CERAMICSINTERNATIONAL

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http://dx.doi.org/0272-8842/& 20

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(2015) 8059–8063

Ceramics International 41 www.elsevier.com/locate/ceramint

Dependence of optical and electrical properties on Ag thicknessin TiO2/Ag/TiO2 multilayer films for photovoltaic devices

Jun Ho Kima, Han-Kyeol Leeb, Jin-Young Nab, Sun-Kyung Kimb, Young-Zo Yooc,Tae-Yeon Seonga,n

aDept. of Materials Science and Engineering, Korea University, Seoul 136-713, KoreabDept. of Applied Physics, Kyung Hee University, Gyeonggi 446-701, KoreacDuksan Hi-Metal Co. Ltd., Yeonam-dong, Buk-gu, Ulsan 683-804, Korea

Received 25 February 2015; received in revised form 1 March 2015; accepted 1 March 2015Available online 9 March 2015

Abstract

We report on the formation of highly transparent and low conductance TiO2/Ag/TiO2 multilayer films with high figure of merit (FOM). Theoptical and electrical properties of the multilayer films were investigated as a function of Ag layer thickness. As the Ag thickness increased, thetransmission window narrowed and the transmittance was gradually lowered. The TiO2/Ag/TiO2 multilayer films have the highest transmittanceof 86.3–97% at 591 nm for different Ag thicknesses. The relationship between transmittance and TiO2 thickness was simulated using thescattering matrix method to understand the abnormally high transmittance. As the Ag thickness increased from 15 to 25 nm, the carrierconcentration of the TiO2/Ag/TiO2 samples gradually increased from 6.18� 1021 to 1.07� 1022 cm�3 and the mobility also increased from 16.7to 25.2 cm2/V-s. Meanwhile, the sheet resistance slightly decreased from 6.17 to 2.27 Ω⧸sq with the Ag thickness. The TiO2/Ag/TiO2 multilayer(with Ag thicknesses of 17–21 nm) had Haacke's FOMs of 121� 10�3

–157.2� 10–3 Ω�1.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Ag; Multilayer; Transparent conducting electrode; D. TiO2

1. Introduction

Transparent conducting electrodes (TCEs) are of great techno-logical importance because of their application in optoelectronics,flat panel display, solar cells [1–3]. These devices require hightransmittance and low resistance. In this regard, Sn-doped indiumoxide (ITO) is most frequently used as a TCE in optoelectronicapplications since it has low resistivity (�10�4 Ω cm) and hightransmittance (over 80%) in the visible wavelength region [4].However, the limited supply of indium will result in a rapidincrease in the fabrication costs of future applications. For thisreason, a wide variety of oxides with high transmittance, su-ch as SnO2 [2], ZnO [5], TiO2 [6], and Nb2O5 [7], have beeninvestigated. However, these oxides were found to be inferior to

10.1016/j.ceramint.2015.03.00215 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ82 2 3290 3288; fax: þ82 2 928 3584.ss: tyseong@korea.ac.kr (T.-Y. Seong).

ITO in a combination of the electrical and optical properties.Recently, dielectric/metal/dielectric (D/M/D) multilayers, namely,transparent oxides sandwiching a thin metal film, have beenextensively studied in order to realize the combination of lowresistivity and high transmittance in the visible region. Ag iscommonly used as the middle layer for D/M/D multilayers sinceAg thin films (less than 20 nm thick) have low resistance and hightransmittance in the visible spectrum. Various deposition techni-ques have been used to produce D/M/D structures, includingthermal evaporation [8], chemical vapor deposition [9], spraypyrolysis [10], sol–gel process [11], and sputtering [12–22].Among these techniques, sputtering has been shown to be themost promising technique in terms of the deposition of uniformfilms at a proper deposition rate. Numerous D/Ag/D multilayerfilms have been developed by optimizing the deposition parametersof sputtering, containing transparent oxides of SnO2 [12], Nb2O5

[13], TiInZnO [14], MoO3 [15], WO3 [16], Al-doped ZnO [17],

Fig. 1. XRD patterns obtained from TiO2/Ag/TiO2 multilayer films withvarious Ag layer thicknesses.

Fig. 2. Transmittance spectra of TiO2/Ag/TiO2 multilayer films with variousAg thicknesses.

J.H. Kim et al. / Ceramics International 41 (2015) 8059–80638060

Al2O3 [18], ZrON [19], ZnSnO [20], MoSnO [21], ZnO [22], andZnS [23] multilayers. For example, Mouchaal et al. [23] investi-gated the electrical and optical properties of ZnS/M/ZnS multilayerfilms (with M¼Ag, Cu or Cu/Ag) prepared under vacuum by thesimple joule heating effect. It was shown that the optimumthicknesses of the different layers were 50/45 nm for ZnS, 11 nmfor Ag, 16 nm for Cu and 3 nm/9 nm for Cu/Ag. For the optimizedZnS/Cu/Ag/ZnS, the averaged transmission (between 400 nm and1000 nm) was 85% and the sheet resistance was 5.0Ω/sq. Inaddition, titanium dioxide (TiO2) is a technologically importantmaterial because of its high refractive index (2.3–2.7), hightransmittance in the visible spectrum (above 90%), and goodchemical stability [24]. Thus, TiO2/Ag/TiO2 multilayers have beeninvestigated by many researchers [25–28]. For instance, Dhar andAlford [25], investigating the effect of Ag thickness (6–13 nm) onthe opto-electrical properties of TiO2/Ag/TiO2 multilayers depos-ited on a flexible substrate, showed that the multilayer film with9.5-nm-thick Ag layer gave a sheet resistance of 5.7 Ω⧸sq and anaverage optical transmittance of 90% at 590 nm. Jia et al.[27],investigating the effect of Ag layer thickness on the transmittanceand sheet resistance of TiO2 (10 nm)/Ag/TiO2 (10 nm) multilayers,reported that the multilayer film with 8-nm-thick Ag layer had thelowest sheet resistance (30 Ω⧸sq) and the highest transmittance(�90%) in the 500–700 nm region. In this study, we investigatedthe optical and electrical properties of TiO2/Ag/TiO2 multilayers asa function of Ag thickness. Unlike previous research [29], the effectof thick Ag layers (15–25 nm) on the optical and electricalproperties of 40 nm-thick TiO2 multilayers was investigated.Simulation was performed using the scattering matrix method tounderstand the high transmittance of thick Ag-based multilayers. Afigure of merit (FOM) was used to characterize the performance ofthe multilayers.

2. Experimental procedure

TiO2/Ag/TiO2 multilayer thin films were consecutivelydeposited onto corning eagle XG glass substrates by an RFmagnetron sputtering system. Ceramic TiO2 target (99.999%purity) and pure Ag target (99.99% purity) were used at roomtemperature under a base pressure of less than 1� 10�6 Torr.Before being loaded into the sputtering chamber, the glasssubstrates (1.5� 1.5 cm2) were cleaned with acetone, metha-nol, and deionized water for 15 min per cleaning agent in anultrasonic bath, and finally dried in a N2 stream. Prior todeposition, both the TiO2 and Ag targets were presputtered for30 min to remove contaminants. TiO2 and Ag were depositedusing RF powers of 90 W and 30 W, respectively. Duringsputtering, the glass substrate was constantly rotated at a speedof 12 rpm for TiO2 and 23 rpm for Ag. The thickness of the Agfilms varied from 15 to 25 nm, while the TiO2 film was fixed at40 nm. The crystal structure of the multilayers was examinedwith X-ray diffraction (XRD, ATX-G, Rigaku). Transmittanceof the multilayers was measured with a UV/visible spectro-meter (UV-1800, Shimadzu). Hall measurements by the vander Pauw technique were performed with a magnetic field of0.55 T (HMS 3000, Ecopia). The four-point-probe techniquewas used for sheet resistance measurements.

3. Results and discussion

Fig. 1 shows XRD patterns obtained from the TiO2/Ag/TiO2

multilayer films with various Ag layer thicknesses. The as-deposited TiO2 sample was found to be amorphous. All of themultilayer samples have peaks at 2θ¼38.21, 44.41, 64.61, and77.61 that correspond to the (111), (200), (220), and (311)planes of Ag (JCPDS No. 87-0720) respectively. It can be seenthat the intensity of the peaks increases with increasing the Agthickness. This indicates that the crystallinity becomesimproved with the Ag thickness.Fig. 2 shows the transmittance spectra of the TiO2/Ag/TiO2

multilayer films with various Ag thicknesses. Regardless of the Agthicknesses, the transmittance reaches a global maximum around590 nm and then gradually decreases with increasing wavelength.For example, the TiO2/Ag/TiO2 multilayer films have the highesttransmittance of 86.3–97% at 591 nm for different Ag thicknesses.The transmission window narrows and gradually becomes loweredas the Ag thickness increases. Subsequently, the samples showlower transmittance at the absorption edge and infra-red regionwith increasing the Ag thickness.Fig. 3 shows the carrier concentration and Hall mobility of the

TiO2/Ag/TiO2 multilayer films as a function of the Ag thickness.The carrier concentration gradually increases with increasing Agthickness; it increases from 6.18� 1021 to 1.07� 1022 cm�3 as the

Fig. 3. The carrier concentration and Hall mobility of TiO2/Ag/TiO2 multi-layer films as a function of Ag thickness.

Fig. 4. The resistivity and sheet resistance of TiO2/Ag/TiO2 multilayer films asa function of Ag thickness.

Fig. 5. Calculated FOM (φTC) of TiO2/Ag/TiO2 multilayer films as a functionof Ag film thickness.

J.H. Kim et al. / Ceramics International 41 (2015) 8059–8063 8061

Ag thickness increases from 15 to 25 nm, respectively. However,the mobility slightly increases from 16.7 to 25.2 cm2/V-s as the Agthickness increases. Mobility behavior can be explained byscattering mechanisms such as phonon scattering, grain-boundaryscattering, surface scattering, interface scattering, and ionized-impurity scattering [30,31]. The amorphous TiO2 films areundoped and have constant thickness (40 nm), and so, interfacescattering would be dominant at the TiO2/Ag interfaces.

Fig. 4 shows the resistivity and sheet resistance of the TiO2/Ag/TiO2 multilayer films as a function of the Ag thickness. The sheetresistance slightly decreases from 6.17 to 2.27Ω⧸sq as the Agthickness increases. The resistivity also decreases with increasingthe Ag thickness; it decreases from 6.05� 10�5 to 2.30�10�5Ω-cm as the Ag thickness increases from 15 to 25 nm.The resistivity is inversely proportional to the mobility and thecarrier concentration [32]. Thus, with the mobility and the carrierconcentration increasing by a factor of 1.5–1.72 with increasing Agthickness, this means that the resistivity is dominated by thecombined effect of the mobility and carrier concentration, as shownin Fig. 3.

Fig. 5 shows calculated FOM (φTC) of the TiO2/Ag/TiO2

multilayer films as a function of the Ag film thickness. φTCwas calculated using the equation defined by Haacke [33],φTC ¼ T10

av=Rs where Rs is the sheet resistance and Tav is theaverage optical transmittance. Tav can be estimated using therelation, Tav ¼

RVðλÞTðλÞdλ= R VðλÞdλ, where T(λ) is the trans-

mittance and V(λ) is the photopic luminous efficiency functiondefining the standard observer for photometry [12,34]. As shown in

Fig. 5, the FOM reaches a maximum at 19 nm and then graduallydecreases with increasing the Ag thickness. All the samples showfairly high FOM. In particular, the 19-nm-thick Ag multilayer givesthe highest FOM of 157.2� 10�3 Ω�1. As the sheet resistanceslightly changes from 4.7 to 2.67 Ω⧸sq with increasing Agthickness, the high FOMs can be attributed to the major contribu-tion of high transmittance.It was shown that the TiO2/Ag/TiO2 samples (with Ag thickness

in the range 15–25 nm) yielded high optical transmittance at550 nm. To understand the unusually high transmittance, therelationship between transmittance and TiO2 thickness was simu-lated using the scattering matrix method [35]. The unconventionaltransmission properties of dielectric–metal–dielectric–oxide multi-layer films are readily illustrated by phasor diagram in whichreflected partial waves are represented in the complex plane[36–38]. To construct the phasor diagram, all available reflectedpartial waves in our TiO2/Ag/TiO2 multilayer films were exploredand grouped by the numbers of their passing the TiO2 layer(i.e., the reflected partial waves in the 2-ϕ group pass the TiO2

layer twice), as shown in Fig. 6(a). No reflected partial waveexperiencing the metal layer more than four times was included inthe groups because of the large extinction coefficient (k) of the Aglayer (e.g., k¼3.3 at λ¼550 nm). For 0-ϕ to 6-ϕ groups, eachbasis, the sum of the reflected partial waves in the same groupaccounting the Fresnel equations [35], was represented in thecomplex plane (straight lines in Fig. 6(b)). For this phasorrepresentation, the wavelength of incident light was 550 nm andthe thickness of the Ag layer was 18 nm. The other higher ϕ termswere not considered because of their negligibly small Fresnelcoefficients. Then, each basis is rotated by 0, 2ϕ, 4ϕ, and 6ϕ,where ϕ is determined by the thickness (d) of the TiO2 layer:ϕ¼ 2πd � nTiO2=λ (dashed lines in Fig. 6(b)). For the localmaximum (d¼40 nm) and minimum (d¼80 nm) transmissionconditions, the phasor diagrams were constructed (Fig. 6(c) and(d)). At d¼40 nm, the trajectory of phasors was marginallydeviated from the origin, which accounts reduced reflectance. Onthe contrary, at d¼80 nm, each phasor from different groups wasadded constructively, which results in augmented reflectance. Thesimulation result (Fig. 7) exhibits that for the 18 nm-thick Ag layer,the transmittance of the TiO2/Ag/TiO2 sinusoidally varies with theTiO2 thickness and maximum at the TiO2 thickness of 40 nm.Thus, the unusual transmittance of the TiO2/Ag/TiO2 multilayer

Fig. 6. (a) Schematic diagram illustrating several possible optical paths in TiO2/Ag/TiO2 films. Each optical path is grouped by how many times the reflected partialwave passes the TiO2 layer. (b) Total phasor sum (i.e., basis) for each group with zero (straight lines) and non-zero (dashed lines) TiO2 thicknesses. Phasor diagramsfor (c) the maximum and (d) minimum transmission conditions. For these calculations, the optical constants (n, k) of TiO2 and Ag at λ¼550 nm are (2.26, 0) and(0.35, 3.26), respectively.

Fig. 7. Variation of transmittance as a function of TiO2 thickness.

J.H. Kim et al. / Ceramics International 41 (2015) 8059–80638062

films can be understood as a direct consequence of the anti-reflection effect accompanied by complex optical media. Thetransmission is maximized or minimized at specific oxide thick-nesses by the orientation (i.e., destructive or constructive) ofphasors, and primarily dictated by the phasors from the 0-ϕ and2-ϕ groups. An optimum thickness of TiO2 for the maximumtransmission is nearly unchanged over a narrow range of metalthicknesses because a metal thickness slightly changes theamplitude of the phasors (e.g., 2b and 2c in Fig. 6(a)) assignedto optical paths experiencing the Ag layer. Furthermore, we expectthat other combinatorial sets of dielectric–metal–dielectric

multilayer films comprising different materials may provide amore enhanced transmission over a broad range of wavelengths. Itis, however, worth noting that the calculated transmittance issomewhat lower than the measured one (Figs. 2 and 7). This maybe attributed to the fact that unlike the real samples [39], themultilayer film employed in the model is assumed to have flat andsmooth surfaces and interfaces. In addition, the refractive index ofthe amorphous TiO2 film might be also different from the valueused in the simulation.

4. Summary and conclusions

The effect of Ag layer thickness on the optical and electricalproperties of the TiO2/Ag/TiO2 multilayer films was investigated.The transmittance and transmission windows were dependent onthe Ag thickness. The TiO2/Ag(19 nm)/TiO2 multilayer films hadthe highest transmittance at around 590 nm, which was attributedto the anti-reflection effect including complex optical media. As theAg thickness increased, the carrier concentration graduallyincreased while the sheet resistance slightly decreased. The TiO2

(40 nm)/Ag (19 nm)/TiO2 (40 nm) multilayer produced the highestHaacke's FOM. The results show that the TiO2/Ag/TiO2 multilayerfilms can be used as transparent multilayer electrodes for displayapplications.

J.H. Kim et al. / Ceramics International 41 (2015) 8059–8063 8063

Acknowledgments

This work was supported by Korea Evaluation Institute ofIndustrial Technology (Grant no.10049601). S.-K.K. acknowl-edges support of this work by Basic Science Research Programthrough the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT & Future Planning(NRF-2013R1A1A1059423).

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