MgxZn1–xO Multilayers As High-Performance

Jan 11, 2016 - (8) Because of these advantages, DMD-based TCL layers, such as ... Ag layer on MgxZn1–xO using the ion beam pretreatment, the film th...
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MgxZn1−xO/Ag/MgxZn1−xO Multilayers As High-Performance Transparent Conductive Electrodes Hyo-Ju Lee,† Jang-Won Kang,† Sang-Hyun Hong,‡ Sun-Hye Song,† and Seong-Ju Park*,†,‡ †

School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea

‡

S Supporting Information *

ABSTRACT: We report on the optical and electrical properties of MgxZn1−xO/Ag/MgxZn1−xO transparent conductive electrodes. The transmittance and sheet resistance of MgxZn1−xO/Ag/MgxZn1−xO multilayers deposited at room temperature were strongly dependent on the thickness and surface morphology of Ag layer. The optical absorption edge of MgxZn1−xO/Ag/MgxZn1−xO showed a blue shift with increasing Mg composition due to the increased band gap of Mg x Zn 1− x O. The Haack figure of merit value of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O with a 14 nm-thick Ag layer, which has a sheet resistance of 6.36 Ω/sq and an average transmittance of 89.2% at wavelengths in the range from 350 to 780 nm, was 69% higher than that of a ZnO/Ag/ZnO multilayer electrode. These results indicate that MgxZn1−xO/Ag/ MgxZn1−xO multilayers, which also show low surface roughness, can be used as highly conductive transparent electrodes in various optoelectronic devices operating over a wide wavelength region. KEYWORDS: transparent electrode, MgZnO, multilayers, band gap engineering, surface roughness

T

Ag/GMZO have been investigated by many research groups.9−14 The DMD-based TCL layers have shown high transparency in the visible wavelength region. But their optical property was not investigated in the UV wavelength region by controlling the bandgap of dielectric layer. In addition, the control of film thickness of metal layer in DMD-based TCL is also very important because the transparency of DMD-based TCL decreases exponentially with increasing the metal film thickness. It is known that the evaporated metal films deposited on dielectric layer initially form island structure and then become continuous thin films with a thickness of 10−25 nm.15,16 Because the electrical and optical properties of thin metal layers critically depend on the continuity and morphology of metal film, it is important to deposit very thin continuous metal films with an appropriate surface morphology in order to realize the highly transparent and conducting multilayer electrode. In this study, we show that high-performance MgxZn1−xO/ Ag/MgxZn1−xO multilayers TCL can meet the requirements of various TCL applications. The optical and electrical properties were enhanced by controlling the wetting property of Ag layer on MgxZn1−xO using the ion beam pretreatment, the film thickness and surface morphology of Ag layer, and the Mg composition in the MgxZn1−xO layer.

he transparent conducting layer (TCL) is a very important material in a variety of devices and systems, including solar cells, light-emitting diodes (LEDs), and flat panel displays.1,2 TCLs should have high transparency in the visible wavelength region and low resistivity, and indium tin oxide (ITO) has been widely used for TCLs. However, ITO is a high-cost material because of the comparative rarity of indium and it is also mechanically brittle. ZnO has attracted considerable attention as a possible alternative to ITO because it is both highly conductive and transparent in the visible wavelength region.3 However, high growth temperatures and postannealing processes are necessary to obtain high-quality ZnO.3,4 And many applications of ZnO and ITO to flexible substrates and devices are also limited by these high temperature processes. Recently, it was reported that a symmetrical dielectric/metal/dielectric (DMD) multilayer structure has high electrical conductivity and optical transmittance in the visible wavelength region, even though the layers are grown at room temperature.5 Zhang et al. reported that the transmittance of the ZnO/Ag/ZnO structure is enhanced by surface plasmon polaritons because the free carriers at the interface between the metal and the dielectric layer can be coupled with and then decoupled from the light via surface plasmon resonance.6,7 Han et al. also found that the conductivity of ZnO/Ag/ZnO multilayers is improved by increasing the Ag layer thickness.8 Because of these advantages, DMD-based TCL layers, such as ITO/Ag/ITO, ITO/Au/ITO, GZO/Ag/GZO, ZnO/Au/ZnO, ZTO/Ag/ZTO, and GMZO/ © XXXX American Chemical Society

Received: October 19, 2015 Accepted: January 11, 2016

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DOI: 10.1021/acsami.5b09974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Dielectric layers of MgxZn1−xO (50 nm) were deposited on a glass substrate at room temperature by radio frequency (RF) magnetron sputtering using dual targets. A pure ZnO target and a ZnO target mixed with 20 wt % MgO were used to control the Mg composition in the MgxZn1−xO layers. The Mg composition in the MgxZn1−xO layers was calculated from optical band gap of as-grown MgxZn1−xO (see Figure S1). To enhance the wettability at the interface between MgxZn1−xO and Ag layer, the surface of the MgxZn1−xO bottom layer was pretreated using the energetic Ar ions provided by a gridless end-Hall ion source. Ar gas was introduced at a flow rate of 2 SCCM into an electron beam evaporation chamber as a process gas and the working pressure of the chamber was maintained at 2.0 × 10−4 Torr. The discharge voltage of the ion source was 160 V and the average ion energy was 100 eV.17,18 The Ag layer was then deposited on the bottom MgxZn1−xO dielectric layer by electron beam evaporation. Finally, the MgxZn1−xO (50 nm) top layer was deposited on the Ag/MgxZn1−xO/glass structure by RF magnetron sputtering. The electrical properties of the MgxZn1−xO/Ag/MgxZn1−xO multilayers were measured using the Hall measurement system at room temperature. The optical transmittance of MgxZn1−xO/Ag/MgxZn1−xO multilayers was measured using a UV−vis spectrophotometer (Agilent 8453) with a blank glass substrate as the reference in the spectral range of 300−800 nm. The work functions of ZnO/Ag/ZnO and Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O multilayers were measured KP 6500 Digital Kelvin probe (McAllister Technical Services). The crystalline properties of as-grown MgxZn1−xO and MgxZn1−xO/Ag/MgxZn1−xO multilayers were measured by X-ray diffraction (XRD) with a Cu Kα radiation at room temperature (see Figures S2 and S3). The surface morphology of the Ag films on MgxZn1−xO layers and the MgxZn1−xO/Ag/ MgxZn1−xO multilayer films was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figure 1a, b shows SEM images of 10 and 14 nm-thick Ag film on Mg0.28Zn0.72O (50 nm) without ion beam pretreatment on Mg0.28Zn0.72O bottom layer. The 10 nm thick Ag film shows large islands on Mg0.28Zn0.72O. As the thickness of Ag layer is increased to 14 nm, the Ag islands are connected to each other. In the case of Ag layer on Mg0.28Zn0.72O with ion beam pretreatment, the 10 nm-thick Ag film shows a continuous film and the 14 nm thick Ag film forms a continuous Ag layer with Ag nanoparticles on the Ag surface as shown in Figure 1c, d. Figure 1e shows the sheet resistance of Ag/Mg0.28Zn0.72O (50 nm) with various Ag layer thicknesses on Mg0.28Zn0.72O bottom layer with and without ion beam pretreatment. A sheet resistance could not be measured on the Ag layer with a thickness below 12 nm on the surface of MgxZn1−xO without ion beam pretreatment because Ag forms discrete islands. However, the sheet resistance of Ag/Mg0.28Zn0.72O was dramatically decreased after ion beam pretreatment on Mg0.28Zn0.72O. Low energy ion beam treatment on substrate was reported to disrupt the surface of substrate, leaving dangling bonds.19 The initially evaporated Ag atoms on Mg0.28Zn0.72O after ion beam treatment can be immobilized by reacting with dangling bonds on the surface of Mg0.28Zn0.72O and this reaction can enhance the wetting property of Ag on Mg0.28Zn0.72O. To optimize electrical and optical properties of MgxZn1−xO/ Ag/MgxZn1−xO multilayers, we investigated MgxZn1−xO/Ag/ MgxZn1−xO with ion beam pretreatment. Figure 2 shows SEM images of the Ag layer deposited on Mg0.28Zn0.72O (50 nm) films. The Ag layer thickness was varied from 4 to 16 nm. The

Figure 1. SEM image of (a) 10 nm and (b) 14 nm thick Ag layers on Mg0.28Zn0.72O without ion beam pretreatment, (c) 10 nm and (d) 14 nm thick Ag layers on MgxZn1−xO with ion beam pretreatment, and (e) sheet resistance of Ag/Mg0.28Zn0.72O (50 nm) with various Ag layer on Mg0.28Zn0.72O bottom layer thickness with and without ion beam pretreatment.

morphology of a 4 nm-thick Ag layer shows discrete islands on the Mg0.28Zn0.72O film, as shown in Figure 2a. As the Ag layer thickness increased from 4 to 16 nm, the Ag islands were gradually transformed from aggregated islands into a continuous Ag layer, as shown in Figure 2. Figure 2e shows the Ag thickness-dependent sheet resistance and sheet carrier concentration in the Mg0.28Zn0.72O (50 nm)/Ag/Mg0.28Zn0.72O (50 nm) structure. The sheet resistance of the MgZnO/Ag/ MgZnO multilayer TCL can be expressed using the following equation5 1 1 2 = + Rs RAg RMgZnO where Rs, RAg, and RMgZnO are the sheet resistances of the MgZnO/Ag/MgZnO multilayer TCL, the Ag film, and the MgZnO layer, respectively. If the sheet resistance of MgZnO is much higher than that of the Ag film, the sheet resistance of the MgZnO/Ag/MgZnO multilayer TCL can largely be determined by the sheet resistance of the Ag film. The sheet resistance of the 4 nm-thick Ag film could not be measured because the Ag film consists of discrete island structures. Figure 2e shows that the sheet resistance of Mg0.28Zn0.72O/Ag/ Mg0.28Zn0.72O decreases dramatically as the Ag film thickness increases from 8 to 16 nm, because the Ag islands coalesce and electrical conduction channels are then formed. The sheet carrier concentration of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O increased with increasing Ag film thickness, as shown in Figure 2e. The free electrons in the Ag film can easily be injected into B

DOI: 10.1021/acsami.5b09974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. SEM images of (a) 4, (b) 8, (c) 12, and (d) 16 nm thick Ag layers on Mg0.28Zn0.72O, and (e) sheet resistance and sheet carrier concentration of Mg0.28Zn0.72O (50 nm)/Ag (x nm)/Mg0.28Zn0.72O (50 nm) multilayers with different Ag layer thicknesses.

Figure 3. (a) Transmittance spectra and (b) optical band gap of Mg0.28Zn0.72O (50 nm)/Ag (x nm)/Mg0.28Zn0.72O (50 nm) structures with different Ag thicknesses.

the Mg0.28Zn0.72O layer because the contact between Ag and Mg0.28Zn0.72O behaves like an ohmic contact.8 Figure 3 shows the optical transmittance spectra and the (αhν)2 plots of the Mg0.28Zn0.72O (50 nm)/Ag/Mg0.28Zn0.72O (50 nm) structure as a function of the Ag layer thickness. Figure 3a shows that the transmittance of Mg0.28Zn0.72O/Ag/ Mg0.28Zn0.72O in the visible range increases with increasing Ag thickness up to 14 nm. The transmittance of Mg0.28Zn0.72O/ Ag/Mg0.28Zn0.72O with a 4 nm-thick Ag layer is lower than that of Mg0.28Zn0.72O (100 nm) alone because of the surface scattering of light that is incident on the Ag particles. However, the transmittance of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O increased with increasing Ag film thickness up to 8 nm because the surface scattering of the incident light can be reduced by the Ag layer transformed from particles into a continuous film. In the case of a 14 nm-thick Ag layer, the transmittance of Mg0.28Zn0.72O (50 nm)/Ag(14 nm)/Mg0.28Zn0.72O (50 nm) with ion beam pretreatment is higher than that of the Mg0.28Zn0.72O (100 nm) layer in the broad wavelength range of 355−650 nm. This enhanced optical transmittance is observed in all samples independent of Mg composition (see Table S2) but the enhancement is not observed in Mg0.28Zn0.72O (50 nm)/Ag (14 nm)/Mg0.28Zn0.72O (50 nm) which has a smooth Ag film (see Figure S4). Several research groups reported that the transmittance in the metal film is enhanced when nanoparticles and nanoholes are randomly distributed over the metal film.20,21 Enhancement of transmittance by surface plasmon is not observed in the smooth Ag film because surface plasmon is not coupled with light due to difference of wave vectors between light of surface plasmons.

However, in the case of Ag films with nanoparticle on the Ag surface and nanohole in the Ag film, incident light can couple with surface plasmon and couple out into air via a Bragg scattering process.21 However, the transmittance of the Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O multilayer decreased over a broad wavelength range as the Ag film thickness increased to more than 14 nm because of the increased reflection of the Ag film. We calculated the optical band gap energies of the Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O multilayers using a Tauc relation:9 αhν = A(hν − Eg )1/2

Here, α is the absorption coefficient, A is a constant, hν is the photon energy and Eg is the optical band gap. As shown in Figure 3b, the optical band gap energy of the Mg0.28Zn0.72O/ Ag/Mg0.28Zn0.72O multilayers is estimated to be 3.80 eV, and the optical band gap decreases to 3.71, 3.66, 3.64, 3.62, and 3.6 eV with increasing Ag layer thickness. This reduction in the optical band gap of the Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O multilayers with increasing Ag thickness is mainly due to the increased sheet carrier concentration, as shown in Figure 2e. Increased sheet carrier concentration in Mg0.28Zn0.72O/Ag/ Mg0.28Zn0.72O multilayers was previously reported to reduce the band gaps of degenerate MgZnO layer with free-carrier densities above the Mott critical density.22−24 To optimize the Ag layer thickness, we also calculated the Haack figure of merit (ΦH) value using the following equation25 C

DOI: 10.1021/acsami.5b09974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ΦH =

10 Tav Rs

short wavelength region with increasing Mg composition in the MgxZn1−xO layers due to the increased band gap of the MgxZn1−xO layer. As shown in Figure 4b, the optical band gap of MgxZn1−xO/Ag/MgxZn1−xO increased from 3.2 eV (for x = 0) to 3.6 eV (for x = 0.28). These results show that the transmittance of the MgxZn1−xO/Ag/MgxZn1−xO structure in the UV wavelength region can be increased by using MgxZn1−xO layers with high Mg compositions. We also measured the work function of ZnO/Ag/ZnO and Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O. The work function of ZnO/ Ag/ZnO and Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O is 4.3 and 4.18 eV, respectively, and the decrease in work function is attributed to the shift of conduction band to vacuum level by increase of optical band gap in Mg0.28Zn0.72O.26 Table 2 shows the sheet resistance, transmittance at 550 nm, average transmittance (in the 350−780 nm range), and the

Here, Rs is the sheet resistance and Tav is the average transmittance in the wavelength range from 350 to 780 nm. Table 1 summarizes the values of Rs, Tav, and ΦH of the Table 1. Sheet Resistance, Optical Transmittance, and Figure of Merit of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O Multilayer Films As a Function of Ag Thickness Ag thickness (nm) Rs (Ω/sq) Tav (%) (350−780 nm) ΦH × 10−3(Ω−1)

8

12

14

16

47.4 74.7 1.14

8.48 88.3 34.0

6.36 89.2 50.1

6.07 84.8 31.7

Table 2. Sheet Resistance, Optical Transmittance at 550 nm, Average Transmittance, and Figure of Merit of MgxZn1−xO/ Ag/MgxZn1−xO Multilayer Films with Different Mg Compositions

Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O structure as a function of the Ag thickness. Table 1 shows that the maximum ΦH of 50.1 × 10−3 Ω−1 can be obtained for Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O when the Ag layer thickness is 14 nm. Figurs 4a, b shows the optical transmittance spectra and the optical band gap of MgxZn1−xO (50 nm)/Ag (14 nm)/

Mg composition Rs (Ω/sq) T (%) (550 nm) Tav (%) (350−780 nm) ΦH × 10−3 (Ω−1)

0

0.08

0.19

0.28

6.35 93.8 84.6 29.6

6.31 95.3 87.4 41.2

7.09 94.8 88.7 42.5

6.36 94.2 89.2 50.1

Haack figure of merit value for the MgxZn1−xO/Ag/MgxZn1−xO structure with various Mg compositions. The sheet resistance of MgxZn1−xO/Ag/MgxZn1−xO is quite similar in all the samples. However, the average transmittance increased with increasing Mg composition, while the transmittance at 550 nm was similar for all samples, regardless of the Mg composition. The increase in the average transmittance is attributed to the increased transmittance of MgxZn1−xO in the UV region due to Mg alloying with ZnO. The Haack figure of merit value of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O increased by 69% when compared with that of the ZnO/Ag/ZnO structure, indicating that the MgxZn1−xO/Ag/MgxZn1−xO multilayer can be used as a TCL in devices that require high transmittance over a broad wavelength region. Figure 5 shows AFM images of MgxZn1−xO (50 nm)/Ag (14 nm)/MgxZn1−xO (50 nm) with various Mg compositions. Organic devices are well-known to use thin organic layers because organic materials have low charge mobilities. Therefore, electrodes with low surface roughness are required for a proper operation of organic devices because high surface roughness of electrode can cause direct current flow between the cathode and the anode after the deposition of the thin active polymer layer.27,28 As shown in Figure 5, the root-meansquare (RMS) roughness of the MgxZn1−xO/Ag/MgxZn1−xO multilayer decreases with increasing Mg composition. The RMS roughness of 6.2 nm of the ZnO/Ag/ZnO structure is much higher than the roughness values of the MgxZn1−xO/Ag/ MgxZn1−xO multilayers. The high RMS roughness of ZnO/Ag/ ZnO is mainly due to preferential ZnO growth in the direction of the c-axis of wurtzite ZnO. However, MgxZn1−xO/Ag/ MgxZn1−xO shows much lower RMS roughness of 1.1 nm at x = 0.28 because MgO suppresses the vertical growth of ZnO, and also promotes lateral growth.29

Figure 4. (a) Transmittance spectra and (b) optical band gap of MgxZn1−xO (50 nm)/Ag (14 nm)/MgxZn1−xO (50 nm) multilayers for various Mg compositions.

MgxZn1−xO (50 nm) as a function of the Mg composition. As shown in Figure 4a, the transmittance of MgxZn1−xO/Ag/ MgxZn1−xO is similar to that of ZnO/Ag/ZnO in the visible wavelength region from 450 to 750 nm. However, the absorption edge of MgxZn1−xO/Ag/MgxZn1−xO shifts to a D

DOI: 10.1021/acsami.5b09974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the “Basic Research Projects in High-tech Industrial Technology” Project through a grant provided by GIST in 2015 and “INNOPOLIS technology commercialization services” Project through a grant provided by INNOPOLIS foundation.

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Figure 5. AFM images of (a) ZnO/Ag/ZnO, (b) Mg0.08Zn0.92O/Ag/ Mg 0.08 Zn 0.92 O, (c) Mg 0.19 Zn 0.81 O/Ag/Mg 0.19 Zn 0.81 O, and (d) Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O multilayers.

We have investigated the characteristics of MgxZn1−xO/Ag/ MgxZn1−xO multilayer transparent electrodes grown on glass substrates at room temperature. We found that Mg0.28Zn0.72O/ Ag/Mg0.28Zn0.72O has high average transmittance of 89.2% over a broad wavelength region (350−780 nm) and low sheet resistance of 6.36 Ω/sq when a 14 nm thick Ag layer is embedded in Mg0.28Zn0.72O (100 nm). The figure of merit of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O is 69% higher than that of the ZnO/Ag/ZnO structure. The RMS roughness of the multilayers is reduced from 6.2 nm for ZnO/Ag/ZnO to 1.1 nm for Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O. These results indicate that MgxZn1−xO/Ag/MgxZn1−xO multilayers, which are indiumfree, can be used as a high-performance TCL in applications such as UV LEDs, touch panels, solar cells, and flexible organic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09974. Calculation of Mg composition of as-grown MgxZn1−xO, XRD pattern of as-grown MgxZn1−xO and MgxZn1−xO/ Ag/MgxZn1−xO, and optical transmittance spectra of MgxZn1−xO/Ag/MgxZn1−xO with and without ion beam pretreatment (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. E

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DOI: 10.1021/acsami.5b09974 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX