Solvent-mediated threshold voltage shift in solution-processed transparent oxidethin-film transistorsYong-Hoon Kim, Hyun Soo Kim, Jeong-In Han, and Sung Kyu Park Citation: Applied Physics Letters 97, 092105 (2010); doi: 10.1063/1.3485056 View online: http://dx.doi.org/10.1063/1.3485056 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/97/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of oxygen vacancies on the bias illumination stress stability of solution-processed zinc tin oxide thin filmtransistors Appl. Phys. Lett. 105, 023509 (2014); 10.1063/1.4890579 Impact of dopant species on the interfacial trap density and mobility in amorphous In-X-Zn-O solution-processed thin-film transistors J. Appl. Phys. 115, 214501 (2014); 10.1063/1.4880163 Solution-processed zinc-indium-tin oxide thin-film transistors for flat-panel displays Appl. Phys. Lett. 103, 072110 (2013); 10.1063/1.4818724 Photo stability of solution-processed low-voltage high mobility zinc-tin-oxide/ZrO2 thin-film transistors fortransparent display applications Appl. Phys. Lett. 102, 123506 (2013); 10.1063/1.4795302 Band transport and mobility edge in amorphous solution-processed zinc tin oxide thin-film transistors Appl. Phys. Lett. 97, 203505 (2010); 10.1063/1.3517502

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Solvent-mediated threshold voltage shift in solution-processed transparentoxide thin-film transistors

Yong-Hoon Kim,1 Hyun Soo Kim,1 Jeong-In Han,2 and Sung Kyu Park3,a�

1Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi 463-816,Republic of Korea2Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, Seoul 100-715,Republic of Korea3Department of Textile Engineering, Convergence Materials and Devices Laboratory,Chonbuk National University, Jeonju 561-756, Republic of Korea

�Received 20 July 2010; accepted 11 August 2010; published online 31 August 2010�

We investigated solvent-mediated threshold voltage �VTH� shift in solution-processed zinc–tin oxide�ZTO� thin film transistors �TFTs�. The ZTO TFTs showed negative VTH shift when exposed tovarious organic solvents such as hexane, isopropanol, and chlorobenzene. Additionally themagnitude of the shift showed a close relationship with the dielectric constant or electronegativityof the solvent molecules. From the experiments, one of the origins of the VTH shift in the transparentoxide TFTs appears to be closely correlated with the dipole interaction of the solvent molecules andZTO back channel surface. © 2010 American Institute of Physics. �doi:10.1063/1.3485056�

Oxide semiconducting materials are now of great inter-est in the field of high functional active-matrix displays asalternatives for conventional amorphous and polycrystallinesilicon �poly-Si� channels. The oxide semiconductors suchas indium–gallium–zinc oxide1–3 and zinc–tin oxide �ZTO��Refs. 4–6� exhibit high carrier mobility even in amorphousstate due to high symmetry of s-orbitals of heavy metal ions.This unique characteristic of oxide semiconductors providesbetter uniformities over laser-crystallized poly-Si thin-filmtransistors �TFTs�. However, the oxide semiconductor basedTFTs usually suffer from large threshold voltage �VTH� shiftduring operation. Back channel effects by water and oxygenmolecules, charge trapping at the semiconductor/gate dielec-tric interface, and charge injection7–11 have been consideredas the several reasons for the VTH shift.

Among them, the back channel effect by gaseous mol-ecules has been considered as one of the main causes of theVTH shift during electrical bias stressing. It should be notedthat the oxides such as zinc oxide �ZnO�, and tin oxide�SnO2� films are sensitive to various molecules and used fordetecting volatile organic compounds, gases, and solventmolecules.12,13 Therefore, it is quite reasonable to considerthe electrical properties of TFTs based on ZnO, SnO2, andtheir composites are influenced by such organic molecules.To minimize the back channel effect �VTH shift�, the interac-tion between external molecules and oxide surface should besuppressed. Typically, in vacuum process, high density pas-sivation layers �etch stop layers� have been achieved usingplasma-enhanced chemical vapor deposition or atomic-layerdeposition.14,15 However, in solution-processed oxide TFTs,introducing the vacuum processes for passivation is rathermeritless. Solution-processed inorganic or organic materialsare more likely to be adopted for passivation layer but insuch case there is some possibility of exposing the oxidesurface to organic solvents, acids, or bases which may alterthe intrinsic electrical characteristics of the device.

In present work, the solvent-mediated VTH shift insolution-processed ZTO TFTs was investigated in order tostudy the relationship between the chemical properties oforganic molecules and variation in the electrical properties ofthe oxide surface. It was observed that the magnitude of theVTH shift has a close relationship with the dielectric constant,or polarity of the organic molecules. Generally, organic mol-ecule with higher dielectric constant induced larger VTHshift.

The ZTO TFTs were fabricated on heavily doped p-typeSi wafer which served as both common gate electrode andsubstrate. As a gate dielectric layer, 200-nm-thick thermallygrown SiO2 layer was used. For the formation of ZTO activelayer, ZTO solution was first prepared by dissolving 0.07 Mtin chloride and 0.07 M zinc chloride powders in acetonitrile.The solution was then spin coated on SiO2 gate dielectric andthermally annealed at 500 °C for 1 h in air ambient achiev-ing a thickness of �30 nm. Next, the channel was patternedby photolithography and wet etching process. After channeldefinition, 100-nm-thick indium–zinc oxide was depositedby rf-magnetron sputter and patterned by liftoff process toform top-contact structure. The channel width and length ofthe devices were 200 �m and 10 �m, respectively.

For the evaluation of solvent-mediated VTH shift in ZTOTFTs, the TFT samples were submerged in each organic sol-vent of hexane, chlorobenzene �CHB�, isopropanol �IPA�,and acetonitrile. After submerging of �1 min, the sampleswere then dried by a stream of dry nitrogen. The current-voltage characteristics of the TFTs were measured underdark and air ambient using Keithley 4200-SCS semiconduc-tor parameter analyzer.

Figure 1 shows log �IDS�-VGS and �IDS-VGS curves ofZTO TFTs which were exposed to various organic solvents�with VDS=40 V�. The fresh device means a TFT devicewhich was not exposed to any solvent. The fresh device hadVTH of 6.3 V �VON of �1 V� with field-effect mobility3.0 cm2 V−1 s−1. After solvent exposure, negative VTH shiftwas observed in all cases with small variation in subthresh-old slope �0.92–1.40 V decade−1�. The measured VTH after

a�Author to whom correspondence should be addressed. Electronic mail:skpark1004@jbnu.ac.kr.

APPLIED PHYSICS LETTERS 97, 092105 �2010�

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exposing to hexane, CHB, IPA, and acetonitrile were �0.4 V,�14.2 V, �35.6 V, and �41.2 V, respectively. As noticed,the magnitude of the VTH shift depends on the dielectricconstant of the organic solvent. Generally, in n-type oxideTFTs, negative VTH shift is observed when the carrier con-centration or the number of carriers in the channel is in-creased, whether by compositional effect, back channeleffect, or by channel thickness effect.7,16,17 In these experi-ments, the compositional effect and channel thickness effectare minimal because the Zn:Sn composition ratio is fixed andthe thickness variation in ZTO channel by spin-coating isnegligible. Therefore, the back channel effect is expected tobe the most case. The interactions of water/oxygen mol-ecules and the oxide back channel during the gate bias-stresshave been reported previously.7,18–20 Also, it was suggestedthat these interactions caused a significant VTH shift duringelectrical bias stressing. In n-type oxide semiconductors suchas SnO2, the adsorption of oxygen molecules on the surfacemight cause formation of depletion layer, decreasing the con-ductivity of the film at high interactive conditions.19 Volatileorganic compounds such as methyl alcohol, and IPA are alsoknown to interact with the oxide surface and change the con-ductivity of the film.21,22 Surface chemical reaction �etching�of IPA and ZnO nanowires was also reported.23 The chemicalreaction creates higher surface defect sites for oxygen mol-ecule adsorption which extends the depletion layer in the

ZnO nanowire resulting in positive VTH shift. However, thereported VTH shift behaviors are opposite to what was ob-served in our study. It may be attributed that submerging thedevices for �1 min is rather short time for such reaction toinfluence the VTH shift �compared to 15–30 days of submerg-ing in IPA in Ref. 23�. Therefore, although it is not fullyunderstood but the adsorption of solvent molecules on theZTO back channel surface may be one of the main causes forthe VTH shift in our experiments.

The magnitude of the VTH shift, as indicated earlier, isdependent on the dielectric constant of the submerging sol-vent. Such result can be explained by terms of polarity orelectronegativity of the adsorbed solvent molecules. As illus-trated in Fig. 2, when solvent molecules are adsorbed on theback channel surface, extra electron carriers can be accumu-lated near the channel surface possibly due to electronegativ-ity of the solvent molecules. If the adsorbed solvent mol-ecule has higher polarity �higher dielectric constant� and thushigher electronegativity �Fig. 2�b��, then the number of ac-cumulated carriers near the surface will be larger comparedto when solvent molecule with lower polarity is adsorbed�Fig. 2�a��. This is possibly due to more band bending nearthe back channel surface caused by adsorption of high-polarsolvent molecules as described in Fig. 2�c�. For the resultsdescribed here, it is likely that solvent with higher dielectricconstant will induce more carriers on the surface �more VTHshift�. In Fig. 3 and Table I, the relationship between thedielectric constant of the solvent and the corresponding VTH�VON� variation is demonstrated. High dielectric constantsolvent such as IPA �dielectric constant 18� and acetonitrile

FIG. 1. �Color online� The �a� log �IDS�-VGS and �b��IDS-VGS curves of ZTO TFTs exposed to various or-ganic solvents �VDS=40 V�. The channel width andlength of the TFTs are 200 �m and 10 �m,respectively.

FIG. 2. �Color online� Schematics showing charge accumulation at the backchannel surface by adsorption of �a� low polarity and �b� high polaritymolecules. �c� Energy band diagrams of ZTO channel layer near the backchannel surface.

FIG. 3. �Color online� The dependence of threshold voltage �VTH� andturn-on voltage �VON� on dielectric constant of submerging solvents.

092105-2 Kim et al. Appl. Phys. Lett. 97, 092105 �2010�

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�dielectric constant 38.5� caused more negative VTH shift,whereas hexane �dielectric constant 1.88� has shown a littleeffect. This result clearly indicates that the dielectric constantof the adsorbed molecule has a strong relationship with themagnitude of the VTH shift.

In summary, the solvent-mediated VTH shift in solution-processed ZTO TFTs was investigated in this study. TheZTO TFTs showed negative VTH shift when exposed to or-ganic solvents and the magnitude of the VTH shift appearedto be closely related to the dipole interaction of the solventmolecule and oxide back channel. This work demonstratesthat solution-processed ZTO TFTs with appropriate passiva-tion layers can provides a possible path to low-cost electron-ics and high definition sensor applications.

This research was supported by a grant �Grant No.F0004023-2010-33� from Information Display R&D Center,one of the Knowledge Economy Frontier R&D Programfunded by the Ministry of Knowledge Economy of Koreangovernment.

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TABLE I. The dielectric constant of organic solvents and electrical properties of ZTO TFTs after submergingin the solvents.

Solvent Dielectric constantVON

�V�VTH

�V�Mobility

�cm2 V−1 s−1�Subthreshold slope

�V decade−1�

Fresh sample ¯ �1 6.3 3.0 0.95Hexane 1.88 �7 �0.4 2.6 0.92Chlorobenzene 5.62 �22 �14.2 3.7 1.05IPA 18.0 �46 �35.6 2.7 1.34Acetonitrile 37.5 �51 �41.2 2.9 1.40

092105-3 Kim et al. Appl. Phys. Lett. 97, 092105 �2010�

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