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† Author to whom all correspondence should be addressed:E-mail: khkim@gachon.ac.kr

Copyright 2013 KIEEME. All rights reserved.This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

pISSN: 1229-7607 eISSN: 2092-7592DOI: http://dx.doi.org/10.4313/TEEM.2013.14.1.9

TRANSACTIONS ON ELECTRICAL AND ELECTRONIC MATERIALS

Vol. 14, No. 1, pp. 9-11, February 25, 2013

Properties of AZO/Ag/AZO Multilayer Thin Film Deposited on Polyethersulfone Substrate

Yu Sup Jung, Yong Seo Park, and Kyung Hwan Kim

Department of Electrical Engineering, Gachon University, Seongnam 461-701, Korea

Received November 7, 2011; Revised August 27, 2012; Accepted August 27, 2012

The AZO/Ag/AZO multilayer films were deposited on polyethersulfone (PES) substrate by using facing target sputtering methods at room temperature. The AZO/Ag/AZO multilayer films with polymer substrate had advantages, such as low sheet resistance, high optical transmittance in visible range and stable mechanical properties. From the results, the AZO/Ag/AZO multilayer films (50/12/50 nm) demonstrated a sheet resistance of 11 Ω/ and average transmittance of 87% in visible range (wavelength of 380-770 nm). Moreover, the multilayer showed stable mechanical properties compared to the single-layered AZO sample during the bending test due to the existence of the ductile Ag metal layer.

Keywords: AZO/Ag/AZO, Multilayer, Polyethersulfone, Facing target sputtering

Regular Paper

1. INTRODUCTION

Transparent conducting oxide films are used by applications in microelectronic systems, such as flat panel display and thin film solar cells [1,2]. Among the known TCO materials, indium tin oxide (ITO) material has been used extensively, as it yields excellent elec-trical properties. However, ITO thin films have disadvantages, such as toxicity, high price and instability in hydrogen plasma. Zinc oxide (ZnO) is a wide optical band gap material with a wurtzite hexagonal structure, which exhibits a change in conductivity when doped with Al, Ga, and In [3]. However, the resistivity of these thin films is high and mechanical flexural properties are low. In order to improve the electrical and optical properties of transparent conducting films, the multilayer films consisting of a metal layer and semiconductor layers have been studied, such as ZnS/Ag/ZnS [4] ITO/Ag/ITO [5], and ZnO/Ag/ZnO [6]. For ITO/Ag/ITO multilayer films, ITO lacks

the high refraction that requires reducing the strong reflection from the Ag layer [4]. Due to the high refraction of ZnO, ZnO/Ag/ZnO multilayer films with high transmittance and conductivity have been studied [6]. In this paper, multilayer (AZO/Ag/AZO) films were prepared on polyethersulfon (PES) substrate at room temperature by using the Facing targets sputtering method. Moreover, the multi-layers exhibit stable mechanical properties compared to the single-layered TCO sample during the bending test due to the existence of the ductile metal layer.

2. EXPERIMENTS

Figure 1 shows the diagram of the facing targets sputtering (FTS) system used in the multilayer thin film deposition process. The FTS system was designed to array the two targets to face each other and so form high density plasma between the targets. The FTS system was able to restrain the bombardment of the substrate by high-energy particles because the position of the substrate was located outside of the plasma. Consequently, the FTS system suppresses the substrate damage caused by high-energy particles, such as electrons and partial ions [7-10]. In this work, Ag targets were installed on top

Won-Jae LeeDepartment of Electronic Engineering, Gachon University, Seongnam 461-701, Korea

Trans. Electr. Electron. Mater. 14(1) 9 (2013): Y. S. Jung et al.10

and the AZO targets were installed on the bottom. The chamber was evacuated to 2.6 × 10-4 Pa before the film deposition began using a rotary pump and a turbo molecular pump. Before each run, the target was pre-sputtered in a pure argon atmosphere for 10 min in order to remove the natural surface oxide layer of the Ag and AZO targets. The PES substrate was ultrasonically cleaned using isopro-pyl alcohol (IPA) and DI-water at 30 min and was blown dry with N2 gas. More details about the sputtering conditions are given in Table 1. The thicknesses of multilayer thin films were measured by using a surface profiler (alpha-step). Electrical properties of the films were measured by using a four-point probe. Optical transmittance and structural properties were measured by using UV/VIS spectrometer, X-ray diffractometer, and field emission scanning electron micros-copy (FE-SEM). Finally, mechanical flexural strength of the single-layered AZO and the multilayer were measured by using a bending tester.

3. RESULTS AND DISCUSSION

Electrical and optical properties of the multilayer should be excel-lent. Therefore, we measured sheet resistance and optical transmit-tance in visible range (380-770 nm wavelength). Figure 2 shows the optical transmittance of the AZO/Ag/AZO multilayer in visible range

as a function of AZO thicknesses. The maximum average transmit-tance is 87% with the AZO thickness of 50 nm. The AZO layers play the role of anti-reflective coatings in the multilayer films. This result is consistent with that reported by Leftheriotis et al [11]. As the AZO thickness increases further to 100 nm, the average transmittance decreases. Figure 3 shows the sheet resistance and average opti-cal transmittance in visible range of the multilayer as a function of AZO thicknesses. The sheet resistance of multilayer films shows 21, 11, 12 and 14 Ω/ with 25, 50, 75 and 100 nm AZO thicknesses, re-spectively. The average optical transmittance of multilayer films in visible range shows 84, 87, 86, and 82 with 25, 50, 75 and 100 nm top and bottom AZO thicknesses, respectively. Figure 4 shows the figure of merits (=FOM) suggested by Haacke [12] as a function of the AZO thicknesses. In various applications of transparent conductive films, the optical and electrical properties of the films are very important. Haacke [12] proposed a figure of merit followed by:

(1)

Where T is the average optical transmittance and Rsh is the electri-cal sheet resistance of the TCOs. This equation can be used to com-pare the performance of transparent conductive films. The maxi-mum value (= 2.2 × 10-2 Ω-1) is found when the AZO thickness was 50 nm (T: 0.87 and Rsh: 11 Ω/).

10

TCSh

TR

φ =

Fig. 1. Diagram of the facing targets sputtering (FTS) system used in the multilayer film deposition process.

Fig. 3. The sheet resistance and average optical transmittance in the visible range of the multilayer as a function of AZO thicknesses.

Fig. 2. The optical transmittance of the AZO/Ag/AZO multilayer in visible range as a function of AZO thicknesses.

Fig. 4. The figure of merits (=FOM) suggested by Haacke[12] as a function of the AZO thicknesses.

11Trans. Electr. Electron. Mater. 14(1) 9 (2013): Y. S. Jung et al.

Figure 5 shows the XRD patterns of multilayer films as a function of AZO thicknesses. For all samples, the ZnO(002) and Ag(111)peaks are observed, and the intensity of the (111) peak is much weaker than the (002) peak. As the AZO thickness increases from 25 to 100 nm, the intensity of the (002) peak increases and the intensity of the (111) peak increases slowly, which suggests that the crystallinity of the AZO and Ag layer is simultaneously improved. The AZO thick-ness dependence of crystal size for the AZO/Ag/AZO multilayer films is investigated by FE-SEM analysis. Figure 6 shows the micro-graphs of AZO/Ag/AZO multilayer films as a function of AZO thick-nesses. When the AZO thickness increases from 25 to 100 nm, an increase in the size of crystallite is observed. Figure 7 demonstrates the mechanical flexural strength of single-layered AZO and multi-layer films. The resistance of single-layered AZO thin film rapidly increased above 100 cycle. However, the resistance of the multilayer was constant. Moreover, the multilayers showed more stable me-chanical properties in comparison to the single-layered AZO sample during the bending test due to the existence of the ductile Ag metal layer.

4. CONCLUSIONS

Electrical, structural, and optical properties of the AZO/Ag/AZO multilayer structure on polyethersulfone substrate were investigated

as a function of top and bottom AZO thicknesses. From the results, the sheet resistance and average optical transmittance of AZO/Ag/AZO films with a thickness of 50/12/50 nm are 11 Ω/ and 87%, and the figure of merit exhibits 2.2 × 10-2 Ω-1. Moreover, the multilayers demonstrated more stable mechanical properties compared to the single layered AZO sample during the bending test due to the exis-tence of the ductile Ag metal layer. Thus, AZO/Ag/AZO multilayer thin films could apply flexible transparent electrodes.

ACKNOWLEDGMENTS

This work was supported by the Gachon University research fund of 2013 (GCU-2013-R040).

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Fig. 5. The XRD patterns of multilayer films as a function of AZO thicknesses.

Fig. 6. The micrographs of AZO/Ag/AZO multilayer films as a func-tion of AZO thicknesses.

Fig. 7. Mechanical flexural strength of single-layered AZO and the multilayer with the bending tester.