Influence of target to substrate spacing on the
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Influence of target to substrate spacing on the
properties of ITO thin films
Aldrin Antony, M. Nisha, R. Manoj, M.K. Jayaraj
*
Optoelectronics Device Laboratory, Department of Physics, Cochin University of Science & Technology, Kochi 682022, Kerala, India
Received 3 October 2003; received in revised form 3 October 2003; accepted 16 October 2003
Abstract
Indium tin oxide thin films were deposited at room temperature on glass substrates by RF magnetron sputtering. The
structural, electrical and optical properties of the films showed a dependence on target to substrate spacing and annealing
temperature. Films deposited with a target to substrate spacing of 4 cm showed the lowest resistivity of 3:07 Â 10
Ã3
O cm and
maximum band gap of 3.89 eV on annealing at a temperature of 250 8C under high vacuum for 1 h.
# 2003 Elsevier B.V. All rights reserved.
PACS: 81.40.Ef; 81.40-z; 78.66
Keywords: Indium tin oxide; RF magnetron sputtering; Annealing
1. Introduction
Thin films of indium oxide doped with tin (ITO) are
commonly used in various applications such as dis-
plays, image sensors and solar cells owing to their
outstanding properties [1]. These films show high
transmission in the visible region and a low electrical
resistivity. Their high conductivity results from the
non-stoichiometry produced by oxygen deficiency and
the introduction of tin as dopant [2]. Because of its
well-matched work function, it is the most commonly
used material acting as hole injector and transparent
conducting anode in organic light emitting diodes [3].
ITO films can be prepared by a wide variety of
techniques such as plasma enhanced metallorganic
chemical vapor deposition (PEMOCVD) [4], ion
assisted deposition [5], sputtering [6], pulsed laser
deposition (PLD) [7], dip coating [8] etc. Sputtering
is one of the effective methods to obtain good quality
ITO thin films. It is superior in both its controllability
and resultant uniformity of the films deposited on a
large area substrate [9]. Reports also show that good
quality polycrystalline ITO films can be grown at
room temperature by adopting PLD technique coupled
with laser irradiation of substrate [10]. Production of
low resistivity films at room temperature is of impor-
tance in high performance flat panel displays (FPDs)
which use heat sensitive substrates such as polymers.
In the present study, ITO films were prepared at
room temperature by RF magnetron sputtering of
ITO target. The deposition was carried out for target
to substrate spacing (T-S spacing) of 4, 6 and 8 cm.
The films were then annealed under high vacuum
(2 Â 10
Ã5
mbar) for 1 h. The structural, electrical
Applied Surface Science 225 (2004) 294“301
*
Corresponding author. Tel.: þ91-484-2577404;
fax: þ91-484-2577595.
E-mail address: mkj@cusat.ac.in (M.K. Jayaraj).
0169-4332/$ “ see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsusc.2003.10.017Page 2

and optical properties were investigated as a function
of T-S spacing and annealing temperature.
2. Experimental
ITO films were deposited on to glass substrates by
RF magnetron sputtering at room temperature using
an ITO target (5 cm diameter) containing 95 wt.% of
In
2
O
3
and 5 wt.% of SnO
2
. Deposition rate was found
to decrease almost linearly from 35 to 12 A/min as
target to substrate distance was increased from 4
to 8 cm. The sputtering time was adjusted in such a
way that all the films studied had the same thickness
irrespective of the substrate to target distance. The
base pressure in the chamber was 5 Â 10
Ã6
mbar.
Sputtering was carried out in argon atmosphere at
a pressure of 0.01 mbar with RF power of 20 w.
The films prepared at room temperature were then
annealed at various temperatures ranging from 100 to
300 8C in high vacuum (2 Â 10
Ã5
mbar) for 1 h.
The thickness of the films was determined by
Tolansky interference technique. In the present
study all the measurements were performed on the
films having the thickness of 2500 A. Electrical
measurements were carried out using a Hall measure-
ment system (Model MMR technology H-50) which
employs four-probe in Vanderpauw configuration.
Transmission spectra of the samples were recorded
using a UV-Vis-NIR spectrophotometer (Hitachi U
3410). The crystallinity of the films were analysed
using an X-ray diffractometer using the Cu Ka radia-
tion (1.5414 A).
3. Results and discussion
Fig. 1 shows the XRD pattern of the ITO films
deposited on glass substrates at various target to
substrate spacings (T-S spacing ¼ 4, 6 and 8 cm).
The substrates were not preheated intentionally. How-
ever, the substrate temperatures rouse upto 55 8C
during deposition when the T-S spacing was 4 cm,
45 8C when the T-S spacing was 6 cm and 40 8C when
the T-S spacing was 8 cm. From the XRD pattern it is
evident that the as deposited films are polycrystalline
even though the crystallization temperature of ITO is
150 8C [11]. All of them showed a peak at 2y ¼ 30
,
which correspond to (2 2 2) plane of In
2
O
3
[12]. This
is because of the greater kinetic energy of the sputtered
Fig. 1. XRD patterns of as deposited ITO films at various target to substrate spacings.
A. Antony et al. / Applied Surface Science 225 (2004) 294“301
295Page 3

particles reaching the substrate surface. Generally,
sputtered particles have kinetic energies of several
electron volts. This kinetic energy enhances the sur-
face migration of sputtered particles arriving at the
substrate surface and the crystallinity of the films are
greatly affected by them. Thus it is possible to deposit
Fig. 2. Variation of (2 2 2) peak intensity and grain size with target to substrate spacing.
Fig. 3. XRD pattern of ITO films prepared with a target to substrate spacing of 4 cm at room temperature and annealed at various
temperatures.
296
A. Antony et al. / Applied Surface Science 225 (2004) 294“301Page 4

polycrystalline films even at room temperature by
sputtering [13]. The crystallinity of ITO films showed
a dependence on T-S spacing. With increase in T-S
spacing the kinetic energy of the sputtered particles
reaching the substrate surface decreases. This retards
the surface migration of sputtered particles and hence
reduces the crystallinity of the films.
The grain size of the films as calculated from
Scherrerâ„¢s formula [14] is in agreement with the above
result. The variation of grain size and the (2 2 2) peak
intensity with T-S spacing is shown in Fig. 2. The
decrease in grain size with increase in T-S spacing
confirms the degradation in crystallinity with T-S
spacing.
Fig. 3 shows the XRD pattern of the ITO films
deposited at a T-S spacing of 4 cm as a function of
annealing temperature. The films showed a peak at
2y ¼ 30
corresponding to (2 2 2) plane and 2y ¼ 51
which corresponds to (4 4 0) plane of In
2
O
3
. With
increase in annealing temperature the intensity of the
(2 2 2) peak increased which is in agreement with the
literature [15].
The crystallinity, transparency, electrical resistivity
and mobility of the films prepared at T-S spacing of 6
and 8 cm showed similar variations on annealing as
that of the variation in these properties for films
deposited at a T-S spacing of 4 cm. However, better
film properties viz. lower resistivity, higher crystal-
linity and better transparency were observed for the
films deposited at T-S spacing of 4 cm in comparison
with the other two T-S spacing. So the discussion
regarding the variation of film properties on annealing
is limited to the case of T-S spacing of 4 cm only.
The transmission spectra of the ITO films for
various T-S spacings are shown in Fig. 4. All the films
irrespective of T-S spacing were highly transparent.
Fig. 4. Transmission spectra of ITO thin films prepared at various target to substrate spacings. Inset shows the variation of bandgap with target
to substrate spacing.
A. Antony et al. / Applied Surface Science 225 (2004) 294“301
297Page 5

The average transmission in the visible region of the
electromagnetic spectrum was >85%.
The bandgap of the ITO films were calculated from
the transmission spectra. By assuming a parabolic
band structure for the material, the absorption coeffi-
cient and bandgap can be related by the expression
ahn ¼ Aðhn-E
g
Þ
1=N
where E
g
is the band gap energy
and a is the absorption coefficient corresponding to
frequency n [16]. The constant N depends on the
nature of electronic transition. In the case of ITO
films N is equal to 2, for direct allowed transition.
The bandgap of ITO films were determined from the
plot of (ahn)
2
versus hn by extrapolating the linear
portion of the curve to ahn equal to 0. In the present
study it was found that the bandgap increases with
increase in target to substrate spacing (inset of Fig. 4).
The effect of annealing on the optical properties of
ITO films was also studied. The annealed films exhib-
ited high transmission in the visible region with long
tail in the IR region. It was seen that the reflecting edge
shift towards the lower wavelength region on anneal-
ing the film at high temperatures in vacuum (Fig. 5).
The shift in reflecting edge is due to increase in carrier
concentration introduced by the oxygen deficiencies
created during annealing.
The band gap of ITO films increased with annealing
temperature, showed a maximum value at 250 8C
(3.89 eV) and then decreased. The variation of band
gap with annealing temperature is shown in the inset
of Fig. 5. The increase in band gap can be explained on
the basis of Burstein-Moss effect [17]. Burstein-Moss
shift is proportional to carrier concentration. Increase
in carrier concentration with increase in annealing
temperature results in band gap widening.
The electrical characteristics of the films also
showed dependence on target to substrate spacing.
Fig. 6 gives the variation of resistivity ® and mobility
(m) with T-S spacing. The decrease in mobility (m) and
Fig. 5. Transmission spectra of ITO films annealed at various temperatures. Inset shows the variation of bandgap as a function of annealing
temperature.
298
A. Antony et al. / Applied Surface Science 225 (2004) 294“301Page 6

the increase in resistivity ® with T-S spacing is
related to the degradation in crystallinity of the films
with T-S spacing. At greater T-S spacing, the smaller
grains increase thegrain boundary scattering of carriers
and hence reduce the mobility of the films, which result
in higher resistivity.
The resistivity and sheet resistance of the ITO films
were found to decrease with increase of annealing
Fig. 6. Mobility (m) and resistivity ® of ITO films deposited at various target to substrate spacings.
Fig. 7. Sheet resistance (R
s
) and resistivity ® of ITO films annealed at various temperatures.
A. Antony et al. / Applied Surface Science 225 (2004) 294“301
299Page 7

temperature (Fig. 7). The lowest resistivity of
3:07 Â 10
Ã3
O cm and sheet resistance of 110 O/sq.
was obtained for the film prepared with a target to
substrate distance of 4 cm and annealed at 250 8C. The
mobility of the ITO films increased with the increase
of annealing temperature whereas carrier concentra-
tion was maximum for ITO films annealed at 250 8C
(Fig. 8).
In ITO, oxygen deficiency is one of the reasons for
high conductivity. Oxygen deficiencies induce free
electrons as conduction carriers [18]. Vacuum anneal-
ing creates oxygen deficiency and this reduces the
resistivity of the ITO films. The increase in carrier
concentration, mobility and crystallinity of the films is
also responsible for the decrease in resistivity. The
films annealed at 250 8C showed a preferred orienta-
tion in the (2 2 2) plane and showed the minimum
resistivity.
4. Conclusion
Tin doped indium oxide films were prepared at room
temperature by RF magnetron sputtering. The effect of
target to substrate spacing and annealing temperature
on the structural, electrical and optical properties was
investigated. The films are found to show a preferential
orientation in the (2 2 2) plane. Highly transparent and
conducting films were obtained for a target to substrate
spacing of 4 cm. The X-ray diffraction pattern of the
ITO films annealed at 250 8C shows the maximum
peak intensity for the (2 2 2) plane. The resistivity of
the film decreased with the annealing temperature and
resistivity is minimum at 250 8C. The carrier concen-
tration and band gap of the film increased with anneal-
ing temperature.
Acknowledgements
The authors wish to thank Department of Science
and Technology for the financial support.
References
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300
A. Antony et al. / Applied Surface Science 225 (2004) 294“301Page 8

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