Superlattices and Microstructures 59 (2013) 21–28

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o ca t e / s u p e r l a t t i c es

Bendable thin-film transistors based on sol–gelderived amorphous Ga-doped In2O3

semiconductors

0749-6036/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.03.026

⇑ Corresponding authors. Tel.: +82 10 9755 2315.E-mail addresses: sjeong@krict.re.kr (S. Jeong), youngmin@krict.re.kr (Y. Choi).

Sunho Jeong a,⇑, Ji-Yoon Lee a, Moon-Ho Ham b, Keunkyu Song c, Jooho Moon c,Yeong-Hui Seo a, Beyong-Hwan Ryu a, Youngmin Choi a,⇑a Advanced Materials Division, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong-gu, Daejeon 305-600,Republic of Koreab School of Materials Science and Engineering and Department of Nanobio Materials and Electronics, Gwangju Institute of Scienceand Technology, Gwangju 500-712, Republic of Koreac Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro Seodaemun-gu, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:Received 15 November 2012Received in revised form 21 March 2013Accepted 28 March 2013Available online 16 April 2013

Keywords:BendableTransistorSol–gelSemiconductor

a b s t r a c t

Bendable thin-film transistors (TFTs) are demonstrated based onsol–gel-derived amorphous Ga-doped In2O3 (GIO) that can bethermally converted into a device-quality semiconducting layer at300 �C, which is compatible with a plastic polyimide (PI) substrate.The device performance of the GIO TFTs is studied through the inves-tigation on the electrical parameters (including mobility, thresholdvoltage, off-current, and subthreshold swing) of the devices as afunction of Ga composition. With increasing Ga composition up to36 mol%, the mobility decreases from 1.4 to 0.08 cm2 V s�1 withsluggish reduction in the Ga compositional range between 0 and12 mol%, and the threshold voltage shifts from�21.6 to 13.5 V. Boththe off-current and subthreshold swing decreases with a dramaticvariation at Ga composition of 12 mol%. From the overall analysis,it is concluded that the incorporation of 12 mol% Ga enables forthe GIO semiconducting layer with the best electrical performance.In addition, the bending characteristics of GIO TFTs, prepared on aSiO2/ITO/PI substrate, are analyzed with device performancevariations depending on the bending radius. It is demonstratedthat the device performance is maintained with acceptableelectrical characteristics under a bending radius of 10 mm.

Crown Copyright � 2013 Published by Elsevier Ltd. All rightsreserved.

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1. Introduction

Amorphous metal oxide semiconductors are promising candidates as transparent semiconductorsfor thin-film transistor (TFT) circuitry due to their favorable field-effect mobility, excellent environ-mental/thermal stability, high optical transparency, and high uniformity in large-scale fabricationapplications [1]. Furthermore, solution-processed amorphous metal oxide semiconductors offer manyadvantages, including simplicity, low cost, and high throughput, facilitating the fabrication of high-performance, low-cost electronics. Solution-processed metal oxide semiconductors, however, have amajor drawback, that is, a high-temperature annealing process for device-quality semiconductor films[2–4], which limits the use of plastic films as a substrate for flexible electronics.

Recently, a fully flexible ZnO thin-film transistor with the field-effect mobility of 0.03 cm2 V�1 s�1

was reported based on the use of an amine-hydroxo chemical complex [5], which is the most viablesolution-processable precursor in terms of low-temperature processability [6]. However, it suffersfrom limits in adjusting the chemical composition to enhance device performance; except for Zn, otherfunctional elements do not easily form the ammine-hydroxo complex, and the successfully synthe-sized chemical complex is not thermally converted into corresponding oxides at low temperatures.In contrast, metal salt-based sol–gel precursor solutions not only allow a device-quality metal oxidesemiconducting layer to be formed via a thermally activated chemical conversion reaction, but alsooffer the availability on versatile chemical compositions that improve device performance and biasstability [7,8]. The major drawback, a high-temperature annealing, has been resolved through therecent chemical/physical approaches for creating the metal oxide skeleton at low temperatures[9–13]. However, to date, metal salt-based, bendable metal oxide TFTs have been rarely reporteddue to the lack of optimization for low-temperature processable precursor solutions. Furthermore,the bending characteristics of TFTs prepared on plastic substrates have not been investigated.

In this study, we present the design of a metal nitrate-based Ga-doped In2O3 (GIO) precursor solu-tions which can generate device-quality metal oxide semiconductors at a low temperature compatiblewith a plastic polyimide substrate, and demonstrate the feasibility of bendable GIO TFTs in conjunc-tion with an investigation of their bending characteristics.

2. Experimental details

GIO precursor solutions were synthesized using indium nitrate hydrate and gallium nitrate hydrate[9]. The precursor solutions with a concentration of 0.2 M were prepared in 2-methoxyethanol (99%)with monoethanolamine (99%) as a stabilizer. The chemical compositions of GIO precursor solutionwere In:Ga = 94:6, 88:12, 76:24, and 64:36 in molar ratio. Before the GIO semiconducting film isadopted as a channel layer for bendable TFTs, the devices, with a configuration of bottom gate andtop contact, were fabricated on a rigid substrate to optimize the device performance of GIO TFTs asa function of Ga composition. Each precursor solution was spin-coated on a doped silicon substratewith a 300 nm-thick thermal SiO2 layer, followed by annealing at 300 �C for 30 min. Then, 50 nm-thickAl source and drain electrodes were deposited via thermal evaporation through shadow masks. Thechannel length and width were 100 and 1000 lm, respectively. According to the device performances,which will be discussed later, the 12 mol%-Ga doped In2O3 semiconducting layer showed the bestelectrical characteristics; thus, Bendable GIO TFTs on a plastic polyimide substrate were fabricatedby depositing 12 mol% Ga-doped In2O3 precursor solution on a SiOx (PECVD grown at 300 �C, thickness(t) = 270 nm)/ITO (t = 50 nm)/PI (t = 50 lm) substrate. The spin-coated GIO precursor layer wasannealed at 300 �C for 30 min, followed by thermal evaporation of the Al source/drain electrodes withchannel dimensions identical to those of TFTs on a rigid substrate.

The electrical performance of the transistors was analyzed in ambient condition using an AgilentE5270B source-measure unit. Saturation mobilities were extracted from the slope of (drain cur-rent)1/2 versus gate voltage derived from the device transfer plot. The crystal structures were analyzedwith a grazing angle X-ray diffraction (XRD) using Cu Ka radiation on a Rigaku ATX-G thin-filmdiffraction workstation. The grain size and surface morphology were observed by atomic force

Fig. 1. (a) Transfer characteristics for GIO TFTs on a rigid SiO2=Siþ substrate as a function of Ga composition in precursorsolutions. (b) Output characteristic GIO (12 mol% Ga) TFTs on a rigid SiO2=Siþ substrate.

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microscope (Digital Instruments, NanoScope IV). The cross-sectional microstructure of solution-pro-cessed, bendable GIO TFT was investigated by a high-resolution transmission electron microscopy(HR-TEM, JEM-2100F, JEOL), and the sample was prepared using a focused ion-beam technique.

3. Results and discussion

For low-temperature annealed oxide semiconductors based on metal-salt-based sol–gel chemistry,the chemical structure should be tailored toward an oxide framework containing less hydroxide at alow temperature, which can be derived through the doping of Ga with a high bonding strength withoxygen ions [9]. In addition, the presence of impurities due to incomplete decomposition of anions inprecursors critically determines device performances [12]. In this regard, the addition of Ga in metaloxide semiconductors derived from nitrate precursors, the anions of which are completely decom-posed at low temperatures, is a viable approach. In2O3 is a proper oxide semiconductor showing a rea-sonable electrical performance; thus, Ga-doped In2O3 oxide semiconductor derived from nitrate

Fig. 2. (a) XRD results and (b) AFM images for IGO semiconductors with different Ga composition. The surface roughness of IGOsemiconductors with 6, 12, 24 and 36 mol% of Ga composition are 0.5, 0.6, 0.5, 0.7 nm, respectively.

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precursors could be one of promising candidate for low-temperature annealed, solution-processableoxide semiconductors.

Fig. 1a shows the transfer characteristics for GIO TFTs with various Ga compositions ranging from 6to 36 mol%. The electrical performance of Ga-doped oxide semiconductors varies significantly depend-ing on the Ga composition. Fig. 1b shows the output characteristic with a clear pinch-off and drain cur-rent saturation behavior for the GIO (12 mol% Ga) TFT. As shown in Fig. 2, all of IGO semiconductorswere an amorphous phase and the surface morphology did not vary significantly regardless of Ga com-position. Thus, the crystalline structure and morphological property are not a determining factor fordevice performances of IGO TFTs. In the viewpoint of chemical structure, the addition of Ga leads tothe reduction of oxygen vacancy that acts as a source for generating charge carriers as well as facili-tates the metal oxide framework with less hydroxide [9]. This implies that, the electrical mobilitywould be improved as a result of reduced hydroxide content, but the addition of Ga also has theadverse influence on electrical mobility owing to lowered oxygen vacancy concentration [9]. However,as seen in the variation of mobility depending on the Ga composition (Fig. 3a), the negative contribu-tion of the reduced oxygen vacancy concentration is predominant in GIO semiconductors derived fromnitrate precursors. The mobility decreased from 1.4 to 0.08 cm2 V�1 s�1, as the Ga compositionincreased up to 36 mol%. This predominant role of the reduced charge carrier concentration is also

Fig. 3. The extracted electrical parameters for GIO TFTs on a rigid SiO2=Siþ substrate as a function of Ga composition: (a)variation of mobility and threshold voltage (VTH), and (b) variation of off-current (Ioff) and subthreshold swing (SS).

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confirmed by the fact that the threshold voltage shifts from �21.6 to 13.5 V as the Ga composition in-creases. For oxide semiconductors with less oxygen vacancies (i.e. charge carriers), more charge car-riers need to be accumulated in the channel region to bend the conduction band enough to reach thethreshold voltage [14], which shifts the threshold voltage toward a positive bias. If the contribution bythe chemical structure with less hydroxide has a critical role in device performance, the thresholdvoltage should shift toward a negative bias. In n-type semiconductors, the hydroxyl groups are wellknown trap sites for electrons [15]; thus, as the number of trap sites, i.e., hydroxyl groups, decreases,a lower gate bias is required to reach the threshold voltage due to the lack of states to be filled by thecharge carriers.

In terms of the off-current and subthreshold swing, the device performance was significantlyenhanced by incorporating 12 mol% Ga as shown in Fig. 3b. The off-current and subthreshold swingwere improved drastically from 4 � 10�8 to 2 � 10�10 A and from 11.1 to 3.4 V/dec, respectively.The improvement of both electrical parameters is due to the aforementioned chemical structural evo-lution involved in the reduction of oxygen vacancy and hydroxide. Oxygen vacancy is the main sourcefor charge carrier generation, and hydroxide acts as a site for charge conduction in oxides by thePoole–Frenkel mechanism [16]. Therefore, the evolution of the chemical structure with less oxygen

Fig. 4. (a) A photograph and (b) cross-sectional high-resolution transmission electron microscopy (HRTEM) image and selectivediffraction pattern for bendable 12 mol% Ga-doped In2O3 TFTs fabricated on a plastic polyimide substrate. The inset is amagnified HRTEM image.

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vacancy and hydroxide increases the electrical resistance of channel layer, resulting in the suppressionof off-current. The subthreshold swing is associated with the density of trap states at the interfaceadjacent to a gate dielectric, which is mainly attributed to the hydroxyl group as well as positivelycharged oxygen vacancy in solution processed oxide semiconductors [17]. This implies that as theGa composition increases, the subthreshold swing would be improved owing to the gradual reductionof density of trap states. Therefore, taking into the consideration all of electrical parameters, includingelectrical mobility, off-current, and subthreshold swing, the 12 mol% Ga-doped In2O3 semiconductinglayer exhibited the best device performance, when annealed at 300 �C, which is compatible to a plasticpolyimide substrate.

For demonstrating the applicability of sol–gel derived, low-temperature annealed GIO semiconduc-tors as a bendable channel layer, bendable GIO TFTs were fabricated by depositing 12 mol% Ga-dopedIn2O3 precursor solution on a PECVD grown SiOx/ITO/PI substrate. The complete bendable GIO TFTs ona plastic PI substrate is shown in Fig. 4a. The cross-sectional HRTEM image of Fig. 4b reveals that thesolution-processed, bendable GIO TFTs on a plastic substrate possess ultrathin (8 nm-thick), coherentinterfaces without interlayer defects and/or delamination between semiconductor and dielectric layer.The amorphous-like phase, preferred for bendable functional thin films, of the GIO layer is confirmedby the selective area diffraction pattern.

The transfer characteristics of bendable GIO TFTs in relation to the bending radius are shown inFig. 5a, and the variation of representative electrical parameters, mobility and off-current, are shownin Fig. 5b. When compressive force is not applied, the mobility and off-current for the bendable GIOTFTs were measured to be 0.36 cm2 V�1 s�1 and 5 � 10�9 A, respectively. In comparison to GIO TFTon a SiO2=Siþ substrate, the slight degradation of both mobility and off-current in bendable GIO TFTsresults from the thickness variation of the gate dielectric on a rough PI substrate as well as the pooroxide quality of the low-temperature deposited SiOx dielectric. The insufficient supply of thermalenergy during a deposition process leads to an imperfect chemical structure with undesirableSi–OH and Si–H bonds [18]. Until the bending radius decreased to 8 mm, the mobility did not

Fig. 5. (a) Transfer characteristics and (b) variation of mobility and off-current for bendable GIO TFTs as a function of bendingradius.

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noticeably vary in the range between 0.36 and 0.44 cm2 V�1 s�1, showing the bendable TFT character-istics. This implies that the GIO oxide semiconductor is active without the critical degradation of elec-trical performance, when the compressive force with a bending radius of 8 mm is applied. In fact, themobility did not vary even after a repeated bending test. However, the off-current abruptly increasedto 6 � 10�8 A under the bending radius of 8 mm, and under the bending radius of 4 mm, the devicewas inactive due to the breakdown of the gate dielectric layer. The degradation of the off-currentand the electrical breakdown of gate dielectric are associated with the increment of the leakage cur-rent by the gradual electrical failure of the low-temperature deposited SiOx gate dielectric layer. Evenwith the imperfection of SiOx layer grown by PECVD at a low temperature, the electrical propertiesshould be enough to be applied as a gate dielectric for TFT. In the case of TFT employing the ZnO semi-conducting layer derived from an amine-hydroxo chemical complex, the device operates well withslightly increased off-current [5]. The amine-hydroxo chemical complex does not accompany the

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reactive by-product during a oxide film formation process at elevated temperature. However, inmetal-salt-based sol–gel chemistry, the chemical conversion of precursors for metal oxide frameworkformation occurs via thermally activated chemical reaction involving reactive chemical moieties,which thermally and chemically deteriorate the electrical/mechanical property of underlying gatedielectrics in bottom-gate architecture devices. Thus, for fully bendable TFTs based on sol–gel-derivedmetal oxides, the development of low-temperature processable, chemically robust, bendable gatedielectric layers is a prerequisite. Alternatively, the device architecture with top-gate configurationcould offer the possibility for fully bendable TFTs employing the metal-salt sol–gel-derived metaloxides.

4. Conclusions

In summary, bendable, solution-processed metal oxide TFTs were fabricated based on sol–gel-derived Ga-doped In2O3 semiconductors activated by annealing at 300 �C. The device performanceof GIO TFTs prepared on a rigid SiO2=Siþ substrate was analyzed as a function of the Ga composition,and the best device performance, with the mobility of 1.0 cm2 V�1 s�1, off-current of 2 � 10�10 A, andsubthreshold swing of 3.4 V/dec, was achieved for 12 mol% Ga-doped In2O3 TFTs. The bendable TFTswere successfully demonstrated by adopting the optimized GIO semiconductor as a channel layerfor transistors prepared on SiO2/ITO/PI substrate, exhibiting the mobility of �0.4 cm2 V�1 s�1 undera bending radius of 10 mm.

Acknowledgements

This work was supported by a project funded by the Ministry of Knowledge Economy (KK-1102-B0), and by a grant from the Industrial Source Technology Development Program funded by the Min-istry of Knowledge and Economy (TS-101-39). It was partially supported by the Mid-Career ResearcherProgram through an NRF grant funded by the MEST (No. 2009-0086302). M.H. Ham acknowledge sup-port by the WCU program through an NRF Grant (R31-10026) of MEST.

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