Journal of the Korean Physical Society, Vol. 65, No. 10, November 2014, pp. 1555∼1558

Influence of a Self-assembled Monolayer on Indium-Zinc-OxideSemiconductor Thin-film Transistors

Yong Suk Yang,∗ In-Kyu You, Sung-Hoon Hong, Ju-hyeon Park and Ho-Gyeong Yun

Electronics and Telecommunications Research Institute (ETRI), Daejeon 305-700, Korea

Young Hun Kang, Changjin Lee and Song Yun Cho

Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Korea

(Received 1 December 2013, in final form 1 May 2014)

The fabrication of an active-matrix liquid-crystal display by using printing processes offers thepotential to reduce the number of photolithography steps and the manufacturing costs. In thisstudy, we prepare the indium-zinc-oxide (IZO) thin-film transistors (TFTs) by using non-vacuumprocesses such as inkjet printing. The self-assembled monolayers of hexadecanethiol (HDT) on thesurface of the oxide semiconductor prior to the inkjet printing of Ag were employed to modify theelectric barrier between the IZO and the printed Ag. The field-effect mobility of the IZO TFTswith 0.5-mM HDT treatments and with the inkjet-printed Ag electrodes that were investigated byusing their current-voltage characteristics was approximately 0.36 cm2/Vs.

PACS numbers: 77.84.Bw, 73.61.Ph, 72.80.LeKeywords: Inkjet printing, Indium zinc oxide, Thin film transistors, Self-assembled monolayers, Work func-tion, Electric barrierDOI: 10.3938/jkps.65.1555

I. INTRODUCTION

Recently, there has been great interest in adaptingthin-film transistors (TFTs) made of metal oxide semi-conductors. In particular, the oxide system that con-sists of In2O3 and ZnO has shown promising electricalperformance for active layer of TFTs with a high mo-bility, a low off current, and an expected good unifor-mity compatible with a liquid-crystal display (LCD) [1,2]. The devices and the manufacturing techniques forTFT-LCDs using solution-processable materials are alsobeing studied for applications of flexible display in whichlow-cost and low-temperature manufacture is required[3–5]. However, fabricating high-performance TFTs atlower temperatures by using a solution processes suchas printing is difficult. One of the development direc-tions for fabricating TFTs by using solution processes isto make not only semiconductor ink materials but alsometal-semiconductor junctions with low work functions[6,7].

In this study, the indium-zinc-oxide (IZO) semicon-ductor TFTs were prepared using solution processessuch as spin coating and inkjet printing. To modifythe interface between the semiconductor and the metals,we coated self-assembled monolayers (SAMs) of hexade-

∗E-mail: jullios@etri.re.kr

canethiol (HDT) on the surface of the IZO semiconductorprior to the inkjet printing of Ag. The electrical prop-erties of the untreated and the HDT-treated IZO TFTswere investigated by using their current-voltage charac-teristics.

II. EXPERIMENTAL METHODS

The IZO thin films were fabricated on a SiO2(300nm)/Si substrate by using spin coating. Zinc acetylacet-onate hydrate (Zn(C5H7O2)2 ·xH2O) and indium nitratehydrate (In(NO3)3 ·xH2O) (all from Sigma-Aldrich) wereemployed for the preparation of the IZO precursor solu-tions [8]. The total concentration of the IZO precur-sor was fixed at 0.1 M, and the molar ratio of indiumnitrate hydrate to zinc acetylactonate hydrate was 1:1.The SiO2/Si substrate, which was a heavily p-type dopedand thermally oxidized silicon wafer, was prepared as asubstrate after cleaning sequentially with acetone andisopropyl alcohol for 10 min during each step and thenrinsing with de-ionized water. The IZO solution was thenfiltered using a 0.1-μm syringe filter and spin-coated at4000 rpm onto the substrate. The IZO/SiO2(300 nm)/Sifilms were annealed on a hot plate at 350 ◦C for 2 h. Theself-assembly was performed by immersing the samplesinto a 0.5-mM solution of HDT in ethanol for 12 h at

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Fig. 1. (Color online) (a) Schematic image of the inkjetdrop-on-demand fabrication and (b) the results for the mea-sured and the designed channel lengths of the source-drainelectrode.

room temperature. Then, the samples were thoroughlyrinsed with ethanol and 2-proanol, and dried with a N2

flow.Ag as a source-drain electrode was formed in the sam-

ple by using an inkjet printing process with commercialAg inks based on organosilver compounds. The Ag inkswere produced by InkTec (Republic of Korea) [9]. Theinkjet drop-on-demand fabrication of the source-drainelectrode consisted of a basic setup utilizing a heatedfluid reservoir coupled to a piezoelectric ceramic throughwhich a machined nozzle dispensed uniform polymerdroplets onto the substrate mounted on xyz microcon-trollers, as illustrated in Fig. 1(a). After the printing,the samples were annealed at 150 ◦C for 30 min. Figure1(b) shows the results for channel length in the printedsource-drain electrodes. In the case of no treatment, thechannel length of printed Ag in the IZO films was nar-rower than that of the designed pattern. Otherwise, thesamples with HDT treatment almost did not show anywetting behavior of the ink material. That is, the HDTtreatment enhanced the hydrophobicity of the surface ofIZO. To compare the characteristics of the IZO TFTs,we deposited Al as a source-drain electrode in situ on

Fig. 2. (Color online) AFM images of (a) the untreatedand (b) the HDT-treated IZO films.

IZO/SiO2(300 nm)/Si by using a conventional thermalevaporator at a pressure of below 10−4 Pa. The electri-cal property of IZO TFTs and the work functions of Al,Ag, and IZO were measured using a semiconductor char-acterization system (Keithley 4200) and a Kelvin probemethod, respectively.

III. RESULTS AND DISCUSSION

Figures 2(a) and (b) show AFM images of the un-treated and the HDT-treated IZO films, respectively.These images show that the IZO films were composedof fine small grains and that the surface roughness wasincreased after the HDT treatments. Clusters of HDTSAMs were formed in the surface after the treatment, asshown in Fig. 2(b). The root-mean-square roughnessesof the untreated and the HDT-treated IZO films are ap-proximately 0.9 and 1.7 nm, respectively.

The output characteristics of the IZO TFTs under var-ious gate-to-source voltages (VG) ranging from 0 to 40 Vare shown in Fig. 3. The evaporated Al metal was usedas a source-drain electrode. The length and the width ofthe electrode channel were approximately 100 μm and 20

Influence of a Self-assembled Monolayer on Indium-Zinc-Oxide· · · – Yong Suk Yang et al. -1557-

Fig. 3. (Color online) Output characteristics of IZO TFTsunder various VG ranging from 0 to 40 V.

mm, respectively. During the measurement, the drain-to-source voltage (VDS) was varied from 0 to 40 V. Aclear distinction between the linear and the saturationregions was observed around a VDS = 20 V.

Figures 4(a)−(c) illustrate the transfer characteristicsof the saturation region (VDS = 40 V) for IZO TFTswith several source-drain electrodes. The metals of thesource-drain electrode were evaporated Al (Fig. 4(a)),inkjet-printed Ag (Fig. 4(b)), and inkjet-printed Ag withHDT treatment (Fig. 4(c)). The lengths of the inkjet-printed Ag electrode in the bare and the HDT-treatedIZO TFTs were approximately 110 and 280 μm, respec-tively. The width of the inkjet-printed Ag electrode wasapproximately 15 mm. We extracted the field effect mo-bility (μ) and the threshold voltage (Vth) based on thestandard MOSFET equation [10].

The μ and the Vth in the IZO TFTs with the evapo-rated Al electrodes were determined to be approximately2 cm2/Vs and 1 V, respectively. In addition, the μ andthe Vth in the case of the inkjet-printed Ag electrodeswere approximately 0.02 cm2/Vs and 1.9 V, respectively.We surmise that the difference in the transfer characteris-tics for the IZO TFTs fabricated with different electrodescan be attributed to an aspect of the charge carrier in-jection between the semiconductor and the metal elec-trode. The value of the bulk band gap of IZO is taken asapproximately 3 eV, and the IZO material pins slightlyabove the charge neutrality level, resulting, according tothe previous studies, in a negative surface charge fromoccupied acceptor surface states and an upward bendingof the bands relative to the Fermi level [11,12]. The workfunction of the printed Ag metal is generally known tobe larger than that of Al [13]. The work function of theprinted Ag is expected to be further increased because ofthe electron injection barrier between the semiconductorand the metal electrode. In Fig. 4(c), the μ and theVth for the IZO TFTs with the HDT treatments andthe inkjet-printed Ag electrodes were determined to be

Fig. 4. (Color online) Transfer characteristics of the IZOTFTs with source-drain electrodes of (a) evaporated Al, (b)inkjet-printed Ag, and (c) inkjet-printed Ag with the HDTtreatments. The VDS is 40 V.

approximately 0.36 cm2/Vs and 1.5 V, respectively.To understand these results, we use a Kelvin probe

method to investigate the work functions of the Al, IZO,and Ag, and of the SAMs of IZO and Ag that were mod-ified by HDT. The IZO and Ag were prepared by usingspin-coating and inkjet printing, respectively. The mo-lar concentrations of HDT in ethanol were 0.2 and 0.5mM. The Kelvin probe method measures the differencein the electrostatic potential, that is established betweenthe surfaces of two conductors brought into contact. Theabsolute value of the work function reported here for theKelvin probe measurements was calculated by assumingthe value of the work function of a gold electrode to be

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Table 1. Work functions and error ranges measured in theAl, IZO, and Ag films modified and unmodified with HDTSAMs.

Sample Work Function (eV) Error range

Evaporated Al film 3.70 ± 0.0081

No treatment 4.40 ± 0.0052

IZO films 0.2 mmol 4.40 ± 0.0083

0.5 mmol 4.30 ± 0.0087

No treatment 4.7 ± 0.023Printed

0.2 mmol 4.483 ± 0.027Ag films

0.5 mmol 4.468 ± 0.024

Fig. 5. (Color online) Schematic structures of (a) the un-treated and (b) the HDT-treated IZO TFTs. Band diagramsof (c) the untreated and (d) the HDT-treated IZO TFTs.

5.1 eV. The properties of the work function and the errorrange measured in the unmodified Al, IZO, and Ag filmsand in the modified with HDT SAMs are given in Table 1.When the density of the HDT solution was increased, thework function of the inkjet-printed Ag films decreased, asdid the work function of the IZO films. The decreased Agwork function for the HDT SAMs was already reportedby de Boer et al. [14]. Figures 5(a) and (b) describethe schematic structures of the untreated and the HDT-treated IZO TFTs, respectively, and Figs. 5(c) and (d)describe their related band diagrams. The applied molarconcentration of HDT was 0.5 mM. The upward bend-ings of the IZO bands relative to the Fermi level weredrawn by referring to the results of the work functions,as shown in Table 1. When the HDT SAMs in the IZOTFTs were treated, the difference in the work functionsbetween the IZO and the Ag, that is, ΔE, decreasedfrom 0.30 to 0.17 eV. This was caused by the molecu-lar dipole at the Ag/HDT interface, as suggested in aprevious work [14]. The SAM based on alkanethiols isconsidered to be a dipole layer in which the total dipolemoment arises from two internal dipoles because of theAg metal-sulfur charge-transfer interaction and the com-position of the HDT monolayer itself [14]. We surmisedthat the decrease of ΔE was mainly the cause of the

decrease of the electrical barrier and, thus, the improve-ment of the electric property caused by increasing theinjection of electrons.

IV. CONCLUSIONS

In this study, we investigated the electric characteris-tics of IZO TFTs at the HDT treatments by using I-Vmeasurements. The μ and the Vth in the IZO TFTs withthe HDT treatments and the inkjet-printed Ag electrodeswere approximately 0.36 cm2/Vs and 1.5 V, respectively.To study the effect of the HDT treatments, we measuredthe work functions of the IZO and metals under vari-ous conditions. Under the 0.5-mM HDT treatments, thevalue of ΔE decreased by approximately 0.17 eV, theseimproving of the electric property by increasing the in-jection of electrons.

ACKNOWLEDGMENTS

This study was supported by the Development ofNew Materials and Solution Process for LCD Backplanesfunded by the Korea Research Council for Industrial Sci-ence and Technology (ISTK, B551179-09-06-00).

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