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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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