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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.).

2 A.K. Kulkarni et al. / Thin Solid Films 308309 (1997) 17


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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.

3A.K. Kulkarni et al. / Thin Solid Films 308309 (1997) 1 7


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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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