CERAMICSINTERNATIONAL

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

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(2015) 14805–14810

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

Al-doped ZnO/Ag/Al-doped ZnO multilayer films with a high figure of merit

Jun Ho Kima, Yoon-Jong Moonb, Sun-Kyung Kimb, Young-Zo Yooc, Tae-Yeon Seonga,n

aDepartment of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of KoreabDepartment of Applied Physics, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea

cDuksan Hi-Metal Co. Ltd., Yeonam-dong, Buk-gu, Ulsan 683-804, Republic of Korea

Received 14 July 2015; received in revised form 31 July 2015; accepted 1 August 2015Available online 10 August 2015

Abstract

In this study, we investigated the effects of the Al-doped ZnO (AZO)-layer thickness on the optical and electrical properties of AZO/Ag/AZOmultilayer films deposited on glass substrates at room temperature. The optimal AZO/Ag/AZO (36 nm/19 nm/36 nm) multilayer sample exhibiteda transmittance of approximately 93% at 550 nm. As the AZO-layer thickness increased from 9 to 45 nm, the carrier concentration graduallydecreased from 1.87� 1022 to 6.36� 1021 cm�3, while the sheet resistance slightly increased from 3.86 to 4.47 Ω sq�1 and the charge mobilityincreased from 24.15 to 25.42 cm2 V�1 s�1. The samples had smooth surfaces with a root mean square (RMS) roughness ranging from 0.40 to1.23 nm. The Haacke figure of merit (FOM) was calculated for the samples as a function of the AZO-layer thickness. The optimal AZO/Ag/AZOmultilayer film had the highest FOM of 99.9� 10�3 Ω�1.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Al-doped ZnO; Ag; Multilayer film; Transparent conducting electrode

1. Introduction

Thin films of transparent conducting oxides (TCOs) are ofgreat technological importance because of their use in photo-electric devices, such as displays and solar cells. Indium tinoxide (ITO) is the most commonly used TCO because of itsexcellent electrical and optical properties [1,2]. However, ITOsuffers from thermal instability and is composed the rare andexpensive element In. Thus, to replace the expensive ITO, avariety of TCOs with low sheet resistances and high transmit-tances has been extensively investigated [3–8]. However, theoptoelectrical properties of these TCOs were found to beinferior to that of ITO. Thus, a thin metal film sandwichedbetween two TCO films, i.e., TCO/metal/TCO (TCO/M/TCO)multilayers, has been widely studied. Ag films have frequentlybeen used as the middle layer for TCO/M/TCO multilayersbecause of its low resistance and high transmittance in thevisible spectrum [9–18]. For example, the electrical and opticalproperties of SnO2/Ag/SnO2 films prepared on quartz glass

10.1016/j.ceramint.2015.08.00115 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).

substrates were investigated as functions of the Ag- and SnO2-layer thicknesses [9]. It was reported that a SnO2/Ag/SnO2 film(layer thicknesses¼50 nm/5 nm/50 nm) had a maximum fig-ure of merit (FOM) of 60� 10�3 Ω�1, sheet resistance of9.67 Ω sq�1, resistivity of 1.0� 10�4 Ω cm, and an averagetransmittance of 94.8% in the visible spectrum. The transmit-tance and sheet resistance of TiO2 (10 nm)/Ag/TiO2 (10 nm)multilayers were also studied as a function of the Ag-layerthickness [17]. The TiO2/Ag (8 nm)/TiO2 multilayer filmsexhibited the lowest sheet resistance (30 Ω sq�1) and highesttransmittance (approximately 90%) over 500–700 nm. Inaddition, ZnO/Ag/ZnO multilayers have been widely studiedby many researchers because of their excellent characteristics,such as easy fabrication, high thermal stability, and durabilityagainst hydrogen plasma [19–22]. For example, the effects ofthe thickness of the Ag layer (dAg) and upper ZnO layer (dZnO)on the physical properties of ZnO/Ag/ZnO multilayer electro-des were investigated [19]. The ZnO/Ag/ZnO (56.7 nm/dAg/56.7 nm) multilayer films exhibited a sheet resistance of lessthan 5 Ω sq�1 when dAg¼18.8 nm. On the other hand, theZnO/Ag/ZnO (56.7 nm/8.9 nm/dZnO) multilayer films yieldedsheet resistances of less than 6.5 Ω sq�1, regardless of dZnO.

Fig. 1. XRD patterns of the AZO/Ag/AZO multilayer films as a function ofdAZO.

J. Ho Kim et al. / Ceramics International 41 (2015) 14805–1481014806

Hajj et al. [20] reported that ZnO/Ag/ZnO (35 nm/10 nm/20 nm) multilayer electrodes had a sheet resistance of 6 Ωsq�1, transmittance of approximately 80% in the visiblespectrum, and FOM of 16.5� 10�3 Ω�1. Furthermore, Miaoet al. [23] investigated the electrical and optical properties ofAl-doped ZnO (AZO)/Ag/AZO multilayer films prepared withradio-frequency (RF) magnetron sputtering as functions of dAgand the AZO-layer thickness (dAZO). They reported that theAZO/Ag/AZO (30 nm/10 nm/30 nm) samples had the highesttransmittance of 80.5% in the visible spectrum. Sahu et al. [24]investigated the optical and electrical properties of AZO/Ag/AZO multilayers prepared with electron beam evaporation as afunction of dAg. They reported that the multilayers had sheetresistances as low as 5.34 Ω sq�1, transmittances of approxi-mately 90%, and a FOM of 65� 10�3 Ω�1 at a wavelength of450 nm. Crupi et al. [25] also studied the optoelectricalproperties of AZO/Ag/AZO multilayers prepared by RFmagnetron sputtering as a function of dAg. They reported thatthe highest FOM of approximately 9� 10�3 Ω�1 wasobtained when a dAg¼9.5 nm. In this study, we also investi-gated the optical and electrical properties of AZO/Ag/AZOmultilayers, but as a function of dAZO. The AZO/Ag/AZOmultilayer films were deposited with RF magnetron sputteringat room temperature. A FOM was estimated for each sample tocharacterize the performance of the multilayers.

2. Experimental procedures

The AZO/Ag/AZO multilayer thin films were consecutivelydeposited onto glass substrates (Corning Eagle XG) with anRF magnetron sputtering system. Ceramic AZO targets (ZnO:Al2O3¼98:2 wt%, 99.99% purity) and pure Ag targets(99.99% purity) were used at room temperature under a basepressure of less than 1� 10�6 Torr. Before being loaded intothe sputtering chamber, the glass substrates (1.5� 1.5 cm2)were cleaned in an ultrasonic bath with acetone, methanol, anddeionized water for 15 min per cleaning agent, and finally

dried in a N2 stream. Prior to deposition, both the AZO and Agtargets were pre-sputtered for 30 min to remove any contami-nants. The AZO and Ag layers were deposited with RF powersof 90 and 30 W, respectively. During the sputtering process,the glass substrate was constantly rotated at a speed of 12 rpmfor the AZO layers and 24 rpm for the Ag layer. The thicknessof the top and bottom AZO layers (dAZO) was varied from 9 to45 nm, while the thickness of the Ag layer was kept constant at19 nm, which was chosen based on previous studies [26]. Thethicknesses of the multilayer films were determined with high-resolution transmission electron microscopy (HR-TEM, JEM-ARM 200F, JEOL). Hall measurements were performed withthe van der Pauw method using a magnetic field of 0.55 T(HMS 3000, Ecopia). The four-point-probe technique wasused to measure the sheet resistances of the samples. Thetransmittance of the multilayer films was measured with a UV/Visible (UV/Vis) spectrometer (UV-1800, Shimadzu). A bareglass substrate was used as the reference when measuring theoptical transmittance of the multilayer samples. The crystalstructure of the multilayer films was determined with X-raydiffraction (XRD, ATX-G, Rigaku). In addition, the surfacemorphology of the multilayer films was characterized withatomic force microscopy (AFM, XE-1000, Park Systems).

3. Results and discussion

The crystal phases and structures present in the AZO and Aglayers were characterized with XRD. Fig. 1 shows the XRDpatterns of the AZO/Ag (19 nm)/AZO multilayer films as afunction of dAZO. All of the multilayer samples have peaks at2θ¼34.21 and 62.61 that correspond to the (002) and (103)planes of ZnO, respectively (JCPDS No. 36-1451). In addition,the intensity of these peaks increases as dAZO increases, whichindicates that the crystallinity is improved by thicker AZOlayers. Moreover, apart from the ZnO peaks, no peakscorrespond to either Al or Al-based compounds. This impliesthat the films do not contain any secondary phases [27].Furthermore, the strong (002) peaks are indicative of thepreferred c-axis orientation of the AZO layers [28]. All of themultilayer samples also have peaks at 2θ¼38.31, 44.31, and64.51, which correspond to the (111), (200), and (220) planesof Ag, respectively (JCPDS No. 04-0783). However, unlikethe results of a previous study [19], all of the peak intensitiesfor the sample with dAZO¼9 nm, such as the Ag(111), Ag(200), and Ag(220) peaks, are weaker than that of the othersamples. The reason for this is not currently known.Fig. 2 shows the transmittance spectra of the AZO/Ag/AZO

multilayer films as a function of dAZO. For all of the samples,the transmittance reaches a global maximum and then itgradually decreases with increasing wavelength. The transmit-tance at 550 nm was estimated to be 60%, 77%, 90%, 93%,and 86% for the samples with dAZO¼9, 18, 27, 36, and 45 nm,respectively. In addition, the transmission window widens andgradually shifts towards lower energies as dAZO increases.To improve our understanding of the dependence of the

transmittance on dAZO, we conducted three-dimensional finite-difference time-domain (3D-FDTD) simulations [29,30]. For

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these 3D-FDTD simulations, a spatial resolution of 1 nm wasimposed on all three orthogonal coordinates (x-, y-, and z-axes). A normal plane wave with λ=300–800 nm was gener-ated above the simulated TCO/M/TCO multilayer films andthe transmittance was detected in a glass substrate, which isidentical to the measurement conditions of this study (Fig. 3(a)). The dispersive optical constants of the Ag and AZOlayers were applied to the 3D-FDTD simulations [29]. Thesimulated transmittance spectra (Fig. 3(b)) are in good agree-ment with the measured spectra (Fig. 2). First, the simulatedspectra shift to longer wavelengths with increasing dAZO.Second, the average transmittance over blue-to-red wave-lengths (λ¼450–750 nm) is maximized at dAZO¼36 nm.Third, the transmittance abruptly drops in the UV region(λo400 nm). These three characteristics of the TCO/M/TCOmultilayer films can be explained by an interference model thatconsiders the complex refractive indices of the Ag and AZOlayers [31–33]. In general, when a material with a large

Fig. 2. Transmittance spectra of the AZO/Ag/AZO multilayer films withvarious dAZO values.

Fig. 3. (a) Schematic of the 3D-FDTD simulation model. (b) Calculated tran

extinction coefficient (i.e., a metal) is sandwiched by twodielectrics, the destructive condition for reflected wavesexhibits a complicated behavior compared to the simplecondition required for an anti-reflection coating, which is wellsupported by a complex phasor diagram [31]. The very hightransmittance (490%) of the TCO/M/TCO multilayer films,even with an optically thick Ag layer (19 nm), will provide ascheme for fabricating the next generation of transparentelectrodes with high transmittances and low sheet resistances.Fig. 4 shows the carrier concentration and Hall mobility of

the AZO/Ag/AZO multilayer films with various dAZO values.The carrier concentration gradually decreases as dAZOincreases; it decreases from 1.87� 1022 to 6.36� 1021 cm�3

as dAZO increases from 9 to 45 nm, respectively. The chargemobility slightly increases from 24.15 to 25.42 cm2 V�1 s�1

as dAZO increases. Mobility characteristics are typicallydescribed by scattering mechanisms, such as phonon scatter-ing, grain-boundary scattering, surface scattering, interfacescattering, and ionized-impurity scattering [34]. As a resultof the preferred orientation of the samples improving withincreasing dAZO (Fig. 1), the enhanced charge mobility may beascribed to the improved preferred orientation.Fig. 5 shows the sheet resistance and resistivity of the AZO/

Ag/AZO multilayer films with various dAZO values. All of thesamples have similarly low sheet resistances that range from3.86 to 4.47 Ω sq�1. On the other hand, the resistivity increasesas dAZO increases from 9 to 45 nm; it is 1.31� 10�5 and3.95� 10�5 Ω cm when dAZO¼9 and 45 nm, respectively. Theresistivity is inversely proportional to the charge mobility andcarrier concentration [35]. Thus, by considering that the chargemobility only varies slightly with increasing dAZO, the increas-ing resistivity of the samples can be attributed to the dominantcontribution of the carrier concentration.Fig. 6 shows the FOM (φTC) of the AZO/Ag/AZO multi-

layer films with various dAZO values. φTC was calculated usingthe equation defined by Haacke [36], φTC¼ (Tav)

10/Rs, whereRs is the sheet resistance and Tav is the average optical

smittances of TCO/M/TCO multilayer films as a function of dAZO.

Fig. 4. Carrier concentration and Hall mobility of the AZO/Ag/AZO multi-layer films with various dAZO values.

Fig. 5. Resistivity and sheet resistance of the AZO/Ag/AZO multilayer filmswith various dAZO values.

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

Fig. 7. Surface roughness results of the AZO/Ag/AZO multilayer films, whichwere obtained with AFM, as a function of dAZO. The inset shows an AFMimage of the multilayer film with dAZO¼36 nm.

J. Ho Kim et al. / Ceramics International 41 (2015) 14805–1481014808

transmittance over 450�750 nm. Tav can be estimated with therelation, Tav ¼

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

transmittance and V(λ) is the photopic luminous-efficiencyfunction for defining the standard observer for photometry

[9,37]. Tav was estimated to be 58%, 74%, 87%, 91%, and85% when dAZO=9, 18, 27, 36, and 45 nm, respectively. Themultilayer sample with dAZO=36 nm has the highest FOM of99.9� 10�3 Ω�1. The FOM is significantly lower for thesamples with dAZO=9 and 18 nm (1.2� 10�3 and 13.6� 10�3

Ω�1, respectively). As the sheet resistance does not changesignificantly with dAZO, the high FOM when dAZO=36 nm canbe attributed to the dominant contribution of the high opticaltransmittance.The surface morphologies of the AZO/Ag/AZO multilayer

films with various dAZO values were characterized with AFM.The surfaces of the multilayer samples are smooth, with theRMS roughness of the samples ranging from 0.40–1.23 nm(Fig. 7). For example, the sample with dAZO¼36 nm has asmooth surface, as shown in the inset of Fig. 7, which has anRMS roughness of 1.23 nm. The surface morphology of theTCO layers is one of the most important parameters thatinfluence the coating process of organic photovoltaic devices;the surface smoothness of the underlying TCO layer signifi-cantly affects the uniformity of the organic layer, and there-fore, the performance of the devices.The results show that the AZO/Ag/AZO samples have high

optical transmittances over 450�750 nm. The large differencebetween the refractive indices of Ag and the dielectric layers cancause efficient plasmon coupling, which results in a high visibletransmittance (480%) [38]. The realization of a high transmit-tance with our AZO/Ag/AZO multilayer films may also beattributed to the surface plasmon resonance in the Ag layerwhen using the optimal dAZO of 36 nm. The global transmit-tance maximum of the AZO/Ag/AZO samples is blue-shifted asdAZO decreases (Fig. 2), and the carrier concentration increases(Fig. 4), which is consistent with the general behavior of TCOs[39,40]. As the carrier concentration increases, the absorptionedge of the transmission window for TCOs slightly shifts withrespect to the photon absorption edge [40]. Furthermore, theaverage transmittance for wavelengths above 700 nm increasedas dAZO increased, which may be explained in terms of the

J. Ho Kim et al. / Ceramics International 41 (2015) 14805–14810 14809

plasmon-absorption-dependent reflections that are caused by theincreased carrier concentration [9].

4. Conclusions

The optical and electrical properties of AZO/Ag/AZOmultilayer films deposited on glass substrates were investi-gated as a function of dAZO. The transmission windowwidened and shifted towards lower energies as dAZO increased.The AZO/Ag/AZO (36 nm/19 nm/36 nm) multilayer samplehad the highest transmittance at 550 nm. As dAZO increased,the carrier concentration gradually decreased, while the chargemobility and sheet resistance varied insignificantly. The multi-layer sample with dAZO¼36 nm exhibited the highest value forthe Haacke FOM. With the smooth surface morphology, theresults of this study show that AZO/Ag/AZO multilayer filmshave the potential to be used as transparent multilayerelectrodes for display applications.

Acknowledgments

This work was supported by the Brain Korea 21 program,which is funded by the Ministry of Science, ICT, and FuturePlanning, Korea, and the Korea Evaluation Institute of IndustrialTechnology (Grant no. 10049601: Development of outputcoupling conductive substrate with light extraction efficiencyup to 2.0 times). S.-K.K. was supported by the Basic ScienceResearch Program through the National Research Foundation ofKorea (NRF), which is funded by the Ministry of Science, ICT,and Future Planning (NRF-2013R1A1A1059423).

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