Effect of substrate temperature on the growth of ITO thin films
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Effect of substrate temperature on the growth of ITO thin films
M. Nisha, S. Anusha, Aldrin Antony, R. Manoj, M.K. Jayaraj
*
Optoelectronics Device Laboratory, Department of Physics, Cochin University of Science and Technology,
Kochi 682 022, Kerala, India
Received 2 November 2004; received in revised form 20 February 2005; accepted 20 February 2005
Abstract
Indium tin oxide (ITO) thin films were deposited onto glass substrates by rf magnetron sputtering of ITO target and the
influence of substrate temperature on the properties of the films were investigated. The structural characteristics showed a
dependence on the oxygen partial pressure during sputtering. Oxygen deficient films showed (4 0 0) plane texturing while
oxygen-incorporated films were preferentially oriented in the [1 1 1] direction. ITO films with low resistivity of
2.05 Â 10
Ã3
V cm were deposited at relatively low substrate temperature (150 8C) which shows highest figure of merit of
2.84 Â 10
Ã3
square/V
# 2005 Published by Elsevier B.V.
PACS: 81.40.Ãz; 78.66.Ãw; 81.40.Ef
Keywords: Transparent conducting oxides; Indium tin oxide; Rf magnetron sputtering
1. Introduction
Transparent conducting oxides (TCOs) have been
widely used as transparent electrode for flat panel
displays including liquid crystal displays, organic
light emitting diodes and plasma displays. Among the
various TCOs tin doped indium oxide (ITO) is widely
used due to its low electrical resistivity and compat-
ibility with fine patterning processes [1]. ITO is an n-
type degenerate wide bandgap semiconductor. The
degeneracy is caused by both oxygen vacancy and
substitutional tin created during deposition [2]. As a
degenerate semiconductor, ITO can be used as the
window layer in n
+
“p heterojunctions [3]. Because
ITO films have good efficiency for hole injection into
organic materials, they have been widely utilized as
the anode contact for organic light emitting diodes
(OLEDs) [4]. ITO thin films can be prepared by a wide
variety of techniques like plasma enhanced metal
organic chemical vapour deposition (PEMOCVD) [5],
ion assisted deposition [6], pulsed laser deposition
(PLD) [7], dip coating [8], ion beam sputtering [9], rf
magnetron sputtering [10], reactive thermal evapora-
tion, [11] etc. Most of the preparation methods involve
elsevierlocate/apsusc
Applied Surface Science xxx (2005) xxx“xxx
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* Corresponding author. Tel.: +91 484 2577404;
fax: +91 484 2577595.
E-mail address: mkj@cusat.ac.in (M.K. Jayaraj).
0169-4332/$ “ see front matter # 2005 Published by Elsevier B.V.
doi:10.1016/j.apsusc.2005.02.115
APSUSC 12683 1“6Page 2

UNCORRECTED PROOF
a relatively high substrate temperature in order to
obtain thin films with a reasonably high conductivity
and transmittance. However, some ITO based devices
such as amorphous silicon photovoltaic devices and
flexible electro optical devices demand the deposition
of ITO at low substrate temperature (<200 8C) [12].
Magnetron sputtering offers the possibility to prepare
ITO thin films at low processing temperature and on
large areas [13].
In this paper, we report the influence of substrate
temperature, oxygen partial pressure and fluorine
doping on the properties of sputtered ITO thin films.
2. Experimental
In the present study the Indium tin oxide films were
deposited by rf magnetron sputtering of ITO target
containing 95 wt.% of In
2
O
3
and 5 wt.% of SnO
2
. The
target used for sputtering was prepared from In
2
O
3
(99.99% pure) and SnO
2
(99.999% pure) powders.
The powders were mixed in a mechanical shaker for
1 h pressed into a pellet of two-inch diameter and then
sintered at 1300 8C for 6 h in air.
The rf sputtering was carried out in a vacuum
chamber in which high vacuum of the order of
2 Â 10
Ã5
mbarwas createdbymeansofanoildiffusion
pump backed by a rotary pump. The rf power was
delivered to the target material by an rf generator
(13.56 MHz) through an impedance matching network.
Glass slides of dimension 2.5 cm  1 cm was used as
thesubstrates.Thesubstrateswere keptabovethe target
at a distance of 4 cm, which was found to be the
optimumforthegrowthofgoodqualitycrystallinefilms
[14]. Initially the substrate was heated to the required
temperature. After attaining the required substrate
temperature high purity argon gas was allowed to flow
into the chamber and it was so adjusted by a mass flow
controller maintaining the argon pressure at 0.01 mbar.
The films were deposited at an rf power of 30 W. The
target was pre-sputtered for 10 m before each deposi-
tion in order to remove any contaminants and to
eliminateanydifferentialsputteringeffects.Bykeeping
allotherparametersthesame,thesputteringwascarried
out for various substrate temperatures ranging from
room temperature to 300 8C. For films deposited at
room temperature, the temperature of the substrate
increasedfrom20 to50 8C duringdeposition.But when
the deposition was carried out onto preheated
substrates, the temperature of the substrate was
maintained at the specified value by controlling power
into the heating coil. The sputtering time was adjusted
suchthatalltheresultingfilmsusedinthisstudywereof
thickness 220 nm.
The structural characterisation of the films were
carried out using a Rigaku X-ray diffractometer with
Cu Ka radiation (l = 1.5414 A). The optical transmis-
sion was taken in the wavelength range of 200“
2500 nm using a Hitachi U-3410 model UV“VIS“NIR
spectrophotometer. The electrical characterisation
was done by measuring the resistivity using Vander
Pauw four-probe method and the carrier type and
concentration was determined using the Hall mea-
surement system (H-50, MMR Technologies Inc.).
3. Results and discussion
Fig. 1 shows the X-ray diffraction (XRD) pattern of
the ITO thin films deposited at various substrate
temperatures. All the films are polycrystalline. They
crystallized in the cubic bixbyite structure of indium
oxide. The growth of the films showed preferred
orientation depending on the substrate temperature.
The films deposited onto unheated substrates showed
reflection corresponding to (2 2 2) and (4 4 0) planes
[15]. A substrate temperature of 100 8C resulted in
films with a prominent (4 0 0) peak indicating a
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Fig. 1. XRD pattern of ITO thin films deposited at various substrate
temperatures.Page 3

UNCORRECTED PROOF
preferred orientation along [1 0 0] direction. The films
grown at all other substrate temperatures do not show
any preferential orientation as seen from the XRD
pattern. It was also observed that all the films
deposited onto heated substrates showed (4 0 0) plane
texturing. With an increase in substrate temperature
above 100 8C, a decrease in intensity of (4 0 0) peak
and an increase in intensity of (2 2 2) peak was
observed (Fig. 2). On the other hand, post deposition
annealing of ITO films deposited at room temperature
resulted in films showing (2 2 2) and (4 4 0) diffrac-
tion peaks [14]. The film annealed at 250 8C was
preferentially oriented in the [1 1 1] direction. None of
the films showed (4 0 0) plane texturing on annealing.
The change in orientation of the films from (2 2 2)
to (4 0 0) plane is related to the deposition conditions.
The variation of orientation from (1 1 1) to (1 0 0) at
higher substrate temperatures is related to the energy
of the sputtered particles reaching the substrate
surface [16]. Energy of the sputtered particles on
the substrate surface should attain a certain value in
order to form thin film with (1 0 0) orientation.
Sputtering at higher substrate temperature satisfies this
criterion and it result in the (1 0 0) orientation.
The influence of oxygen incorporation on the
structural properties of the films were studied by
depositing the films at various oxygen partial
pressures. The deposition was carried at a substrate
temperature of 150 8C and an rf power of 30 W.The
XRD pattern (Fig. 3) shows that in the absence of
oxygen, the films orient randomly. Oxygen incorpora-
tion leads to films oriented in the (1 1 1) direction.
According to Kim et al. (1 0 0) orientation is related to
oxygen deficiency [17]. The change in orientation of
the films from (1 0 0) to (1 1 1) texture results from the
incorporation of oxygen into the films.
Fluorine doping in ITO thin films enhanced the
crystallinity (Fig. 3). Doping was carried out by
placing indium fluoride (InF
3
) pellets on the erosion
area of the target. The deposition was carried out at an
rf power of 30 W and a substrate temperature of
150 8C. Incorporation of fluorine resulted in films with
(1 0 0) preferred orientation. This may be due to the
substitutional incorporation of fluorine in the place of
oxygen which arises because their radii are compar-
able. The resulting oxygen deficiency leads to (1 0 0)
oriented films.
The lattice parameter of the films were calculated
using the equation
n
2
d
2
¼
h
2
þ k
2
þ l
2
a
2
An increase in lattice parameter of ITO thin films with
substrate temperature was observed (Fig. 4). The
increase in lattice parameter is attributed to the
increase in repulsive forces arising from the extra
positive charge of the tin cations. Tin is incorporated
into In
2
O
3
lattice as Sn
4+
. In the oxidised state, the
interstitial oxygen anion charge compensate the mate-
rial [18]. As those oxygen anions are removed, which
M. Nisha et al. / Applied Surface Science xxx (2005) xxx“xxx
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Fig. 2. Variation of the ratio of peak intensity of (2 2 2) and (4 0 0)
planes with substrate temperature.
Fig. 3. XRD pattern of fluorine doped ITO thin films and those
deposited at various oxygen pressures. Substrate tempera-
ture = 150 8C and rf power = 30 W.Page 4

UNCORRECTED PROOF
is the case with higher substrate temperatures the
repulsive forces increase, leading to an enlargement
of the unit cell.
In the present case, the increase in substrate
temperature resulted in oxygen deficient films. This
was confirmed by the carrier density measurements,
which will be discussed along with the electrical
properties of the films. Similar effect was observed in
thin films prepared at various oxygen concentrations
(inset of Fig. 4). The lattice constant decreased with
increase in oxygen concentration.
The transmission spectra (Fig. 5) of the films
deposited at various substrate temperatures shows that
the films are highly transparent in the visible region of
the electromagnetic spectrum. The average transmis-
sion in the visible range was greater than 80%. The
transmission in the higher wavelength region decreased
with increase in substrate temperature. This is because
at high deposition temperature carrier concentration
increases because of oxygen deficiency. Higher carrier
concentration results in higher reflection in long
wavelength region. Inset of Fig. 5 shows the absorption
and cut off for glass substrate.
The optical bandgap of the films were determined
by extrapolating the linear portion of the hn versus
(ahn)
2
curve to (ahn) = 0 (inset of Fig. 6). The
absorption coefficient, a, was determined from the
relation, I = I
0
exp(Ãat) where t is the thickness of the
sample, I the transmitted intensity at a particular
wavelength and I
0
the maximum transmitted intensity
which is taken to be 100%. This relation gives a = (1/
t)ln(I
0
/I).
Inthepresentstudyithasbeenfoundthatbandgapof
ITO films increased with increase in substrate
temperature (Fig. 8). The increase in bandgap may
be due to an increase in carrier concentration with
substratetemperatureasaresultofwhichtheabsorption
edge shifts towards the near UV range [19]. The
increase in bandgap with carrier concentration can be
explained on the basis of Burstein“Moss effect.
Assuming that the conduction band and valence band
are parabolic in nature and that B“M shift is the
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Fig. 4. Variation of lattice parameter of ITO thin films with sub-
strate temperature. Inset shows the variation of lattice parameter of
ITO thin films deposited at a substrate temperature of 150 8C with
oxygen concentration.
Fig. 5. Transmission spectra of ITO thin films deposited at various
substrate temperatures. Inset shows the transmission and cut off for
glass substrate.
Fig. 6. Variation of bandgap of ITO thin films with substrate
temperature. Inset shows a typical plot of hn vs. (ahn)
2
for ITO
film deposited at a substrate temperature 150 8C.Page 5

UNCORRECTED PROOF
predominant effect, we can write E
g
= E
g0
+ DE
BÃM
g
where E
g0
istheintrinsicbandgap and DE
BÃM
g
theB“M
shift due to filling of low lying levels in the conduction
band [20]. An expression for B“M shift is given by
DE
BÃM
g
= (h
2
/8p
2
m
Ã
vc
)(3p
2
n)
2/3
where n is the carrier
concentration and m
Ã
vc
the reduced effective mass of the
carriers.FromthisexpressionitisclearthatB“Mshiftis
directly proportional to n
2/3
. However, at very high
carrier concentrations it is seen that there is bandgap
narrowing due to electron-electron scattering and
electron“impurity scattering.
The electrical properties of the ITO thin film
depend on the film composition and deposition
parameters such as substrate temperature, oxygen
pressure, etc. There is a trade off between the carrier
density and carrier mobility for achieving low
resistivity [21]. In the present study it was found that
the resistivity and sheet resistance decreased with
increase in substrate temperature and became a
minimum at a temperature of 150 8C and then
increased on further increase of substrate temperature
(Fig. 7). The carrier mobility and carrier density
increased with increase of substrate temperature and
showed their maximum value around 150 8C and then
decreased (Fig. 8a).
The increase in mobility may be due to better
crystallinity of the film, which increases with the
increase in substrate temperature. The decrease in
resistivity with increase in substrate temperature can
also be explained by the fact that the crystallite
grain size increases significantly with the increase in
deposition temperature, thus reducing grain boundary
scattering and increasing conductivity. The decrease
in resistivity was also associated with the observed
increase in carrier mobility. For the film grown at
higher substrate temperatures >200 8C the resistivity
was found to increase again. This increase may be due
to the contamination of the films by alkali ions from
glass substrates [22].
The increase in carrier concentration may be due to
an increase in diffusion of Sn atoms from interstitial
locations and grain boundaries into the In cation sites.
Since the Sn atom has a valency of 4 and In is trivalent,
the Sn atoms act as donors in ITO films. Hence the
increase in diffusion with substrate temperature results
in higher electron concentration.
The figure of merit (F) proposed by Haake [23] for
transparent conductors for photovoltaic applications
is given by F = T
10
a
/R
s
where T
a
is the average
transmittance in the visible range and R
s
the sheet
resistance of the film. The highest value of figure of
merit was observed for the film deposited at a substrate
temperature of 150 8C(Fig. 8b). The figure of merit for
commercial ITO thin film was 5.9 Â 10
Ã2
square/V.
4. Conclusion
Indiumtinoxidethinfilmswere depositedontoglass
substratebyrfmagnetronsputteringatvarioussubstrate
temperatures. The film orientation showed a depen-
dence on the processing parameters such as substrate
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Fig. 7. Variation of resistivity and sheet resistance of ITO thin films
with substrate temperature.
Fig. 8. Variation of (a) carrier mobility, carrier density and (b) figure
of merit of ITO thin films with substrate temperature. The figure of
merit for commercial ITO film was 5.9 Â 10
Ã2
square/V.Page 6

UNCORRECTED PROOF
temperature and oxygen pressure during sputtering.
The films deposited onto preheated substrates showed a
(4 0 0) plane texturing. Oxygen incorporation led to
orientationofthefilmsinthe(2 2 2)planeparalleltothe
substrate surface. Minimum resistivity of 2 Â 10
Ã3
V cmwithhighestfigureofmeritof2.8 Â 10
Ã3
square/
V was obtained for the films deposited at a sufficiently
lower substrate temperature of 150 8C using pure argon
as sputtering gas.
Acknowledgements
This work was supported by Department of Science
and Technology, Government of India. MKJ wish to
thank Kerala State Council for Science, Technology
and Environment for the financial assistance under
SARD programme. One of the author (NM) thanks
Council of Scientific and Industrial Research for
Junior Research Fellowship.
References
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Hosono, Appl. Phys. Lett. 76 (2000) 2740.
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[4] H. Kim, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H.
Kafafi, D.B. Chrisey, Appl. Phys. Lett. 74 (1999) 3444.
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378 (2000) 81.
[18] G.G. Gonzalez, J.B. Cohen, J.-H. Hwang, T.O. Mason, J. Appl.
Phys. 87 (2001) 2550.
[19] C.G. Granqvist, A. Hultaker, Thin Solid Films 411 (2002) 1.
[20] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G.
Granqvist, Phys. Rev. B 37 (1988) 10244.
[21] H. Kim, J.S. Horwitz, Appl. Phys. Lett. 78 (8) (2001) 1050.
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