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Electrical, optical and structural characteristics of indium-tin-oxide thin filmsdeposited on glass and polymer substrates
A.K. Kulkarnia ,*, K.H. Schulzb, T.-S. Lima, M. Khanb
aDepartment of Electrical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USAbDepartment of Chemical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
Abstract
The sheet resistance, optical transmittance and microstructure of tin-doped indium oxide (ITO) thin films (50 100-nm thick) rf sputter
deposited on polymer substrates are investigated using a four-point probe, spectrophotometer, X-ray diffractometer and a transmission
electron microscope (TEM). Sheet resistances vary from 250 Q/sq. to 170 kQ/sq. Sheet resistances for the ITO films on polycarbonate
substrates are at least an order of magnitude higher than those ITO films deposited on glass substrates at the same time. Annealing ITO
films on polycarbonate substrates at 100C in air for 1 h decreased the sheet resistances significantly (almost by 50%). The X-ray diffraction
data indicate polycrystalline films with grain orientations predominantly along (222) and (400) directions. TEM photographs show two
distinct regions of growth: a dense growth close to the substrate and a sparse growth away from the substrate. The vertical growth is
columnar and rod shaped. Changes in the ITO film sheet resistance either due to the types of substrate used or due to annealing can be
correlated to the grain size and grain orientation. 1997 Elsevier Science S.A.
Keywords: ITO; Sheet resistance; Transmittance; Grain orientation
1. Introduction
Indium tin oxide, commonly referred to as ITO, is a
degenerate n-type semiconducting material that has wide
applications in optics and optoelectronics. These applica-
tions include flat panel display devices [1,2], heat reflecting
mirrors [3], and heterojunction solar cells [4]. High electri-
cal conductivity ( 2 104 Q1 cm1), and high transpar-
ency ( 90% in the visible spectrum range) of this material
have been the focus of research throughout the world. Most
of the research on ITO thin films is concentrated on the
simultaneous improvement of the conductivity and the
transparency of the ITO thin films deposited on glass sub-strates by a variety of techniques such as rf sputtering [4,5],
electron beam deposition [6], chemical vapor deposition [7],
and spray pyrolysis [8]. Attempts are also made to deter-
mine the fundamental properties of ITO based on the energy
band diagram so that optimum theoretical values of conduc-
tivity and transparency can be obtained [9,10]. However,very little work is reported on the ITO thin films deposited
on polymer substrates since these substrates have low ther-
mal stability [11,12]. Modern applications employing ITO
films require light polymer substrates such as plastics for
use in liquid crystal display devices. In this paper, we report
on the sheet resistance, optical transmittance and micro-
structure of the ITO thin films rf sputter deposited on poly-
mer substrates.
2. Fabrication and processing
Details of the rf sputtering technique and system, and
sample preparation are given elsewhere [13]; only high-
lights of the fabrication and processing methods are given
here. A water cooled pressed In2O3 target with 10 wt.%
SnO2is used inside a Perkin-Elmer model 2400 rf sputtering
system. Three types of substrates were used: (i) glass, (ii)
PET (poly ethylene terephthalate), and (iii) polycarbonate.
These substrates were pre-cleaned with 2-propanol in an
ultrasonic cleaner for approximately 6 min. During deposi-
tion, the oxygen partial pressure was varied from 0% to 20%
Thin Solid Films 308309 (1997) 17
0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reserved
PI I S 0 0 4 0 -6 0 9 0(9 7 )0 0 5 2 6 -9
* Corresponding author. Tel.: +1 906 4872773; fax: +1 906 4872949;
e-mail: [email protected]
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of the total pressure ( 2.67 Pa or 20 mTorr) to achieve the
highest conductivity and transparency. Deposition para-
meters such as target bias and deposition time were held
constant during deposition. A few of the samples were
annealed at 100C in air for 1 h. Table 1 shows the details
of sample fabrication and annealing.
Table 1
Fabrication, processing and sheet resistances of ITO thin films
Sample Substrate Deposition
time (min)
RF
power (W)
Oxygen partial
pressure (%)
Sheet resistancea (Q/sq.)
Before annealing After annealing
1C Polycarbonate 30 50 0 6.15 103 5.26 103
2C Polycarbonate 30 50 15 170 103 78.3 103
2D Glass 30 50 15 129 103
3C Polycarbonate 30 50 21 131 103 75.6 103
3D Glass 30 50 21 18.8 103 31.1 103
4C Polycarbonate 20 100 5 23.5 103
4D Glass 20 100 5 998 2.10 103
5B PET 30 100 ? 250
5C Polycarbonate 30 100 ? 587
5D Glass 30 100 ? 293
aThe error on the sheet resistance is about 3%.
Fig. 1. TEM pictures of sample 2C. (a) Electron diffraction patterns. (b) Before annealing (RS 170 kQ/sq.). (c) After annealing (RS 78.3 kQ/sq.).
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3. Experimental results
3.1. Sheet resistance measurements
The sheet resistances of as-deposited and annealed sam-
ples were measured by a standard four-point probe techni-
que and are listed in Table 1. Several interesting obser-
vations are made based on the oxygen partial pressure, the
type of substrate and post deposition annealing. Sheet resis-tances increase with increasing oxygen partial pressure on
both glass and polymer substrates. The increase in sheet
resistances is significant in the ITO films deposited on poly-
carbonate substrates with 015% of oxygen partial pressure.
The sheet resistances of the ITO films deposited on glass
substrates are lower than those deposited on polycarbonate
substrates. After annealing in air at 100C for 1 h, the sheet
resistances of the ITO films on glass substrates increased in
both samples 3D and 4D whereas the sheet resistances of the
ITO films deposited on polymer substrates decreased in all
three samples 1C, 2C and 3C. This decrease is substantial
(54% and 42%) for samples 2C and 3C, respectively, which
are fabricated under high oxygen partial pressures (15% and21%, respectively).
3.2. Transmission electron microscope (TEM)
measurements
The TEM used to characterize the structure of ITO thin
films is a JEOL 4000 TEM operated at 200-kV accelerating
voltage. Fig. 1a shows the electron diffraction ring patterns
of sample 2C before annealing. As seen in Fig. 1a, the
electron diffraction ring patterns give four strong lines
with intensities similar to those expected from the JCPDS
(Joint Committee on Powder Diffraction Standards) card on
In2O3. No appreciable changes are observed in line intensi-
ties upon tilting the sample by 10, indicating the absence of
extreme preferred orientations. Fig. 1b shows the plan view
of sample 2C (before annealing) showing clearly the colum-
nar vertical growth with voids in between columns. The
column length is approximately 60 nm and the columns
appear to coalesce at the bottom where the film joins the
substrate. The sheet resistance of this sample is 170 kQ/sq.
After annealing in air at 100C for 1 h, the sheet resistance
drops to 78.3 kQ/sq.; the TEM picture of the annealed sam-
ple is shown in Fig. 1c. Here it is possible to observe parallel
(222) and (112) fringes crossing multiple columns, suggest-
ing grain growth beyond the vertical columns resulting in a
multiple columnar growth.
Vertical columnar growth is very apparent in the TEM
picture of sample 4C shown in Fig. 2. As shown in the
figure, the individual columns are single crystals with a
fringe separation of 0.29 nm corresponding to the (222)
planes. Fig. 3 shows the cross-sectional view of the ITO/polymer interface. The ITO film is separated from the poly-
mer during specimen preparation, but several polymer liga-
ments remain attached to the ITO film base (indicated by
arrows) which would imply good adhesion. The ITO film
appears to be quite dense at the base.
Fig. 2. TEM picture of annealed sample 4C. Inset shows columnar mor-
phology.
Fig. 3. TEM cross-sectional view of the ITO film/polymer interface. The
length of the line at the top is 10 nm.
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3.3. X-ray diffraction measurements
The X-ray diffraction (XRD) patterns were obtainedusing an XDS 2000 Diffractometer. The X-ray source
used is Cu Ka radiation (l = 0.154 nm) with a graphite
monochromator (2a = 26.6). Fig. 4a shows XRD data on
sample 4C (as-deposited ITO on polymer) which has a sheet
resistance of 23.5 kQ/sq. As seen in this figure, the peaks are
small and broad indicating growth in (222), (400), (440) and
(622) orientations. The normalized peak intensities for these
peaks are 100, 100, 28 and 14, respectively and FWHM (full
width at half maximum) values are 0.703, 0.600, 1.076
and 1.633, respectively. In Fig. 4b, the X-ray data on sam-
ple 4D (as-deposited ITO on glass substrate) shows a sharp
peak for (400) orientation and small broad peaks for (222),
(440) and (622) orientations. The normalized intensities for
(222), (400), (440) and (622) peaks are 30, 100, 15 and 15
and the FWHMs for the corresponding peaks are 0.749,
0.592, 1.093 and 1.593, respectively. The sheet resis-
tance of this sample is approximately 1 kQ/sq.
In Fig. 5a,b,c, we compare the X-ray diffraction data and
sheet resistances of three different ITO samples deposited
on PET (5B), polycarbonate (5C) and glass (5D) substrates
in the same run under exactly the same deposition condi-
tions. The sheet resistances of samples 5B, 5C and 5D are
250 Q/sq., 587 Q/sq. and 293 Q/sq., respectively. Fig. 5a
shows a very sharp peak at (400) orientation and almost
negligible peaks for other orientations. The peaks centered
at 2v values equal to 47 and 54 are due to the PET sub-strate. Similarly, Fig. 5b shows a sharp peak only for (400)
orientation and very small and broad peaks for (222), (440)
and (622) orientations. On the other hand, the XRD data of
an ITO sample deposited on glass substrate shown in Fig. 5c
shows not only a sharp peak for (400) orientation but also
for (320) orientation corresponding to the JCPDS orienta-
tions of ITO rather than In2O3.
Comparing the X-ray diffraction plots of Fig. 4a and 5a
(samples 4C and 5C), it is obvious that the decrease in
sheet resistance from 23.5 kQ/sq. to 587 Q/sq. is quite sig-
nificant and is due to a highly oriented film in sample 5C
versus a poorly oriented film in sample 4C. The FWHM
values of the (400) peaks in samples 4C and 5C are
0.600 and 0.145, respectively. Similarly, the XRD plots
of samples 4D and 5D show substantial differences with a
corresponding decrease in the sheet resistance from 1 kQ/sq.
to 293Q/sq. The FWHM values of the (400) peaks in sam-
ples 4D and 5D are 0.592 and 0.114, respectively. These
results strongly suggest the importance of oriented growth
(particularly along (400) direction) and large grain sizes
(low FWHMs) in improving the conductivity of ITO thin
films. Similar results are also observed by other investiga-
tors [14].
Fig. 4. X-ray diffraction data on (a) sample 4C (before annealing, RS 23.5 kQ/sq.), and (b) sample 4D (before annealing,RS 1 kQ/sq.).
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3.4. Optical transmission measurements
The optical measurements of the ITO samples were car-
ried out using a HP8451A Diode Array Spectrophotometer.
Fig. 6 shows the transmission spectra of a bare polycarbo-
nate substrate and samples 2C, 3C and 5C which are ITO
films deposited on polycarbonate substrates. As seen in the
figure, the transmittance is roughly 60%, 69% and 64%
(with respect to the bare polycarbonate substrate) for sam-
ples 2C, 3C and 5C, respectively, in the wavelength range
from 0.39mm to 0.77mm (visible spectrum).
4. Discussion
4.1. Sheet resistance
The lowest sheet resistivities of the ITO thin films depos-
Fig. 5. X-ray diffraction data on (a) sample 5B (before annealing, R S 250 Q/sq.), (b) sample 5C (before annealing,R S 587 Q/sq.), and (c) sample 5D
(before annealing, R S 293 Q/sq.).
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ited on heated glass substrates reported in the literature are
of the order 2 104 Q cm. Previous researchers working onsubstrates other than glass such as mylar, polyester, teflon
and other polymer substrates have reported resistivity
values in the range of 1 103 Q cm, which is an order of
magnitude higher than the resistivity of the ITO films depos-
ited on glass [11,12]. The resistivity values reported here on
unheated polymer substrates are comparable to the values
reported by other researchers [11,12]. The dependence of
the resistivity on the oxygen partial pressure is a well known
experimental result and is explained on the basis of oxygen
deficiency in the film (each oxygen vacancy gives rise
to two conduction electrons) [7]. Increasing oxygen con-
tent of the films either by increasing the partial pressureof oxygen during the growth or annealing the samples in
air or oxygen should decrease the oxygen vacancies lead-
ing to less conductive films. However, a minimum in the
resistivity of the ITO thin films deposited on glass substrates
is reported between 15 and 20% of oxygen partial pressure
by a few investigators indicating an improvement in the
crystallinity of the films (mobility of the carriers is depen-
dent on crystallinity) [15]. Our experimental results on ITO
deposited on polycarbonate substrates did not show a max-
imum in the conductivity as a function of oxygen partial
pressure.
An interesting observation can be made on the effect
of annealing the ITO films deposited on glass and poly-
mer substrates. As shown in Table 1, the sheet resistance
of these ITO films decreased by 50% in samples 2C and
3C after annealing in air at 100C for 1 h. This improve-
ment in conductivity is attributed to an increase in the
grain size of the film as observed in these TEM pictures
(Fig. 1b, c) of sample 2C. As seen in these figures, the
voids are significantly less in the annealed sample. The
X-ray diffraction data (not shown) on annealed sample
3C showed a peak at 2v 30.58 indicating (222) pre-
ferred orientation for this film. On the other hand, the
sheet resistances of the ITO films deposited on glass
substrates (samples 3D and 4D) increased significantly
after annealing in air at 100C for 1 h. This is an
expected result since the annealing of these ITO films
decreased the number of vacancies resulting in more resis-
tive films.
4.2. Microstructure
The grain orientations, grain size and grain boundaries
determine the quality of the thin films suggesting a depen-
dence of the electrical and optical properties on the growth
parameters [16]. The TEM results shown in Figs. 1,2 and 3
clearly show the columnar growth of the ITO films on poly-
carbonate substrates. However, the vertical columns are
separated by voids at certain places and it appears that the
voids may be responsible for the high sheet resistances of
unannealed samples. The decrease in voids as well as
increase in grain size from a few nanometers to a few tens
of nanometers after annealing in air at 100C for 1 h is
speculated to be the cause for the decrease in the sheetresistance. The two distinct regions of growth observed in
the TEM pictures (dense growth close to the substrate and
sparse growth away from the substrate) have also been
reported [17].
The multiple orientations observed in TEM pictures
are substantiated by XRD results. The X-ray diffraction
results shown in Figs. 4 and 5 indicate mainly four different
orientations (222), (400), (440) and (622) which are nor-
mally observed for ITO films [18]. Our XRD results
strongly suggest that highly oriented films have lowest
sheet resistances. The lower FWHM values are a good indi-
cation of the crystallinity of the films resulting in lowersheet resistances.
5. Conclusions
The ITO thin films are deposited on glass, PET and poly-
carbonate substrates by rf sputtering technique. The sheet
resistances vary from 250 Q/sq. to 170 kQ/sq. depending
upon oxygen partial pressure during the growth and the
types of substrate used. Annealing in air at 100C for 1 h
resulted in either an increase or decrease in sheet resistance
values depending on the types of substrate. Optical trans-
mittance is 6070% in the visible region for the samples
studied here. The TEM results indicate vertical columnar
growth with multiple orientations. The grain sizes range
from a few nanometers to a few tens of nanometers and
control the sheet resistances of the films. A significant
decrease in the sheet resistances of the ITO films on poly-
carbonate substrates after annealing is attributed to a corre-
sponding change in the grain size and the absence of voids.
Our results suggest that the low resistive ITO thin films are
obtained in the films that are highly oriented in the (400)
direction.
Fig. 6. Optical transmission spectra on a bare polycarbonate substrate and
samples 2C, 3C and 5C.
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Acknowledgements
The authors gratefully acknowledge the support of the
3M corporation, GE plastics, Steve Hackney for assistance
with TEM work and Edward Laitila for assistance with
XRD work.
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