Ag multilayer films fabricated by magnetron sputtering for flexible electronics

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Acta Materialia 61 (2013) 5429–5436 www.elsevier.com/locate/actamat

Transparent conductive Sb-doped SnO2/Ag multilayer films fabricated by magnetron sputtering for flexible electronics Shihui Yu a, Weifeng Zhang b, Linngxia Li a,⇑, Dan Xu a, Helei Dong a, Yuxin Jin a b

a School of Electronic and Information Engineering, Tianjin University, Tianjin 300072, People’s Republic of China Key Laboratory of Photovoltaic Materials of Henan Province and School of Physics and Electronics, Henan University, Kaifeng 475004, People’s Republic of China

Received 20 April 2013; received in revised form 23 May 2013; accepted 24 May 2013 Available online 20 June 2013

Abstract The effects of an embedded silver layer and substrate temperature on the electrical and optical properties of Sb-doped SnO2 (ATO)/ silver (Ag) layered composite structures on polyethylene naphthalate substrates have been investigated. The highest conductivity of ATO/Ag multilayer films was obtained with a carrier concentration of 1.5  1022 cm3 and a resistivity of 2.4  105 X cm at the optimum Ag layer thickness and substrate temperature. The photopic averaged transmittance and Haacke figure of merit are 81.7%, and 21.7  103 X1, respectively. In addition, a conduction mechanism is proposed to elucidate the mobility variation with increased Ag thickness. We also describe the influence of substrate temperature on the structural, electrical and optical properties of the ATO/Ag multilayer films, and propose a mechanism for the changes in electrical and optical properties at different substrate temperatures. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Semiconductors; Multilayer thin films; Electrical properties; Growth temperature; Mechanism

1. Introduction Transparent conducting oxide (TCO) films have been widely used as transparent conducting electrodes in many optoelectronic and electro-optical devices such as solar cells and flat panel displays [1–5]. Sn-doped In2O3 (ITO) thin film is the most widely used TCO film due to its low resistivity, high transparency and high work function [6,7]. However, the conductivity of ITO thin film is limited by its semiconductor mechanism, these materials are expensive due to the cost of indium. Therefore, it is important to develop cheap and high-performance TCO thin films. Impurity-doped tin oxide (SnO2) thin films have received much attention as TCO films [8–13] because they are nontoxic, and use cheap and abundant elements. Sb and F are the most commonly used dopants for photovol⇑ Corresponding author. Tel./fax: +86 2227402838.

E-mail address: [email protected] (L. Li).

taic devices. Sb has been shown to be effective and low cost, and hence is an attractive material [13]. However, inherent limitations in the conductivity need to be further improved by using multilayer structures. Although three-layer structures have been widely reported, there are to date few reports about double-layer structures, despite the latter being cheaper to produce. Many papers on multilayer films on glass substrates have been reported [14–18]. However, there is little in the literature about multilayer coatings on flexible substrates, and no Sb-doped SnO2 (ATO)/metal multilayer films on polymer substrates have yet been reported. In this work, we used polyethylene naphthalate (PEN) as a flexible substrate, due to its superior optical properties compare to other materials [19,20]. We selected Ag as the metal layer because of its low resistivity. In this work, magnetron sputtering was used to deposit ATO/Ag multilayer films by radiofrequency (RF) sputtering of ATO and direct current (DC) sputtering of Ag on

1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.05.031

S. Yu et al. / Acta Materialia 61 (2013) 5429–5436

PEN substrates. These multilayer films had the highest conductivity yet reported in the literature, while retaining transmittance values acceptable for optoelectronic applications. We investigated the conduction mechanism and optical properties of the multilayer films deposited at various thicknesses of Ag layers. The role of substrate temperature on the structural, electrical and transmission properties of the films are also investigated. 2. Experimental First, Ag thin films were deposited onto PEN substrates using a metal Ag target by DC magnetron sputtering. Then, ATO thin films were directly deposited on the Ag layer using a Sb-doped SnO2 ceramic target by RF magnetron sputtering. Bulk Sb-doped SnO2 ceramic targets were prepared using a solid-state reaction method from SnO2 powder (99.5% purity) and Sb2O3 powder (99.9% purity). The powders were pressed to form pellets with a molar mixture ratio of Sb2O3:SnO2 = 0.1:0.8, which were sintered at 1400 °C for 5 h in air. The Ag target was 99.99% pure. The targets were 5.0 cm in diameter and 0.4 cm thick. The PEN substrates were ultrasonically cleaned in acetone for 30 min, rinsed in absolute ethyl alcohol and subsequently dried before the deposition. The base pressure of the sputter system before each deposition was approximately 3  104 Pa. An ATO layer was deposited by RF magnetron sputtering at 50 W, at a deposition rate of about 12.5 nm min1. The ATO layer was approximately 40 nm thick. High-purity (99.999%) Ar and O2 were introduced through separate mass flow controllers. The total pressure during sputtering was maintained at 0.5 Pa, and the Ar/O2 ratio was 14:1. The Ag layer was deposited by DC magnetron sputtering at 40 W, using a deposition rate 1 nm s1, at a pressure of 0.5 Pa in a pure Ar atmosphere. There was no break in the vacuum at any stage during the preparation of the films. The thickness of the Ag layer was varied between 3 and 13 nm and that of the substrate temperature was varied between room temperature and 200 °C. The thickness of the ATO and Ag layers was estimated based on the deposition time and deposition rate. The substrate temperature was measured using a thermocouple gauge and a hot cathode gauge. The variation of substrate temperature during deposition was maintained within ±1 °C. X-ray diffraction (XRD) patterns were collected on a DX-2500 diffractometer with Cu Ka radiation (k = ˚ ). The surface morphologies were investigated by 1.5418 A scanning electron microscopy (JEOL JSM-7600F, Akishima, Tokyo, Japan). Hall measurements by the van der Pauw technique were done using an Ecopia HMS 3000. The four-point probe technique was used for sheet resistance measurements. Optical transmittance spectra and absorption spectra were obtained on an ultraviolet– visible–near infrared (UV–Vis-NIR) spectrophotometer (Varian Cary 5000) in the wavelength range 300–800 nm.

3. Results and discussions 3.1. Effect of the Ag layer thickness on the properties of ATO/Ag multilayer films 3.1.1. Optical properties Fig. 1 shows the transmission spectra in the wavelength range 300–800 nm for the ATO/Ag multilayer films on PEN substrate with different Ag film thicknesses deposited at room temperature (25 °C). The average optical transmittance Tav can be computed as follows: R V ðkÞT ðkÞdk T av ¼ R ; ð1Þ V ðkÞdk where T(k) is the transmittance and V(k) is the standard photopic luminous efficiency function [21]. As can be seen, the optical transmittance of ATO thin film without any Ag layer is above 90% in the visible wavelength range (400– 800 nm). After insertion of the 3 nm Ag layer, the transmittance of the multilayer films drops to 81%, and then increases slightly to 84% as the Ag layer thickness increases from 3 to 7 nm, and noticeably decreases to 65% with a further increase in Ag layer thickness to 13 nm. The initial increase in transmission spectra is attributed to a decrease in the scattering of the aggregated Ag islands. When the Ag layer thickness is thin (<7 nm), the Ag islands are discontinuous, and the low transmittance is attributed to light scattering at the ATO/Ag interface in isolated islands of Ag. However, increasing Ag thickness leads to an improvement in the transmittance because the near-continuous Ag layer has lower scattering loss. As the Ag layer thickness increases further, the Ag layer becomes continuous, resulting in an increase in its plasmon absorption and reflectivity [22], and therefore, the transmittance decreases. The change in photon energy (hv) and optical absorption coefficient (a) for the ATO/Ag multilayer films with different Ag film thicknesses deposited at room tempera-

100

80

Transmittance (%)

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60 0 nm Ag 3 nm Ag 40

5 nm Ag 7 nm Ag 9 nm Ag

20

11 nm Ag 13 nm Ag

0 300

400

500

600

700

800

Wavelength (nm) Fig. 1. Optical transmittance spectra of ATO/Ag multilayer films deposited at room temperature (25 °C) as a function of Ag layer thickness.

S. Yu et al. / Acta Materialia 61 (2013) 5429–5436

ture are displayed in Fig. 2. The optical band gap (Eg) was calculated by using following relation [23]:

3.1.2. Electrical properties The dependence of electrical properties of ATO/Ag multilayer films deposited at room temperature on the thickness of the Ag layer are displayed in Fig. 3. For the asdeposited ATO layers in this study, the resistivity and sheet resistance values are 5.08  102 X cm and 1312 X/sq., respectively, for 40 nm ATO thin film. For the ATO/Ag structure on PEN substrate, as shown in Fig. 3a, the resistivity decreases from 1.09  103 to 1.58  105 X cm as the Ag layer thickness is increased from 3 to 13 nm. The sheet resistance also decreases from 258.4 to 3.9 X/sq. The sheet resistance is approximately equal to the resistivity/thickness of the films [27], and therefore the sheet resistance curve follows a similar trend to that of sheet resistance as a function of Ag layer thickness. The decrease in resistivity can be understood by inspection of the changes in carrier concentration and mobility as shown in Fig. 3b. The increase in the carrier concentration with

0 nm Ag 3 nm Ag 5 nm Ag 7 nm Ag

α2 (a.u)

9 nm Ag 11 nm Ag 13 nm Ag

3.2

3.4

3.6

3.8

4.0

4.2

Energy (eV) Fig. 2. The a2 vs. hm relation for ATO/Ag multilayer films at different Ag layer thicknesses.

ρ (Ω cm) 10-3 102

10-4 101

10-5

Sheetrestance, Rsh (Ω/sq.)

103

(a) 0

3

6

9

12

The thickness of Ag layer (nm)

(b)

18

1022

15 12 9

1021

-3

n (cm )

6

Mobility (cm2/V-s)

where a and hm represent the absorption coefficient and the incident radiation energy, respectively, and C is a constant. The values of the direct optical band gap (Eg) were determined by extrapolation of linear regions of the plots to zero absorption (ahm = 0). The optical band gap of the as-deposited ATO thin film obtained from Fig. 2 is 3.77 eV. For the ATO/Ag multilayer films, as shown in Fig. 2, increasing the Ag thickness resulted in a remarkable decrease in its optical band gap (Eg); a similar trend has been reported by Alford’s group [24,25] for ZnO/Ag/ZnO multilayer films and ITO/Ag/ITO multilayer films, and by Zhizhen Ye’s group [26] for GZO/Ag/GZO multilayer films. Those researchers believe that many-body effects cause downward shifting of the conduction band and upward shifting of the valence band, resulting in the band gap shrinking.

Rsh(Ω/sq.) Resistivity, ρ (Ω cm)

ð2Þ

10-2

Carrier concentration (cm-3)

2

ðahmÞ ¼ Cðhm  EgÞ;

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2

μ (cm /V-s) 1020 0

3

6

9

3

12

The thickness of Ag layer (nm) Fig. 3. (a) Resistivity, sheet resistance and (b) Hall mobility, carrier concentration of the ATO/Ag multilayer films deposited at room temperature (25 °C) as a function of Ag layer thickness.

the inclusion of Ag layer can be understood based on the Schottky theory. Ag has a work function of WM = 4.3 eV [28] and ATO has a work function of WS = 4.84.9 eV [29]. Consequently an Ohmic contact forms at the metal– oxide interface and there is an accumulation of electrons in the ATO layer. As the work function U difference is large, there is a significant injection of electrons into the ATO layer. Due to this electron transfer, the conduction and valence bands of ATO curve downward, until they achieve a thermodynamic equilibrium, where the Fermi level crosses the interface in a straight line. In this case, there is no barrier to electron flow between the Ag and ATO. Hence, electrons are easily injected from the Ag layer into the ATO layer. According to research by the Alford group [25], the variation in mobility with increasing Ag thickness can be explained by the combined effects of interface scattering and grain-boundary scattering. The Hall mobility of multilayer films with 3–5 nm Ag layer thickness is lower than that of the single ATO thin films. As shown in Fig. 3b, the mobility drops to 3.34 cm2 V1 s1 for the multilayer film with a 3 nm Ag layer, which suggests that interface scattering (i.e. scattering of carriers at the ATO/Ag and Ag/ATO interfaces) is the primary factor. For higher Ag thicknesses (5–13 nm), the Ag layer becomes near-continuous or continuous, and the interface regions become a

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smaller fraction of the total layer thickness; therefore the interface scattering decreases. As a consequence, the mobility of the multilayer films increases with the increase of Ag layer thickness.

uTC ¼

T 10 av ; Rsh

200°C

Intensity (a.u.)

3.1.3. Figure of merit In various applications of transparent conductive films, the optical and electrical properties of the films are very important. Ideally, both optical transmittance and electrical conduction should be as large as possible. However, their interrelation excludes the simultaneous achievement of maximum transmittance and conduction in most cases. We estimated a figure of merit uTC for the films defined as [30]:

(211)

(101)

150°C

100 °C

Room temperature 20

30

40

50

60

70

2θ (deg.)

ð3Þ

where uTC is the figure of merit, Tav is the transmittance (considering the application of the films in display devices, we use the average transmittance) and Rsh is the sheet resistance. This equation can be used to compare the performance of transparent conductive films. The figures of merit obtained are 0.5  103, 7.5  103, 14.3  103, 18.6  103, 7.3  103 and 3.4  103 X1 for the 3, 5, 7, 9, 11 and 13 nm Ag layers, respectively. The best figure of merit is obtained when the Ag layer thickness is 9 nm. 3.2. Effect of the substrate temperature on the properties of ATO/Ag (10 nm) multilayer films According to the above results, we found that the ATO/ Ag multilayer films exhibited the best figure of merit uTC at an Ag thickness of 9 nm. Hence we decided to fix the Ag thickness at 9 nm to investigate the properties of the ATO/Ag multilayer films deposited at various substrate temperatures. As the PEN substrates are not capable of withstanding high temperatures, the substrate temperatures were limited to below 200 °C. 3.2.1. Structural properties Fig. 4 shows XRD patterns of the ATO/Ag (9 nm) multilayer films deposited at substrate temperatures ranging between room temperature and 200 °C. The XRD patterns indicate that the structure of the multilayer films is strongly dependent on substrate temperature. However, no Ag peak was ever observed; this may be because the Ag layer was too thin, similarly to the result previously reported by Yu Sup Jung [31]. The ATO/Ag (9 nm) multilayer films prepared at room temperature show no characteristic XRD peaks, indicating the amorphous nature of these films. When the ATO/Ag multilayer films were deposited at 100 °C, it is quite clear from Fig. 4 that these films possessed a poorly polycrystalline structure oriented along the (1 0 1) and (2 1 1) planes. As the substrate temperature is increased, it is noted that a relatively strong intensity peak corresponding to the (1 0 1) and (2 1 1) planes is observed. According to the results, it is easy to infer that

Fig. 4. XRD patterns of ATO/Ag (9 nm) multilayer films at different substrate temperatures.

high substrate temperature can greatly improve the crystallization of ATO layer. These phenomena can be explained as follows. For the multilayer films deposited at low substrate temperature, there exist many oxygen vacancies, thus causing the chemical composition of the ATO thin films to be non-stoichiometric, and resulting in the ATO thin films being poorly crystalline. In addition, the low substrate temperature might induce defects in the ATO thin films, which influence the nucleation and growth of the thin films, thus leading to the poor crystalline quality. As the substrate temperature increases, the energy of the surface species increases and they migrate through the lattice, becoming deposited at appropriate lattice sites to form a more stoichiometric crystalline phase [32]. Fig. 5 shows micrographs of ATO/Ag multilayer films deposited at various substrate temperatures. As shown in Fig. 5a, the surface morphology of multilayer films deposited at room temperature is an non-compact structure, indicating the amorphous nature of the films, which is in good agreement with the above XRD analysis results. As the substrate temperature rises from room temperature to 100 °C (Fig. 5b), the surface morphology transforms to a compact and smooth structure. With a further increase in substrate temperature up to 200 °C (Fig. 5c), some grains are formed and are uniformly distributed on the surface, which indicates that the ATO layers have better crystallinity. Furthermore, it is found that the average size of the crystalline grains increases with increase in substrate temperature. On increasing the substrate temperature the surface adatom mobility of the species increases, leading to a coalescence of smaller grains [33]. The increase in substrate temperature resulted in significant change in the cross-sectional scanning electron microscopy (SEM) image of ATO/Ag. The interface can be clearly seen in the cross-section images shown in Fig. 5d and e, indicating that the adhesion of multilayer films deposited at 100 °C is stronger than those deposited at room temperature.

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Fig. 5. SEM images of the cross-section of ATO/Ag (9 nm) multilayer films deposited at different temperatures: (a) plan view, room temperature (25 °C); (b) plan view, 100 °C; (c) plan view, 200 °C; (d) cross-section, room temperature; (e) cross-section, 100 °C.

3.2.2. Electrical and optical properties The dependence of sheet resistance and resistivity of the ATO/Ag (9 nm) multilayer films on substrate temperature is shown in Fig. 6a. As can be seen, both sheet resistance and resistivity initially decrease with substrate temperature, reach a minimum at 100 °C, and increase with a further increase in substrate temperature up to 200 °C. The initial decrease in sheet resistance and resistivity is attributed to an increase in both carrier concentration and Hall mobility. However, for the ATO/Ag multilayer films deposited at higher substrate temperature (>100 °C), both the free carrier concentration and Hall mobility significantly decrease, and, as a result, both the resistivity and sheet resistance decrease. The variation of carrier concentration and Hall mobility with substrate temperature is shown in Fig. 6b. The substrate temperature also affects the optical properties of the ATO/Ag multilayer films. The transmission

spectra in the wavelength range of 300–800 nm were measured for the ATO/Ag multilayer films deposited at various substrate temperatures, as shown in Fig. 7a. The average optical transmittance in the visible range (400–800 nm) decreases slightly from 81.9% to 81.7% as the substrate temperature increases from room temperature to 100 °C and noticeably decreases to 73.3% with a further increase in substrate temperature up to 200 °C. The band gap energies of the ATO/Ag (9 nm) multilayer films at different substrate temperatures are determined according to Eq. (2). Fig. 7b shows the a2 vs. hm relation. It is observed that the band-gaps do not show much variation with an increase in the substrate temperature below 100 °C; the band gaps are about 3.7 eV. With a further increase in the substrate temperature to 200 °C, the optical band gap slightly decreases to 3.6 eV. This shift of band gap with substrate temperature (i.e. carrier concentration)

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5.0x10-5 12 -5

4.0x10

9 3.0x10-5

(a)

6

80

Transmittance (%)

Resistivity, ρ (Ω cm)

15

Rsh(Ω/sq.)

Sheetrestance, Rsh (Ω/sq.)

100 ρ (Ω cm)

6.0x10-5

60

40

Room Temperature 100 ºC 150 ºC 200 ºC

2.0x10-5 50

100

150

200

20

Substrate Temperature (°C)

300

22

1.4x10

16

1.2x1022

15

1.0x1022

14

μ (cm /V-s) 2

12 50

100

150

400

500

600

700

800

Wavelength (nm)

13

n (cm-3)

8.0x1021

(a)

0

Room Temperature 100 ºC 150 ºC 200 ºC α2 (a.u)

(b)

17

Mobility (cm2/V-s)

Carrier concentration (cm-3)

1.6x1022

200

Substrate Temperature (°C) Fig. 6. (a) Resistivity, sheet resistance and (b) Hall mobility, carrier concentration of the ATO/Ag (9 nm) multilayer films deposited at various substrate temperatures.

(b) 3.2

can be explained by the Burstein–Moss shift [5,9]; the absorption edge shifts towards lower energy with a decrease of carrier concentration. ATO is reported to have an optical band gap of 3.4–4.1 eV [13,34]. The optical band gap obtained from Fig. 7b is within the range for ATO band gap, though it is more advantageous to utilize the combined band gap of the ATO + PEN structure because PEN is known to have a band gap of 3.2 eV [35]. 3.2.3. Discussion of the mechanism The change in both the electrical and optical properties of the ATO/Ag (9 nm) multilayer films deposited at substrate temperatures ranging between room temperature and 200 °C can be interpreted in terms of the change in the interior structure of these multilayer films. The schematic drawing in Fig. 8 shows the mechanism of the change of both the electrical properties and optical properties. The ATO/Ag (9 nm) multilayer films deposited at 100 °C have a poor polycrystalline structure (there exists a large amount of amorphous ATO thin film), confirmed by XRD in Fig. 4. Hence, the arrangement of tin oxide molecules is disordered, and the three-dimensionally distributed grain boundaries in the ATO layers result in a lower effective diffusion coefficient [36], Therefore, the penetration of the Ag molecules, which causes the degradation of the electrical properties in ATO/Ag (9 nm) multilayer films, was

3.4

3.6

3.8

4.0

Energy (eV) Fig. 7. (a) Optical transmittance spectra and (b) optical band gap of the ATO/Ag (9 nm) multilayer films prepared at various substrate temperatures.

(a)

ATO layer Ag atoms Ag layer PEN Substrate

(b)

ATO layer Ag atoms Ag layer PEN Substrate

Fig. 8. Schematic illustrations of (a) retarded diffusion of Ag atoms into noncrystalline ATO layer, and (b) easy penetration of Ag atoms into ATO layer with a crystal structure.

retarded by the amorphous structure, as shown in Fig. 8a. Since few Ag atoms are incorporated within the ATO layer, the Ag layer remains continuous. As we

S. Yu et al. / Acta Materialia 61 (2013) 5429–5436

observe no significant change in the carrier concentration and optical properties of the ATO/Ag (9 nm) multilayer films deposited at 100 °C compared to those deposited at room temperature (as shown in Figs. 6 and 7), but rather an improvement in the mobility, this is consistent with the picture that increasing the substrate temperature removes deep-level oxygen vacancy charged defects (2+ state) of ATO thin film [37]. The improved mobility can thus be attributed to reduced ionized defect scattering centers in the ATO layer, although grain boundary passivation by oxygen at 100 °C is another possible explanation. With a further increase in the substrate temperature to 200 °C, the ATO/Ag (9 nm) multilayer films exhibit better crystalline quality, confirmed by XRD in Fig. 4. In this case, the structure of the ATO layer was not quite effective at hindering the diffusion, and Ag atoms could easily diffuse into the ATO layer through the grain boundaries between the columns as shown in Fig. 8b. In addition, at higher substrate temperatures (>100 °C), Ag atoms have enough energy to diffuse though the ATO layer and O atoms diffuse through the Ag layer, resulting in oxidation of the metal layer [38]. The diffusion and oxidation of numerous Ag atoms leads to destruction of Ag layer, and the Ag layer then becomes discontinuous or even disappears. As a result, the carrier concentrations of multilayer films decrease noticeably, due to the intragrain cluster scattering. Both the discontinuous Ag layer and the Ag-incorporated ATO layer lead to scattering losses, and the transmittance of ATO/Ag (9 nm) multilayer films decreases. The figures of merit uTC, which were determined according to Eq. (3), are 18.6  103, 21.7  103, 6.1  103 and 2.6  103 X1 for the ATO/Ag multilayer films deposited at room temperature, 100, 150 and 200 °C, respectively. The highest figure of merit is obtained when the substrate temperature is 100 °C. According to the above results, the ATO/Ag multilayer films exhibited the best figure of merit uTC (21.7  103 X 1 ) at the optimum Ag thickness (9 nm) and a substrate temperature of 100 °C; this is higher than the uTC of AZO/Ag/AZO [16,39], GZO/Cu [40], ITO/Au/ITO [41], ZnO/Cu/ZnO [22] and TCO single-layer films. A higher uTC results in better-quality TCO films [42,43], suggesting that ATO/Ag multilayer films have better optical and electrical properties, making them suitable for practical applications. 4. Conclusion ATO/Ag multilayer films were deposited on PEN substrates by RF and DC magnetron sputtering. We investigated the structural, electrical and optical properties of these multilayer films deposited at various Ag thicknesses and substrate temperatures. As the Ag layer thickness increases, both the resistivity and sheet resistance decrease. As the substrate temperature increases, the resistivity and sheet resistance initially decrease and then increase. In brief, the thickness of the Ag layer and substrate tempera-

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ture play an important role in controlling the electrical, optical and structural properties of these multilayer films. The XRD patterns show that the crystallinity of the samples improves with increasing substrate temperature. The highest-conductivity ATO/Ag multilayer films are obtained with a carrier concentration of 1.5  1022 cm3 and a resistivity of 2.4  105 X cm at the optimum Ag layer thickness and substrate temperature. The photopically averaged transmittance and Haacke figure of merit are 81.7%, and 21.7  103 X1, respectively. Acknowledgments This work was supported financially by the Program for New Century Excellent Talents in University (NCET), the 863 Program (2007AA03Z423), the China Postdoctoral Science Foundation and the Natural Science Foundation of China (Grant No. 2012IRTSTHN004). References [1] Chen CN, Chen CP, Dong TY, Chang TC, Chen MC, Chen HT, et al. Acta Mater 2012;60:5914. [2] Yua JS, Jung GH, Jo J, Jung Kim S, Kim JW, Kwak SW, et al. Sol Energy Mater Sol Cells 2013;109:142. [3] Noirfalise X, Godfroid T, Guisbiers G, Snydersa R. Acta Mater 2011;59:7521. [4] Pust SE, Worbs J, Jost G, Hu¨pke J. Sol Energy Mater Sol Cells 2013;113:106. [5] Kang Y, Jeon SH, Son YW, Lee YS, Ryu M, Lee S, et al. Phys Rev Lett 2012;108:196404. [6] De S, Higgins TM, Lyons PE, Doherty EM, Nirmalraj PN, Blau WJ, et al. ACS Nano 2009;3:1767. [7] Feigenbaum E, Diest K, Atwater HA. Nano Lett 2010;10:2111. [8] Consonni V, Rey G, Roussel H, Doisneau B, Blanquet E, Bellet D. Acta Mater 2013;61:22. [9] Yu S, Ding L, Xue C, Chen L, Zhang WF. J Non-Cryst Solids 2012;358:3137. [10] Gokulakrishnan V, Parthiban S, Jeganathan K, Ramamurthi K. J Mater Sci 2011;46:5553. [11] Huang X, Yu Z, Huang S, Zhang Q, Li D, Luo Y, et al. Mater Lett 2010;64:1701. [12] Huang Y, Li G, Feng J, Zhang Q. Thin Solid Films 2010;518:1892. [13] Montero J, Herrero J, Guille´n C. Sol Energy Mater Sol Cells 2010;94:612. [14] Longa G, Geng Y. Appl Surf Sci 2012;263:546. [15] Yu SH, Jia CH, Zheng HW, Ding LH, Zhang WF. Mater Lett 2012;85:68. [16] Wu HW, Yang RY, Hsiung CM, Chu CH. Thin Solid Films 2012;520:7147. [17] Crupi I, Boscarino S, Strano V, Mirabella S, Simone F, Terrasi A. Thin Solid Films 2012;520:4432. [18] Lee HM, Lee YJ, Kim IS, Kang MS, Heo SB, Kim YS, et al. Vacuum 2012;86:1494. [19] Chen Y, Li H, Li M. Nature Sci Rep 2012;2:622. [20] Sierros KA, Cairns DR, Abell JS, Kukureka SN. Thin Solid Films 2010;518:2623. [21] Driscoll WG, Vaughan W. Handbook of optics. New York: McGraw-Hill; 1978. [22] Sivaramakrishnan K, Alford TL. Appl Phys Lett 2009;94:052104. [23] Kim H, Auyeung RCY, Pique´ A. Thin Solid Films 2008;516:5052. [24] Han H, Theodore ND, Alford TL. J Appl Phys 2008;103:013708. [25] Indluru A, Alforda TL. J Appl Phys 2009;105:123528. [26] Gong L, Lu J, Ye Z. Thin Solid Films 2011;519:3870.

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