Thin Solid Films 547 (2013) 225–229

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Effect of thickness and substrate temperature on the properties of transparentTi-doped In2O3 films grown by direct current magnetron sputtering

Dong-Ju Kim a, Bong-sueg Kim b, Han-Ki Kim a,⁎a Department of Advanced Materials Engineering for Information and Electronics, Kyung-Hee University, 1 Seocheon-dong, Yongin-si, Gyeonggi-do 446-701, South Koreab Z-TEC Co., Ltd., Star-Biz Building #105, 4th Gumi National Industrial Complex, Sandong-myeon, Gumi-si, Gyeongsanbuk-do, 730-853, South Korea

⁎ Corresponding author. Tel.: +82 31 201 2462.E-mail address: imdlhkkim@khu.ac.kr (H.-K. Kim).

0040-6090/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tsf.2013.02.073

a b s t r a c t

a r t i c l e i n f o

Available online 5 March 2013

Keywords:Ti-doped In2O3DC magnetron sputteringResistivityBandgap

We reported on the effect of film thickness and substrate temperature on the properties of TiO2-doped In2O3(TIO) films deposited on a glass substrate by direct current (DC) magnetron sputtering for touch panel appli-cations. The electrical, optical, surface and structural properties of TIO films grown at room temperature weresignificantly dependent on its thickness. At optimized TIO thickness (480 nm), the resistivity of the TIO filmdecreased with increasing substrate temperature due to effective activation of Ti dopant and crystallization ofthe In2O3 matrix. Furthermore, the increase in substrate temperature during DC sputtering leads to increaseof optical bandgap of the TIO films due to Burstein–Moss effect, which is caused by change in carrier concen-tration of TIO films. At optimized conditions, the TIO film shows resistivity of 1.947 × 10−4 Ω-cm and opticaltransmittance of 85.3% comparable conventional indium tin oxide films.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conducting oxide (TCO) films have been widelyemployed in wide range of optoelectronic applications, such as flatpanel displays, photovoltaics, light emitting diodes, and touch panelsdue to its low resistivity and high transmittance in a visible wavelengthregion. Among In2O3, ZnO, SnO2, TiO2-based TCOmaterials, In2O3-basedTCO films, including Sn-doped In2O3 (ITO) and Zn-doped In2O3, and Zn,Sn co-doped In2O3 are most widely investigated due to predominantelectrical and optical properties of In2O3 [1–5]. In particular, most re-search have been devoted to the study of ITO films with intension oftheir utilization in flat panel displays and photovoltaics in more than50 years because they exhibited a remarkable combination of low resis-tivity (~10−4 Ω-cm) and high transmittance (>80%) [6]. As an alterna-tive dopant of Sn for the In2O3, several dopant such as Si, Ge, Ti, Te, Mg,F, Ca, Cu, S, As, Sb, Sr, Bi and B have been suggested [7–11]. In particular,Ti-doped In2O3 (TIO) has a great attention for application in touch paneldisplays due to stable crystalline structure and high stability against hu-midity. As discussed by Hest et al., one carrier is generated for every Tifor doping concentration between 1and 3 at.% in TIO film unlike thecase of Sn dopant [12]. They also reported that the maximum mobilityof the TIO film increased up to 83.3 cm2/V-s much higher than that ofITO. Abe et al. also suggested that Ti doped In2O3 had a low resistivity(2.1–3.0 × 10−4 Ω-cm) and high transmittance in the near infrared re-gion [13]. Although the possibility of Ti dopant in the In2O3 film as TCOmaterials has been suggested, the investigation of the effect of thickness

rights reserved.

and substrate temperature on the properties of the TIO film is stilllacking.

In this work, we investigated the effect of thickness and substratetemperature on the electrical, optical, surface and structural proper-ties of the DC sputtered TIO films. With increasing thickness from24 to 720 nm, sheet resistance and optical transmittance of the TIOfilms were measured to correlate the electrical and optical propertiesof the TIO film with its thickness. At optimized thickness (480 nm),the effect of substrate temperature during DC sputtering on the elec-trical, optical, surface and structural properties of the TIO film wasfurther investigated. Based on calculated figure of merit values, weoptimized the thickness and substrate temperature of the TIO films.

2. Experimental details

The Ti doped In2O3 (TIO) films were deposited on glass substrateby means of a specially designed DC magnetron sputtering system.Using the 3 inch TIO ceramic target as a sputtering source, TIO filmswere sputtered on glass substrate as a function of thickness and substratetemperature. During TIO sputtering, target-to-substrate distance waskept constant at 80 mm. Firstly, the TIO films were sputtered on a glasssubstrate with a dimension of 25 × 25 mm2 as a function of thicknessat room temperature. By adjusting the sputtering time, we controlledthe thickness of the TIO film at constant Ar flow rate of 15 sccm,workingpressure of 0.399 Pa (3 mTorr), and DC power of 60 Wwithout additionof reactive oxygen gas. The sputtering time was varied from 1 min to30 min and the resulting film thickness is of 24 nm to 720 nm, respec-tively. Secondly, to investigate the effect of substrate temperature onthe properties of the TIO films, 480 nm thick TIO films were depositedon glass substrate as a function of substrate temperature from 100 °C

http://dx.doi.org/10.1016/j.tsf.2013.02.073mailto:imdlhkkim@khu.ac.krhttp://dx.doi.org/10.1016/j.tsf.2013.02.073http://www.sciencedirect.com/science/journal/00406090

226 D.-J. Kim et al. / Thin Solid Films 547 (2013) 225–229

to 550 °C at same sputtering conditions. The substrate temperature wasintentionally heated by halogen lamp heater. The sputtering rate andthickness of the TIOfilmswere obtainedbymeans of a stylus profilometer(Tencor Alpha-step 250). The electrical properties of the TIO films weremeasured by the Hall measurements at room temperature as a functionof the film thickness and substrate temperature. In addition, the opticaltransmittance of the TIO films was measured by UV/Visible spectrometerin a range of 220–800 nm. The structural properties of the TIO filmswerestudied by X-ray diffraction (XRD-2θ mode, D/Max 2500) with Cu Kα(λ = 1.54059 Å) radiation. Furthermore, the surface of the TIO filmswas analyzed by field emission scanning electron microscopy (FESEM;Hitachi-S4800) with operating voltage of 5 kV.

3. Results and discussion

Fig. 1a shows sheet resistance and resistivity of the as-depositedTIO films as a function of the film thickness. During DC sputtering ofthe TIO target, the DC power, working pressure, and Ar flow rate werekept constant at 60 W, 0.399 Pa (3 mTorr), and 15 sccm, respectively.At 24 nm thickness, the TIO film shows fairly high sheet resistance of275 Ω/square and resistivity of 6.463 × 10−4 Ω-cm. However, the120 nm thick TIO film shows abruptly decreased sheet resistance of55 Ω/square and resistivity of 6.146 × 10−4 Ω-cm. Note that sheet re-sistance of TIO films above 120 nm thickness is similar (~50 Ω/square)regardless of film thickness due to increase of resistivity. Fig. 1b showsthe optical transmittance of the as-deposited TIO films with increasingfilm thickness. As can be seen, that as the TIO film thickness increases,variation of the optical transmittance occurred due to interference phe-nomena. Regardless of the film thickness, all TIO films exhibited a fairlyhigh optical transmittance in a visible wavelength range. The TIO film

24 120 240 480 720

45

60

260

280

Sheet Resistance Resistivity

Thickness [nm]

Sh

eet

Res

ista

nce

[O

hm

/sq

.]

10-5

10-4

10-3

10-2

Res

isti

vity

[o

hm

-cm

]

(a)

300 400 500 600 700 8000

20

40

60

80

100

Tra

nsm

itta

nce

[%

]

Wavelength [nm]

Sub. Temp : RToC 24nm 120nm 240nm 480nm 720nm

(b)

Fig. 1. (a) Electrical properties of TIO film grown at room temperature by DC sputteringon glass substrate at constant DC power of 60 W, working pressure of 0.399 Pa(3 mTorr) and Ar flow rate of 15 sccm as a function of film thickness. (b) Optical trans-mittance spectra of TIO films as a function of thickness.

with a thickness between 120 and 480 nm showed the high transmit-tance nearly 90% at a wavelength of 550 nm. Due to the variation ofthe optical transmittance caused by optical interference, it is difficultto determine the exact relationship between TIO thickness and opticaltransmittance. However, the shift of the absorption edge of the TIOfilms towards longer wavelengths with increasing thickness is appar-ent, indicating bandgap narrowing [10,14].

Fig. 2a showed the sheet resistance and optical transmittance ofthe TIO films grown at a substrate temperature of 550 °Cwith increasingthickness. To decide the optimized thickness of TIO films, we comparedthe sheet resistance and optical transmittance of the ITO films grown athigh substrate temperature of 550 °C. Compared to the as-depositedTIO film in Fig. 1a, the TIO film grown at substrate temperature of550 °C showed a significantly reduced sheet resistance. In addition, theTIO film showed a decreased sheet resistance with increasing film thick-ness. The 480 nm and 720 nm thick TIO films showed the low sheet re-sistances of 4.0 and 3.3 Ω/square, respectively. However, the increase offilm thickness led to a decrease in optical transmittance of the TIO film at550 nm wavelength. Based on sheet resistance (Rsh) and optical trans-mittance (T) of the TIO films, we calculated the figure of merit value(T10/Rsh) of the TIO films with increasing film thickness as shown inFig. 2b. With increasing the thickness, the TIO film showed an increasein the figure of merit value. Except the 720 nm thick TIO film, the 120,240, and 480 nm thick TIO films showed a similar high figure of meritvalues due to low sheet resistance and high optical transmittance. There-fore, we decided the 480 nm-thick TIO film as an optimized thickness toinvestigate the effect of substrate temperature.

Fig. 3 shows XRD plot of the TIO films grown at room temperaturewith increasing thickness. In the case of the TIO film with thickness of24 and 120 nm, similar XRD plots were observed. There is only a broadglass substrate peak indicating that the TIO film had an amorphous

0

5

10

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55Sub. temp. : 550 oC

Sheet ResistanceTransmittance

Thickness [nm]

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hm

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.]

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]

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Fig

ure

of

Mer

it [

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oh

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]

Thickness [nm]

Figure of merit

(b)

24 120 240 480 720

120 240 480 720

Fig. 2. (a) Sheet resistance and optical transmittance of TIO films grown at substratetemperature of 550 °C with increasing thickness. (b) Figure of merit value of the TIOfilms as a function of film thickness.

10 20 30 40 50 60 70 800

3500

7000

(211

)

(622

)

(440

)

(400

)

(222

)

720 nm

480 nm

240 nm

120 nmInte

nsi

ty [

arb

.un

its]

2θ [Degree]

24 nm

Fig. 3. XRD plots of the TIO films grown at room temperature with increasing filmthickness from 24 nm to 720 nm.

Fig. 4. Surface and cross-sectional FESEM images of the TIO films with increasing filmthickness; (a) 24 nm, (b) 120 nm, (c) 240 nm, (d) 480 nm, and (e) 720 nm.

227D.-J. Kim et al. / Thin Solid Films 547 (2013) 225–229

structure. However, the 240 nm thick TIO film shows only a peak at2θ = 34.47° (400), indicative of crystalline structure with (400) pre-ferred orientation. Further increase in thickness leads to appearance ofadditional XRD peaks as well as (400) peak. The XRD plots of the720 nm thick TIO film exhibited several peaks at 2θ = 20.80° (211),29.71° (222), 34.47° (400), 49.57° (440), and 59.07° (622), indicativeof a bixbyite structure of the TIO film. It was noteworthy that the TIOfilm grown at room temperature showed a strongly (400) preferred ori-entation, unlike conventional ITOfilmswith (222) preferred orientation[15]. The different preferred orientation of the TIO film with thicknessabove 240 nm could be attributed to the process ambient effect [16].

The dependence of the surface morphology of the TIO films on thefilm thickness was investigated by FESEM as shown in Fig. 4. At thick-nesses of 24 nm and 120 nm, both TIO films in Fig. 4a and b show asimilar surface morphology because the structure of both samples iscompletely amorphous as expected from XRD results. However, thesurface and cross-sectional FESEM images of the 240 nm thick TIOfilm in Fig. 4c exhibited crystalline feature. The surface of the 240 nmthick TIO film shows square shape grain indicative of (400) preferredorientation. With increasing thickness, the surface and cross-sectionalFESEM image shows clear crystalline feature as shown in Fig. 4d and e.The TIO films with 480 and 720 nm thicknesses showed a [100] pre-ferred columnar structure that consists of small sub-grains like sputter-deposited ITO films discussed by Shigesato et al. [17].

To investigate the substrate temperature effect on electrical, opti-cal, and structural properties of TIO film, we deposited the TIO filmwith a constant thickness of 480 nm as a function of substrate tem-perature. Fig. 5 shows the Hall measurement results of the 480 nmthick TIO films as a function of substrate temperature from 100 °Cto 550 °C. It is clearly shown that the increase of substrate tempera-ture lead to a significant reduction of sheet resistance and resistivityof the TIO films due to effective activation of Ti dopant and crystalli-zation of In2O3 matrix at high substrate temperature. The TIO filmgrown at 550 °C substrate temperature showed the lowest sheet resis-tance of 4.0 Ω/square and resistivity of 1.95 × 10−4 Ω-cm. The low re-sistivity of the TIO film grown at high substrate temperature could beattributed to high mobility and carrier concentration as shown inFig. 5b. As expected from the resistivity of the TIO film, the increase ofthe substrate temperature leads to an increase of carrier concentrationand mobility. With increasing substrate temperature, the Ti4+ dopantmore effectively substitute the In3+ and increase the carrier concentra-tion [12]. In addition, the crystallization of the TIO with increasing sub-strate temperature increases the mobility. Therefore the combinedeffect of the activation of Ti dopant and the crystallization of In2O3 ma-trix resulted in the decrease in the resistivity of the TIO film with in-creasing substrate temperature.

Fig. 6a shows optical transmittance spectra of the TIO film grownas a function of substrate temperature. Upper panels exhibited the

pictures of the TIO films sputtered at different substrate tempera-tures. All TIO films show a high optical transmittance in a visiblewavelength region regardless of the substrate temperature. However,the shift of the absorption edge of the TIO films towards shorterwavelengths with increasing substrate temperature is apparent, indi-cating bandgap widening. The optical energy band gap of the TIOfilms was evaluated from the variation of absorption coefficient (α)as a function of photon energy (hν). The energy band gap of thefilms was determined using the relation [15],

α hνð Þ ¼ C hν−Eg� �1=2

;

where C is a constant that depends on the material, and Eg is its opti-cal band gap. The optical band gap can be deduced by extrapolatingthe straight line portion of this plot to the energy axis as shown inFig. 6b. By increasing the substrate temperature, the bandgap energy

100 200 300 400 500 550

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0

5x1021

4x1021

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Carrier concentrationMobility

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rier

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nce

ntr

atio

n [

cm-3

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Substrate temperature [oC]

(b)

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Mo

bili

ty [

cm2 /

Vs]

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4550556065707580

Thickness :480 nm Sheet resistanceResistivity

Substrate temperature [oC]

Sh

eet

resi

stan

ce [

oh

m/s

q.]

(a)

10-4

10-3

Res

isti

vity

[o

hm

-cm

]

Fig. 5. Hall measurement results of the 480 nm thick TIO films grown at different sub-strate temperature. (a) Sheet resistance and resistivity of the TIO films. (b) Carrier con-centration and mobility of the TIO films as a function of substrate temperature.

2.0 2.5 3.0 3.5 4.0 4.5 5.00.0

6.0x1029

4.0x1029

2.0x1029

α2 [

cm-2

]

Energy [eV]

RT oC 100 oC 200 oC 300 oC 400 oC 500 oC 550 oC

(b)

300 400 500 600 700 800

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Tra

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[%

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Wavelength [nm]

RT oC 100 oC 200 oC 300 oC 400 oC 500 oC 550 oC

(a)

Fig. 6. (a) The optical transmittance spectra of the 480 nm thick TIO films as a functionof substrate temperature. Upper panel shows pictures of the TIO films deposited onglass substrate. (b) Square of the absorption coefficient as a function of photon energy.

0

6000

12000

550 oC

400 oC

300 oC

200 oC

100 oC

(422

)

(400)

(222

)

Inte

nsi

ty [

arb

.un

its]

(211

)

RT oC

10 20 30 40 50 60 70 80

2θ [Degree]

Fig. 7. XRD plots of the 480 nm thick TIO film with increasing substrate temperatureduring DC sputtering process.

228 D.-J. Kim et al. / Thin Solid Films 547 (2013) 225–229

of the TIO film was shifted to the higher energy. It is clearly seen thatthe optical band gap of the TIO films increases linearly from 3.4 eV to3.75 eV with increasing substrate temperature. The increase of carrierconcentration in the TIO film with increasing substrate temperature isclosely related to the band gap widening of the TIO film. The band gapwidening as a result of the increase in the carrier concentration is re-lated to the Burstein–Moss (BM) effect [13,15].

Fig. 7 shows the XRD plots of the TIO films as a function of sub-strate temperature. As we explained in Fig. 3, the TIO film grown atroom temperature showed (400) preferred orientation due to effectof Ar ambient during sputtering. With increasing substrate tempera-ture, the peak intensity of (400) significantly reduced. The TIO filmsgrown at substrate temperature above 300 °C showed strong (211)and (222) peaks indicating preferred orientation similar to structureof conventional ITO film. As discussed by Thilakan et al., the preferentialorientation in the bixbyite structure was dependent on the oxygen con-centration in the film [16]. Therefore, it was thought that the changedoxygen concentration in the TIO filmwith increasing substrate temper-ature could affect the preferred orientation of the TIO films.

Fig. 8 shows surface FESEM images of the TIO film with increasingsubstrate temperature. It was clearly shown that sub-grain size of theTIO film increased with increasing substrate temperature. At roomtemperature, the TIO film shows fairly smooth surface even thoughit has a (400) preferred orientation. However, increase of the substratetemperature up to 550 °C increase in the sub-grain size nearly 107 nm.Sub-grain size increase of the TIO film with increasing substrate tem-perature could be explained by enhanced self-diffusion of sputteredatoms on the surface of substrate. The general microstructure of thesputter-deposited TCO film could be zone model [18]. The zone modeldescribed that themicrostructure of sputtered filmswas closely relatedto the substrate temperature (Tsubstrate/Tmelting). Based on the substrate

temperature, the structure zone could be divided into four regimes, inwhich the microstructure of sputtered film is critically affected byshadowing, surface diffusion, bulk diffusion and desorptionwith increas-ing substrate temperature. The Tsubstrate/Tmelting for TIO film grown at asubstrate temperature of 550 °C is about 0.48, which corresponds tothe zone where surface and self-diffusion become active. This is consis-tent with our microstructure of TIO films grown by DC magnetronsputtering.

image of Fig.�5

Fig. 8. Surface FESEM images of the 480 nm thick TIO films with increasing substratetemperature; (a) room temperature, (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 400, and(f) 550 °C.

229D.-J. Kim et al. / Thin Solid Films 547 (2013) 225–229

4. Conclusions

The effect of thickness and substrate temperature on the propertiesof TIO film grown by DC sputtering was investigated. In case of the TIOfilm grown at room temperature, the resistivity and optical transmit-tance were affected by thickness of the TIO films. With increasing thethickness, evolution of the crystalline structure lead to the change ofelectrical and optical properties. In addition, increase of substrate tem-perature during DC sputtering process influenced on the quality of theTIO films. At substrate temperature of 550 °C, the TIO film showed the

sheet resistance of 4.0 Ω/square, resistivity of 1.947 × 10−4 Ω-cm,and optical transmittance of 85.3%. Furthermore, the increase of sub-strate temperature led to increase of bandgap of the TIO film due to theBM effect which is critically affected by carrier concentration. Based onHall measurement, UV–visible spectrometry, XRD, and FESEM examina-tions, it was found that the electrical, optical, structural, and surfaceproperties of the DC sputter grownTIOfilms clearly depend on the thick-ness and substrate temperature during sputtering process.

Acknowledgments

This work was supported by grant no. 10031768 from theMinistryof Knowledge Economy (MKE) and Fundamental R&D Program forCore Technology of Materials funded by the Ministry of KnowledgeEconomy, Republic of Korea.

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Effect of thickness and substrate temperature on the properties of transparentTi-doped In2O3 films grown by direct current magnetron sputtering1. Introduction2. Experimental details3. Results and discussion4. ConclusionsAcknowledgmentsReferences