CERAMICSINTERNATIONAL

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

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(2016) 3473–3478

Ceramics International 42 www.elsevier.com/locate/ceramint

Highly flexible Al-doped ZnO/Ag/Al-doped ZnO multilayer films depositedon PET substrates at room temperature

Jun Ho Kima, Da-Som Kimb, Sun-Kyung Kimb, Young-Zo Yooc, Jeong Hwan Leed,Sang-Woo Kimd, Tae-Yeon Seonga,n

aDepartment of Materials Science and Engineering, Korea University, Seoul 02841, Republic of KoreabDepartment of Applied Physics, Kyung Hee University, Gyeonggi-do, 17104, Republic of Korea

cDuksan Hi-Metal Co. Ltd., Yeonam-dong, Buk-gu, Ulsan 44252, Republic of KoreadSchool of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea

Received 25 August 2015; received in revised form 5 October 2015; accepted 27 October 2015Available online 3 November 2015

Abstract

We investigated the effects of the Ag layer thickness on the electrical and optical properties of AZO (36 nm)/Ag/AZO (36 nm) multilayer filmsthat were deposited on polyethylene terephthalate (PET) substrates using a radio frequency magnetron sputtering method. The AZO/Ag/AZOfilms had transmittances over 74–89% at 550 nm. The relationship between the transmittance and the Ag layer thickness was investigated withthree-dimensional finite-difference time-domain (3D FDTD) simulations to understand high transmittance. As the Ag layer thickness increasedfrom 15 to 23 nm, the carrier concentration increased from 5.84� 1021 to 9.66� 1021 cm�3, while the sheet resistance decreased from 10.15 to3.47 Ω sq�1. The Haacke figure of merit (FOM) was calculated for the samples with various Ag layer thicknesses; it was a maximum at 19 nm(43.9� 10�3 Ω�1). The resistance change for the 100 nm-thick ITO only films was unstable even after 5 cycles, while that of the AZO (36 nm)/Ag (19 nm)/AZO (36 nm) film remained constant up to 1000 cycles.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Al-doped ZnO; Ag; Transparent conducting electrode; Flexible device

1. Introduction

Transparent conductive oxide (TCO) thin films play animportant role in photoelectric devices such as organic photo-voltaic cells (OPVs) and organic light-emitting diodes(OLEDs) [1–3]. Sn-doped indium oxide (ITO) is used widelyas a transparent electrode material in display and optoelectricdevices due to its high transparency and low resistivity [4]. Forexample, ITO is a common anode for OLEDs since it formsgood energy-level alignment, thereby providing the efficienthole injection into the organic layers [5–7]. On the one hand,the development of flexible displays requires mechanicallyrobust flexible TCOs. However, ITO fails under relatively

10.1016/j.ceramint.2015.10.14615 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).

lower mechanical stress as compared with other devicecomponents [8,9]. Furthermore, the rare and expensive elementIn will increase the production costs of device application[10,11]. Therefore, it is important to develop low-cost TCOfabricated at low temperatures, which can replace the expen-sive ITO. To realize such TCOs, oxide/metal/oxide (O/M/O)structures have been actively investigated [12–14]. The O/M/Ostructures normally consist of a thin Ag layer (7–20 nm thick)sandwiched between two oxides (20–70 nm thick) with highdielectric constants. For O/M/O structures, a visible transmit-tance of larger than 90% can be realized due to a largedifference in the refractive indices of Ag and oxide layers[15,16] and good conductivity can be also achieved due toconductive Ag [17–19]. In particular, Al-doped ZnO (AZO)films are of particular interest because of low cost, nontoxicity,and chemical stability against H plasma environments.

Fig. 1. A cross-section TEM image of an AZO/Ag/AZO (36 nm/19 nm/36 nm)multilayer film grown on a PET substrate.

J.H. Kim et al. / Ceramics International 42 (2016) 3473–34783474

However, the realization of the desirable optoelectronicproperties of AZO films requires high substrate temperaturesand/or post-growth annealing [20,21], which hampers theirapplications for flexible polymer substrtaes. To solve theproblem, AZO/Ag/AZO multilayer films have been widelyinvestigated by many researchers [14,17,22–31]. For example,Miao et al. [24] investigated the electrical, optical and infraredreflectance properties of AZO/Ag/AZO films prepared on glasssubstrates as a function of Ag layer thickness and showed thatAZO (30 nm)/Ag (10 nm)/AZO (30 nm) films had the highestaverage visible transmittance of 85.4%. However, AZO(30 nm)/Ag (15 nm)/AZO (30 nm) had the lowest sheetresistance of 3.2 Ω sq�1 and the highest infrared reflectionrate of 97% in far infra-red region. Guillén et al. [17],investigating the electrical and optical properties of AZO/Ag/AZO films deposited on polymer substrates, showed that AZO(40 nm)/Ag (10 nm)/AZO (40 nm) films had a sheet resistanceof 6 Ω sq�1 and maximum transmittance around 85% near600 nm. Crupi et al. [25], investigating the thermal stabilityand optical and electrical properties of AZO (20 nm)/Ag/AZO(30 nm) films as a function of Ag film thickness, reported thatthe AZO/Ag (19 nm)AZO films had the lowest sheet resistanceof about 2.5 Ω sq�1 whereas the AZO/Ag(6.3 nm)AZO filmshad the highest transmittance of about 80%. In this study, theoptical and electrical properties of AZO/Ag/AZO multilayerfilms were also investigated as a function of Ag layer thicknessand compared with those of ITO films. The AZO/Ag/AZOfilms were deposited on polyethylene terephthalate (PET)substrates by an RF sputtering system at room temperature.Haacke figure of merit (FOM) was calculated to characterizethe performance of the multilayers. The mechanical flexibilityof the samples was investigated.

2. Experimental procedures

AZO/Ag/AZO multilayer thin films were deposited on PETsubstrates by an RF magnetron sputtering system. CeramicAZO target (ZnO:Al2O3¼98:2 wt%, 99.99% purity) and pureAg target (99.99% purity) were used at room temperatureunder a base pressure of less than 1� 10�6 Torr. Before beingloaded into the sputtering chamber, the PET substrates(2.0� 2.0 cm2) were cleaned with methanol and deionizedwater for 15 min per cleaning agent in an ultrasonic bath, andfinally dried in a N2 stream. Prior to deposition, both the AZOand Ag targets were presputtered for 30 min to removecontaminants. The AZO and Ag layers were deposited withRF powers of 90 and 30 W, respectively. During the sputterdeposition, the PET substrates were constantly rotated at aspeed of 12 and 24 rpm for the AZO and Ag layers,respectively. The thickness of the AZO layer (dAZO) was fixedconstant at 36 nm, while the thickness of the Ag films (dAg)was varied from 15 to 23 nm. The thicknesses of the multilayerfilms were determined with high-resolution transmission elec-tron microscopy (HR-TEM, JEM-ARM 200F, Jeol). Forexample, Fig. 1 shows a bright field TEM image obtainedfrom a AZO/Ag (19 nm)/AZO multilayer film. It is noted thatthe thickness of the Ag layer is somewhat irregular. Hall

measurements were performed with the van der Pauw methodusing a magnetic field of 0.55 T (HMS 3000, Ecopia). Thefour-point-probe technique was used to measure the sheetresistances of the samples. The transmittance of the multilayerfilms was measured with a UV/visible spectrometer (UV-1800,Shimadzu). A bare PET substrate was used as the referencewhen measuring the optical transmittance of the multilayersamples. The crystal structure of the multilayers was char-acterized with X-ray diffraction (XRD, ATX-G, Rigaku). Themechanical flexibility of the samples was analyzed with abending test system (ZBT-200, Z-tec). The samples wereclamped between two parallel semicircular-plates. One platewas mounted to the shaft of a motor, while the other was fixedto a supporter. The distance of the stretched mode was 80 mmand that of the bent position was 45 mm. Finally, the resistanceof the samples during the bending test was measured using amultimeter.To verify the measured transmittance spectra of the AZO/

Ag/AZO multilayer films, three-dimensional finite-differencetime-domain (3D FDTD) simulations were performed [32,33].In the FDTD simulations, dAZO was fixed at 36 nm. On the onehand, dAg also varied from 15 to 23 nm with a step of 2 nm,which is the same as the measurement conditions. For accuratesimulations, a spatial resolution of 1 nm was applied to allthree orthogonal axes and dispersive optical constants wereadapted for Ag [34] and AZO [35].

3. Results and discussion

Fig. 2 shows the XRD patterns obtained from the AZO/Ag/AZO multilayer films as a function of dAg. It is noted that apartfrom the PET, all of the peaks are fairly weak. All of themultilayer samples have weak peaks at 2θ¼34.21 thatcorrespond to the (002) plane of ZnO (JCPDS no. 36-1451).The multilayer samples also have weak peaks at 2θ¼38.31,corresponding to the (111) planes of Ag (JCPDS no. 04-0783).Apart from the characteristic peak of ZnO, no other phases,such as Al or any Al compounds, were observed. This findingindicates that there is no phase segregation or any secondary

Fig. 2. XRD patterns obtained from AZO/Ag/AZO multilayer films as afunction of Ag layer thickness.

Fig. 3. The transmittance spectra obtained from ITO only and AZO/Ag/AZOmultilayer films.

Fig. 4. The carrier concentration and Hall mobility of AZO/Ag/AZO multi-layer films with various Ag layer thicknesses.

Fig. 5. The resistivity and sheet resistance of AZO/Ag/AZO multilayer filmswith various Ag layer thicknesses.

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phase and that Al was not incorporated into ZnO lattice [36].The reference ITO film contains peaks at 2θ¼35.41, corre-sponding to the (400) plane of In2O3 (JCPDS no. 06-0416). Itis shown that the intensity of the Ag (111) peaks remainsalmost unchanged although its thickness increased from 15 to23 nm, as shown in the inset. The reason for this behavior isnot currently known. A similar feature was also observed byother researchers [37].

Fig. 3 shows the transmittance spectra of the ITO (100 nmthick) and AZO/Ag/AZO multilayer films with various dAg.Regardless of dAg, the transmittance reaches a global max-imum over 480–505 nm and then it gradually decreases withincreasing wavelength. For example, the AZO/Ag/AZO multi-layers films with different Ag layer thicknesses have transmit-tances of 79–91% at 500 nm. The transmittance at 550 nm ismeasured to be 89%, 89%, 87%, 81%, and 74% for thesamples with dAg¼15, 17, 19, 21, and 23 nm, respectively,while it is 88.7% for ITO. The transmission window narrowsand the transmittance gradually decreases with increasing dAg.Consequently, the thicker samples show lower transmittance atthe absorption edge and infra-red region.

Fig. 4 shows the carrier concentration and Hall mobility of theAZO/Ag/AZO multilayer films with various Ag layer thicknesses.The carrier concentration gradually increases with increasing dAg.As dAg increases from 15 to 23 nm, the concentration increases

from 5.84� 1021 to 9.66� 1021 cm�3, and the mobility alsoincreases from 15.5 to 23.5 cm2 V�1 s�1.Fig. 5 exhibits the resistivity and sheet resistance of the

AZO/Ag/AZO multilayer films with various Ag layer thick-nesses. The sheet resistance slowly decreases from 10.15 to3.47 Ω sq�1 as dAg increases from 15 to 23 nm. It is noted thatthe as-deposited reference ITO has a sheet resistance of145.5 Ω sq�1. The resistivity also decreases from6.87� 10�5 to 2.74� 10�5 Ω cm with increasing dAg. Theresistivity is inversely proportional to the mobility and thecarrier concentration [38]. This implies that the carrierconcentrations play a dominant role in reducing the resistivityof the multilayer films.The optical and electrical properties of TCOs are very

important for their use in photonic and optoelectronic deviceapplications. In other words, such applications require as hightransmittance and low sheet resistance as possible. FOM iscommonly used to investigate the relationship between opticaland electrical properties of TCOs. A FOM, φTC was calculatedfor all samples using the equation defined by Haacke [39],

φTC¼ Tav10

Rs, where Rs is the sheet resistance and Tav is the

average optical transmittance. Tav is defined as,

Tav ¼R

VðλÞTðλÞdλRVðλÞdλ , where T(λ) is the transmittance and V(λ) is

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the photopic luminous efficiency function defining the stan-dard observer for photometry [40,41]. Tav (over 450–750 nm)was estimated to be 88%, 88%, 85%, 79%, and 73% for the15, 17, 19, 21, and 23 nm-thick AZO/Ag/AZO films, respec-tively, while ITO had 88%. As can be seen in Fig. 6, FOMreaches a maximum at 19 nm and then rapidly decreases withincreasing dAg. In particular, the multilayer sample withdAg¼19 nm has the highest FOM (43.9� 10�3 Ω�1), whichis much larger than that of ITO (2.01� 10�3 Ω�1).

Fig. 7 shows the dependence of resistance on bending cyclein the ITO only (100 nm thick) and optimal AZO (36 nm)/Ag

Fig. 6. FOM (φTC) of AZO/Ag/AZO multilayer films as a function of Ag layerthickness.

Fig. 7. The change in resistance of (a) ITO single and (b) AZO/Ag/AZOmultilayer films on a PET substrate as a function of cycle. The insets showbending testers used in this experiment.

Fig. 8. (a) Calculated transmittance spectra of AZO/Ag/AZO multilayer films as a fulayer was fixed at 36 nm. (b) Calculated transmittance spectra of a Ag only film a

(19 nm)/AZO (36 nm) multilayer films. The inset exhibits thebending tester used in this study. The resistance change can bedefined as R�Ro/Ro, where Ro is the initial resistance and R isthe measured resistance after bending. The ITO only filmshows very unstable feature even after five cycles, indicatingthe formation and propagation of cracks. On the contrary, thevalue of R�Ro/Ro for the AZO/Ag/AZO multilayer filmremains almost constant. This electrical stability (i.e. bendingstability) is due to the presence of the ductile Ag layer withhigher failure strain (4–50%) [42].Fig. 8 exhibits the calculated transmittance spectra of AZO/

Ag/AZO and Ag only films as a function of dAg. The insetsillustrate the stacking structures of the samples. As shown inFig. 8(a), irrespective of dAg, the transmittance reaches a globalmaximum over 475–495 nm and then it gradually decreaseswith increasing wavelength. The dAg dependence is in agree-ment with the experimental results (Fig. 3). Although theoverall transmittance decreases monotonically with increasingdAg, the transmittance is higher than 85% even with opticallythick Ag layers (i.e. dAg¼15–19 nm), particularly at blue-to-green wavelengths. Such high transmittance from O/M/Omultilayer films is a direct consequence of the destructiveinterference between partial reflected waves [43–45], whichcan be illustrated by a complex phasor diagram [38]. Theinterference features from the AZO/Ag/AZO films are moreevident when compared with Ag only films on PET substrates(Fig. 8(b)). For the Ag only films, the transmittance is less than60% at λ of longer than 450 nm and decreases gradually withincreasing wavelength. The high transmittance at near UVwavelengths (λo400 nm) is due to absence of the AZO layersbecause ZnO-based materials have large absorption at thesewavelengths [45]. Taken together, the top and bottom dielec-trics in the O/M/O films generate complex partial reflectedwaves, which enhance the transmittance. This indicates that thetransmittance of O/M/O multilayer films can be furtherincreased at specific wavelengths by tailoring the thicknessand refractive index of dielectrics [43].

4. Summary and conclusions

The opto-electrical properties of AZO/Ag/AZO multilayerfilms deposited on PET substrates were investigated as a

nction of the Ag layer thickness. For the simulations, the thickness of the AZOs a function of the Ag layer thickness.

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function of the Ag layer thickness. The AZO/Ag (19 nm)/AZOmultilayer films had the highest transmittance of 89.7% at498 nm, which was attributed to the anti-reflection effectincluding complex optical media. As the Ag thicknessincreased, the carrier concentration gradually increased, whilethe sheet resistance slightly decreased. The AZO (36 nm)/Ag(19 nm)/AZO (36 nm) films had the highest Haacke's FOMmuch higher than that of ITO only samples. The optimal AZO/Ag/AZO electrode exhibited considerably improved mechan-ical flexibility compared to the ITO only film. These resultsshow that AZO/Ag/AZO (36 nm/19 nm/36 nm) sample can beused as an important transparent multilayer electrode inflexible photovoltaic and photonic devices.

Acknowledgments

This work was supported by the Korea Evaluation Instituteof Industrial Technology (Grant no. 10049601) and WC300R&D (Grant no. S2317456) through Korea Institute forAdvancement of Technology (KIAT) which is funded by theSmall and Medium Business Administration, Korea. S.-K.K.was supported by the Basic Science Research Program throughthe National Research Foundation of Korea (NRF), which isfunded by the Ministry of Science, ICT, and Future Planning(NRF-2013R1A1A1059423).

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