2-methoxyethanol and annealed at 250 �C. In this process,

organic compounds can be introduced to improve the resulting layer morphology. The thermal

behaviors of the self-combustive AlOx precursors are investigated and compared with those of

noncombustive AlOx precursors and combustive urea–AlOx precursors to evaluate the generation of

exothermic heat at a relatively low annealing temperature. The AlOx dielectrics prepared from self-

combustive precursors have uniform and smooth surfaces, low leakage current densities, and high

dielectric constants above 8.7. They also exhibit excellent insulating properties and no breakdown at

high electric fields. Furthermore, the ZnO TFTs prepared to confirm the operation of the AlOx gate

dielectrics show a good mobility of 24.7 cm2 V�1 s�1 and an on/off ratio of 105. It is believed that the

AlOx dielectrics prepared using the self-combustion reaction at a low temperature can form a good

interface facilitating the growth of desirable ZnO crystal structures, which leads to a considerable

improvement in ZnO TFT performance.

1. Introduction

The gate dielectric is a critical element affecting the electricalperformance of thin-lm transistors (TFTs). To obtain high-performance gate dielectrics for TFTs that can provide highcapacitance without relying on ultrathin lm thicknesses, high-k dielectric materials have been actively pursued to replace SiO2

(dielectric constant �3.9). The high capacitance and the thickerlayer also allow for efficient charge injection into the transistorchannels and reduce the direct-tunneling leakage currents.Such advantages have motivated intense research into thedevelopment of zirconium oxide (ZrO2), hafnium oxide (HfO2),and aluminum oxide (AlO3) as high-k dielectric materials.1–3

However, gate dielectric layers made of these materials must

rch Institute of Chemical Technology, 141

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

terials, Korea University of Science and

ajoen 305-350, Republic of Korea

(ESI) available: Transfer and outputs and different composition ratios andlOx thin lms, TEM cross-sectionalors with different composition ratios,8. See DOI: 10.1039/c4tc00874j

hemistry 2014

still be fabricated using vacuum processes, which have highcosts for processing equipment and facilities.

Recently, numerous research studies into solution-processedTFTs have focused on the development of soluble high-k gatedielectric materials for high-performance TFTs.4–10 Solution-processability can lead to several advantages including print-ability, the possibility of large-area deposition, low-cost devicefabrication, and compatibility with mechanically exiblesubstrates.11–14 Among many potential materials, Al2O3 thinlms are well known as high-k gate dielectrics because of theirlow interfacial trap density with oxide semiconductors and arelative permittivity of �9.15 For example, a sodium b-aluminagate dielectric prepared using a sol–gel reaction at 830 �C toproduce an oxide TFT showed a mobility of 28 cm2 V�1 s�1.16

Furthermore, an AlOx gate dielectric deposited using a multiple-spin-coating procedure and annealed at 300 �C had an oxideTFT mobility of 33 cm2 V�1 s�1.17 Despite these high reportedmobilities for oxide TFTs, however, the high processingtemperature andmultiple spin-coating processes lead to certainlimitations.

As an alternative, the solution combustion reaction is an easyand time-saving production process based on thermochemicalconcepts used in the propellant and explosive elds. Recently,combustion chemistry has been utilized for the synthesis of

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nano-oxides for electronic applications. The advantages of thisprocess derive from the fact that no special equipment isrequired to provide additional heat, because the combustionreaction of simple precursor compounds itself generates heatthat can be used to form the metal oxide.18,19

Research into combustion reactions for TFT fabrication hasmostly been conducted using metal oxides such as In2O3, ZTO,IZO, and ITO.20,21 However, quite recently the addition of urea asa fuel for combustion to form gate dielectric materials has beenintroduced.22 Currently, this approach has several limitationsthat should be solved, such as low combustion efficiency andthe resulting high leakage current of the resulting dielectric. Inparticular, the low combustion efficiency can result in insuffi-cient heat supply to the internal system, which leads to unsat-isfactory dielectric properties. Moreover, it is quite difficult tonely tune the amount of urea needed to realize the optimalcombustion reaction.

Here, we suggest a promising and novel method to synthe-size solution-processable gate dielectrics by self-combustion atlow temperatures. Our facile, efficient self-combustion processovercomes the shortcomings of previous combustiveapproaches by simply mixing two metal precursors that containcoordinated fuel and oxidizer ligands. We believe that thissimple process can provide more stable TFTs and betterdielectric performance because of its high combustive effi-ciency. In addition, hybridization of the dielectric material withorganic polymers can be achieved by carrying out a sol–gelreaction with the AlOx precursors to provide more electricallystable and ne-tuned properties in a high-k gate dielectric layer.In the basic synthesis, the AlOx gate dielectrics for oxide TFTswere obtained by solution combustion from a mixture of AlOx

and organic precursors at a low temperature of 250 �C. Toconrm the dielectric properties of the AlOx prepared using thiscombustion system with organic components for use in oxideTFTs, ZnO TFTs were fabricated by spin-coating ZnO precursorson the AlOx lms and annealing at 250 �C. Moreover, the crys-talline structures of the ZnO on the AlOx lms prepared fromcombustive precursors were compared with those on lmsprepared from noncombustive precursors and combustiveprecursors with urea to investigate the inuence of the gatedielectric layer on the TFT performance.

2. Results and discussion

The combustion reaction between organic fuels (e.g., acetyla-cetone, urea, carbohydrazide, and citric acid) and an oxidizer(e.g., nitrate) can lower the effective annealing temperaturerequired for the formation of metal oxide compounds fromoxide precursors. When a fuel–oxidizer pair is moderatelyheated, a highly intense exothermic reaction occurs. Theevolved heat can accelerate the formation of metal hydroxidesor alkoxides into the corresponding metal oxide frameworks(M–O–M) at sufficiently low external temperatures. In otherwords, in the self-combustion system, the energy required forcomplete conversion of the precursors into oxides can bereplenished by the exothermic energy from the combustiveprecursor reaction. Previous studies have demonstrated that

5696 | J. Mater. Chem. C, 2014, 2, 5695–5703

metal nitrates (as both a metal source and an oxidizer) andacetylacetone (AcAc, as both a chelating agent and a fuel) are themost effective among the various oxidizers and fuels for self-combustion.23,24

To investigate the thermal behavior of the AlOx precursorduring the self-combustion reaction, thermogravimetric differ-ential thermal analysis (TG-DTA) was conducted. To compare thethermal properties of combustive precursors with those of non-combustive precursors, TG-DTA data were plotted for self-combustive AlOx precursors, the Al(NO3)3$9H2O and Al(C2H5O2)3pairs, and the noncombustive precursors Al(NO3)3$9H2O andAl(C2H5O2)3 (Fig. 1). Furthermore, to analyze and compare thethermal behavior of a conventional combustive system, acombustive precursor composed of Al(NO3)3$9H2O and urea asan external fuel was investigated. The DTA data for the self-combustive AlOx precursor containing acetylacetonate (fuel) andnitrate (oxidizer) ligands show a single, intense exothermic peakat 170 �C in Fig. 1(a) that corresponds exactly to the abrupt massloss. On the other hand, the combustive precursor with ureashows a negligible exothermic peak and mass loss at around250 �C. These results demonstrate that the combustion efficiencyis remarkably lower and the exothermic temperature is muchhigher than those of the self-combustive precursor. This meansthat quite a small amount of heat can lead to M–O–M formationin the urea-based combustion system. In the case of the non-combustive precursor, the Al(NO3)3$9H2O precursor, a gradualmass loss is completed at around 400 �C without any exothermicpeaks, which is possibly due to the decomposition of organicligands. Even though the TGA data for the Al(C2H5O2)3 precursorsshow abrupt thermal decomposition at a relatively low temper-ature of 250 �C, Fig. 2(d) shows that they do not undergo anexothermic reaction. This also implies that the Al(C2H5O2)3precursor is a good candidate for producing AlOx gate dielectrics.However, when the AlOx gate dielectric for a ZnO TFT is preparedusing Al(C2H5O2)3 precursors at 250 �C, the electrical perfor-mance of the resulting ZnO TFT is particularly poor, as shown inthe ESI Fig. S1.† This means that noncombustive AlOx precursorsrequire a high temperature of over 400 �C for the completeconversion of the precursors into the M–O–M lattice. In addition,this result supports the conclusion that the exothermic heat froma self-combustive AlOx precursor can assist the conversion intocorresponding oxides at low temperatures.

When only the AlOx precursor was deposited without anyorganic ingredients, the obtained AlOx gate dielectric layershowed a quite poor surface morphology (Fig. 2): the surfaceof the lm was cracked and numerous pores generatedduring annealing. The combustive AlOx precursor with ureabut no other organic components also shows an impropersurface morphology for the gate dielectric layer. These poorsurface properties can render the AlOx gate dielectriclayers prepared from only the AlOx precursor difficult for usein actual devices because of their high leakage currentsand charge-trapping phenomena. To improve the layermorphology, organic components such as (3-glycidoxy-propyl)trimethoxysilane (GPTMS) can be introduced into theoxide network as polymeric binders using the sol–gelmethod. To fabricate a stable gate dielectric layer with a

This journal is © The Royal Society of Chemistry 2014

Fig. 1 TG-DTA curves of combustive and noncombustive precursors: (a) self-combustive AlOx precursor, (b) combustive precursor with urea, (c)noncombustive Al(NO3)3$9H2O, and (d) noncombustive Al(C2H5O2)3.

Fig. 2 Atomic force microscopy (AFM) step profiler images and surface scanning electron microscopy (SEM) images of AlOx thin films preparedby (a and b) self-combustion and (c and d) combustion with urea.

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smooth surface, the combustive AlOx precursor can bemixed with GPTMS, the epoxy moiety of which polymerizesduring the annealing process (Scheme 1).

The dielectric characteristics of the self-combustive AlOx

dielectric lms and their dependence on the organic content(GPTMS) were evaluated by measuring the quantitative leakagecurrent and dielectric constant. ESI Fig. S2† shows typicalcurrent density vs. electrical eld and capacitance vs. frequencycurves for metal–insulator–metal (MIM) structures fabricatedfrom AlOx insulators with various amounts of GPTMS. The plotdemonstrates that GPTMS-only lms exhibit the highest

This journal is © The Royal Society of Chemistry 2014

dielectric constant, and higher GPTMS contents result in higherdielectric constants of the AlOx insulators in the range of 8.7–12.Even though the GPTMS lm shows a high dielectric constant of16 at 100 Hz, this lm is unsuitable as a gate dielectric becauseof its unstable nature in all frequency ranges. Furthermore,GPTMS-only lms have high leakage current densities becauseof their nonuniform and rough surface morphologies. Thisresult means that the dense network structure produced bythe sol–gel reaction of GPTMS together with the hybridization ofthe AlOx precursors is necessary to obtain a satisfactory surfacestructure for the nal gate dielectric lm. When the ratio of the

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Scheme 1 Schematic diagram of the AlOx gate dielectric film prepared by self-combustion of metal precursors mixed with organic compounds.

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AlOx precursor to GPTMS is 1 : 8, the dielectric constant of theAlOx lms is stable in all frequency ranges. When the ratio ofthe AlOx precursor to GPTMS decreases to 1 : 30 or 1 : 45, thedielectric properties of the obtained lms are unstable, espe-cially in the low and high frequency ranges. Consequently, theoperating zone of TFTs fabricated with these GPTMS ratios islimited to specic frequency ranges. The dielectric constants ofAlOx insulators prepared without GPTMS could not bemeasured because of lm cracking. Therefore, the ratiobetween the AlOx precursor and GPTMS was chosen to be 1 : 8to obtain the optimal TFT characteristics. The leakage currentdensities for lms with thicknesses of d ¼ 200 nm preparedfrom AlOx precursors with GPTMS are around 10�8 to 10�9 Acm�2 up to E z 2 MV cm�1. In particular, AlOx insulatorsprepared with a 1 : 8 ratio of the AlOx precursor to GPTMS showexcellent leakage current densities of 3.92 � 10�9 A cm�2. Allthe electrical properties obtained with different AlOx pre-cursor : GPTMS ratios are summarized in Table 1. The excellentinsulating properties of the lms obtained from combustiveAlOx precursors hybridized with an organic component aresuperior to those of the commonly used solution-processablehigh-k gate dielectric materials. For the comparison with theconventional combustive precursor, AlOx insulators were

Table 1 Electrical properties of films obtained from combustive AlOx pr

Combustiveprecursor

Molar ratio(AlOx precursor :GPTMS)

Dielectricconstant(@ 100 Hz)

C(p(@

Self-combustive 0 : 1 16.0 701 : 8 8.7 381 : 30 10.9 471 : 45 12.0 531 : 0

Combustive with urea 1 : 8 4.4 11

a Extrapolated to a frequency of 1 Hz.

5698 | J. Mater. Chem. C, 2014, 2, 5695–5703

prepared by combustion with urea using a 1 : 8 ratio of thecombustive precursor to GPTMS with annealing at 250 �C. Fig. 3shows typical current density vs. electric eld and capacitancevs. frequency curves from the GPTMS-containing AlOx lmsformed by self-combustion and combustion with urea, respec-tively. The dielectric constant of the AlOx insulators formed bycombustion with urea is around 4.4, which is much lower thanthat of the AlOx formed by self-combustion. In addition, thelayer formed by combustion with urea shows a poor leakagecurrent density of 2.80 � 10�6 A cm�2, which results from thesparse AlOx lattice structure. We believe that this structuraldefect originates from the formation of a imsy M–O–M struc-ture because not enough exothermic heat was supplied to thesystem owing to the low combustion efficiency of the combus-tive precursor with urea (ESI, Fig. S3†).

To demonstrate the feasibility of the combustive AlOx

precursor for forming a gate dielectric layer, ZnO TFTs with abottom-gate top-contact structure were fabricated from spin-coated ZnO semiconductors on AlOx gate dielectrics that wereannealed at 250 �C in air. Al source–drain electrodes of 100 nmthickness were subsequently deposited on the ZnO channellayers. The electrical characteristics of the AlOx-based TFTs withsemiconducting ZnO active layers were investigated. The

ecursors mixed with various amounts of GPTMS

apacitanceF mm�2)100 Hz)

Capacitancea

(pF mm�2)(@ 1 Hz)

Leakage currentdensity (A cm�2)(@ 2 MV cm�1)

Thickness(nm)

0 — — 2004 426 3.29 � 10�9 2004 752 6.20 � 10�9 2000 868 1.40 � 10�8 200

N/A0 110 2.80 � 10�6 250

This journal is © The Royal Society of Chemistry 2014

Fig. 3 (a) Current density vs. electrical field and (b) dependence of capacitance on the frequency for AlOx thin films with GPTMS prepared by self-combustion and combustion with urea.

Fig. 4 (a) Transfer and (b) output characteristics of the ZnO TFTsfabricated from AlOx gate dielectrics with 1 : 8 ratios of the self-combustive AlOx precursor to GPTMS. (c) Transfer and (d) outputcharacteristics of the ZnO TFTs fabricated from AlOx gate dielectricswith 1 : 8 ratios of the AlOx precursor with urea to GPTMS. (e) Deviceuniformity of the ZnO TFTs fabricated from AlOx gate dielectricsprepared with the self-combustive AlOx precursor.

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thickness of the ZnO channel layer was determined to beapproximately 10 nm from cross-sectional transmission elec-tronmicroscopy (TEM) images of the ZnO TFT (ESI, Fig. S4†). Torealize high eld effect mobilities and high on/off current ratiosfrom ZnO TFTs with AlOx gate dielectrics, an equivalent amountof fuel and oxidizer is necessary (ESI, Fig. S5 and S6†). When thecontent of fuel is higher than that of the oxidizer or vice versa,the combustion process may not proceed to completion,resulting in a reduced M–O–M and an increased M–OHcomposition. Therefore, the AlOx thin lms' chemical compo-sition between the oxidizer and the fuel for the proper electricalperformance of the corresponding TFTs was determined to be1 : 1. Fig. 4 shows the output and transfer curves of ZnO TFTswith AlOx gate dielectric layers prepared with a 1 : 8 ratio of thecombustive AlOx precursor to GPTMS. The eld-effect mobilityof ZnO TFTs with AlOx gate dielectrics prepared from combus-tive precursors with urea is 2.48 � 10�3 cm2 V�1 s�1, and theiron/off ratio of 104, with a large hysteresis. Such poor electricperformance of ZnO TFTs with AlOx gate dielectrics prepared bycombustion with urea can be attributed to insufficient dielectricproperties of the gate dielectric layer due to the low combustionefficiency of the precursors. In contrast, ZnO TFTs with AlOx

gate dielectrics prepared from self-combustive precursorsexhibit an improved eld-effect mobility of 24.7 cm2 V�1 s�1 andan on/off ratio of 105. Furthermore, these ZnO TFTs show goodoperating stability and excellent mobility with good switchingcharacteristics. The output curve of this ZnO TFT has clearpitch-off characteristics and good current saturation.

In addition, we investigated the device uniformity, which isof substantial importance for practical use of TFTs. To measurethe uniformity of ZnO TFTs, 20 TFT units were fabricated with1 : 8 ratios of the self-combustive AlOx precursor to GPTMSusing an annealing temperature of 250 �C. The performance ofthe TFT units was measured, and standard deviations wereobtained for the mobility (m ¼ 23.1 � 5.22 cm2 V�1 s�1) andturn-on voltage (Von ¼ 7.87 � 3.4 V) (Fig. 4(e)). It was found thatthe ZnO TFTs with AlOx gate dielectrics prepared by self-combustion exhibit good device uniformity and reproducibleyields. The mobility, threshold voltage, and the on/off ratio aresummarized in Table 2. The mobility values for different self-combustive AlOx precursor/GPTMS ratios are in the range of8.97–24.7 cm2 V�1 s�1, as calculated by extrapolating the

This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. C, 2014, 2, 5695–5703 | 5699

Table 2 Characteristics of ZnO TFTs fabricated from AlOx gatedielectrics with different amounts of GPTMS

Combustiveprecursor

Molar ratio(AlOx precursor :GPTMS)

Mobilitya

(cm2 V�1 s�1)Thresholdvoltage (V)

On/offratio

Self-combustive 1 : 0 N/A1 : 8 24.7 6.35 1.71 � 105

1 : 30 13.9 8.14 2.35 � 105

1 : 45 8.97 8.32 4.07 � 105

Combustivewith urea

1 : 8 2.48 � 10�3 7.11 1.00 � 105

a Calculated from capacitance values extrapolated to a frequency of1 Hz.

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capacitance values to a frequency of 1 Hz.16 Although the ZnOTFTs with other self-combustive AlOx precursor/GPTMS ratiosoperated well, their output characteristics are unstable, asshown in the ESI, Fig. S7.†

To further investigate the nanocrystalline structures of theAlOx gate dielectrics prepared from combustive precursors,TEM analysis was performed. As shown in Fig. 5, no AlOx

nanocrystallites were present in lms prepared by eithercombustion with urea or without combustion. However, theAlOx nanocrystallites were uniformly embedded in the AlOx gatedielectric layer prepared by self-combustion. All of the

Fig. 5 TEM images of the AlOx gate dielectric films prepared from the(a) self-combustive AlOx precursor with GPTMS (with insets showingan AlOx nanocrystallite in the film and the diffraction pattern), (b)combustive AlOx precursor with urea with GPTMS, (c) noncombustiveAl(NO3)3$9H2O precursor with GPTMS, and (d) noncombustiveAl(C2H5O2)3 AlOx precursor with GPTMS.

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nanocrystallites possess a lattice structure, as shown in the TEMimage in Fig. 5(a) (inset). We believe that the strong combustionreaction of the self-combustive precursor provides sufficientexternal heat to generate AlOx nanocrystallites, whereas theweak or absent combustion reactions of the other precursors donot generate enough energy to form crystal structures. Theexistence of the AlOx nanocrystalline lattice further proves thatthe bottom AlOx structure facilitates the vertical crystal growthof ZnO crystals when the self-combustive AlOx precursors areused. The crystal lattice mismatch between ZnO and AlOx isreduced by the existence of nanocrystalline AlOx beneath theZnO layer, so this environment is more favorable for the verticalZnO crystal growth. The surface morphology of the ZnO lmsgrown on the AlOx lms was analyzed using an atomic forcemicroscope (AFM). Fig. 6 shows the surface characteristics ofthe ZnO lms on the AlOx lms with and without GPTMSprepared by self-combustion. The roughness of the gatedielectric affects the surface morphology of the channel layer inthe bottom-gate TFTs. The root mean square (RMS) roughnessvalue of the ZnO lm on the AlOx lm with GPTMS is 1.19 nm,which means that the surface is much denser and smootherthan that of the ZnO lm without GPTMS, which has an RMSroughness of 10.4 nm. The improved surface morphology of theZnO lms proves that there is a synergetic function of the AlOx

gate dielectric with GPTMS that enhances the carrier mobilityand overall performance of ZnO TFTs.

To investigate the possible reason for the signicantlyenhanced eld-effect mobility of ZnO TFTs containing AlOx gatedielectrics produced from self-combustive precursors at lowannealing temperatures, the crystallinity of the ZnO depositedon the obtained AlOx layer was analyzed using the thin-lmdiffraction method (Fig. 7). For comparison, AlOx gate dielec-trics were prepared from combustive and noncombustiveAl(C2H5O2)3 and Al(NO3)3$9H2O precursors with GPTMS ratiosof 1 : 8 at 250 �C, respectively. The ZnO lms with AlOx gatedielectrics prepared using these combustive and non-combustive precursors show amorphous-like diffractionpatterns with no signicant diffraction peaks, whereas relativelystrong polycrystalline peaks of ZnO are clearly apparent in theZnO lms on AlOx gate dielectrics prepared from self-combus-tive precursors. Interestingly, the intensity of the (002) peak isrelatively stronger than that of the (100) and (101) peaks in theZnO lm prepared on the AlOx gate dielectric prepared fromthe self-combustive precursor, indicating that the direction ofthe ZnO crystals is preferentially with the c-axis oriented alongthe surface of the AlOx gate dielectric. The preferential orien-tation of the (002) direction indicates vertical growth of the ZnOcrystals, which is advantageous for the enhancement the TFTmobility because it provides a favorable charge conductionpathway. It is believed that the more crystalline nature of theAlOx layer due to the additional heat supplied by the self-combustive precursors can affect the microstructure of theadjacent ZnO layer (ESI, Fig. S8†), as compared to the case forthe layers produced from the combustive precursor with urea orthe noncombustive precursor. During the annealing of the ZnOlayer, the more crystalline structure of the AlOx prepared fromself-combustive precursors beneath the ZnO layer can reduce

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Fig. 6 AFM images of ZnO films on AlOx thin films (a) without GPTMS and (b) with GPTMS.

Fig. 7 XRD patterns of the ZnO thin films on AlOx gate dielectricsprepared from three different precursors after annealing at 250 �C: (a)self-combustive AlOx precursor with GPTMS, (b) combustive AlOx

precursor with urea with GPTMS, (c) noncombustive Al(NO3)3$9H2Owith GPTMS, and (d) noncombustive Al(C2H5O2)3 with GPTMS.

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the amount of interruption of the crystal formation due tolattice mismatching, which ultimately leads to the more crys-talline character of the grown ZnO.25–28 Consequently, the self-combustion reaction of the AlOx precursors results in a morecrystalline structure of the nal AlOx dielectric annealed at a lowtemperature, which yields a favorable semiconducting ZnOcrystal structure for good TFT characteristics.

3. Conclusions

We successfully fabricated an AlOx gate dielectric using a self-combustion reaction in a solution process at a low temperatureof 250 �C. The self-combustion of the AlOx precursors wasinvestigated in terms of their thermal behaviors, and thecombustive AlOx precursors were converted into high-k AlOx

This journal is © The Royal Society of Chemistry 2014

gate dielectrics at low temperatures by the self-combustionreaction. In addition, organic components (GPTMS) wereincorporated into the soluble AlOx precursors using the sol–gelmethod to improve the surface properties, which can provide awell-dened interface between the gate dielectric and the activelayer. The electrical characteristics of the AlOx gate dielectricswith various amounts of GPTMS were analyzed and compared.The dielectric constants and capacitances of AlOx gate dielec-trics with AlOx precursor : GPTMS ratios higher than 1 : 8 areunstable in the low- and high-frequency ranges. The AlOx gatedielectric layer prepared using a 1 : 8 ratio of combustive AlOx

precursor to GPTMS exhibits quite stable insulating propertieswith a leakage current density of 1.80 � 10�8 A cm�2. Further-more, AlOx dielectrics prepared from self-combustive precur-sors exhibit a high dielectric constant of 8.7. In comparison, thelayer prepared from the combustive AlOx precursor composedof Al(NO3)3$9H2O and urea as an additional fuel, which hasbeen studied previously, shows quite unsatisfactory dielectricproperties and a high leakage current due to the low combus-tion efficiency. The ZnO TFTs with high-k AlOx prepared fromself-combustive precursors show an excellent eld-effectmobility of 24.7 cm2 V�1 s�1 and an on/off ratio of 105. Suchsignicant enhancement of the mobility of ZnO TFTs due to thevertical growth of the ZnO crystal structure can be attributed tothe crystalline structure of the AlOx gate dielectric preparedfrom the combustive precursors beneath the ZnO layer. Thiscrystallinity was promoted by the heat provided by thecombustion reaction. Hence, these electrically stable AlOx gatedielectrics prepared using low-temperature processing demon-strate the potential application of the proposed system in theeld of exible printed electronics.

4. Experimental sectionInstrumentation

A TG-DTA (SDT 2060, TA Instruments, USA) was used tomeasure the thermal behaviors of the AlOx precursors, whichwere dried at 70 �C for 24 h. Samples of about 10 mg to 15 mgwere heated from room temperature to 700 �C at a heating rateof 10 �C min�1 in ambient air in all cases. An X-ray diffrac-tometer (XRD; RIGAKU Co., Japan) was used to identify thecrystal structures of the ZnO thin lms coated on the AlOx gatedielectrics and AlOx thin lms. XRD measurements were

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performed using the thin lm diffraction technique, in whichsamples were xed at a low angle of 3� to the X-ray beam duringthe 2q scan of the detector. The source power was 60 mA/40 kV,which is stronger than the power used in normal XRD. Toinvestigate the nanocrystalline structure of the AlOx lm, TEM(JEOL Ltd., Japan) was employed. The TEM samples wereprepared by spin-coating the AlOx precursor onto a copper grid.The prebaking and annealing processes were conducted in thesame manner used for MIM device fabrication described below.The cross-sections of the ZnO TFTs were also investigated byTEM. The cross-sectional samples were cut by using a focusedion beam (JEOL Ltd., Japan). The surface morphology of theAlOx lms and ZnO thin lms was investigated using a SEM(Philips XL30S FEG, The Netherlands) and an AFM (NanoscopeSystem Co., Korea) in an area of 3 mm2 in noncontact mode.Before the analysis of the lms, platinum lms with a thicknessof approximately 100 nm were deposited on the prepared lmswith a coating machine (E-1030, Hitachi Ltd., Japan).

Preparation of self-combustive and noncombustive AlOx

precursor solutions

For the combustive AlOx precursor solutions, aluminum nitratenonahydrate (Al(NO3)3$9H2O, 99.997%) and aluminum acety-lacetonate (Al(C2H5O2)3, 99%) were dissolved in anhydrous2-methoxyethanol. All chemicals for this procedure wereobtained from Sigma-Aldrich. The noncombustive AlOx

precursor solutions were prepared with equivalent concentra-tions of 0.05 M Al(NO3)3$9H2O and 0.05 M Al(C2H5O2)3 in 10 mlof anhydrous 2-methoxyethanol. The solutions were thoroughlystirred for more than 2 h at room temperature to prepare thehomogeneous precursor solution. The total concentration ofAlOx precursor solution was xed at 0.1 M. To improve thesurface properties of the AlOx lms, GPTMS solutions wereadded into the AlOx precursor solutions in various molar ratios.The GPTMS solutions were prepared by hydrolyzing GPTMS in2-propanol by adding a dilute aqueous solution of HNO3 andstirring at room temperature for 1 h.29 For the comparison ofself-combustive AlOx precursor solutions with noncombustiveprecursors for AlOx gate dielectrics, two types of solutions wereprepared by separately dissolving 0.1 M Al(NO3)3$9H2O and 0.1M Al(C2H5O2)3 in 10 ml of anhydrous 2-methoxyethanol.

Preparation of urea-combustive AlOx precursor solutions

The combustive precursor solutions with urea were prepared asdescribed in the literature.22

Preparation of ZnO precursor solutions

For the ZnO precursors, 0.1 M zinc hydroxide (Zn(OH)2, 98%,Junsei Co., Japan) was dissolved in 10 ml of ammoniumhydroxide (NH4OH, �20%, OCI Co., Korea) and stirred for lessthan 24 h in an ice bath.30

Fabrication of MIM structures

To measure the electrical characteristics of the AlOx gatedielectric lms, MIM structures consisting of Au/AlOx/indium-

5702 | J. Mater. Chem. C, 2014, 2, 5695–5703

tin-oxide (ITO) layers were fabricated under various conditions.The AlOx precursor solutions were deposited on ITO-coatedglass substrates using a spin-coating method. Prior to deposi-tion, the ITO-coated glass substrates were consecutively cleanedby ultrasonication in a detergent solution, acetone, and hotisopropyl alcohol. The ITO-coated glass substrates were thendried and treated with ozone plasma for 20 min. The AlOx

precursor solutions were spin-coated at 4000 rpm for 30 s andprebaked on a hot plate at 130 �C for 10 min to remove theresidual solvent. Aer prebaking, the AlOx lms were annealedat 250 �C for 1 h in a vacuum oven. The circular electrodes (100nm thickness, Au) were thermally evaporated through apatternedmetal shadowmask. The area of the circular electrodewas 25 mm2.

Fabrication and electrical measurement of ZnO TFTs

A bottom-gate top-contact design was used as a typical devicestructure for the ZnO TFTs. AlOx precursor solutions were spin-coated and annealed on ITO in the same manner describedabove to form a gate dielectric layer. The ZnO precursor solu-tions were then deposited on the obtained AlOx gate dielectricusing a spin-coating method. Prior to deposition, the AlOx gatedielectric was treated with ozone plasma for 20 min. The ZnOprecursor solutions were spin-coated at 1000 rpm for 45 s andprebaked on a hot plate at 90 �C for 5 min to remove anyresidual solvents. Aer pre-baking, the ZnO thin lms wereannealed at 250 �C for 1 h on a hot plate in ambient air. The topcontact source and drain electrodes (120 nm thickness, Al) werethermally evaporated through a patterned metal shadow mask.The ratio of the channel width to length was 60 (width: 3000 mm;length: 50 mm).

The electrical characteristics of the ZnO TFTs were measuredin air using an Agilent semiconductor parameter analyzer4155C. The measurements were typically performed using acontinuous method, and the transfer curve was recorded beforethe output curve. The eld-effect mobility (mlin) and thethreshold voltage (Vth) were calculated for the linear regionusing eqn (1),

ID ¼ W

LmlinCiðVG � VthÞVds (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 the Ministry of Science, ICT, and the Future Plan-ning and Korea Research Council for Industrial Science andTechnology of the Republic of Korea (Grant B551179-09-06-00).

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