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TiO2 multilayer films for photovoltaic devices
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8059–8063 www.elsevier.com/locate/ceramint
Dependence of optical and electrical properties on Ag thickness in 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 a
Dept. of Materials Science and Engineering, Korea University, Seoul 136-713, Korea b Dept. of Applied Physics, Kyung Hee University, Gyeonggi 446-701, Korea c Duksan 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 2015 Available 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). The optical and electrical properties of the multilayer films were investigated as a function of Ag layer thickness. As the Ag thickness increased, the transmission window narrowed and the transmittance was gradually lowered. The TiO2/Ag/TiO2 multilayer films have the highest transmittance of 86.3–97% at 591 nm for different Ag thicknesses. The relationship between transmittance and TiO2 thickness was simulated using the scattering matrix method to understand the abnormally high transmittance. As the Ag thickness increased from 15 to 25 nm, the carrier concentration of the TiO2/Ag/TiO2 samples gradually increased from 6.18 1021 to 1.07 1022 cm 3 and the mobility also increased from 16.7 to 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 technological importance because of their application in optoelectronics, flat panel display, solar cells [1–3]. These devices require high transmittance and low resistance. In this regard, Sn-doped indium oxide (ITO) is most frequently used as a TCE in optoelectronic applications since it has low resistivity ( 10 4 Ω cm) and high transmittance (over 80%) in the visible wavelength region [4]. However, the limited supply of indium will result in a rapid increase in the fabrication costs of future applications. For this reason, a wide variety of oxides with high transmittance, such as SnO2 [2], ZnO [5], TiO2 [6], and Nb2O5 [7], have been investigated. However, these oxides were found to be inferior to n
Corresponding author. Tel.: þ82 2 3290 3288; fax: þ 82 2 928 3584. E-mail address: [email protected] (T.-Y. Seong).
http://dx.doi.org/10.1016/j.ceramint.2015.03.002 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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 been extensively studied in order to realize the combination of low resistivity and high transmittance in the visible region. Ag is commonly used as the middle layer for D/M/D multilayers since Ag thin films (less than 20 nm thick) have low resistance and high transmittance in the visible spectrum. Various deposition techniques have been used to produce D/M/D structures, including thermal evaporation [8], chemical vapor deposition [9], spray pyrolysis [10], sol–gel process [11], and sputtering [12–22]. Among these techniques, sputtering has been shown to be the most promising technique in terms of the deposition of uniform films at a proper deposition rate. Numerous D/Ag/D multilayer films have been developed by optimizing the deposition parameters of sputtering, containing transparent oxides of SnO2 [12], Nb2O5 [13], TiInZnO [14], MoO3 [15], WO3 [16], Al-doped ZnO [17],
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Al2O3 [18], ZrON [19], ZnSnO [20], MoSnO [21], ZnO [22], and ZnS [23] multilayers. For example, Mouchaal et al. [23] investigated the electrical and optical properties of ZnS/M/ZnS multilayer films (with M¼ Ag, Cu or Cu/Ag) prepared under vacuum by the simple joule heating effect. It was shown that the optimum thicknesses of the different layers were 50/45 nm for ZnS, 11 nm for Ag, 16 nm for Cu and 3 nm/9 nm for Cu/Ag. For the optimized ZnS/Cu/Ag/ZnS, the averaged transmission (between 400 nm and 1000 nm) was 85% and the sheet resistance was 5.0 Ω/sq. In addition, titanium dioxide (TiO2) is a technologically important material because of its high refractive index (2.3–2.7), high transmittance in the visible spectrum (above 90%), and good chemical stability [24]. Thus, TiO2/Ag/TiO2 multilayers have been investigated by many researchers [25–28]. For instance, Dhar and Alford [25], investigating the effect of Ag thickness (6–13 nm) on the opto-electrical properties of TiO2/Ag/TiO2 multilayers deposited on a flexible substrate, showed that the multilayer film with 9.5-nm-thick Ag layer gave a sheet resistance of 5.7 Ω⧸sq and an average optical transmittance of 90% at 590 nm. Jia et al.[27], investigating the effect of Ag layer thickness on the transmittance and sheet resistance of TiO2 (10 nm)/Ag/TiO2 (10 nm) multilayers, reported that the multilayer film with 8-nm-thick Ag layer had the lowest sheet resistance (30 Ω⧸sq) and the highest transmittance ( 90%) in the 500–700 nm region. In this study, we investigated the optical and electrical properties of TiO2/Ag/TiO2 multilayers as a function of Ag thickness. Unlike previous research [29], the effect of thick Ag layers (15–25 nm) on the optical and electrical properties of 40 nm-thick TiO2 multilayers was investigated. Simulation was performed using the scattering matrix method to understand the high transmittance of thick Ag-based multilayers. A figure of merit (FOM) was used to characterize the performance of the multilayers.
3. Results and discussion Fig. 1 shows XRD patterns obtained from the TiO2/Ag/TiO2 multilayer films with various Ag layer thicknesses. The asdeposited TiO2 sample was found to be amorphous. All of the multilayer samples have peaks at 2θ ¼ 38.21, 44.41, 64.61, and 77.61 that correspond to the (111), (200), (220), and (311) planes of Ag (JCPDS No. 87-0720) respectively. It can be seen that the intensity of the peaks increases with increasing the Ag thickness. This indicates that the crystallinity becomes improved with the Ag thickness. Fig. 2 shows the transmittance spectra of the TiO2/Ag/TiO2 multilayer films with various Ag thicknesses. Regardless of the Ag thicknesses, the transmittance reaches a global maximum around 590 nm and then gradually decreases with increasing wavelength. For example, the TiO2/Ag/TiO2 multilayer films have the highest transmittance of 86.3–97% at 591 nm for different Ag thicknesses. The transmission window narrows and gradually becomes lowered as the Ag thickness increases. Subsequently, the samples show lower transmittance at the absorption edge and infra-red region with 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 Ag thickness; it increases from 6.18 1021 to 1.07 1022 cm 3 as the
2. Experimental procedure TiO2/Ag/TiO2 multilayer thin films were consecutively deposited onto corning eagle XG glass substrates by an RF magnetron sputtering system. Ceramic TiO2 target (99.999% purity) and pure Ag target (99.99% purity) were used at room temperature under a base pressure of less than 1 10 6 Torr. Before being loaded into the sputtering chamber, the glass substrates (1.5 1.5 cm2) were cleaned with acetone, methanol, and deionized water for 15 min per cleaning agent in an ultrasonic bath, and finally dried in a N2 stream. Prior to deposition, both the TiO2 and Ag targets were presputtered for 30 min to remove contaminants. TiO2 and Ag were deposited using RF powers of 90 W and 30 W, respectively. During sputtering, the glass substrate was constantly rotated at a speed of 12 rpm for TiO2 and 23 rpm for Ag. The thickness of the Ag films varied from 15 to 25 nm, while the TiO2 film was fixed at 40 nm. The crystal structure of the multilayers was examined with X-ray diffraction (XRD, ATX-G, Rigaku). Transmittance of the multilayers was measured with a UV/visible spectrometer (UV-1800, Shimadzu). Hall measurements by the van der Pauw technique were performed with a magnetic field of 0.55 T (HMS 3000, Ecopia). The four-point-probe technique was used for sheet resistance measurements.
Fig. 1. XRD patterns obtained from TiO2/Ag/TiO2 multilayer films with various Ag layer thicknesses.
Fig. 2. Transmittance spectra of TiO2/Ag/TiO2 multilayer films with various Ag thicknesses.
J.H. Kim et al. / Ceramics International 41 (2015) 8059–8063
Fig. 3. The carrier concentration and Hall mobility of TiO2/Ag/TiO2 multilayer films as a function of Ag thickness.
Fig. 4. The resistivity and sheet resistance of TiO2/Ag/TiO2 multilayer films as a function of Ag thickness.
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 Ag thickness increases. Mobility behavior can be explained by scattering mechanisms such as phonon scattering, grain-boundary scattering, surface scattering, interface scattering, and ionizedimpurity scattering [30,31]. The amorphous TiO2 films are undoped and have constant thickness (40 nm), and so, interface scattering 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 sheet resistance slightly decreases from 6.17 to 2.27 Ω⧸sq as the Ag thickness increases. The resistivity also decreases with increasing the Ag thickness; it decreases from 6.05 10 5 to 2.30 10 5 Ω-cm as the Ag thickness increases from 15 to 25 nm. The resistivity is inversely proportional to the mobility and the carrier concentration [32]. Thus, with the mobility and the carrier concentration increasing by a factor of 1.5–1.72 with increasing Ag thickness, this means that the resistivity is dominated by the combined effect of the mobility and carrier concentration, as shown in Fig. 3. Fig. 5 shows calculated FOM (φTC) of the TiO2/Ag/TiO2 multilayer films as a function of the Ag film thickness. φTC was calculated using the equation defined by Haacke [33], φTC ¼ T 10 av =Rs where Rs is the sheet resistance and Tav is the average optical Rtransmittance. RTav can be estimated using the relation, T av ¼ VðλÞTðλÞdλ= VðλÞdλ, where T(λ) is the transmittance and V(λ) is the photopic luminous efficiency function defining the standard observer for photometry [12,34]. As shown in
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Fig. 5. Calculated FOM (φTC) of TiO2/Ag/TiO2 multilayer films as a function of Ag film thickness.
Fig. 5, the FOM reaches a maximum at 19 nm and then gradually decreases with increasing the Ag thickness. All the samples show fairly high FOM. In particular, the 19-nm-thick Ag multilayer gives the highest FOM of 157.2 10 3 Ω 1. As the sheet resistance slightly changes from 4.7 to 2.67 Ω⧸sq with increasing Ag thickness, the high FOMs can be attributed to the major contribution 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 at 550 nm. To understand the unusually high transmittance, the relationship between transmittance and TiO2 thickness was simulated using the scattering matrix method [35]. The unconventional transmission properties of dielectric–metal–dielectric–oxide multilayer films are readily illustrated by phasor diagram in which reflected partial waves are represented in the complex plane [36–38]. To construct the phasor diagram, all available reflected partial waves in our TiO2/Ag/TiO2 multilayer films were explored and 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 wave experiencing the metal layer more than four times was included in the groups because of the large extinction coefficient (k) of the Ag layer (e.g., k¼ 3.3 at λ¼ 550 nm). For 0-ϕ to 6-ϕ groups, each basis, the sum of the reflected partial waves in the same group accounting the Fresnel equations [35], was represented in the complex plane (straight lines in Fig. 6(b)). For this phasor representation, the wavelength of incident light was 550 nm and the thickness of the Ag layer was 18 nm. The other higher ϕ terms were not considered because of their negligibly small Fresnel coefficients. Then, each basis is rotated by 0, 2ϕ, 4ϕ, and 6ϕ, where ϕ is determined by the thickness (d) of the TiO2 layer: ϕ ¼ 2πd nTiO2 =λ (dashed lines in Fig. 6(b)). For the local maximum (d¼ 40 nm) and minimum (d¼ 80 nm) transmission conditions, the phasor diagrams were constructed (Fig. 6(c) and (d)). At d¼ 40 nm, the trajectory of phasors was marginally deviated from the origin, which accounts reduced reflectance. On the contrary, at d¼ 80 nm, each phasor from different groups was added constructively, which results in augmented reflectance. The simulation result (Fig. 7) exhibits that for the 18 nm-thick Ag layer, the transmittance of the TiO2/Ag/TiO2 sinusoidally varies with the TiO2 thickness and maximum at the TiO2 thickness of 40 nm. Thus, the unusual transmittance of the TiO2/Ag/TiO2 multilayer
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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 partial wave 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 diagrams for (c) the maximum and (d) minimum transmission conditions. For these calculations, the optical constants (n, k) of TiO2 and Ag at λ¼ 550 nm are (2.26, 0) and (0.35, 3.26), respectively.
multilayer films comprising different materials may provide a more enhanced transmission over a broad range of wavelengths. It is, however, worth noting that the calculated transmittance is somewhat lower than the measured one (Figs. 2 and 7). This may be attributed to the fact that unlike the real samples [39], the multilayer film employed in the model is assumed to have flat and smooth surfaces and interfaces. In addition, the refractive index of the amorphous TiO2 film might be also different from the value used in the simulation.
4. Summary and conclusions Fig. 7. Variation of transmittance as a function of TiO2 thickness.
films can be understood as a direct consequence of the antireflection effect accompanied by complex optical media. The transmission is maximized or minimized at specific oxide thicknesses by the orientation (i.e., destructive or constructive) of phasors, and primarily dictated by the phasors from the 0-ϕ and 2-ϕ groups. An optimum thickness of TiO2 for the maximum transmission is nearly unchanged over a narrow range of metal thicknesses because a metal thickness slightly changes the amplitude of the phasors (e.g., 2b and 2c in Fig. 6(a)) assigned to optical paths experiencing the Ag layer. Furthermore, we expect that other combinatorial sets of dielectric–metal–dielectric
The effect of Ag layer thickness on the optical and electrical properties of the TiO2/Ag/TiO2 multilayer films was investigated. The transmittance and transmission windows were dependent on the Ag thickness. The TiO2/Ag(19 nm)/TiO2 multilayer films had the highest transmittance at around 590 nm, which was attributed to the anti-reflection effect including complex optical media. As the Ag thickness increased, the carrier concentration gradually increased while the sheet resistance slightly decreased. The TiO2 (40 nm)/Ag (19 nm)/TiO2 (40 nm) multilayer produced the highest Haacke's FOM. The results show that the TiO2/Ag/TiO2 multilayer films can be used as transparent multilayer electrodes for display applications.
J.H. Kim et al. / Ceramics International 41 (2015) 8059–8063
Acknowledgments This work was supported by Korea Evaluation Institute of Industrial Technology (Grant no.10049601). S.-K.K. acknowledges support of this work by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1059423). References [1] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Flexible light-emitting diodes made from soluble conducting polymers, Nature 357 (1992) 477–479. [2] Q. Wan, E.N. Dattoli, W. Lu, Transparent metallic Sb-doped SnO2 nanowires, Appl. Phys. Lett. 90 (2007) 222107. [3] A. Dhar, T.L. Alford, Optimization of TiO2/Cu/TiO2 multilayer as transparent composite electrode deposited on flexible substrate at room temperature, ECS Solid State Lett. 3 (2014) N33–N36. [4] H. Hosono, H. Ohta, M. Orita, K. Ueda, M. Hirano, Frontier of transparent conductive oxide thin films, Vacuum 66 (2002) 419–425. [5] M.-S. Oh, S.-H. Kim, T.-K. seong, Growth of nominally undoped p-type ZnO on Si by pulsed-laser deposition, Appl. Phys. Lett. 87 (2005) 122103. [6] S.X. Zhang, S. Dhar, W. Yu, H. Xu, S.B. Ogale, T. Venkatesan, Growth parameter-property phase diagram for pulsed laser deposited transparent oxide conductor anatase Nb:TiO2, Appl. Phys. Lett. 91 (2007) 112113. [7] Ö.D. Coşkun, S. Demirela, The optical and structural properties of amorphous Nb2O5 thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 277 (2013) 35–39. [8] Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, G. Shimaoka, Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation, Appl. Surf. Sci. 142 (1999) 233–236. [9] K. Haga, M. Kamidaira, Y. Kashiwaba, T. Sekiguchi, H. Watanabe, ZnO thin films prepared by remote plasma-enhanced CVD method, J. Cryst. Growth 214 (2000) 77–80. [10] O. Vigil, F. Cruz, G. Santana, L. Vaillant, A. Morales-Acevedo, G. Contreras-Puente, Influence of post-thermal annealing on the properties of sprayed cadmium–zinc oxide thin films, Appl. Surf. Sci. 161 (2000) 27–34. [11] A.E. Jimenez-Gonzalez, J.A.S. Urueta, R. Suarez-parra, Optical and electrical characteristics of Al-doped ZnO thin films prepared by solgel technique, J. Cryst. Growth 192 (1998) 430–438. [12] S. Yu, W. Zhang, L. Li, D. Xu, H. Dong, Y. Jin, Optimization of SnO2/ Ag/SnO2 tri-layer films as transparent composite electrode with high figure of merit, Thin Solid Films 552 (2014) 150–154. [13] A. Dhar, T.L. Alford, Optimization of Nb2O5/Ag/Nb2O5 multilayers as transparent composite electrode on flexible substrate with high figure of merit, J. Appl. Phys. 112 (2012) 103113. [14] H.-H. Kim, E.-M. Kim, K.-J. Lee, J.-Y. Park, Y.-R. Lee, D.-C. Shin, T.-J. Hwang, G.-S. Heo, TiInZnO/Ag/TiInZnO multilayer films for transparent conducting electrodes of dye-sensitized solar cells, Jpn. J. Appl. Phys. 53 (2014) 032301. [15] H. Kim, K.-T. Lee, C. Zhao, L.J. Guo, J. Kanicki, Top illuminated organic photodetectors with dielectric/metal/dielectric transparent anode, Org. Electron. 20 (2015) 103–111. [16] B. Lin, C. Lan, C. Li, Z. Chen, Effect of thermal annealing on the performance of WO3/Ag/WO3 transparent conductive film, Thin Solid Films 571 (2014) 134–138. [17] D. Miao, S. Jiang, S. Shang, Z. Chen, Highly transparent and infrared reflective Al-doped ZnO(AZO)/Ag/AZO multilayer film prepared on PET substrate by RF magnetron sputtering, Vacuum 106 (2014) 1–4.
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[18] J.-A. Jeong, H.-K. Kim, Al2O3/Ag/Al2O3 multilayer thin film passivation prepared by plasma damage-free linear facing target sputtering for organic light emitting diodes, Thin Solid Films 547 (2013) 63–67. [19] J.-H. Song, J.-W. Jeon, Y.-H. Kim, J.-H. Oh, T.-Y. Seong, Optical, electrical, and structural properties of ZrON/Ag/ZrON multilayer transparent conductor for organic photovoltaics application, Superlattice Microstruct. 62 (2013) 119–123. [20] J.-W. Lim, S.-I. Oh, K. Eun, S.-H. Choa, H.-W. Koo, T.-W. Kim, H.-K. Kim, Mechanical flexibility of ZnSnO/Ag/ZnSnO films grown by roll-to-roll sputtering for flexible organic photovoltaics, Jpn. J. Appl. Phys. 51 (2012) 115801. [21] C.-H. Lee, R. Pandey, B.-Y. Wang, W.-K. Choi, D.-K. Choi, Y.-J. Oh, Nano-sized indium-free MTO/Ag/MTO transparent conducting electrode prepared by RF sputtering at room temperature for organic photovoltaic cells, Sol. Energy Mater. Sol. Cells 132 (2015) 80–85. [22] Y.-H. Kim, J.-W. Lee, R.-I. Murakami, Dependences of sputtering times on the structural and electrical properties of ZnO/Ag/ZnO thin films on PET by DC sputtering, IEEE Trans. Nanotechnol. 12 (2013) 991–995. [23] Y. Mouchaal, G. Louarn, A. Khelil, M. Morsli, N. Stephant, A. Bou, T. Abachi, L. Cattin, M. Makha, P. Torchio, J.C. Bernede, Broadening of the transmission range of dielectric/metal multilayer structures by using different metals, Vacuum 111 (2015) 32–41. [24] K. Hashimoto, H. Irie, A. Fujhishima, TiO2 photocatalysis: a historical overview and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269. [25] A. Dhar, T.L. Alford, High quality transparent TiO2/Ag/TiO2 composite electrode films deposited on flexible substrate at room temperature by sputtering, APL Mater. 1 (2013) 012102. [26] J. Kulczyk-Malecka, P.J. Kelly, G. West, G.C.B. Clarke, J.A. Ridealgh, K.P. Almtoft, A.L. Greer, Z.H. Barber, Investigation of Ag diffusion in TiO2/Ag/TiO2 coatings, Acta Mater. 66 (2014) 396–404. [27] J.H. Jia, P. Zhou, H. Xie, H.Y. You, J. Li, L.Y. Chen, Study of optical and electrical properties of TiO2/Ag/TiO2 multilayers, J. Korean Phys. Soc. 44 (2004) 717–721. [28] P.K. Chiu, C.T. Lee, D.Y. Chiang, W.H. Cho, C.N. Hsiao, Y.Y. Chen, B.M. Huang, J.R. Yang, Conductive and transparent multilayer films for low-temperature TiO2/Ag/SiO2 electrodes by E-beam evaporation with IAD, Nanoscale Res. Lett. 9 (2014) 35–39. [29] J.H. Kim, D.-H. Kim, T.-Y. Seong, Realization of highly transparent and low resistance TiO2/Ag/TiO2 conducting electrode for optoelectronic devices, Ceram. Int. 41 (2015) 3064–3068. [30] H. Han, N.D. Theodore, T.L. Alford, Improved conductivity and mechanism of carrier transport in zinc oxide with embedded silver layer, J. Appl. Phys. 103 (2008) 013708. [31] A. Indluru, T.L. Alford, Effect of Ag thickness on electrical transport and optical properties of indium tin oxide–Ag–indium tin oxide multilayers, J. Appl. Phys. 105 (2009) 123528. [32] H. Han, J.W. Mayer, T.L. Alford, Band gap shift in the indium-tin-oxide films on polyethylene napthalate after thermal annealing in air, J. Appl. Phys. 100 (2006) 083715. [33] G. Haacke, New figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086. [34] W.G. Driscoll, W. Vaughan, Handbook of Optics, McGraw-Hill, New York, 1978. [35] E. Hecht, in: Optics, fourth ed., Addison-Wesley, New York, 2002. [36] M.A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M.M. Qazilbash, D.N. Basov, S. Ramanathan, F. Capasso, Ultra-thin perfect absorber employing a tunable phase change material, Appl. Phys. Lett. 101 (2012) 221101. [37] M.A. Kats, R. Blanchard, P. Genevet, F. Capasso, Nanometre optical coatings based on strong interference effects in highly absorbing media, Nat. Mater. 12 (2013) 20–24. [38] F.F. Schlich, R. Spolenak, Strong interference in ultrathin semiconducting layers on a wide variety of substrate materials, Appl. Phys. Lett. 103 (2013) 213112. [39] The thickness of the TiO2/Ag/TiO2 films was examined by high resolution transmission electron microscopy. The results showed that although the individual layers were well defined, the TiO2/Ag interfaces were slightly undulated.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8059–8063 www.elsevier.com/locate/ceramint
Dependence of optical and electrical properties on Ag thickness in 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 a
Dept. of Materials Science and Engineering, Korea University, Seoul 136-713, Korea b Dept. of Applied Physics, Kyung Hee University, Gyeonggi 446-701, Korea c Duksan 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 2015 Available 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). The optical and electrical properties of the multilayer films were investigated as a function of Ag layer thickness. As the Ag thickness increased, the transmission window narrowed and the transmittance was gradually lowered. The TiO2/Ag/TiO2 multilayer films have the highest transmittance of 86.3–97% at 591 nm for different Ag thicknesses. The relationship between transmittance and TiO2 thickness was simulated using the scattering matrix method to understand the abnormally high transmittance. As the Ag thickness increased from 15 to 25 nm, the carrier concentration of the TiO2/Ag/TiO2 samples gradually increased from 6.18 1021 to 1.07 1022 cm 3 and the mobility also increased from 16.7 to 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 technological importance because of their application in optoelectronics, flat panel display, solar cells [1–3]. These devices require high transmittance and low resistance. In this regard, Sn-doped indium oxide (ITO) is most frequently used as a TCE in optoelectronic applications since it has low resistivity ( 10 4 Ω cm) and high transmittance (over 80%) in the visible wavelength region [4]. However, the limited supply of indium will result in a rapid increase in the fabrication costs of future applications. For this reason, a wide variety of oxides with high transmittance, such as SnO2 [2], ZnO [5], TiO2 [6], and Nb2O5 [7], have been investigated. However, these oxides were found to be inferior to n
Corresponding author. Tel.: þ82 2 3290 3288; fax: þ 82 2 928 3584. E-mail address: [email protected] (T.-Y. Seong).
http://dx.doi.org/10.1016/j.ceramint.2015.03.002 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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 been extensively studied in order to realize the combination of low resistivity and high transmittance in the visible region. Ag is commonly used as the middle layer for D/M/D multilayers since Ag thin films (less than 20 nm thick) have low resistance and high transmittance in the visible spectrum. Various deposition techniques have been used to produce D/M/D structures, including thermal evaporation [8], chemical vapor deposition [9], spray pyrolysis [10], sol–gel process [11], and sputtering [12–22]. Among these techniques, sputtering has been shown to be the most promising technique in terms of the deposition of uniform films at a proper deposition rate. Numerous D/Ag/D multilayer films have been developed by optimizing the deposition parameters of sputtering, containing transparent oxides of SnO2 [12], Nb2O5 [13], TiInZnO [14], MoO3 [15], WO3 [16], Al-doped ZnO [17],
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Al2O3 [18], ZrON [19], ZnSnO [20], MoSnO [21], ZnO [22], and ZnS [23] multilayers. For example, Mouchaal et al. [23] investigated the electrical and optical properties of ZnS/M/ZnS multilayer films (with M¼ Ag, Cu or Cu/Ag) prepared under vacuum by the simple joule heating effect. It was shown that the optimum thicknesses of the different layers were 50/45 nm for ZnS, 11 nm for Ag, 16 nm for Cu and 3 nm/9 nm for Cu/Ag. For the optimized ZnS/Cu/Ag/ZnS, the averaged transmission (between 400 nm and 1000 nm) was 85% and the sheet resistance was 5.0 Ω/sq. In addition, titanium dioxide (TiO2) is a technologically important material because of its high refractive index (2.3–2.7), high transmittance in the visible spectrum (above 90%), and good chemical stability [24]. Thus, TiO2/Ag/TiO2 multilayers have been investigated by many researchers [25–28]. For instance, Dhar and Alford [25], investigating the effect of Ag thickness (6–13 nm) on the opto-electrical properties of TiO2/Ag/TiO2 multilayers deposited on a flexible substrate, showed that the multilayer film with 9.5-nm-thick Ag layer gave a sheet resistance of 5.7 Ω⧸sq and an average optical transmittance of 90% at 590 nm. Jia et al.[27], investigating the effect of Ag layer thickness on the transmittance and sheet resistance of TiO2 (10 nm)/Ag/TiO2 (10 nm) multilayers, reported that the multilayer film with 8-nm-thick Ag layer had the lowest sheet resistance (30 Ω⧸sq) and the highest transmittance ( 90%) in the 500–700 nm region. In this study, we investigated the optical and electrical properties of TiO2/Ag/TiO2 multilayers as a function of Ag thickness. Unlike previous research [29], the effect of thick Ag layers (15–25 nm) on the optical and electrical properties of 40 nm-thick TiO2 multilayers was investigated. Simulation was performed using the scattering matrix method to understand the high transmittance of thick Ag-based multilayers. A figure of merit (FOM) was used to characterize the performance of the multilayers.
3. Results and discussion Fig. 1 shows XRD patterns obtained from the TiO2/Ag/TiO2 multilayer films with various Ag layer thicknesses. The asdeposited TiO2 sample was found to be amorphous. All of the multilayer samples have peaks at 2θ ¼ 38.21, 44.41, 64.61, and 77.61 that correspond to the (111), (200), (220), and (311) planes of Ag (JCPDS No. 87-0720) respectively. It can be seen that the intensity of the peaks increases with increasing the Ag thickness. This indicates that the crystallinity becomes improved with the Ag thickness. Fig. 2 shows the transmittance spectra of the TiO2/Ag/TiO2 multilayer films with various Ag thicknesses. Regardless of the Ag thicknesses, the transmittance reaches a global maximum around 590 nm and then gradually decreases with increasing wavelength. For example, the TiO2/Ag/TiO2 multilayer films have the highest transmittance of 86.3–97% at 591 nm for different Ag thicknesses. The transmission window narrows and gradually becomes lowered as the Ag thickness increases. Subsequently, the samples show lower transmittance at the absorption edge and infra-red region with 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 Ag thickness; it increases from 6.18 1021 to 1.07 1022 cm 3 as the
2. Experimental procedure TiO2/Ag/TiO2 multilayer thin films were consecutively deposited onto corning eagle XG glass substrates by an RF magnetron sputtering system. Ceramic TiO2 target (99.999% purity) and pure Ag target (99.99% purity) were used at room temperature under a base pressure of less than 1 10 6 Torr. Before being loaded into the sputtering chamber, the glass substrates (1.5 1.5 cm2) were cleaned with acetone, methanol, and deionized water for 15 min per cleaning agent in an ultrasonic bath, and finally dried in a N2 stream. Prior to deposition, both the TiO2 and Ag targets were presputtered for 30 min to remove contaminants. TiO2 and Ag were deposited using RF powers of 90 W and 30 W, respectively. During sputtering, the glass substrate was constantly rotated at a speed of 12 rpm for TiO2 and 23 rpm for Ag. The thickness of the Ag films varied from 15 to 25 nm, while the TiO2 film was fixed at 40 nm. The crystal structure of the multilayers was examined with X-ray diffraction (XRD, ATX-G, Rigaku). Transmittance of the multilayers was measured with a UV/visible spectrometer (UV-1800, Shimadzu). Hall measurements by the van der Pauw technique were performed with a magnetic field of 0.55 T (HMS 3000, Ecopia). The four-point-probe technique was used for sheet resistance measurements.
Fig. 1. XRD patterns obtained from TiO2/Ag/TiO2 multilayer films with various Ag layer thicknesses.
Fig. 2. Transmittance spectra of TiO2/Ag/TiO2 multilayer films with various Ag thicknesses.
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Fig. 3. The carrier concentration and Hall mobility of TiO2/Ag/TiO2 multilayer films as a function of Ag thickness.
Fig. 4. The resistivity and sheet resistance of TiO2/Ag/TiO2 multilayer films as a function of Ag thickness.
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 Ag thickness increases. Mobility behavior can be explained by scattering mechanisms such as phonon scattering, grain-boundary scattering, surface scattering, interface scattering, and ionizedimpurity scattering [30,31]. The amorphous TiO2 films are undoped and have constant thickness (40 nm), and so, interface scattering 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 sheet resistance slightly decreases from 6.17 to 2.27 Ω⧸sq as the Ag thickness increases. The resistivity also decreases with increasing the Ag thickness; it decreases from 6.05 10 5 to 2.30 10 5 Ω-cm as the Ag thickness increases from 15 to 25 nm. The resistivity is inversely proportional to the mobility and the carrier concentration [32]. Thus, with the mobility and the carrier concentration increasing by a factor of 1.5–1.72 with increasing Ag thickness, this means that the resistivity is dominated by the combined effect of the mobility and carrier concentration, as shown in Fig. 3. Fig. 5 shows calculated FOM (φTC) of the TiO2/Ag/TiO2 multilayer films as a function of the Ag film thickness. φTC was calculated using the equation defined by Haacke [33], φTC ¼ T 10 av =Rs where Rs is the sheet resistance and Tav is the average optical Rtransmittance. RTav can be estimated using the relation, T av ¼ VðλÞTðλÞdλ= VðλÞdλ, where T(λ) is the transmittance and V(λ) is the photopic luminous efficiency function defining the standard observer for photometry [12,34]. As shown in
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Fig. 5. Calculated FOM (φTC) of TiO2/Ag/TiO2 multilayer films as a function of Ag film thickness.
Fig. 5, the FOM reaches a maximum at 19 nm and then gradually decreases with increasing the Ag thickness. All the samples show fairly high FOM. In particular, the 19-nm-thick Ag multilayer gives the highest FOM of 157.2 10 3 Ω 1. As the sheet resistance slightly changes from 4.7 to 2.67 Ω⧸sq with increasing Ag thickness, the high FOMs can be attributed to the major contribution 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 at 550 nm. To understand the unusually high transmittance, the relationship between transmittance and TiO2 thickness was simulated using the scattering matrix method [35]. The unconventional transmission properties of dielectric–metal–dielectric–oxide multilayer films are readily illustrated by phasor diagram in which reflected partial waves are represented in the complex plane [36–38]. To construct the phasor diagram, all available reflected partial waves in our TiO2/Ag/TiO2 multilayer films were explored and 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 wave experiencing the metal layer more than four times was included in the groups because of the large extinction coefficient (k) of the Ag layer (e.g., k¼ 3.3 at λ¼ 550 nm). For 0-ϕ to 6-ϕ groups, each basis, the sum of the reflected partial waves in the same group accounting the Fresnel equations [35], was represented in the complex plane (straight lines in Fig. 6(b)). For this phasor representation, the wavelength of incident light was 550 nm and the thickness of the Ag layer was 18 nm. The other higher ϕ terms were not considered because of their negligibly small Fresnel coefficients. Then, each basis is rotated by 0, 2ϕ, 4ϕ, and 6ϕ, where ϕ is determined by the thickness (d) of the TiO2 layer: ϕ ¼ 2πd nTiO2 =λ (dashed lines in Fig. 6(b)). For the local maximum (d¼ 40 nm) and minimum (d¼ 80 nm) transmission conditions, the phasor diagrams were constructed (Fig. 6(c) and (d)). At d¼ 40 nm, the trajectory of phasors was marginally deviated from the origin, which accounts reduced reflectance. On the contrary, at d¼ 80 nm, each phasor from different groups was added constructively, which results in augmented reflectance. The simulation result (Fig. 7) exhibits that for the 18 nm-thick Ag layer, the transmittance of the TiO2/Ag/TiO2 sinusoidally varies with the TiO2 thickness and maximum at the TiO2 thickness of 40 nm. Thus, the unusual transmittance of the TiO2/Ag/TiO2 multilayer
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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 partial wave 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 diagrams for (c) the maximum and (d) minimum transmission conditions. For these calculations, the optical constants (n, k) of TiO2 and Ag at λ¼ 550 nm are (2.26, 0) and (0.35, 3.26), respectively.
multilayer films comprising different materials may provide a more enhanced transmission over a broad range of wavelengths. It is, however, worth noting that the calculated transmittance is somewhat lower than the measured one (Figs. 2 and 7). This may be attributed to the fact that unlike the real samples [39], the multilayer film employed in the model is assumed to have flat and smooth surfaces and interfaces. In addition, the refractive index of the amorphous TiO2 film might be also different from the value used in the simulation.
4. Summary and conclusions Fig. 7. Variation of transmittance as a function of TiO2 thickness.
films can be understood as a direct consequence of the antireflection effect accompanied by complex optical media. The transmission is maximized or minimized at specific oxide thicknesses by the orientation (i.e., destructive or constructive) of phasors, and primarily dictated by the phasors from the 0-ϕ and 2-ϕ groups. An optimum thickness of TiO2 for the maximum transmission is nearly unchanged over a narrow range of metal thicknesses because a metal thickness slightly changes the amplitude of the phasors (e.g., 2b and 2c in Fig. 6(a)) assigned to optical paths experiencing the Ag layer. Furthermore, we expect that other combinatorial sets of dielectric–metal–dielectric
The effect of Ag layer thickness on the optical and electrical properties of the TiO2/Ag/TiO2 multilayer films was investigated. The transmittance and transmission windows were dependent on the Ag thickness. The TiO2/Ag(19 nm)/TiO2 multilayer films had the highest transmittance at around 590 nm, which was attributed to the anti-reflection effect including complex optical media. As the Ag thickness increased, the carrier concentration gradually increased while the sheet resistance slightly decreased. The TiO2 (40 nm)/Ag (19 nm)/TiO2 (40 nm) multilayer produced the highest Haacke's FOM. The results show that the TiO2/Ag/TiO2 multilayer films can be used as transparent multilayer electrodes for display applications.
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Acknowledgments This work was supported by Korea Evaluation Institute of Industrial Technology (Grant no.10049601). S.-K.K. acknowledges support of this work by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1059423). References [1] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Flexible light-emitting diodes made from soluble conducting polymers, Nature 357 (1992) 477–479. [2] Q. Wan, E.N. Dattoli, W. Lu, Transparent metallic Sb-doped SnO2 nanowires, Appl. Phys. Lett. 90 (2007) 222107. [3] A. Dhar, T.L. Alford, Optimization of TiO2/Cu/TiO2 multilayer as transparent composite electrode deposited on flexible substrate at room temperature, ECS Solid State Lett. 3 (2014) N33–N36. [4] H. Hosono, H. Ohta, M. Orita, K. Ueda, M. Hirano, Frontier of transparent conductive oxide thin films, Vacuum 66 (2002) 419–425. [5] M.-S. Oh, S.-H. Kim, T.-K. seong, Growth of nominally undoped p-type ZnO on Si by pulsed-laser deposition, Appl. Phys. Lett. 87 (2005) 122103. [6] S.X. Zhang, S. Dhar, W. Yu, H. Xu, S.B. Ogale, T. Venkatesan, Growth parameter-property phase diagram for pulsed laser deposited transparent oxide conductor anatase Nb:TiO2, Appl. Phys. Lett. 91 (2007) 112113. [7] Ö.D. Coşkun, S. Demirela, The optical and structural properties of amorphous Nb2O5 thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 277 (2013) 35–39. [8] Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, G. Shimaoka, Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation, Appl. Surf. Sci. 142 (1999) 233–236. [9] K. Haga, M. Kamidaira, Y. Kashiwaba, T. Sekiguchi, H. Watanabe, ZnO thin films prepared by remote plasma-enhanced CVD method, J. Cryst. Growth 214 (2000) 77–80. [10] O. Vigil, F. Cruz, G. Santana, L. Vaillant, A. Morales-Acevedo, G. Contreras-Puente, Influence of post-thermal annealing on the properties of sprayed cadmium–zinc oxide thin films, Appl. Surf. Sci. 161 (2000) 27–34. [11] A.E. Jimenez-Gonzalez, J.A.S. Urueta, R. Suarez-parra, Optical and electrical characteristics of Al-doped ZnO thin films prepared by solgel technique, J. Cryst. Growth 192 (1998) 430–438. [12] S. Yu, W. Zhang, L. Li, D. Xu, H. Dong, Y. Jin, Optimization of SnO2/ Ag/SnO2 tri-layer films as transparent composite electrode with high figure of merit, Thin Solid Films 552 (2014) 150–154. [13] A. Dhar, T.L. Alford, Optimization of Nb2O5/Ag/Nb2O5 multilayers as transparent composite electrode on flexible substrate with high figure of merit, J. Appl. Phys. 112 (2012) 103113. [14] H.-H. Kim, E.-M. Kim, K.-J. Lee, J.-Y. Park, Y.-R. Lee, D.-C. Shin, T.-J. Hwang, G.-S. Heo, TiInZnO/Ag/TiInZnO multilayer films for transparent conducting electrodes of dye-sensitized solar cells, Jpn. J. Appl. Phys. 53 (2014) 032301. [15] H. Kim, K.-T. Lee, C. Zhao, L.J. Guo, J. Kanicki, Top illuminated organic photodetectors with dielectric/metal/dielectric transparent anode, Org. Electron. 20 (2015) 103–111. [16] B. Lin, C. Lan, C. Li, Z. Chen, Effect of thermal annealing on the performance of WO3/Ag/WO3 transparent conductive film, Thin Solid Films 571 (2014) 134–138. [17] D. Miao, S. Jiang, S. Shang, Z. Chen, Highly transparent and infrared reflective Al-doped ZnO(AZO)/Ag/AZO multilayer film prepared on PET substrate by RF magnetron sputtering, Vacuum 106 (2014) 1–4.
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[18] J.-A. Jeong, H.-K. Kim, Al2O3/Ag/Al2O3 multilayer thin film passivation prepared by plasma damage-free linear facing target sputtering for organic light emitting diodes, Thin Solid Films 547 (2013) 63–67. [19] J.-H. Song, J.-W. Jeon, Y.-H. Kim, J.-H. Oh, T.-Y. Seong, Optical, electrical, and structural properties of ZrON/Ag/ZrON multilayer transparent conductor for organic photovoltaics application, Superlattice Microstruct. 62 (2013) 119–123. [20] J.-W. Lim, S.-I. Oh, K. Eun, S.-H. Choa, H.-W. Koo, T.-W. Kim, H.-K. Kim, Mechanical flexibility of ZnSnO/Ag/ZnSnO films grown by roll-to-roll sputtering for flexible organic photovoltaics, Jpn. J. Appl. Phys. 51 (2012) 115801. [21] C.-H. Lee, R. Pandey, B.-Y. Wang, W.-K. Choi, D.-K. Choi, Y.-J. Oh, Nano-sized indium-free MTO/Ag/MTO transparent conducting electrode prepared by RF sputtering at room temperature for organic photovoltaic cells, Sol. Energy Mater. Sol. Cells 132 (2015) 80–85. [22] Y.-H. Kim, J.-W. Lee, R.-I. Murakami, Dependences of sputtering times on the structural and electrical properties of ZnO/Ag/ZnO thin films on PET by DC sputtering, IEEE Trans. Nanotechnol. 12 (2013) 991–995. [23] Y. Mouchaal, G. Louarn, A. Khelil, M. Morsli, N. Stephant, A. Bou, T. Abachi, L. Cattin, M. Makha, P. Torchio, J.C. Bernede, Broadening of the transmission range of dielectric/metal multilayer structures by using different metals, Vacuum 111 (2015) 32–41. [24] K. Hashimoto, H. Irie, A. Fujhishima, TiO2 photocatalysis: a historical overview and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269. [25] A. Dhar, T.L. Alford, High quality transparent TiO2/Ag/TiO2 composite electrode films deposited on flexible substrate at room temperature by sputtering, APL Mater. 1 (2013) 012102. [26] J. Kulczyk-Malecka, P.J. Kelly, G. West, G.C.B. Clarke, J.A. Ridealgh, K.P. Almtoft, A.L. Greer, Z.H. Barber, Investigation of Ag diffusion in TiO2/Ag/TiO2 coatings, Acta Mater. 66 (2014) 396–404. [27] J.H. Jia, P. Zhou, H. Xie, H.Y. You, J. Li, L.Y. Chen, Study of optical and electrical properties of TiO2/Ag/TiO2 multilayers, J. Korean Phys. Soc. 44 (2004) 717–721. [28] P.K. Chiu, C.T. Lee, D.Y. Chiang, W.H. Cho, C.N. Hsiao, Y.Y. Chen, B.M. Huang, J.R. Yang, Conductive and transparent multilayer films for low-temperature TiO2/Ag/SiO2 electrodes by E-beam evaporation with IAD, Nanoscale Res. Lett. 9 (2014) 35–39. [29] J.H. Kim, D.-H. Kim, T.-Y. Seong, Realization of highly transparent and low resistance TiO2/Ag/TiO2 conducting electrode for optoelectronic devices, Ceram. Int. 41 (2015) 3064–3068. [30] H. Han, N.D. Theodore, T.L. Alford, Improved conductivity and mechanism of carrier transport in zinc oxide with embedded silver layer, J. Appl. Phys. 103 (2008) 013708. [31] A. Indluru, T.L. Alford, Effect of Ag thickness on electrical transport and optical properties of indium tin oxide–Ag–indium tin oxide multilayers, J. Appl. Phys. 105 (2009) 123528. [32] H. Han, J.W. Mayer, T.L. Alford, Band gap shift in the indium-tin-oxide films on polyethylene napthalate after thermal annealing in air, J. Appl. Phys. 100 (2006) 083715. [33] G. Haacke, New figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086. [34] W.G. Driscoll, W. Vaughan, Handbook of Optics, McGraw-Hill, New York, 1978. [35] E. Hecht, in: Optics, fourth ed., Addison-Wesley, New York, 2002. [36] M.A. Kats, D. Sharma, J. Lin, P. Genevet, R. Blanchard, Z. Yang, M.M. Qazilbash, D.N. Basov, S. Ramanathan, F. Capasso, Ultra-thin perfect absorber employing a tunable phase change material, Appl. Phys. Lett. 101 (2012) 221101. [37] M.A. Kats, R. Blanchard, P. Genevet, F. Capasso, Nanometre optical coatings based on strong interference effects in highly absorbing media, Nat. Mater. 12 (2013) 20–24. [38] F.F. Schlich, R. Spolenak, Strong interference in ultrathin semiconducting layers on a wide variety of substrate materials, Appl. Phys. Lett. 103 (2013) 213112. [39] The thickness of the TiO2/Ag/TiO2 films was examined by high resolution transmission electron microscopy. The results showed that although the individual layers were well defined, the TiO2/Ag interfaces were slightly undulated.