Transmission electron microscopy study of degradation in transparentindium tin oxide/Ag/indium tin oxide multilayer filmsJin-A Jeong, Han-Ki Kim, Hyun-Woo Koo, and Tae-Woong Kim Citation: Appl. Phys. Lett. 103, 011902 (2013); doi: 10.1063/1.4812815 View online: http://dx.doi.org/10.1063/1.4812815 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i1 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Transmission electron microscopy study of degradation in transparentindium tin oxide/Ag/indium tin oxide multilayer films

Jin-A Jeong,1 Han-Ki Kim,1,a) Hyun-Woo Koo,2 and Tae-Woong Kim2

1Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University,1 Seocheon-dong, Yongin-si, Gyeonggi-do 446-701, South Korea2Next Generation B/P Development Team, OLED Business Samsung Display, Yongin, Gyeonggi 446-711,South Korea

(Received 20 April 2013; accepted 16 June 2013; published online 2 July 2013)

The degradation mechanism and structural evolution of transparent ITO/Ag/ITO (IAI) multilayer

films caused by rapid thermal annealing (RTA) were investigated by high resolution transmission

electron microscopy (HRTEM) and synchrotron X-ray scattering analysis. The IAI multilayer with

low sheet resistance of 9.51 X/square and high transmittance of 88.24% was significantly degraded

after 600 �C RTA. Discontinuity, agglomeration of the embedded Ag layer at the interface region of

the IAI multilayer, and oxygen diffusion through crystalline ITO grain boundaries into Ag layers led

to electrical and optical degradation of the IAI multilayer. Using HRTEM analysis, the

microstructures and interfaces of as-deposited and 600 �C annealed IAI multilayer films were

compared to explain their electrical and optical degradation mechanisms. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4812815]

Rapid advances in high-quality, large area organic light

emitting diodes (OLEDs), and touch screen panels require

transparent conducting electrodes (TCEs) with much lower

resistivity (�10�5 X � cm) and higher optical transparency

(�90%) than conventional indium tin oxide (ITO) films.1–3

Several TCE materials, including Ag nanowire, Ag grids,

graphene, carbon nanotubes, conducting polymers, and ox-

ide-metal-oxide (OMO) electrodes, have been extensively

investigated as substitutes for conventional ITO films.4–9 In

particular, OMO electrodes have attracted great attention

due to their advantages, such as very low resistivity compa-

rable to metal electrodes, high optical transparency in the

visible wavelength region caused by antireflection effect, rel-

atively lower thickness, and superior flexibility. Recently,

Guill�en and Herrero reviewed the current OMO research

results and demonstrated the potential of several OMO elec-

trodes as alternatives to single layer transparent conducing

oxide (TCO) films.10 In our previous works, we suggested

several applications of OMO electrodes in OLEDs, organic

solar cells, flexible random access memories, touch screen

panels, and transparent oxide thin film transistors.11–15

Thermal annealing has been carried out to improve the elec-

trical properties of OMO electrode.16 As discussed by Jung

et al. and Kl€oppel et al., annealed ITO/Ag/ITO (IAI) showed

significantly decreased sheet resistance due to the improved

crystallinity of embedded Ag metal layers between the

ITO.17,18 Although the effect on electrical and optical prop-

erties caused by thermal annealing of OMO electrode has

been reported, an investigation of the exact degradation

mechanism in high temperature annealed OMO electrodes is

still lacking.

In this letter, we report the effect of rapid thermal anneal-

ing (RTA) on the electrical, optical, and morphological prop-

erties of IAI multilayer electrodes. We also suggest a possible

degradation mechanism for high temperature annealed IAI

multilayer electrodes based on high resolution transmission

electron microscopy (HRTEM), synchrotron X-ray scatter-

ing, and X-ray photoelectron spectroscopy (XPS) depth pro-

file analyses. In addition, we compare the microstructure and

interface properties of as-deposited and 600 �C annealed IAI

multilayer electrodes and correlated the electrical and optical

properties in IAI multilayer electrodes.

Both 50 nm thick top and bottom ITO (B-ITO) layers

were prepared on a glass substrate by a DC magnetron sput-

tering system at room temperature. The detailed optimization

process of IAI films was reported in our previous works.8,19

Under optimized ITO sputtering conditions at constant DC

power of 100 W, Ar flow rates of 10sccm, and working pres-

sure of 3 mTorr, a 50 nm-thick B-ITO layer were sputtered

using a 3 in. diameter ITO target (10 wt.% SnO2 doped

In2O3). After sputtering the B-ITO layer, 10 nm thick Ag

layers were deposited on the B-ITO at constant DC power of

100 W, Ar flow rate of 10sccm, and working pressure of

3 mTorr. Finally, a 50 nm-thick top ITO (T-ITO) layer was

sputtered on the Ag film under sputtering conditions identi-

cal to those used for the B-ITO film. After continuous depo-

sition of IAI films, the samples were rapidly thermal

annealed under vacuum as a function of temperature for

5 min. The electrical and optical properties of as-deposited

and annealed IAI multilayer films were measured by Hall

measurement (HL5500PC, Accent Optical Technology) and

UV/visible spectrometer (UV 540, Unicam). The surface

morphology of IAI films was analyzed using a field emission

scanning electron microscope (FESEM; LEO SUPRA 55).

The structure of IAI multilayer films with increasing RTA

temperature was evaluated by synchrotron x-ray scattering at

beam line 5 A of the Pohang Light Source II (PLS-II) in

Korea. The wavelength of probing x-rays was set to 1.243 A

by a double bounce Si (111) monochromator. In addition,

HRTEM (JEM-2100F) and XPS (XPS-PHI5200) depth

profile examinations were carried out to investigate the

a)Author to whom correspondence should be addressed. Electronic mail:

imdlhkkim@khu.ac.kr

0003-6951/2013/103(1)/011902/5/$30.00 VC 2013 AIP Publishing LLC103, 011902-1

APPLIED PHYSICS LETTERS 103, 011902 (2013)

degradation mechanism of annealed ITO/Ag/ITO multilayer

electrodes.

Figure 1 shows the electrical and optical properties of

IAI multilayer films as a function of RTA temperature. The

sheet resistance and resistivity of IAI multilayer electrodes

with increasing RTA temperature are shown in Fig. 1(a).

Although IAI multilayer films were prepared at room temper-

ature, as-deposited IAI films showed a low sheet resistance of

9.51 X/square and resistivity of 1.03� 10�4 X � cm, due to the

existence of metallic Ag interlayers, as in other OMO films.9

The rapidly thermal annealed IAI multilayer films showed a

decrease in sheet resistance and resistivity with increasing

RTA temperature. At RTA temperature of 500 �C, the IAI

multilayer film showed the lowest sheet resistance of 6.7X/square

and resistivity of 7.24� 10�5 X � cm, which are much lower

values than those of conventional ITO films. However, the

600 �C annealed IAI multilayer film showed increased sheet

resistance of 10.08X/square and resistivity of 1.08� 10�4 X � cm.

These values were due to the discontinuity and agglomeration

of metallic Ag layers between the T-ITO and B-ITO layers

and severe diffusion of Ag atoms into ITO layers, as reported

by Jung et al.17 Figure 1(b) shows the carrier concentration

and mobility of IAI multilayer films as a function of RTA

temperature. The increase in RTA temperature from room

temperature to 500 �C led to increases in carrier concentration

of IAI multilayer film from 4.02� 1021 to 6.82� 1021 cm�3.

However, above annealing temperature of 500 �C, the

IAI multilayer film showed a slightly decreased carrier con-

centration. Therefore, decreased resistivity of IAI multilayer

with increasing RTA temperature could be attributed to

the increased carrier concentration caused by activation of

Sn dopants and effective injection of electrons from Ag

layer as previously reported by Yang et al.20 However, the

mobility of the IAI multilayer films gradually decreased with

increasing RTA temperature. Compared with the mobility

(15.1 cm2/V � s) of as-deposited IAI multilayer, the 600 �Cannealed IAI multilayer showed lower mobility of 11.1 cm2/V � sdue to electron scattering in agglomerated Ag islands. Figure

1(c) shows pictures of IAI samples demonstrating color and

transparency with increasing RTA temperature. Up to an

RTA temperature of 300 �C, the IAI multilayer showed high

transparency without changing color. The 500 �C annealed

IAI multilayer was a bluish color due to light scattering at the

agglomerated Ag layer, even though it had the lowest resis-

tivity. Interestingly, the 600 �C annealed IAI exhibited a deep

blue color, indicating severe agglomeration of the Ag layer.

Figure 1(d) shows the optical transmittance of IAI multilayer

films as a function of RTA temperature. As confirmed

by color in Fig. 1(c), up to RTA temperature of 300 �C,

the IAI multilayer demonstrates a similar high optical trans-

mittance. Both as-deposited and 300 �C annealed IAI multi-

layer films showed optical transmittance of 88.24% and

89.84% at 550 nm wavelengths, respectively. However, fur-

ther increases in RTA temperature resulted in significant

decreases in the optical transmittance of the IAI multilayer.

At RTA temperature of 600 �C, the IAI multilayer showed

the lowest optical transmittance of 57.32%, due to light scat-

tering by the agglomerated Ag layer, which was confirmed by

the deep blue color in Fig. 1(c). Based on the sheet resistance

(Rsh) and transmittance (T) of IAI multilayer films, the figure

of merit value (T10/Rsh) for IAI multilayer films was calcu-

lated as a function of RTA temperature to establish the

FIG. 1. (a) Resistivity, sheet resist-

ance, (b) mobility, and carrier concen-

tration of ITO/Ag/ITO multilayer

electrodes as a function of annealing

temperature. (c) Picture shows trans-

parency of ITO/Ag/ITO multilayer

electrodes with increasing annealing

temperature. (d) Optical transmittance,

(e) figure of merit value, and transmit-

tance of ITO/Ag/ITO multilayer elec-

trodes as a function of annealing

temperature at 550 nm wavelength.

011902-2 Jeong et al. Appl. Phys. Lett. 103, 011902 (2013)

temperature limit of IAI multilayer films. As shown in Fig.

1(d), the figure of merit value increased up to 300 �C due to

reduced sheet resistance and high optical transmittance. The

300 �C annealed IAI multilayer showed the highest figure of

merit value of 46.41� 10�3 X�1, which is much higher than

that of conventional crystalline ITO films.21 However, above

an RTA temperature of 300 �C, the figure of merit value for

IAI multilayers significantly decreased due to deceased trans-

mittance. The possible process temperature limit of IAI mul-

tilayers was determined to be 300 �C. Higher RTA

temperature led to the degradation of the IAI multilayer, as

shown in the shade region of Fig. 1(d).

Figure 2 shows the synchrotron x-ray scattering results

of IAI multilayer films as a function of RTA temperature.

The X-ray scattering plot of as-deposited and 200 �C IAI

multilayers showed similar weak ITO (222) and (400) peaks,

as well as a weak Ag (111) peak. This indicates that the IAI

multilayer prepared at low temperature consisted of nano-

crystalline ITO and Ag phases. The identical X-ray scatter-

ing plots indicate that there was no change in microstructure

below 200 �C. However, IAI multilayers annealed at RTA

temperatures above 300 �C exhibited strong crystalline ITO

peaks at Qz: 1.53 (211), 2.16 (222), 2.49 (400), and 3.52

(440), indicating the start of crystallization of the top and

bottom ITO layers with (222) preferred orientation caused

by RTA process. In addition, strong Ag peaks at Qz: 2.67

(111) and 4.13 (220) demonstrated improved crystallinity of

the Ag layer after the RTA process.

HRTEM examination was performed to investigate the

degradation mechanism of annealed IAI multilayers. Figure

3(a) shows a cross-sectional TEM image obtained from

as-deposited IAI multilayer film. The cross-sectional TEM

image shows a well-defined bottom ITO layer, Ag layer, and

top ITO layer without interfacial layers. In the as-deposited

IAI multilayer, the Ag layer exists as a continuously con-

nected layer between the top and bottom ITO layers. The

enlarged image in Fig. 3(b) demonstrates that the Ag layer

with (222) preferred orientation was embedded between

amorphous ITO (a-ITO) layers. The fast Fourier transform

(FFT) pattern in the inset of Fig. 3(b) shows a diffuse ring,

which is a feature of amorphous structures. Until now,

as-deposited IAI multilayers or OMO electrodes have been

reported to consist of amorphous oxide and crystalline

Ag layers.9,19 However, we found that some regions of

as-deposited IAI multilayers consisted of a crystallized

T-ITO layer with (111) orientation on the Ag (222) layer, as

shown in Fig. 3(c). Unlike B-ITO layers, the T-ITO layer

could be crystallized on the crystalline Ag layer because the

crystalline Ag metal acts as a seed for crystallization of the

T-ITO layer. Konno and Sinclair reported that crystallization

of pure amorphous Si can be induced by contact with metals,

such as Al and Ag.22 An enlarged cross-sectional image in

Fig. 3(d) clearly exhibits that the as-deposited IAI multilayer

had an asymmetric structure consisting of amorphous B-ITO

and crystallized T-ITO layer on crystallized Ag layer. FFT

patterns in the inset of Fig. 3(d) show a diffuse ring as well

as weak and strong spots that are evidence for the amorphous

and crystalline structure of ITO and crystalline Ag layers.

Figure 4(a) shows cross-sectional HRTEM images

obtained from 600 �C annealed IAI multilayer films. These

images clearly show that discontinuous Ag islands were

formed between crystalline bottom and top ITO layers. The

increased resistivity and decreased optical transmittance of

the 600 �C annealed IAI multilayer film are attributed to

the formation of Ag islands between ITO layers, which

led to electron and light scattering. The enlarged images in

Fig. 4(b) reveal that the continuous Ag layer in the as-

deposited IAI layer transformed to disconnected Ag islands

due to the agglomeration of Ag atoms during the RTA pro-

cess. In addition, the T-ITO on the Ag layer showed grain

boundaries that act as an oxygen diffusion path from the am-

bient to Ag layer. It is possible that oxygen could diffuse

through this ITO grain boundary into the Ag layer and

degrade the IAI multilayer film. The enlarged image in

FIG. 2. Synchrotron X-ray scattering results obtained from ITO/Ag/ITO

multilayer electrodes as a function of annealing temperature.

FIG. 3. (a) Cross-sectional TEM image and (b), (c), and (d) enlarged

HRTEM image of as-deposited ITO/Ag/ITO multilayer electrode with the

inset showing fast Fourier transformation pattern.

011902-3 Jeong et al. Appl. Phys. Lett. 103, 011902 (2013)

Fig. 4(c) shows that in some regions, B- and T-ITO layer

were connected due to absence of the Ag layer caused by lat-

eral diffusion of Ag atoms to form Ag islands. Direct contact

of T-ITO layer into the B-ITO layer indicates disappearance

of the main conduction path in the IAI multilayer. Because

the sandwiched Ag layer acts as a main conduction path for

electrons and creates an antireflection effect in the IAI multi-

layer, the disappearance of the Ag layer indicates degrada-

tion of the electrical and optical properties of the IAI

multilayer. The FFT pattern in Fig. 4(c) exhibited strong

spots, indicating that both the T- and B-ITO layers com-

pletely transformed from amorphous to crystalline bixbyite

structures after 600 �C RTA. As shown in Fig. 4(d), Ag

atoms diffused into the crystalline top and bottom ITO layers

after RTA. The electrical and optical degradation of 600 �Cannealed IAI are explained by discontinuity of the Ag layer

caused by agglomeration and diffusion of Ag atoms into the

ITO layer.

An XPS depth profile was used to confirm Ag diffusion

into the ITO layer after 600 �C RTA. Figure 5 shows the

XPS depth profile of as-deposited and 600 �C annealed IAI

multilayer films with an inset showing surface FESEM

images. The as-deposited IAI multilayer film in Fig. 5(a),

shows symmetric IAI structure with individual B-ITO, Ag,

and T-ITO layers. The layers are clearly defined without any

interfacial reactions, consistent with the cross-sectional

HRTEM image in Fig. 3(a). Constant atomic percentages of

In, Sn, and O atoms in the T- and B-ITO layers indicate iden-

tical T- and B-ITO layers with the same thickness and com-

position formed by LFTS processes. In addition, the XPS

depth profile showed a sharp interface between the Ag layer

and ITO layers at 27 and 30 min etching time. The surface

FESEM in the inset of Fig. 5(a) shows that the surface of the

T-ITO layer of as-deposited IAI multilayer films is fairly

smooth and featureless without surface defects or protrusion.

This is because the T-ITO layer completely covered the con-

tinuous Ag layer. However, the XPS depth profile of the

600 �C annealed IAI multilayer film shown in Fig. 5(b)

shows broad and decreased Ag intensity, indicating signifi-

cant Ag diffusion into T- and B-ITO layers. A rapid decrease

in the atomic percent of Ag indicates that Ag atoms diffused

into the T- and B-ITO layers and mixed with ITO layers.

This indicates a change in Ag thickness and morphology as

confirmed by Fig. 4(d). The surface FESEM image of the

top-ITO layer in the 600 �C annealed IAI multilayer film in

the inset of Fig. 5(b) exhibits that the top-ITO surface

became rougher and crystallized with clear grain boundaries.

Due to agglomeration of the Ag layer and crystallization of

the T-ITO on the Ag layer, the 600 �C annealed IAI multi-

layer was crystalline. HRTEM examination and XPS depth

profiles demonstrated the degradation mechanism of IAI

multilayers as follows. First, degradation in the electrical

and optical properties of IAI multilayers is closely related to

Ag agglomeration, which disconnects the conduction path of

the Ag layer and scatters incident light. In particular, diffu-

sion of the Ag layer led to disappearance of the Ag layer in

some regions of annealed IAI multilayers. This indicates ab-

sence of the main conduction path and an antireflection

effect. Therefore, the annealed IAI showed increased resis-

tivity and decreased optical transmittance. Secondly, Ag dif-

fusion into T- and B-ITO layers increased resistivity and

sheet resistance of the IAI multilayer. Finally, oxygen diffu-

sion through the grain boundary of the T-ITO layer oxidized

the metallic Ag layer.

In summary, we report the degradation mechanism of

IAI multilayer films caused by a RTA process. At the opti-

mized RTA temperature of 300 �C, IAI multilayer film had a

resistivity of 7.9� 10�5 X � cm and transmittance of 89.94%

at 550 nm wavelength. The IAI multilayer film showed

increased figure of merit values with increasing RTA tem-

peratures up to 300 �C. Further increases in RTA temperature

led to the degradation of IAI multilayer films. Based on

HRTEM and XPS depth profiles, we found that agglomera-

tion of the Ag layer and diffusion of Ag into T- and B-ITO

led to degradation of the electrical and optical properties of

FIG. 4. (a) Cross-sectional TEM image and (b), (c), and (d) enlarged

HRTEM image of 600 �C annealed ITO/Ag/ITO multilayer electrode with

inset showing fast Fourier transformation pattern.

FIG. 5. XPS depth profile of the (a) as-deposited and (b) 600 �C annealed

ITO/Ag/ITO multilayer electrodes. The inset shows the surface FESEM

images of (a) as-deposited and (b) 600 �C annealed ITO/Ag/ITO multilayer

electrodes.

011902-4 Jeong et al. Appl. Phys. Lett. 103, 011902 (2013)

IAI multilayers. Disappearance of the Ag conduction path

and reduced antireflection effect led to a decrease in the

resistivity and increase in optical transmittance of the IAI

multilayer after high temperature annealing.

The authors are appreciated for the financial support

from Core Materials Development Research Program by the

Korean Ministry of Trade, Industry & Energy (Contract No.

10041161).

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